Hydraulic fluids, particularly higher-viscosity grades, are used as hydraulic andlubricating oils in combined systems.. Zinc-free so-called ZAF hydrau-lic fluids achieve damage load stag
Trang 1318 11 Hydraulic Oils
on the three stationary balls is measured or the load on the revolving ball can beincreased until it welds to the other three [11.42] (see Chapter 19)
Polymer-containing lubricants, high-molecular mass polymer molecules, are used asviscosity index improvers to improve the viscosity–temperature behavior of oils As theirmolecular mass increases, these substances become increasingly sensitive to mechan-ical stress such as that which exists between a piston and its cylinder Several tests areused to evaluate shear stability under different conditions [11.42]: DIN 51 350-6, Four-ball test; DIN 51 354-3, FZG test; and DIN 51 382, Diesel fuel injector method
The drop in kinematic viscosity after shearing provides an indication of the manent drop in viscosity which can be expected during operation (see Chapter 19).The relative viscosity reduction due to shearing after 20 h according DIN 51350-6(Determination of shear stability of lubricating oils containing polymers-taperedroller bearing) is implemented in DIN 51524-3 (2006); recommended shear lossbelow 15 %
The Vickers pump test and a variety of other manufacturers’ pump tests realisticallyevaluate the performance of a hydraulic fluid At present however, alternative tests(such as the DGMK 514 project, Mechanical Testing of Hydraulic Fluids) are beingdeveloped [11.50]
The Vickers test serves to determine wear protection in a rotary vane pump The oil to
be tested is circulated through a rotary vane pump at a given temperature and pressure(the test conditions are 140 bar, 250 h, variable temperature, operating fluid viscosity
13 mm2s–1) After completion of the test the ring and vanes are examined for wear(Vickers V-104 C10 or Vickers V-105 C10) The maximum permissible wear values are
< 120 mg for the ring and < 30 mg for the vanes [11.42] (see Chapter 19)
Hydraulic fluids, particularly higher-viscosity grades, are used as hydraulic andlubricating oils in combined systems Dynamic viscosity is the key wear-protectionfactor in hydrodynamic lubrication At low sliding speeds or high pressures underboundary-friction conditions the wear protection offered by a fluid depends on theadditives used (reactive layer formation) These boundary conditions are replicated
by the FZG test
The test is primarily used to determine the boundary performance of lubricants.Defined gear wheels turning at a defined speed are either splash- or spray-lubricatedwith an oil whose initial temperature is recorded The tooth-flank load is increased
in stages and the appearance of the tooth flanks is recorded This is repeated untilthe final 12th load stage: load stage 10, Hertzian pressure at the pitch point1.539 N mm–2; load stage 11, Hertzian pressure at the pitch point 1.691 N mm–2;and load stage 12, Hertzian pressure at the pitch point 1.841 N mm–2 The startingtemperature at load stage 4 is 90 C, the peripheral speed is 8.3 m s–1, the uppertemperature is not defined; and gear geometry A is used
Trang 2319 11.5 Hydraulic System FiltersThe damage load stage as defined by DIN 51 524-2 is at least 10 ISO VG 46hydraulic fluids which do not contain antiwear EP additives normally achieve loadstage 6 (ca 929 N mm–2) [11.42] (see Chapter 19) Zinc-containing hydraulic fluidsnormally achieve damage load stage 10–11 at least Zinc-free so-called ZAF hydrau-lic fluids achieve damage load stage 12 or greater.
11.5
Hydraulic System Filters
Hydraulic oils are used for very many sensitive industrial manufacturing machines.Because of the use of these oils, hydraulic systems are reliable and are designed torun for years The minimum technical requirements of hydraulic fluids according
to DIN, ISO and manufacturers’ specifications are clearly defined and are generallyfulfilled by the fluids presently available on the market These specifications do not,however, refer to good filterability’ – requirements are not defined
In the past most hydraulic and lubricating oil systems in machine tools, presses,stationary and mobile systems were fitted with 25 to 50-lm filters This mesh sizewas adequate to satisfy the requirements of critical system elements such as valves.Critical hydraulic system components include those with narrow passages and lowflow rates Table 11.11 summarizes the typical gaps and passage sizes in a selection
of hydraulic components [11.51–11.54]
If dirt and contaminants are present in the oil, these critical gaps can influencethe function of the system, and wear rates Experts differ on the size and amount ofparticles which constitute a critical situation
11.5.1
Contaminants in Hydraulic Fluids
There are several types and causes of hydraulic fluid contamination The first major ferentiation is between primary and secondary contamination Primary contamination
dif-is that which exdif-isted in the hydraulic circuit before it was commdif-issioned Thdif-is caninclude machining residues, assembly residues, and fresh-oil contaminants The sec-ondary variety is that formed after the system began to operate, e.g mechanicallyabraded material, flow-related abrasion, corrosion, wear and dirt which enters the sys-tem via cylinder seal materials or via tank de-aerating units [11.6, 11.51–11.54]
After comprehensive trials by a leading manufacturer into the effect of nants on the life of roller bearings great value is now placed on the cleanliness andfiltration of oils Purity and the types of additives used have a significant influence onthe life and likelihood of failure of roller bearings and thus a whole system [11.52, 11.53,11.55] Trials conducted within the framework of the FVA 179/1 research project Influ-ence of Foreign Particles on Roller Bearings and Measures to Avoid Them’ also investi-gated this subject The causes of premature roller bearing failures are, above all, inade-quate lubrication, particle contamination, and overloading
Trang 3contami-320 11 Hydraulic Oils
Tab 11.11 Hydraulic component clearance.
