Handbook of Lubrication Episode 2 Part 9 pdf

If for other reasons the sump oil level is higher than required for gearing lubrication, an oil pan can be used to limit the amount of oil that the gear contacts.. From Standard AGMA 251

runs off the gear case, it can be collected in troughs and directed to bearings For very slow gearing using heavy oil, it may be necessary to use scrapers to remove oil from the sides

of the gear rim for use as a bearing lubricant If for other reasons the sump oil level is higher than required for gearing lubrication, an oil pan can be used to limit the amount of oil that the gear contacts The flow of oil into the pan is controlled by small holes below the oil level to eliminate excessive churning and improve efficiency Oil pans are recom-mended for gears with pitch line velocities over 13 m/sec (2500 ft/min) and work well for slow- and moderate speed gearing with pitch line velocities up to 18 m/sec (3500 ft/min) Higher speed gearing will tend to centrifugally throw off most of the oil before it gets into mesh The lighter oil generally used with high-speed gearing tends to compound the problem Higher power losses caused by churning also tend to make splash lubrication impractical for high-speed gearing

Another common means for applying easy flowing liquid lubricants is a force-fed system (Figure 6) In this system oil is taken from the gear case, pumped through a filter, heat exchanger, and pressure relief valve, and delivered back to the unit under pressure The amount of oil delivered to the bearings can be set by an orifice or other flow-control device Oil is applied to the gear mesh by spray nozzles in a manifold

Frictional heat generation may range, typically, from 0.5 to 1% of the transmitted horse-power per mesh for spur or helical gears Sump capacity and oil flow rate can be established for a desired sump temperature (95°C maximum is recommended for petroleum based lu-bricants) Inlet oil temperature and flow rate should be selected to maintain a suitable operating oil viscosity in the mesh For enclosed, industrial gears, inlet oil temperatures of

38 to 54°C and temperature rises in the mesh of 17 to 28°C are typical for circulating systems The amount of oil is controlled by the size of the nozzles and oil pressure Oil velocity must be sufficient to get the oil down into the tooth space before windage and tooth rotation

Table 7 RECOMMENDED AGMA LUBRICANTS (FOR INTERMITTENT METHODS OF APPLICATION LIMITED TO 1500 FT/MIN (8 M/SEC) PITCH LINE

VELOCITY)a

Note: AGMA Viscosity number recommendations listed above refer to gear oils

shown in Table 5

a Feeder must be capable of handling lubricant selected.

b Ambient temperature is temperature in vicinity of the gears.

c Greases are sometimes used in mechanical spray systems to lubricate open gearing A general purpose EP grease of number 1 consistency (NGLI) is preferred Consult gear manufacturer and spray system manufacturer before proceeding.

d Diluents must be used to facilitate flow through applicators.

From Standard AGMA 251.02, Lubrication of Industrial Open Gearing,

Amer-ican Gear Manufacturers Association, Arlington, Va., November 1974 With permission.

deflect and block the spray Spray nozzle pressures of 15 to 50 psi are usually adequate for industrial gearing Advantages of this system are that the oil delivered is of controlled quantity, cleanliness, and temperature The oil spray can flush wear particles and debris from the gear mesh Heat will also be carried away with the excess lubricant In high-speed gearing where excessive lubricant in the mesh is not desirable, high-flow sprays should be directed at the gear tooth surfaces as they leave mesh for maximum cooling Low-flow sprays should be directed at the teeth just before mesh for maximum lubrication Screening

is sometimes installed in high-speed gear cases above the sump oil level to minimize the windage effects on oil foaming There are no practical limiting speeds for force-fed systems Heavy open gear compounds are generally applied by paddle or brush, slush pan or automatic lubricator The manual paddle or brush method is crude, with the distribution of lubricant being erratic

The slush pan method utilizes a pan filled with lubricant into which the gear dips The lubricant is then carried into mesh where excess material is squeezed out and drops back into the pan The method provides for continuous lubrication and applies lubricant evenly across the entire tooth It can be used with gearing with pitch line velocities up to 10 m/ sec (2000 ft/min)

FIGURE 5 Splash lubrication system.

