Machinery Components Maintenance And Repair Episode 2 Part 12 ppsx

The major limiting properties of nitrile are its poor ozone and weather resis- tance and moderate heat resistance.Advantages: • Good balance of desirable properties • Excellent oil and f

What Makes an O-ring

O-rings are manufactured from a variety of elastomers which are blended to form compounds These compounds exhibit unique properties such as resistance to certain fluids, temperature extremes, and life The following section describes the most prominent elastomers and their inher- ent properties.

Nitrite, Buna N, or NBR. Nitrile is the most widely used elastomer in the seal industry The popularity of nitrile is due to its excellent resistance to petroleum products and its ability to be compounded for service over a temperature range of -67° to 257°F (-55°C to 125°C).

Nitrile is a copolymer of butadiene and acrylonitrile Variation in portions of these polymers is possible to accommodate specific require- ments An increase in acrylonitrile content increases resistance to heat plus petroleum base oils and fuels but decreases low temperature flexibility Military AN and MS O-ring specifications require nitrile compounds with low acrylonitrile content to ensure low temperature performance Nitrile

pro-Table 10-15 Elastomer Capabilities Guide

provides excellent compression set, tear, and abrasion resistance The major limiting properties of nitrile are its poor ozone and weather resis- tance and moderate heat resistance.

Advantages:

• Good balance of desirable properties

• Excellent oil and fuel resistance

• Good water resistance

Disadvantages:

• Poor weather resistance

• Moderate heat resistance

Ethylene-Propylene, EP, EPT, or EPDM. Ethylene-propylene compounds are used frequently to seal phosphate ester fire resistant hydraulic fluids such as Skydrol They are also effective in brake systems, and for sealing hot water and steam Ethylene-propylene compounds have good resistance

Table 10-16 Elastomer Capabilities Guide

to mild acids, alkalis, silicone oils and greases, ketones, and alcohols They are not recommended for petroleum oils or diester lubricants Ethylene-propylene has a temperature range of -67°F to 302°F (-55°C

to 150°C) It is compatible with polar fluids that adversely affect other elastomers.

Advantages:

• Excellent weather resistance

• Good low temperature flexibility

• Excellent chemical resistance

• Good heat resistance

Disadvantage:

• Poor petroleum oil and solvent resistance

Chloroprene, Neoprene, or CR. Neoprene is a polymer of chlorobutadiene and is unusual in that it is moderately resistant to both petroleum oils and weather (ozone, sunlight, oxygen) This qualifies neoprene for O-ring service where many other elastomers would not be satisfactory It is also used extensively for sealing refrigeration fluids Neoprene has good com- pression set characteristics and a temperature range of -57°F to 284°F ( -55°C to 140°C).

Advantages:

• Moderate weather resistance

• Moderate oil resistance

• Versatile

Disadvantage:

• Moderate solvent and water resistance

Fluorocarbon, Viton, Fluorel, or FKM. Fluorocarbon combines more resistance to a broader range of chemicals than any of the other elastomers.

It constitutes the closest available approach to the universal O-ring elastomer Although most fluorocarbon compounds become quite hard

at temperatures below -4°F (-20°C), they do not easily fracture, and are thus serviceable at much lower temperatures Fluorocarbon com- pounds provide a continuous 437°F (225°C) high temperature capability.

• Excellent chemical resistance

• Excellent heat resistance

• Good mechanical properties

• Good compression set resistance

Disadvantage:

• Fair low temperature resistance

Silicone or PVMQ. Silicone is a semi-organic elastomer with outstanding resistance to extremes of temperature Specially compounded, it can provide reliable service at temperatures as low as -175°F (-115°C) to as high as 482°F (250°C) continuously Silicone also has good resistance to compression set.

Low physical strength and abrasion resistance combined with high tion limit silicone to static seals Silicone is used primarily for dry heat static seals Although it swells considerably in petroleum lubricants, this

fric-is not detrimental in most static sealing applications.

Advantages:

• Excellent at temperature extremes

• Excellent compression set resistance

Disadvantages:

• Poor physical strength

Fluorosilicone or FVMQ. Fluorosilicones combine most of the attributes of silicone with resistance to petroleum oils and hydrocarbon fuels Low physical strength and abrasion resistance combined with high friction limit fluorosilicone to static seals Fluorosilicones are used primarily in aircraft fuel systems over a temperature range of -85°F to 347°F (-65°C to 175°C).

Advantages:

• Excellent at temperature extremes

• Good resistance to petroleum oils and fuels

• Good compression set resistance

• Poor physical strength

Styrene-Butadiene or SBR. Styrene-butadiene compounds have properties similar to those of natural rubber and are primarily used in the manufac- ture of tires Their use in O-rings has been mostly in automobile brake systems and plumbing Ethylene-propylene, a more recent development,

is gradually replacing styrene-butadiene in brake service Temperature range is -67°F to 212°F (-55°C to 100°C).