Gear pumps (under pressure)
Gear to side plate
Gear tip to housing
0.5–5 0.5–5 Vane pumps
Vane tip to stator
Vane to side plate
0.5–5 (1) 5.0–13 Piston pumps
Piston to cylinder
Cylinder to valve plate
5.0–40 1.5 (0.5)–10 (5) Servo valves
Jets
Splash care
Piston valve (radial)
130.0–450 18.0–63 2.5–8 Control valves
Hydrostatic slide bearings 0.5–100.0 lm
Hydrodynamic slide bearings 1.0–25.0 lm
11.5.2
Oil Cleanliness Grades
Several methods can be used to classify oil cleanliness The best known are ISO 4406and NAS 1638 Determining oil cleanliness according to ISO 4406 involves examiningthe number and size of particles in a 100-mL sample of fluid The number of particles inthe categories > 2 lm, > 5 lm, and > 15 lm are recorded Normally only particles
> 5 lm and > 15 lm are reported (old commonly used practice) The new specificationISO 4406 (December 1999) defines the particles in the categories > 4 lm, > 6 lm, and
> 14 lm The particles can be counted with a microscope or by suitable automatic cle counters ISO 4406 or NAS 1638 defines the maximum permissible contaminationaccording to the type of hydraulic system, how sensitive it is, and which critical compo-nents form part of the system Depending on the operating conditions, the cleanlinesscategories in Table 11.12 are recommended [11.6, 11.17, 11.22, 11.55]
Trang 4parti-321 11.5 Hydraulic System Filters Tab 11.12 Cleanliness categories
Type of system / case of application /
filter size
Cleanliness category
in accordance withISO 4406
Cleanliness category
in accordance withNAS 1638
Against fine soiling and mudding-up of
sensitive systems; servo hydraulics
Heavy duty servo systems, high-pressure
systems with long service life
Proportional valves, industrial hydraulics
with high operating safety
Mobile hydraulics, common mechanical
engineering, medium pressure systems
. tank vent filters to clean any drawn-in air;
. pressure filters to clean the fluid entering the pump;
. top-up filters which filter the hydraulic fluid as it is being fed into the tank;. by-pass filters in the tank circuit to improve the cleanliness levels; and
. return filters fitted to fluid return lines
The filters can be of the cartridge or surface variety Important data are the meshand retention size of the filter (the designation b3 > 200 describes a filter of 3-lmmesh size and a separation rate of 200, i.e only one particle of 200 particles willpass the filter) In addition, the initial pressure difference (DP max 0.1–0.2 bar) andthe maximum output pressure difference (DP max 3–5 bar) in relation to the flowrate, viscosity, and density are of importance The primary filter materials are micro-fiberglass, metal meshes, cellulose paper, and some other constructions Hydraulicfilters consist of an element, a housing, a contamination indicator, and other compo-nents In general the fluid flows from the outside to the inside The selection ofmesh size is a matter of experience and depends on the specific requirements ofcritical components As a rule, hydraulic systems use filter mesh sizes ranging from
3 to 40 lm [11.55, 11.56] When using filters with micron ratings of e.g., 1 lm,
3 lm, and 6 lm, attention has to be taken Especially high molecular components
of the fluids (e.g VI-improvers) and contaminations (e.g grease, corrosion tives) can block the filters Filter blockage can occur if additiv systems are incompa-tible (e.g mix of incompatible zinc-containing and zinc-free additives)
Trang 5preven-322 11 Hydraulic Oils
11.5.4
Requirements of Hydraulic Fluids
High-performance filtering systems make high demands on the filterability ofhydraulic fluids A hydraulic fluid should only generate a small pressure differenceacross the filter after long-term use Base oils and additives should be easily filter-able with filter mesh sizes of 1, 3, 6, and 10 lm Nothing in the fresh fluid shouldcause the filter to block and thus reduce its life (this is examined by special labora-tory tests) Naturally, the purity of the fresh fluid should be low
According to ISO 4406 the cleanliness of drums should be 17/14 (19/15) andexperience shows that the cleanliness of road tankers should be 15/12 (18/14), al-though transport, storage, and environmental influences generally cause the cleanli-ness factor to deteriorate by 2 to 3 categories In practice poor filter life often resultsfrom contamination of the hydraulic fluid with water, dirt, and other fluids, frominadequate maintenance of the system, or from incorrect filter selection Determin-ing the exact cause normally requires expensive laboratory tests [11.55, 11.56]
11.6
Machine Tool Lubrication
11.6.1
The Role of Machine Tools
Machine tools are the most important machines in the metalworking industry With
a share of approximately 20 %, Germany is one of the world’s leading manufacturers
of machine tools In terms of sales, Germany (DM 14 billion) is second to Japan (ca
DM 16 billion) but ahead of the USA (DM 9 billion), Italy (ca DM 6 billion) andSwitzerland (ca 4 billion) [11.57] Machine tools are used for a wide variety of opera-tions including forming, cutting and bending; they are principally for turning,milling, drilling, grinding and machining center They can combine any of these in
a transfer system [11.58] Machine-tool construction is a major sector in engineeringand their share of overall exports for Germany (about 60–70 %) and Japan illustratethe importance of machine tools in national economies [11.59]
11.6.2
Machine Tool Lubrication
This section covers lubricating oils, hydraulic fluids, and greases for machine tools.Apart from cutting fluids, hydraulic oils are volumetrically the most significantgroup of machine lubricants, followed by slideway oils and gear oils Neat or water-miscible cutting fluids or metalworking fluids are covered in Chapters 14 and 15.The lubrication of machine tools is described in DIN 8659-1 and –2 and ISO 5169and ISO 3498 These standards contain requirements which should be observed
Trang 6323 11.6 Machine Tool Lubricationwhen manufacturers and users establishing lube plans These also satisfy therequirements specified in DIN/ISO 5170 (machine tool lubrication systems) [11.60].Lubrication plans should cover all the components in a machine tool which needlubrication These should describe:
. the precise location of all lubrication points;
. the type of lubrication required;
. the lubricant itself according to DIN 8659-1 and –2 and ISO 3498 and thetank volume; and
. the lubrication timetable
The purpose of a lubricating plan as part of routine servicing is to ensure that asufficient quantity of the correct lubricant is applied to the right point at the righttime (VDI Guideline 3009) Machine manufacturers normally supply lubricantrecommendation tables with machines These list the type of lubricant according toDIN 51 502, ISO 6743, and ISO 3498 for each viscosity grade by its brand name Onthe basis of this information, a maintenance plan is created for every machinewhich shows the type of lubricant and the lubrication interval For most machines, amaintenance plan and a lubrication chart are included in the service handbook