Table 8 RECOMMENDED QUANTITIES OF LUBRICANT (FOR INTERMITTENT METHODS OF APPLICATION WHERE PITCH LINE VELOCITY DOES NOT EXCEED 1500 FT/MIN (8 M/SEC) FOR AUTOMATIC, SEMIAUTOMATIC, HAND SPRAY, GRAVITY

FEED, OR FORCED DRIP SYSTEMS)

a The spraying time should equal the time for 1 and preferably 2 revolutions of the gear to ensure complete coverage Periodic inspections should

be made to ensure that sufficient lubricant is being applied to give proper protection.

b Four hours is the maximum interval permitted between applications of lubricant More frequent application of smaller quantities is preferred.

However, where diluents are used to thin lubricants for spraying intervals must not be so short as to prevent diluent evaporation.

From Standard AGMA 251.02, Lubrication of Industrial Open Gearing, American Gear Manufacturers Association, Arlington, Va., November

1974 With permission.

will remove some material from the area of hardest contact and spread the load over more

of the face-width These minor modifications from light, initial wear improve the gear tooth conformity and make it possible for the gears to operate under full-load with less chance of damage Initial wear can be intentionally induced by using a lighter lubricant for a

break-in period How light a lubricant depends on the break-initial load and the desired rate of wear Extreme caution should be exercised and tooth surfaces should be examined frequently to prevent rapid, destructive wear Under normal operating conditions, initial wear may occur and stop by itself when sufficient conformity has been achieved to support the load

Moderate wear would result from increased rate of surface material removal due to more

significant surface irregularities, gear tooth misalignment, dynamic load pulsations, insuf-ficient lubricant viscosity, or any conditions that would cause the gearing to operate under conditions of boundary of mixed film lubrication It could also be caused by abrasive material

in the lubricant This wear would probably not stop by itself but would continue, slowly, over a long period of time Depending on the anticipated life of the gearing, this type of wear may, or may not be acceptable

Heavy wear would involve rapid removal of surface material, destroying the tooth form,

and hindering the smooth operation of the gear set This can be caused, for example, by operating the gears without any lubricant or under conditions of heavy overload or severe misalignment of contacting tooth surfaces This destruction of the tooth form will lead to a very short-service life for the gear set if the causes are not found and corrected Excessive loading is the most common cause of rapid wear, although lubrication receives considerable attention since it is easier to change

Examining a gear set to determine the cause of excessive wear or failure is not always

an easy task The final mode of failure may be the result of previous wear mechanisms of

an entirely different nature A discussion of several common forms of wear may help to establish the probable sequence of events leading to excessive wear or failure in a gear set

Breakage ( Figure 7 ) — Catastrophic tooth failure is caused by a gear tooth being subjected

to loads and, consequently, bending stresses in excess of the endurance limit of the gear material This will cause a tooth or portion of a tooth to break away The breakage usually starts as a crack which propagates with repeated load cycles showing a typical fatigue failure Occasionally, a tooth may fail from a single-load cycle This would not show a fatigue type failure Breakage can be caused by shock loads, loads induced by high vibration, large pieces of debris passing through the gear mesh, or by misalignment causing the tooth load

to be carried by a small portion of the face-width Obviously, lubrication is not a factor and the mechanical defects must be located and corrected