Advantages:

• Good resistance to brake fluids

• Good resistance to water

Disadvantages:

• Poor weather resistance

• Poor petroleum oil and solvent resistance

Polyacrylate or ACM. Polyacrylate compounds retain their properties when sealing petroleum oils at continuous temperatures as high as 347°F (175°C) Polyacrylate O-rings are used extensively in automotive trans- missions and other automotive applications They provide some of the attributes of fluorocarbon O-rings A recent variation, ethylene-acrylate, provides improved low temperature characteristics with some sacrifice in hot oil resistance.

Advantages:

• Excellent resistance to petroleum oils

• Excellent weather resistance

Disadvantages:

• Fair low temperature properties

• Fair to poor water resistance

• Fair compression set resistance

Polyurethane, AU, or EU. Polyurethane compounds exhibit outstanding tensile strength and abrasion resistance in comparison with other elas-

tomers Fluid compatibility is similar to that of nitrile at temperatures up

to 158°F (70°C) At higher temperatures, polyurethane has a tendency

to soften and lose both strength and fluid resistance advantages over other elastomers Some types are readily damaged by water, even high humidity Polyurethane seals offer outstanding performance in high pressure hydraulic systems with abrasive contamination, high shock loads, and related adverse conditions provided temperature is below l58°F (70°C).

Advantages:

• Excellent strength and abrasion resistance

• Good resistance to petroleum oils

• Good weather resistance

Disadvantages:

• Poor resistance to water

• Poor high temperature capabilities

Butyl or IIR. Butyl is a copolymer of isobutylene and isoprene It has largely been replaced by ethylene-propylene for O-ring usage Butyl is resistant to the same fluid types as ethylene-propylene and, except for resistance to gas permeation, it is somewhat inferior to ethylene- propylene for O-ring service Temperature range is -67°F to 212°F (-55°C

to 100°C).

Advantages:

• Excellent weather resistance

• Excellent gas permeation resistance

Disadvantage:

• Poor petroleum oil and fuel resistance

Polysulfide, Thiokol, or T. Polysulfide was one of the first commercial synthetic elastomers Although polysulfide compounds have limited O-ring usage, they are essential for applications involving combina- tions of ethers, ketones, and petroleum solvents used by the paint and insecticide industries Temperature range is -67°F to 212°F (-55°C to 100°C).

• Poor high temperature capabilities

• Poor mechanical strength

• Poor resistance to compression set

Chlorosulfonated Polyethylene, Hypalon, or CSM. Chlorosulfonated ethylene compounds demonstrate excellent resistance to oxygen, ozone, heat, and weathering But their mechanical properties and compression set are inferior to most other elastomers, and they are seldom used to advantage as O-rings Temperature range is -65°F to 257°F (-55°C to 125°C).

poly-Advantages:

• Excellent resistance to weather

• Good resistance to heat

Disadvantages:

• Poor tear and abrasion resistance

• Poor resistance to compression set

Epichlorohydrin, Hydrin, or ECO. Epichlorohydrin is a relatively recent development Compounds of this elastomer provide excellent resistance

to fuels and oils plus a broader temperature range, -65°F to 275°F (-55°C

to 135°C), than nitrile Initial usage has been in military aircraft where the particular advantages of epichlorohydrin over nitrile are of immediate benefit.

Advantages:

• Excellent oil and fuel resistance

• Excellent weather resistance

• Good low temperature resistance

Disadvantage:

• Fair resistance to compression set

Phosphonitrilic Fluoroelastomer, Polyphosphazene, PNF, or PZ. This is another new elastomer family O-rings of phosphonitrilic fluoroelastomer are rapidly accommodating aircraft sealing requirements where the

physical strength of fluorosilicone is inadequate In other regards, the functional characteristics of phosphonitrilic fluoroelastomer and fluo- rosilicone are similar Temperature range is -85°F to 347°F (-65°C to 175°C).

Advantages:

• Excellent oil and fuel resistance

• Wide temperature range

• Good compression set resistance

Disadvantage:

• Poor water resistance

UTEX HTCR® Fluororubber. Typical of many recent elastomeric pounds, this copolymer of tetrafluoroethylene and propylene is too new to

com-be on most charts In application range, it fits somewhere com-between rocarbon (Viton) and Kalrez®.

fluo-HTRC is thermally stable for continuous use in temperatures of 450°F, and depending on the specific application, has serviceability in environ- ments up to 550°F The US manufacturer, UTEX, claims excellent resis- tance to a wide variety of chemical environments Table 10-17 provides

an indication of its chemical resistance Since temperature, concentration, mixtures and elastomer compound selection can affect performance, this chart provides guidelines only.

Table 10-17

Perfluoroelastomer (Kalrez®). Kalrez®O-rings have mechanical properties similar to other fluorinated elastomers but exhibit greater heat resistance and chemical inertness They have thermal, chemical resistance, and elec- trical properties similar to Teflon®fluorocarbon resins but, made from a true elastomer, possess excellent resistance to creep and set.

Generally, Kalrez®O-rings are capable of providing continuous service

at temperatures of 500°–550°F (260°–288°C) and can operate at 600°F (316°C) for shorter periods as long as they are in static service For long- term dynamic sealing duties, an operating temperature of 450°F (232°C) would be a reasonable limit.