Figure 11.17 shows an example of a lubrication plan for a centerless grindingmachine This shows lubricants conforming to DIN 51 502 and ISO 3498, lubrica-tion intervals, tank volume, and the location of all lube points
Lubricant recommendations should be updated every two years to make use ofnew lube developments Technically similar lubricants can often be grouped toenable some lubricant rationalization [11.61]
Machine manufacturers often refer to the lubricant recommendations issued bycomponent manufacturers The recommendations issued by the manufacturers ofhydraulic components, gearboxes, slideways, and linear guides must be observed.For lubrication a machine tool can be divided into a number of major elements:hydraulic unit, gearbox, spindle, slideway, linear system, plain and roller bearingsand finally, cutting zone lubrication In general, a different lubricant is recom-mended for every component, i.e at least seven different types and viscosities oflubricant (excluding the cutting fluid) are required
11.6.3
Machine Tool Components – Lubricants
Most hydraulic equipment is designed to use HLP (HM), HLPD (HG) fluids with
an ISO viscosity between 32 and 46 Running temperatures range from 40 to 60 Cand peak temperatures of 60 to 80 C can occur [11.58, 11.59] Although operatingpressures range from 50 to 100 bar (relatively low), pressures up to 400 bar are used
in clamping fixtures Generally low system pressures are used to avoid chattermarks (compressibility of the fluid) which often occur at higher pressures More-over, higher pressures lead to more leakage and thus lower overall efficiency [11.58].Figure 11.18 shows a list of hydraulic oils used in machine tools (survey of 12 Ger-
Trang 7324 11 Hydraulic Oils
man machine tool manufacturers, 1995) [11.59] HLPD fluids are often used tosolve friction and compatibility problems
Rotary vane and internal gear pumps are used at pressures between 50 and
100 bar Higher pressures are generated by radial and axial piston pumps Externalgear pumps are seldom used because of the noise they generate Figure 11.19 showsthe types of pump used in machine tools (survey of 12 German machine tool manu-facturers, 1995) [11.59]
Figure 11.20 shows the pressures used in machine tools
Actuator valves, sleeve valves, shut-off valves, and throttle valves are used inmachine tools Many valves have hydrodynamic bearings which make them sensi-tive to stick–slip effects, contamination, and deposits [11.58–11.62]
Machine tool hydraulic systems are normally equipped with mesh or fiber filters.Approximately 80 % of machine tool manufacturers use filters in the 5 to 10 lmrange; the remaining 20 % use filters up to 25 lm [11.59]
Depending on the type of valves used, the pressure, and the importance of themachine, the ISO 4406 cleanliness of the fluids should be between 15/11 and 17/13
or lower according to ISO 4406 [11.28, 11.55, 11.59]
Lubricant
Tank Volume
Lubrication Chart Cylindrical Grinding Machine
sight glass sight glass
• a - centralized lubricating system
• b - grinding wheel spindle bearing
• c - slideway ( table )
• d - slideway ( dressing tool )
• e - spindle bearing
• f - worm gear
• g - slideway ( dressing tool )
• h - slideway ( grinding tool )
• 2* - speed 30-45 m/s
• 2 ** - speed 45-60 m/s
Fig 11.17 Lubrication chart of a machine tool.
Trang 8325 11.6 Machine Tool Lubrication
41.0 % HLP 46
18.0 % HLPD 32
21.7 % radial piston pumps
13.0 % axial piston pumps 34.8 % internal
gear pumps
8
Fig 11.19 Hydraulic pumps used in machine tools.
Pressure range
80 % of all machine tools are working in a pressure range between 50 and 100 bar
Minimum viscosity of hydraulic fluids
Vane pumps:
Normally, a viscosity of min 15 mm 2 /s at pressures up to 100 bar is required
-low viscosity fluids are currently being developed
Piston pumps:
Today, they are available for low viscosity fluids - but expensive
need a lot of assembly volume
Fig 11.20 Working conditions used in machine tools.
Trang 9326 11 Hydraulic Oils
Machine tool slideways which guide supports and workpieces are among the mostimportant elements of a machine tool The special demands made on these slide-ways include precision, high performance, low manufacturing costs, and low operat-ing costs The most important features of slideways are:
. low friction, no stick–slip at low feeds and high load-carrying capacity;. low wear and ultimate reliability against seizures;
. torsional stiffness and minimal play; and
. good damping properties to reduce chatter marks on machined surfaces
In general, hydrodynamic, hydrostatic, and roller guides are used Aerostatic andelectromagnetic guides are seldom found in machine tools Hydrostatic guides arelosing popularity because of their price but can still be found on many machines.These days, hydrodynamic and linear roller guides (linear systems) are often used.Hydrodynamic slideways are losing market share because they only enable relativelysmall feed velocities (maximum 0.5 m s–1), often suffer from stick–slip, and aremore expensive to manufacture than linear roller guides The most common mate-rial pairings used in hydrodynamic slideways are cast iron–cast iron, cast iron–plas-tic, cast iron–steel, and steel–plastic Slideway oils should conform to DIN 51 502,ISO 6743-13, and ISO 3498 [11.63] Horizontal slideways are often lubricated withCGLP 68, HG 68 or G 68 slideway oils Inclined or vertical slideways are lubricatedwith CGLP 220, HG 220 or G 220 oils The oil is applied through central systemsand is lost after use Slideway oils are general lubricating oils with additives toimprove oxidation and corrosion protection They also contain anti-wear agents, EPadditives, surface-active substances and often adhesion improvers (tackifyers)
In recent years, roller or linear guides have been fitted increasingly to machinetools In 1995, nine of twelve German manufacturers surveyed used roller linearguides exclusively and four used hydrodynamic and roller guides The lubricantsused should separate the moving parts in the roller in the contact zone which coun-ter-rotate The lubricant should also have damping characteristics in the contact zone(especially when the direction of movement changes) and reliably protect against wearand seizures The lubricant should also form a stable and effective film in a very shorttime Such total-loss lubricants are supplied to the linear guide zones via a central sys-tem CGLP 68 and CGLP 220 grades are often used [11.60] High-viscosity CGLP 220slideway oils which contain surface-active components are often recommended Alter-natively, K2K or similar greases can also be used (see Chapter 16)
Oils for hydrodynamic slideways and linear guides should have the followingproperties [11.58, 11.59]:
. chemical compatibility with all cutting fluids used;
. good demulsification of emulsions, no sticky residues on slideways;
. low coefficient of friction (static and dynamic);
. avoidance of stick–slip (sliding and static friction alternates during stick–slip
on slideways, which can cause chatter marks);