Pitting ( Figure 8 ) — This common form of surface fatigue occurs when small localized

high spots on the tooth surface are overstressed due to high-unit loading When this occurs, small subsurface cracks are formed which propagate to the surface after repeated cycles When an area of material is no longer supported, it leaves the surface of the tooth creating

a small depression or pit Small pits may form in a new gear set as high spots from machining are removed Once enough load bearing surface is in contact, pitting may stop and the surface will begin to “polish over” If this takes place, the pitting is considered “initial” and is not harmful If the pitting continues to spread, leaving increasingly less surface to carry the load, it is considered progressive If the condition is not corrected, material will continue to be removed from the tooth until, eventually, tooth breakage may occur This is not a lubrication failure The condition can be caused by misalignment of the gear teeth, causing a relatively small area to carry the load with resultant high stresses Gear material too soft for the application or operating loads greater than those for which the gearing was designed can also cause pitting The use of a heavier oil may help spread the load over a larger area, but lubricant adjustments will generally give little, if any, help The use of some

EP lubricants or additives may extend the life of the gearing but will not correct the problem

Spalling (Figure 9) — This mechanism is the same as for pitting Large flakes or chips

may be removed from through hardened or case hardened gears from subsurface flaws or stresses caused by improper heat treatment The joining of pits as the metal between them

is removed is also a form of spalling Again, this is not a lubrication problem It is a result

of material defects, excessive load, or other application problems

Plastic flow ( Figure 10 ) — This type of wear represents gear tooth surface deformation

caused by heavy loads stressing the surface material beyond its elastic limit Usually occurring

in softer metals, the surface material may be extruded out along the ends of the teeth and along the tip causing fins to form Prominent ridges at the pitch line or depressions in the dedendum may also be indications of this type of wear If the wear is caused by high vibration or shock load, a heavier lubricant may cushion the load somewhat However, this type of wear is a material failure and lubricant changes will not correct it

Scratching ( Figure 11 ) — This is a type of abrasive wear When hard particles, which

are larger than the oil film thickness separating the gear teeth, pass through the gear mesh, they scratch the gear tooth surfaces in the direction of sliding These particles can be dirt, sand, casting scale, welding slag, gear or bearing material, or any debris which finds its way into the lubrication system The material can be airborne and enter through openings

in poorly fitting enclosures or uncovered inspection ports It may be the result of poorly cleaned housings or rotating elements prior to assembly The debris may also be from wear within the gear unit Laboratory analysis can generally indicate the type of particle material Increasing the lubricant viscosity will increase the film thickness and may ease the problem, but not cure it A better solution is to remove the abrasive particles through finer lubricant filtration, improved maintenance, etc Once the problem is corrected, gear surface deteri-oration will cease

Scoring ( Figure 12 ) — When small local high spots or asperities on mating gear tooth

surfaces break through the oil film and cause metal-to-metal contact, they may weld together

FIGURE 9 Spalling (From Standard AGMA 110.4, Nomenclature of Gear Tooth Failure Modes, American

Gear Manufacturers Association, Arlington, Va., August 1980 With permission.)

1 Fowle, T I., Gear lubrication: relating theory to practice, Lubr Eng., 32, 17, 1976.

2 Fowles, P E., EHL film thickness — practical significance and simple computation Lubr Eng., 32, 166,

1975.

3 Ku, P M., Gear failure modes — importance of lubrication and mechanics, ASLE Trans., T19, 239, 1976.

4 Wedeven, L D., What is EHD?, Lubr Eng., 31, 291, 1975.

5 Lipp, L C., Solid lubricants — their advantages and limitations, Lubr Eng., 32, 574, 1976.

6 Hersey, M D., Gear lubrications, in Theory and Research in Lubrication, John Wiley & Sons, New York,

1966 chap 11.

7 Root, D C., Selecting the right gear oil, Lubr Eng., 32, 8, 1976.

8 Standard AGMA 250.04, Lubrication of Industrial Enclosed Gear Drives, American Gear Manufacturers

Association, Arlington, Va., 1974.

9 Standard AGMA 251.02, Lubrication of Industrial Open Gearing, American Gear Manufacturers

Associ-ation, Arlington, Va., November 1974.

10 Standard AGMA 110.4, Nomenclature of Gear Tooth Failure Modes, American Gear Manufacturers

As-sociation, Arlington, Va., August 1980.

