590 Machinery Component Maintenance and Repair 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 pro- 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 Table 10-15 Elastomer Capabilities Guide Protecting Machinery Parts Against Loss of Surface 591 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. 592 Machinery Component Maintenance and Repair Advantages: • 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 fric- tion limit silicone to static seals. Silicone is used primarily for dry heat static seals. Although it swells considerably in petroleum lubricants, this 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 Protecting Machinery Parts Against Loss of Surface 593 594 Machinery Component Maintenance and Repair Disadvantage: • 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). Protecting Machinery Parts Against Loss of Surface 595 Disadvantages: • Poor high temperature capabilities • Poor mechanical strength • Poor resistance to compression set Chlorosulfonated Polyethylene, Hypalon, or CSM. Chlorosulfonated poly- 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). 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 596 Machinery Component Maintenance and Repair Protecting Machinery Parts Against Loss of Surface 597 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 com- pounds, this copolymer of tetrafluoroethylene and propylene is too new to be on most charts. In application range, it fits somewhere between fluo- rocarbon (Viton) and Kalrez ® . 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 10-17 598 Machinery Component Maintenance and Repair 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 mod- erate swelling. The parts have excellent resistance to permeation by most chemicals. Resistance to attack is especially advantageous in hot, corrosive envi- ronments such as: • 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 Protecting Machinery Parts Against Loss of Surface 599 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 stan- dardized and range in size from an inside diameter of 0.029 in. and a cross Table 10-18 Gland Design Guide INCHES MILLIMETERS O-Ring Section .070 .103 .139 .210 .275 1.78 2.62 3.53 5.33 6.99 Diameter STATIC SEALING A Gland Depth .048 .077 .109 .168 .222 1.22 1.96 2.77 4.27 5.64 .054 .083 .115 .176 .232 1.37 2.11 2.92 4.47 5.89 B Groove Width .090 .140 .180 .280 .370 2.29 3.56 4.57 7.11 9.40 .100 .150 .190 .290 .380 2.54 3.81 4.83 7.37 9.65 R Groove Radius .015 .020 .025 .035 .050 .38 .51 .64 .89 1.27 (Max.) DYNAMIC SEALING A Gland Depth .055 .088 .120 .184 .234 1.40 2.24 3.05 4.67 5.94 .057 .090 .124 .188 .240 1.45 2.29 3.15 4.76 6.10 B Groove Width .090 .140 .180 .280 .370 2.29 3.56 4.57 7.11 9.40 .100 .150 .190 .290 .380 2.54 3.81 4.83 7.37 9.65 R Groove Radius .015 .020 .025 .035 .050 .38 .51 .64 .89 1.27 (Max.) Figure 10-16. Back-up rings used with O-rings. [...]... 155–156 pre-stressing bolts for, 166 167 residual gasket loads on, 166 167 ring gasket, dimensions of, 165 sealing width for, 169 f, 170 selection of, 153–155 semi-confined type of, 154f spiral wound type of, 156–157, 158t, 160 t, 162 t, 164 t, 202 thickness of, 153–154 torque tables for, 157, 158t, 160 t, 162 t, 164 t torque value on, 157, 165 torque wrenches, proper calibration of, 165 unconfined type of, 154f... Surface Finish (Average): Regrinds: Rod and main journals: 16 rms Seal areas: 16 rms All other diameters: 16 32 rms With HVOF Coating—Chrome Carbide or Nickel-Chromium Self-Fluxing Alloy: Rod and main journals: 16 rms Seal areas: 16 rms Gear fits: 16 rms: minimal grinding marks are acceptable Tapered snouts: 16 rms and 85 percent + blue contact All other diameters: 16 32 rms: minimal grinding marks are...600 Machinery Component Maintenance and Repair 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 manufacturers’ catalogs, which are typically configured as shown in Figure 10-17 Note the small differences in gland dimensions They... provided by customer (Text continued on next page) 608 Machinery Component Maintenance and Repair Table 10-A-4 Fusion HVOF Coating Procedure for Repair of Industrial Crankshafts—cont’d 3.0 Cleaning 3.1 Remove all parts that are needed to completely clean and inspect the crankshaft Mark these parts with the fusion job number 3.2 Remove all plugs and check all oil holes for blockage; clear oil passages... 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 Protecting Machinery Parts Against Loss of Surface 601 References 1 Bloch, H P and Geitner, F K., Machinery Failure Analysis and Troubleshooting, Gulf Publishing Company, Houston, Texas,... 84–85, 85f, 87 Machinery maintenance classification of problems of, 4, 5f rotor charts for, 54, 54f rotors in, 46–47, 47f Teflon, risk of use for, 46 vibration monitoring probes for, 46–47 Magnaflux, 515– 516, 609t Magnaglow, 515– 516, 518 Magnetic particle inspection, 56 Maintainability, process machinery, components of, 6, 7f Maintenance central control system for, 15–20 central parts depot for, 28–29... and crated with proper supports in place, per customer requirements (Text continued on next page) 614 Machinery Component Maintenance and Repair Table 10-A-6 Repair Procedure for Piston Rods and Plungers High Velocity Liquid Fuel (HVLF) Tungsten Carbide Procedure for Tafa JP-5000 Repair of Piston Rods and Plungers—cont’d 11 PST will record work performed on rod which includes: a Rod will be stamped on... in, 479, 480 nickel-base brazing filler metal and, 480 repair material in, 481 service shops contacted with, 482, 487 surplus equipment directory in, 487 Central parts depot central data processing system of, 28–29, 29f parts replacement, nonstandard use of, 59–60 spare parts control method for, 28–29 spare parts program for, 28–29, 29f, 35, 45–47, 47f spare parts tabulation list in, 37f Centralized staff,... factors “M” and “Y” for, 168 , 168 f flange configurations for, 153–156, 154f forces acting on, 155f fully confined type of, 154f gasketed connection for, 155, 155f hydrostatic testing of, 166 identification markings for, 157f initial bolt tightening sequence for, 166 installation of, 165 joint design for, 156 leakage, common reasons for, 153, 165 load calculations to effect a seal on, 167 , 169 f, 170 performance... spray and distance A cross section of the coupon is examined at 200¥ Note 8: Typical coating thickness should be used as a reference only The coating thickness for a specific part must be evaluated based on factors such as part material type, part geometry, and type of service Protecting Machinery Parts Against Loss of Surface PST Coating Name Average Diamond Pyramid Hardness (Note 2) 606 Machinery Component . specific part must be evaluated based on factors such as part material type, part geometry, and type of service. 606 Machinery Component Maintenance and Repair Table 10-A-3 Common Repair Methods and. 1.27 (Max.) Figure 10 -16. Back-up rings used with O-rings. 600 Machinery Component Maintenance and Repair section of 0.040 in. to O-rings with an inside diameter of 16 or more in. and a cross section. resistance to petroleum oils and fuels • Good compression set resistance Protecting Machinery Parts Against Loss of Surface 593 594 Machinery Component Maintenance and Repair Disadvantage: • Poor