. good pumpability in central lubrication systems;
Trang 10327 11.6 Machine Tool Lubrication good adhesion to slideways with tacky additives and/or without tacky addi-tives;
. good wear protection (EP and AW additives) FZG ‡ 12;
. good slideway material compatibility;
. good corrosion protection (no black stains on slideways);
. same additive systems as hydraulic oils (i.e zinc-, ash- and silicone oil-free); and. meet the specifications of hydraulic oil if hydraulic and slideway oils share acircuit
The function of spindles is to guide the tool and/or workpiece at the cutting zone Inaddition, spindles should absorb external forces The accuracy and surface quality ofcomponents made on machine tools depends on the static, dynamic, and thermalbehavior of the spindle bearings These are key elements of machine tools Toolspindles can be supported on greased roller bearings, oil-lubricated roller bearings,
or hydrodynamic plain bearings Roller bearings have almost completely replacedplain bearings Oil-lubricated roller bearings are normally fed total-loss oil from acentral system or via an oil-mist system Often, low-viscosity CL/CLP general lubri-cating oils according to DIN 51 517 or ISO VG 5–22 FC and FD spindle oils accord-ing to ISO 6743-2 are used Spindle oils must lubricate and cool They have to pro-tect against steel and copper corrosion, and be oxidation-stable Depending on theapplication, lubricants with AW/EP additives are used The spindle speed, defined asthe product of rpm (min–1) average bearing diameter (mm) determines whether aspindle should be lubricated with oil or with grease [11.31, 11.58, 11.64]
Gearboxes are designed to convert and transfer movement and forces – they areunits which transmit energy Gearboxes in machine tools serve to reduce drivespeed to the feed velocity of supports, etc The gearboxes can have fixed or selectableratios Speed adjustments are often made with synchronized or non-synchronizedmotors The different gearboxes include spur, worm, crown-wheel and pinion or pla-netary types [11.58] The stress on machine tool gearboxes is relatively small andISO VG 68 to 320 CLP (DIN 51 517 – dated January 2004), CKC or CKD (ISO 6743/6) gear and general lubricating oils are often used Worm drives are often lubricatedwith polyglycol-based CLP PG or CKE gear oils Synthetic, polyalphaolefin-basedCLP HC or CKT oils are used for thermally stressed gearboxes [11.31, 11.58, 11.60].The bearings most often found in gearboxes are plain and roller bearings, al-though plain bearings are seldom used in machine tools The most popular typesare ball and cylindrical roller bearings The corresponding lubricants are generallubricating oils or specific gear oils (see Chapter 10)
Trang 11328 11 Hydraulic Oils
11.6.4
Machine Tool Lubrication Problems
Many different oils and greases are used in a machine tool Hydraulic, gear, way, and spindle oils, and greases, are the most important groups An importantpoint is the required compatibility of the lubricants with each other Leaks can causelarge amounts of hydraulic oils to contaminate cutting fluids (every year 3 to 4 timesthe volume of the hydraulic system enters the cutting fluid circuit [11.59]) At thesame time, metalworking fluids can enter the hydraulic oil circuit via cylinders, etc.Slideway oils as total loss lubricants and metalworking fluids are in close contactand must, therefore, be compatible Developments in the area of universal oils(chemically related lubricants) which eliminate the problem of poor compatibilityare currently a high priority Initial developments of fluid families, i.e hydraulic,gear, slideway, spindle, and metalworking oils which share common additivepackages but which are available in different viscosities have already been tested inpractice [11.59, 11.65] Low-viscosity neat oils have already overtaken water-miscibleemulsions (the trend is oil instead of emulsion) [11.5, 11.65] Future developmentswill concentrate on Unifluid systems which consist of one low viscosity oil whichserves as cutting fluid, slideway oil, and hydraulic fluid This concept, however,requires the redesign of components such as pumps to handle such low-viscosityfluids The development of compatible systems could save large amounts of moneynow spent on the expensive monitoring and maintenance of currently-used water-based cutting emulsions (see Chapter 14)
Base Fluids
Typical hydraulic fluids are composed of 95–98 % base fluids and 2–5 % additives
As already mentioned above, the largest contributor to base fluids are mineral oilsrefined from crude oil (mainly paraffinic and naphthenic compounds and hydrocracked base oils) Other base fluids used are, basically, fully and partially saturatedesters, polyglycols, polyalphaolefins (PAO), and alkylates
Trang 12329 11.6 Machine Tool LubricationAdditives
The most important additives for hydraulic fluids are listed in Tab 11.13
Tab 11.13 Most important types of additive in hydraulic fluids, and their chemistry.
Antioxidants (AO) Phenolic and aminic AO, zinc dialkyldithiophosphate
(ZnDTP) Copper deactivators Nitrogen compounds (triazoles), dimercaptothiadiazoles
Steel/iron corrosion inhibitors Carboxylic acid derivatives, sulfonates, succinic acid
compounds Anti-wear additives (AW) Esters, ZnDTP
Extreme pressure additives (EP) Phosphorus and sulfurous compounds, thiophosphates,
sulfurized hydrocarbons (active and inactive) Friction modifiers Fatty acids, polar compounds, esters
Detergents/dispersants Ca and Mg phosphates, sulfonates, phenates,
polyiso-butylensuccinimide Antifoam agents Silicone oil, silicone oil-free, polymethylsiloxane
Viscosity index improvers (VII) Polymethacrylates
Pour-point depressants (PP) Polymethacrylates
Tackifiers Polyisobutylenes, polar compounds
Approximately 70 % of mineral oil-based hydraulic fluids in Europe are taining fluids, although an increasing number of zinc-free hydraulic fluids arebecoming available These fluids are formulated without zinc dialkyldithiopho-sphate (ZnDTP) as multipurpose additive Although this has a large affect on theperformance of hydraulic oils, it is important to state that the element content itselfhas no decisive effect on the quality of hydraulic fluids Zinc-free hydraulic fluidsusually contain similar or even lower amounts of phosphorus and sulfur than Zn-containing fluids (Tab 11.14) Traditional Group I mineral base oils have an incor-porated sulfur content that contributes positively to anti-wear performance andsynergistic functions, e.g as a radical scavenger for oxidation stability Additionally acertain aromatic content that is typical for Group I base oils improves solubility ofadditives and ageing products New hydraulic fluids using modern base oils (hydro-treated, hydro-cracked or PAO) have a much lower or zero sulfur and aromatic con-tent and therefore need consequently higher additive treat rates to balance the short-fall of base fluid related sulfur and aromatic content
Trang 13zinc-con-330 11 Hydraulic Oils
Tab 11.14 Variation of element content of zinc-containing and zinc-free hydraulic fluids.