The chemical resistance of Kalrez®O-rings is outstanding When using specially formulated compositions, little or no measurable effect is found

in almost all chemicals, excepting fluorinated solvents which induce erate swelling The parts have excellent resistance to permeation by most chemicals.

mod-Resistance to attack is especially advantageous in hot, corrosive ronments such as:

envi-• Polar solvents (ketones, esters, ethers)

• Strong commercial solvents (tetrahydrofuran, dimethyl formamide, benzene)

• Inorganic and organic acids (hydrochloric, nitric, sulfuric, trichloroacetic) and bases (hot caustic soda)

• Strong oxidizing agents (dinitrogen tetroxide, fuming nitric acid)

• Metal halogen compounds (titanium tetrachloride, diethylaluminum chloride)

• Hot mercury/caustic soda

• Chlorine, wet and dry

• Inorganic salt solutions

• Fuels (aviation gas, kerosene, JP-5, Jet Fuel, ASTM Reference Fuel C)

• Hydraulic fluids, synthetics and transmission fluids

• Heat transfer fluids

• Oil well sour gas (methane/hydrogen sulfide/carbon dioxide/steam)

• Steam

Back-Up Rings. Back-up rings, as shown in Figure 10-16, are often used

to prevent extrusion in high pressure applications, or to correct problems such as spiral failure or nibbling They are sometimes used in normal pressure range applications to provide an added measure of protection or

to prolong O-ring life These devices also permit the use of a wider clearance gap when close tolerances are impossible to maintain.

A back-up ring is simply a ring made from a material harder than the O-ring, designed to fit in the downstream side of the groove and close to

the clearance gap to provide support for the O-ring Quite often, O-rings are used as back-up rings, even though back-up rings do not perform any sealing function.

O-Ring, Back-Up Ring, and Gland Dimensions. O-ring sizes have been dardized and range in size from an inside diameter of 0.029 in and a cross

stan-Table 10-18 Gland Design Guide

O-Ring Section 070 103 139 210 275 1.78 2.62 3.53 5.33 6.99Diameter

Figure 10-16 Back-up rings used with O-rings.

section of 0.040 in to O-rings with an inside diameter of 16 or more in and a cross section of 0.210 or more in Installation dimensions vary with duty and application and the user may find it easy to consult manufactur- ers’ catalogs, which are typically configured as shown in Figure 10-17 Note the small differences in gland dimensions They depend on whether the O-ring will be axially squeezed, radially squeezed, or will perform dynamic piston and rod sealing duty.

To calculate your own gland design, refer to Table 10-18, “Gland Design Guide.”

Figure 10-17 A sampling of O-ring and gland dimensions.

1 Bloch, H P and Geitner, F K., Machinery Failure Analysis and

Trou-bleshooting, Gulf Publishing Company, Houston, Texas, Third

Edition, 1997, Pages 42–57.

2 Locke, J J., “Cobalt Alloy Overlays in a Petro-Chemical Refinery,”

Cobalt, 1974, Vol 2, Pages 25–31.

3 Mendenhall, M D., “Shaft Overlays Proven Effective,” Hydrocarbon

Processing, May 1980, Pages 191–192.

4 Tribology Handbook, edited by M J Neale, John Wiley & Sons, New

York-Toronto, 1973, Page E13.

5 Chrome Plating, sales brochure by Exline, Inc., Salina, Kansas 67401.

6 Pyles, R., “Porous Chromium in Engine Cylinders,” Transactions of the ASME, April 1944, Pages 205–214.

7 Tichvinsky, L M and Fischer, E G., “Boundary Friction in Bearings

at Low Loads,” Transactions of the ASME, Vol 61, 1939.

8 Reference 6, Page 206.

9 Reference 6, Page 210.

10 Vacca, A P., “Extended Periods of Overhaul of Diesel Machinery,”

The Motor Ship, January 1964.

11 Mollerus, A P H J., “Wear Data of Cylinder Liners,” study ted to Ingenieursbureau Lemet Chromium H., Van Der Horst N V., November 1964.

submit-12 Stinson, K W., Diesel Engineering Handbook, Diesel Publications

Inc., Stamford, Connecticut, 1966.

13 Eichenour, C and Edwards, V H., “Electromechanical Metallizing

Saves Time in Rebuilding Engine Parts,” Plant Services, December

Appendix 10-A

Part Documentation Record

Table 10-A-1 Typical Part Documentation Record Sheet

THE INTENT OF THIS FORM IS TO RECORD SPECIFIC PART CHARACTERISTICSTHAT WILL BE USED FOR FUTURE EVALUATION

PART IDENTIFICATION _ UNIQUE ID CUSTOMER CUSTOMER P.O _INSPECTOR DATE PST JOB NO DRAWING NO _HARDNESS COATING TYPE OAL MAJOR DIAMETER _LENGTH OF COATING FROM THE CROSSHEAD END OF THE SHAFT LENGTH OF COATING

MAGNAFLUX—ACCEPTED/REJECTED

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