MECHANICAL SHAFT COUPLINGS

Michael M Calistrat

INTRODUCTION

A mechanical shaft coupling is that part of a machine which transmits torque from one shaft to another There are significant differences between different types of mechanical shaft couplings, and for a better understanding of their working principles we should first categorize them First, shaft couplings can be divided into rigid and flexible couplings

Rigid couplings — usually in the form of a long collar or a couple of bolted flanges,

perform the torque transmission in the most efficient way Unfortunately they can be used only when the two connected shafts are perfectly aligned Rigid couplings do not require lubrication

Flexible couplings — transmit torque without slip, and accommodate misalignment

be-tween the driving and driven shafts.1They too can be divided into two categories depending

on the means used to accommodate the misalignment: couplings using the flexing of one or more of their components, and couplings that use sliding of two or more of their components Some couplings use both of these methods in their design The couplings that accommodate misalignment only through flexing do not require lubrication and fall outside the scope of this manual Couplings that use sliding for accommodating misalignment must be lubricated

in order to minimize wear

Nonlubricated, or dry, couplings use either an elastomer or one or more thin metal disks which flex in order to accommodate misalignment Since useful life of these flexing elements

is limited by such factors as fatigue, fretting corrosion, or elastomer aging, they have to be replaced periodically Besides that, elastomer couplings are usually larger and heavier than lubricated couplings with similar ratings Hence, the need for lubrication is compensated in many instances by less maintenance and/or smaller size Dry couplings are used mainly in fractional and low-horsepower applications ( < 200 kW or 300 hp); lubricated couplings are particularly popular in large-horsepower applications

DESCRIPTION

Although there is an endless variety of lubricated shaft couplings, three designs are most frequently found: the gear, chain, and the steel grid couplings, shown in Figures 1 to 3, respectively

Gear Couplings

The gear coupling (Figure 1) has five major components: two hubs, two sleeves, and in some cases a spacer (not shown) It also has a number of bolts, nuts, lockwashers, and two

or more seals The spacer is omitted when the separation between the connected shafts is small The hubs have a row of external teeth with involute profile; the sleeves have matching internal teeth Each gear mesh acts as a spline; to accommodate misalignment, the external teeth are slightly thinner than the space between the internal teeth The space thus generated between the teeth is called the backlash and it allows the hub teeth to assume an angular position, as shown in Figure 4

The need for lubrication can be understood by considering the sliding motion of the hub teeth on the sleeve teeth Figure 5 illustrates a section through half of a gear coupling, i.e.,

a hub and a sleeve Because the coupling is misaligned, the centerline of the hub teeth, BB, does not coincide with the centerline of the sleeve teeth, AA Hence, the lower hub tooth

of coupling is used in applications having short shaft separation The principle of operation

is similar to the gear couplings when considering the sprockets as hubs and the double-row chain as the two sleeves Only when used with a cover can the chain couplings benefit from the effect of centrifugal forces in the lubricant From a torsional point of view, the chain couplings are less stiff than gear couplings

Steel Grid Couplings

Steel grid couplings are even more flexible, torsionally, than chain couplings They also operate similarly to a gear coupling, having two toothed hubs and a sleeve in a form of a convoluted spring steel band Because of the special profile of the teeth, the steel grid flexes under torque, as shown in Figure 7

To accommodate misalignment the hub teeth slide over the steel grid just as in a gear coupling, A split cover is always provided in order to retain both the steel grid and the lubricant within the coupling Similar to chain couplings, the steel grid couplings can be used only when the gap between the shafts is small

DESIGN AND MATERIALS

The Gear Coupling

The three major types of gear couplings are

1 Standard — medium speeds and medium-torque couplings

2 Spindle — low speeds and high-torque couplings

3 High performance — high speeds and medium-torque couplings

FIGURE 6 Centrifugal forces in couplings.