Sulfur (base oil) 300–1000 ppm 300–1000 ppm
Selected characteristics of hydraulic fluids will be discussed in subsequent sections Theuse of Zn-free or Zn-containing additives usually makes a remarkable difference to char-acteristics and performance and can also crucially affect whether a hydraulic fluid isformulated with demulsifying or detergent/dispersant characteristics
Although the Vickers vane pump test is a traditional and important test for lic fluids, the test results obtained are not sufficiently differentiating for state-of-the-art hydraulic fluids Other commonly used EP/AW test methods include use ofthe Shell four ball tester (DIN 51 530-1,2), use of the Brugger test machine(DIN 51 347-2), and use of the FZG test rig (DIN 51 354-2) Tests on hydraulic fluidscontaining different additives (all ISO VG 46) have been conducted with these meth-ods The results showed that Zn-free hydraulic oils of the detergent/dispersant type(DD) usually have better EP performance than traditional Zn-containing demulsify-ing hydraulic oils and other hydraulic oils (Tab 11.15)
hydrau-A relatively new test rig for hydraulic fluids is the so-called FE 8 of Fhydrau-AG for thedetermination of roller bearing wear and friction coefficients Although this proce-dure is normally used for greases and gear oils (DIN 51 819-3), some OEMs request
a “pass’’ for their in-house approval of hydraulic oils This test sequence is passedmainly by Zn-free formulated hydraulic fluids One can, in general, state that phos-phorus/sulfur chemistry without Zn is more suitable for high-EP requirements (e.g.hydraulic systems with incorporated gear drives) Fluids formulated with Zn-basedadditives perform well in the mixed friction applications where only medium EP butgood AW is required Their multifunctional anti-wear performance is thereforeusually better that of than Zn and ash free fluids
Some OEMs set minimum limits for the dirt-carrying properties of hydraulic oils AFuchs in-house test reported by DaimlerChrysler [11.66] entails use of a paper chro-matographic method in which a colloidal graphite dispersion is applied to a paperstrip that is dipped in the lubricant Migration of the graphite under specified condi-tions is reported The in-house limit of the OEM stipulates a minimum migrationdistance of 40 mm Only hydraulic fluids with selected dispersant additives and ele-vated treat rates pass this test (Fig 11.21)
Trang 14331 11.6 Machine Tool Lubrication Tab 11.15 Typical EP/AW test results for different types of hydraulic fluid.
Zn-containing, DD, synthetic base oil
Zn-free, synthetic base oil, demulsifying
DD conventional
DD conventional
Trang 15A series of tests using an inclination tribometer have been performed at SKC technik in Coburg (Germany) The SKC test procedure was originally developed forslide-way oils and determines the coefficients of friction of the fluids for a combina-tion of plastic and steel Static coefficients of friction for detergent/dispersanthydraulic fluids were between 0.130 and 0.177, lower than those of demulsifyingfluids (0.214) Excellent values of 0.088 to 0.11 where achieved with synthetic esters,because of to their polar characteristics.
to a severalfold increase in fluid lifetime
Tab 11.16 Examples of TOST test results.
Minimum test results to fulfill DIN 51524
Mineral oil, demulsifying, low treat rate
Mineral oil, DD, medium treat rate
Mineral oil, DD, high treat rate
Hydro cracked base oil, DD, high treat rate
is also being introduced Some OEMs require a maximum shear loss of 15–20 % inthis test Because the viscosity of conventional HVLP oils based on standard VIimprovers drops by 30–50 % at 100 C, conventional HVLP oils must be reformu-lated by using different additive chemistry, which increases the cost An alternativemethod of formulation is to use base oils which already have in-built natural, shear-stable, high VI Test results can be found in Tab 11.17
Trang 16333 11.6 Machine Tool Lubrication Tab 11.17 Test results of high-VI hydraulic oils: Tapered roller bearing shear-stability test DIN 51 350-6 (20 h, 60 C, 40 mL)
PAOHVLP 46
HEES saturatedHVLP 46
Mineral oil,conventionalHVLP 46
Hydro crackedHVLP 68
In many stationary hydraulic applications multi-grade engine oils are used instead
of regular hydraulic fluids SAE 10W40 and SAE 15W40 are very common Theseproducts have viscosities >95 cSt at 40 C and are formulated with polymers subject
to high shear losses that result in critical decreases in viscosity
The filterability of Zn-free hydraulic oils depends very much on the type of try used A special multi-pass test rig has been designed for dynamic testing Differ-ent pressures in the presence of 1 % water (for better differentiation) are recordedover time Depending on the formulation strategy extremely different filtrationbehavior can be observed Under the practical conditions used with this test riglong-lasting and filterable hydraulic fluids have been developed Test results for avariety of Zn and ash-free hydraulic fluid formulations are shown in Fig 11.22 Inpractice, the compatibility of Zn-containing and Zn-free hydraulic fluids shouldusually be tested before they are mixed
Electrostatic phenomena usually depend on the fluids used and on the filter als used for deep filtration Because electrostatic charges in fluids depend mainly onthe conductivity of the fluids, the risk potential of electrostatic charges in hydraulicfluids depends very much on the additives used (Tab 11.18) Metal-containing fluidswith zinc, calcium, or magnesium compounds as additives are believed to be freefrom problems, because their conductivity is >300 pS m–1 The additives conductelectrostatic charges to the equipment housing Although Zn-free and metal-freehydraulic fluids behave differently and do not conduct electrostatic charges, properselection of Zn-free additives and advanced treatment procedures enable adequateconductivity to be achieved Needless to say, for pure HL oils without any EP/AWadditives conductivity is very low and must therefore be regarded as critical AVDMA working group is actively investigating this phenomenon [11.67]
Trang 17Fig 11.22 Multi-pass filterability test results for different
Zn-free hydraulic fluids (8.7 L min -1 , 65 C, 100 bar,
15 L volume, 3 mm/6 mm).
Tab 11.18 Typical conductivity data for different types of hydraulic oil.
Mineral oil, demulsifying, Zn-containing 200 pS m –1 800 pS m –1
Mineral oil, DD, Zn-containing 470 pS m –1 2000 pS m –1
Mineral oil, DD, Zn-containing 8.000 pS m 40 000 pS m Mineral oil, demulsifying, Zn and ash-free 4 pS m –1 17 pS m –1
Mineral oil, DD, Zn and ash-free 140 pS m–1 690 pS m–1Mineral oil, DD, Zn and ash-free 360 pS m –1 1100 pS m –1
When low-performance zinc and ash-free hydraulic oils are used instead of taining fluids a new phenomenon, micro scratching, very small scratches equallydistributed all around the piston in the axial direction, is occasionally observed
zinc-con-Some discoloration also occurs and surface roughness reaches peaks of 2 mm pared with 0.1 mm for undamaged surfaces Examples of damaged and undamaged
Trang 18com-335 11.6 Machine Tool Lubricationpistons are shown in Fig 11.23 The fluid-related reason for this micro scratching isthe use of zinc and ash-free hydraulic oils with low EP/AW performance A specialpiston test rig at Busak + Shamban in Stuttgart (Germany) has been used to charac-terize different characteristics of hydraulic oils with regard to micro scratching[11.68] The conclusion was that Zn-containing hydraulic fluids and Zn-free hydrau-lic fluids containing high levels of selected EP/AW additives do not usually lead tomicro scratching Film forming properties also affect the behavior positively anddetergent/dispersant type hydraulic fluids are therefore recommended in combina-tion with zinc and ash-free chemistry.
Fig 11.23 Pistons without and with micro scratches.
European Eco-Label “Margerite’’ with EU Directive 2005/360/EC was introduced in
2005 For biodegradable hydraulic fluids it is based on technical specificationISO 15 380 Besides requirements for biodegradability, sustainability is alsorequested – 50 % of the raw materials must be derived from renewable materials
ISO 6743-4 also was recently updated in respect of technical requirements formineral oil-based hydraulic fluids
The variety of available hydraulic fluids are vast, as are their individual technicalrequirements Each application, with its ambient conditions, must therefore be con-sidered in detail when selecting the optimum hydraulic fluid The optimumdepends on many different fluid characteristics It may be the choice between Zn-containing and zinc and ash-free hydraulic fluids, synthetic base oil or mineral baseoil, high additive performance or not, demulsifying properties or detergent/disper-sant fluid characteristics The proper choice affects the performance, lifetime, avail-ability, and cost effectiveness of machinery and its hydraulic equipment Hydraulicfluids are important liquid machine-tools
Trang 19con-Hydraulic oils are key elements in machines and machine tools must therefore
be included in the planning of plant and equipment to reflect the different ties offered by the various types of fluid
proper-Tab 11.19 Important properties of hydraulic fluids (viscosity class ISO VG 46).
Mineraloil
Emulsion/
solution
Polymersolution
Syntheticester
Syntheticester
idesrapeseed oil
ca 0–80 (low)
140–190 (very good)
200–220 (very good) Average compression
Trang 20337 11.7 Summary Tab 11.19 Continued.
Mineraloil
Emulsion/
solution
Polymersolution
Syntheticester
Syntheticester
idesrapeseed oil
HEES*
HETG
Pourpoint [ C] < –18 0 < –30 < –18 < –30 ca –25 Bunsen coefficient,
Relative costs of the
fluid [ %]
dependant
on centration
con-200–300 800–900 300–600 200–250
Market share [ %] ca 88 < 1 ca 4–5 < 1 < 1/2–3 ca 1–2
Literature: Properties according to VDMA 24317, August 1982
Hydrostatische Pumpen und Motoren, J u M Ivantysyn,
Vogel Verlag, Germany
Grundlagen der Fluidtechnik, H Murrenholt, IFAS, Aachen,
Trang 2112.1
Air Compressor Oils
Wolfgang Bock and Georg Lingg
Compressors increase the pressure of air or any gaseous medium in one or morestages and thus transfer energy to the medium As the volume of the medium isreduced, its temperature and density increase
There are two types of compressor: displacement compressors and dynamic pressors In displacement compressors, the gaseous medium is drawn into a cham-ber, compressed and expelled by a reciprocating piston The principle of dynamiccompressors is that turbine wheels accelerate a medium which is then abruptly de-accelerated [12.1] In the past 25 years, traditional piston compressors have increas-ingly been replaced by rotary compressors and in particular screw compressors sothat the market share of screw and rotary vane compressors is presently about
com-> 60 % The reasons for this are the light weight and compact dimensions of rotarycompressors, there low-noise and vibration-free operation and their reliability.Rotary compressors are generally characterized by the constant, relatively lower-pressurization of larger volumes of air while piston compressors provide pulsatinghigher-pressurization of smaller volumes
Compared to piston compressors in which the oil primarily lubricates the bearings,pistons, cylinders and valves, the oil in oil-flooded screw and rotary vane compressorsalso has the additional function of cooling and sealing It is possible to differentiatebetween air and gas compressors, vacuum pumps and refrigerant compressors by ana-lyzing the function of the oil Figs 12.1 and 12.2 show a breakdown of compressorsaccording to their construction and to their operative range [12.2, 12.3]
12
Compressor Oils
Lubricants and Lubrication 2nd Ed Edited by Th Mang and W Dresel
Copyright 2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim
Trang 22339 12.1 Air Compressor Oils
Trang 23340 12 Compressor Oils
12.1.1
Displacement Compressors
Reciprocating piston compressors increase the pressure of the medium by ing the volume of its chambers The air or gas is compressed and displaced by therising and falling of a piston in a sealed cylinder The compression and pumping ofthe medium results from the periodic (oscillating) change in volume Each stage ofthe process is controlled by inlet and outlet valves The compressor is cooled either
decreas-by air circulating around the ribs attached to the cylinder head or decreas-by water whichflows through a jacket around the cylinder [12.2, 12.3]
In reciprocating piston compressors, the piston is connected to the crankshaft by a necting rod In general, the cylinder and the drive share the same splash lubricationfrom oil in the crankcase In larger compressors, the pistons are driven by cross-headrods (cross-head compressors, single or double action) In such compressors, the drive
con-is lubricated by crankcase splash and separately from the cylinders [12.2]
The cylinders in a piston compressor represent the most difficult task for thelubricant and ultimately decide the choice of lubricants The lubricant’s primarytasks are the reduction of friction and wear, sealing the compression chambers andprotection against corrosion The peak stress occurs at the TDC and the BDC (topand bottom dead center) At these points, there is a danger of the lubricant filmtearing and allowing metal-to-metal contact The oil is also subject to enormousstress resulting from the high temperatures created when the medium is com-pressed (which can cause oxidation and lead to deposits) and in the case of air, the
A
C B
A = one stage reciprocating piston compressors (air cooled)
B = two stage reciprocating piston compressors (air cooled)
C = one stage screw compressors (oil flooded)
D = two stage (double acting) reciprocating piston compressors (water cooled)
E = four stage turbo compressor (free of oil)
Figure: Mahle Druckluft, Germany Fig 12.2 Classification of compressors according to their operative range.
Trang 24341 12.1 Air Compressor Oilsoxygen enrichment The cleanest possible air or gas should be compressed becausecontaminants can accelerate oxidation and wear (especially the water content of theair/gas and other contamination – e.g aggressive gases from the surroundings caninfluence the performance of the used lubricant in an extremely negative way) Inthe case of drive unit lubrication, the lubrication of the bearings is of primary impor-tance.
Piston compressors are available in lubricated-with-oil and oil-free versions mally, lubricants based on mineral oil according to DIN 51 506 – VCL, VDL (orPAO- or diester-based lubricants) are used with viscosity grades of ISO VG 68 toISO VG 150 Mobile compressors are often lubricated with monograde engine oil(SAE 20, SAE 30, SAE 40) [12.4, 12.5] Small to medium-sized piston compressorsare used for pressures up to 10 bar
In these compressors, the volume of the pressure chambers varies periodicallybetween two extremes The eccentric cylindrical rotor which is located in the cylind-rical sleeve of the housing has sliders (normally steel or PTFE) fitted in grooveswhich alter the star-shaped chamber between the rotor and the housing When therotor turns, centrifugal force presses the sliders against the wall of the housing Dur-ing rotation, the enlargement of the chamber on the inlet side draws air in and asthe volume gets smaller the medium is compressed until it is expelled through theoutlet The inlet and compression steps are controlled by slots in the housing
In single-action operation, pressures of up to 10 bar are possible and up to 16 bar
in double-action operation Volumes can reach 80 m3min–1 The advantages ofrotary piston compressors are compactness, continuous flow and vibration-freeoperation compared to piston compressors [12.1, 12.2, 12.6]
The pressure chambers of rotary piston compressors are cooled and lubricated bytotal-loss systems or by direct oil injection The lubrication of rotary piston compres-sors is similar to the lubrication of the cylinders in reciprocating piston compressorsinsofar as the lubricant is subject to high outlet temperatures in both cases In thecase of oil-injected and oil cooled rotary piston compressors, a quantity of oil is con-tinuously injected into the compressor chambers The quantity of the oil is suchthat the outlet temperature does not exceed 100 C – 110 C At the same time, itseals the pistons against the housing and protects against wear The cooling of themedium results in an increase in compression performance Cooling and sealingincrease the volumetric efficiency and thus the overall efficiency of the compressor.The oils used are normally VCL or VDL according to DIN 51 506 with an ISO VGbetween 68 and 150 or monograde SAE 20, SAE 30, SAE 40 engine oils Rotary pis-ton compressors are mostly used for vehicle and railroad applications (includingroad tankers) Outlet pressures are mostly less than 10 bar [12.1, 12.6] Figure 12.3shows the lubrication circuit of a rotary vane compressor (oil cooled)
Trang 25342 12 Compressor Oils
Screw compressors have two counter-rotating axial shafts (screws) and use the placement principle One of the two screws is the compressor and the other is anidler and both revolve in the same housing The inlet area has a large cross-sectionand volume As the two shafts revolve, the volume gets smaller and compressiontakes place The compressed medium then leaves the housing the outlet The com-pressor screw has a helical convex cross section while the idler has a helical concavecross section The two rotors of oil-free screw compressors are geared to each other
dis-so that their surfaces never touch On the other hand, the rotors in oil-flooded screwcompressors contact each other and thus do not require the geared coupling.The advantages of screw compressors are compact size, minimum vibration andcontinuous flow [12.1, 12.2, 12.7, 12.8]
In oil-injected screw compressors, the oil has a lubricating, sealing and cooling tion The lubricant is injected into the pressure chamber between the rotors at about3–4 bar It then forms a hydrostatic and a hydrodynamic lubricating film The oilFig 12.3 Lubrication circuit of a rotary vane compressor (oil cooled).
Trang 26func-343 12.1 Air Compressor Oilstherefore lubricates the meshing rotors and the plain and roller bearings which arepart of the geared coupling Furthermore, it seals the gaps between the rotor and thehousing It also helps absorb heat and dissipate this via radiators The temperature
of the compressed air of about 80 C to 100 C is adjusted by the quantity of oilinjected Downstream oil separators (normally cartridge filters) remove the oil fromthe air Residual oil quantities of 1–3 mg m–3of air can be achieved The separatedoil de-aerates in tanks, is then filtered and cooled from about 80 C to 50 C As theoil is one the pressure side of the screw compressor (e.g up to 10 bar), this pressurecan be used to re-inject the oil
As the viscosity of the oil is of primary importance to elasto-hydrodynamic cation and thus for the mechanical stability of the film, it must be matched to start-
lubri-up and normal running conditions As a rule, ISO VG 46 lubricants cover mostmanufacturer’s recommended viscosity thresholds of about > 10 mm2s–1at operat-ing temperature to about 500 mm2s–1 when starting-up This range also satisfiesmost applications in central Europe Higher viscosity ISO VG 68 oils or syntheticester-PAG or PAO-based lubricants are used in countries with high ambient tem-peratures In recent years, lubricants based on hydrocracked oils (so called group IIIoils) have found increasing acceptance Screw compressor oils have mild EP/AW
performance, FZG failure load stage ‡ 10 are typically required In relation to their size
and weight, the volumes achievable with screw compressors are excellent Screw pressors are primarily used for mobile applications as well as for industrial applicationssuch as in the glass and paper industries and general industry Pressures of up to about
com-10 bar and higher are possible with screw compressors [12.4, 12.7, 12.8]
Roots compressors normally consist of two symmetrical, figure-of-eight-shapedrotors in a housing The counter-rotating rotors are driven by external gears and donot touch The oil’s sole task is to lubricate the rotors’ gears and bearings The bene-fits of this type of compressor are oil-free air, large volumes and low vibration [12.2]
Recommended lubricants include DIN 51 517 CL and CLP or HD SAE oils in theviscosity grades ISO VG 68 and ISO VG 100 [12.4]
com-The advantages of turbo compressors are high volumes, minimum vibration andoil-free air [12.1, 12.2]
Trang 27344 12 Compressor Oils
The oils for this type of compressor lubricates bearings, radial shaft seals and bly gears via a positive-feed circuit In some cases, the bearings are lubricated withgrease Ideally, the same lubricant should be used for the compressor and its drive.Most often, DIN 51 515 TDL 32, TDL 46 and TDL 68 turbine oils or TDL-EP grades(EP–Extreme Pressure Additives) are used
possi-Turbo compressors are principally used for creating compressed air in mines andindustrial manufacturing plants [12.3, 12.4]
12.1.3
Preparation of Compressed Air
The oil injected into oil-cooled screw and rotary piston compressors is alwaysremoved from the compressed air The oil which is mixed with the highly com-pressed air is removed and collected in single or multistage downstream oil separa-tors Before the oil is re-circulated, it is filtered and cooled Depending on the specif-
ic requirements, the compressed air may then pass a number of subsequent ment stages such as refrigerant dryers or absorption dryers (to reduce the watercontent coming from the humidity of the air/gas) Very low residual oil quantities inthe air can be achieved by the fitting of a series of in-line oil separators [12.6–12.8]
Gases often contain acidic components such as SO2or NOx If standard compressoroils were used for such applications, the lubricating oil would soon become over-acidified To counter this, lubricants are used for such applications which containhighly alkaline additives These components can neutralize the acidic components
in the gas In these cases, it is recommended that monograde engine oils (20W-20,30W, 40W) with high alkaline reserves (high TBN) are used [12.4]
When inert gases are compressed, the same rules as for air compressors should beused [12.2, 12.4]
Trang 28345 12.1 Air Compressor Oils
Hydrocarbons such as ethane, propane etc are easily soluble in mineral oil Thiscauses the viscosity of the lubricating oil to fall if mineral oil-based products areused For this reason, higher viscosity mineral oils such as ISO VG 100 and ISO VG
150 must be used in piston compressors whose crankcases are subject to low inletpressures (1–3 bar) In the case of screw compressors (high pressure; 10–15 bar),ISO VG 68, 100, 150, and 220 ester- or polyglycol-based lubricants with lower hydro-carbon solubility are recommended [12.2, 12.4]
Vacuum pumps are compressors whose inlet is connected to the chamber where thevacuum is created VDL compressor oils can be used for low vacuums Greater vacuumsrequire synthetic oils with low vapor pressures (mostly synthetic ester oils) The lubri-cant selection must consider if the medium to be extracted is not air, but for example arefrigerant In such cases, a compatible refrigeration oil can be used [12.4]
12.1.5
Characteristics of Compressor Oils
Compressors whose chambers are lubricated pose particular safety problems if air
or aggressive gases contact the lubricant The selection of the most suitable lubricantdepends on the type of compressor in question, the pressures involved, the outlettemperatures and the type of air/gas being compressed Piston compressors whichgenerate the highest pressures are particularly problematic Turbo compressorswhich only have lubricated bearings and non-lubricated pressure chambers pose theleast problems Rotary and screw compressors with outlet pressures under 10 barand correspondingly low outlet temperatures are examples of average compressorlubrication application Table 12.1 shows an overview of normally used compressoroils
In general, reciprocating piston compressors need lubricants with higher ity (ISO VG 100 or ISO VG 150), extremely low carbon residues, and no or mild EP/AW-performance additives Screw compressors need lubricants of lower viscosity(ISO VG 46 or 68) with excellent oxidation stability and mild/high AW/EP perfor-mance additives
viscos-12.1.6
Standards and Specifications of Compressor Oils
DIN 51 506 describes the classification and requirements of lubricating oils whichare used in reciprocating piston compressors with oil-lubricated pressure chambers(also for vacuum pumps) Lubricants for screw and oil-injected rotary vane andscrew compressors are not included in DIN 51 506 [12.5] Table 12.2 shows the clas-sification of air compressor oils according to DIN 51 506 Tables 12.3 and 12.4 con-tain the minimum requirements of air compressor oils according to DIN 51 506
Trang 29Sliding vane
Turbo sors (axial and
HC-Oils PAO
TDL 32 TDL 32 EP Synth Oils
HC-Oils PAO POE
TDL 46 TDL 46 EP Synth Oils
ISO VG 68
(SAE 20W-20)
MO PAO Diester
MO HC-Oils PAO POE
MO Diester HC-Oils
TDL 68 TDL 68 EP Synth Oils ISO VG 100
(SAE 30)
MO PAO Diester
MO Diester HC-Oils ISO VG 150
(SAE 40)
MO PAO Diester
MO = mineral oil; PAO = polyalphaolefin;
HC = hydrocrack oil (so-called group III oils);
POE = biologically degradable polyolesters
Diester, Polyolester and PAO:
for very hard working conditions,
increase of service intervals is possible
HC Oils (so-called group III oils): for medium
and hard working conditions
MO: for normal and medium working
conditions
Lubricants for Roots-compressors: HL, CL,
CLP; ISO VG 100-150, DIN 51 524, DIN
51 517
Lubricants for vacuum pumps: ISO VG 68-150
a) Total-loss lubrication: HD-monograde motor oils HD 20W-20, HD 30, HD 40
b) For oil-injected compressors in mobile equipment (e.g railways, buses): multi- grade motor oils (e.g 10 W 40, 15 W 40) For oil-injected compressors in stationary units: turbine oils according to DIN
51 515 TDL, air compressor oils according
to DIN 51 506 VCL, VDL For hard working conditions: monograde motor oils HD 20W-20, HD 30, MIL 2104 D
c) Turbine oils according to DIN 51 515 TDL or TDL-EP with extreme pressure additives
Trang 30347 12.1 Air Compressor Oils
According to this standard, such lubricants are pure mineral oils or mineral oils withadditives to increase oxidation stability aging resistance and corrosion protection Theclassification of the lubricants depends on the expected outlet temperatures and the gen-eral application DIN 51 506 differentiates between lubricants for mobile applicationsand stationary applications with reservoirs Principal differences between the listedgroups: VB/VBL, VC/VCL as well as VDL are the use of oxidation and corrosion inhibi-tors, aging stability and residue formation and the quality of the base oils (cuts) Thedifference between group VB/VBL and group VC/VCL lies in the aging behavior (forma-tion of residues coke after air-induced aging) Group VDL oils have to pass a more diffi-cult aging test (formation of Conradson coke after air-induced aging in the presence offerrous oxide) DIN 51 506 VDL oils display the best thermal and oxidation stability andform the least residues The selection criteria in DIN 51 506 were adopted into the 1983ISO/DP 6521 draft [12.5, 12.9] Table 12.5 defines the requirements of air compressoroils for reciprocating piston compressors Instead of mineral oil-based lubricants, syn-thetic compressor oils based on hydrotreated (group III base oils) or polyalphaolefin orester oils have found increasing commercial acceptance (long-life oils)
The selection criteria for screw compressor and piston compressor oils differgreatly In oil-flooded rotary vane and screw compressors, the injected oil is con-stantly in contact with the 80–100 C hot medium gas or air being compressed Thecompressed medium and the oil are well mixed and the oil has to be separated bydownstream separators and filters This places special demands on the lubricant
On the whole, these include low foaming, excellent air release and good tion (separation) of condensed water In addition of course, the specifications regard-
demulsifica-ing wear protection (FZG ‡ 10, DIN 51 354), minimum formation of deposits, good
corrosion protection etc also apply Most manufacturers of oil-injected rotary andscrew compressors issue their own lubricant specifications A draft of ISO/DP 6521for oil-injected screw compressors has existed since 1983 Table 12.6 describes therequirements of air compressor oils for screw compressors
Tab 12.2 Classification of air compressor oils according to DIN 51 506 – Table 1, September 1985.
Maximum compressed air temperature ( C)
(mobile) equipment for brakes,signals and tippers
For compressors with storagetanks and pipe network systems
a) Rotary multi-vane compressors designed for a once-through lubrication
can be operated at compressor end temperatures of up to 180 C using
lubricating oils doped in the same manner as motor lubricants or doped
compressor oils, provided that the requirements specified for VCL lubricating
oils in table 2 are complied with.