Handbook of Lubrication Episode 2 Part 9 doc

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 554 CRC Handbook of Lubrication Table 7 RECOMMENDED AGMALUBRICANTS (FOR INTERMITTENTMETHODS OFAPPLICATION 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. 539-564 4/10/06 5:02 PM Page 554 Copyright © 1983 CRC Press LLC 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). Volume II 555 FIGURE 5. Splash lubrication system. 539-564 4/10/06 5:02 PM Page 555 Copyright © 1983 CRC Press LLC 556 CRC Handbook of Lubrication 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) 539-564 4/10/06 5:02 PM Page 556 Copyright © 1983 CRC Press LLC Volume II 557 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. 539-564 4/10/06 5:02 PM Page 557 Copyright © 1983 CRC Press LLC 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 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 wearwould 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 wearwould 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. Adiscussion 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. Volume II 559 539-564 4/10/06 5:02 PM Page 559 Copyright © 1983 CRC Press LLC 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. Abetter 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 Volume II561 FIGURE 9.Spalling. (From Standard AGMA110.4, Nomenclature of Gear Tooth Failure Modes, American Gear Manufacturers Association, Arlington, Va., August 1980. With permission.) 539-564 4/10/06 5:02 PM Page 561 Copyright © 1983 CRC Press LLC REFERENCES 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. 564 CRC Handbook of Lubrication 539-564 4/10/06 5:02 PM Page 564 Copyright © 1983 CRC Press LLC MECHANICALSHAFTCOUPLINGS Michael M. Calistrat INTRODUCTION Amechanical 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. 1 They 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 kWor 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. GearCouplings 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 Volume II 565 565-579 4/10/06 5:06 PM Page 565 Copyright © 1983 CRC Press LLC 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, thesteel 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, Asplit 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 GearCoupling 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 Volume II569 FIGURE 6. Centrifugal forces in couplings. 565-579 4/10/06 5:06 PM Page 569 Copyright © 1983 CRC Press LLC [...]... by the formula: 2 p = ρ 2 (R1 − R2) / 2 2 where ρ = specific mass, ω = angular velocity (rad/sec), R1 = major radius of the annulus, and R2 = minor radius of the annulus Example: For a coupling of Figure 4, with a 1 52- mm (6 in.-) pitch diameter, operating at 3600 rpm, the pressure generated by the centrifugal forces is 410 kPa (60 psi) Methods of Lubrication The two basic methods of lubrication are... corrosion can reduce the fatigue resistance of the coupling sleeves, which in turn can Copyright © 198 3 CRC Press LLC 565-5 79 4/10/06 576 5:07 PM Page 576 CRC Handbook of Lubrication FIGURE 12 Oil circuit for continuous lubrication FIGURE 13 Oil flow in damless couplings result in catastrophic failures The designers of lubrication systems are generally aware of the problems that impurities in the oil... couplings can transmit high power, however, the amount of power loss can be considerable For instance, a coupling transmitting 20 ,000 hp at 99 .9% efficiency will absorb 15 kW, which it will transform into heat If this heat is not dissipated it will damage the coupling The oil used for the lubrication and cooling of couplings is taken from the lubrication system of the machines connected by the coupling In most... to bridge this distance High-Performance Couplings In many cases these couplings (Figure 11) operate at speeds in excess of 10,000 rpm and Copyright © 198 3 CRC Press LLC 565-5 79 4/10/06 574 5:07 PM Page 574 CRC Handbook of Lubrication COUPLING LUBRICATION Whether the coupling is of the gear, chain, or steel grid type, it accommodates misalignment through a sliding motion between the hub (or sprocket)... to find The most important conclusion of one study is that the wear rate of a type gear coupling is greatly influenced by the viscosity of the base oil of the grease: the higher the viscosity, the lower the wear rate .2 This phenomenon might explain why many coupling manufacturers prefer to recommend a NLGI No 2 grease over a No 1 or 0 However, the penetration of a grease seldom reflects its base oil... a grease that has a small percentage of soap or fillers Oil as a Coupling Lubricant Oil lubrication is used almost exclusively in high-speed gear couplings As mentioned previously, the flow of oil is necessary primarily to remove the heat generated by friction.5 The efficiency of a good-quality, high-performance gear coupling is excellent, usually better than 99 .9% Since high-performance couplings can... coupling When the quantity of sludge in a coupling becomes excessive, it will (1) impair axial movement of one shaft with respect to the other, (2) corrode the teeth and accelerate their wear rate, and (3) reduce or completely stop the flow of oil, with high temperatures and high wear rates as a consequence Sludge can be of two types: a moist sludge with the appearance and consistency of a NLGI No 3 grease,... oil and soap is no longer suitable for coupling lubrication First, the oil will then leak out of the coupling, particularly when the coupling is at a standstill Second, the centrifugal force will force the denser soap and fillers away from the axis of rotation and eventually coat the inside surfaces of the coupling housings (or sleeves) In Figures 1, 2, and 3 one can see that the hub teeth are located... flow is insufficient, much of the tooth surface remains nonlubricated Following the manufacturers’ recommendations for the oil flow is very important for these types of Copyright © 198 3 CRC Press LLC 565-5 79 4/10/06 5:07 PM Page 577 Volume II 577 FIGURE 14 Oil flow through nozzles (Courtesy of Koppers Co., Inc., Pittsburgh, Pa.) couplings While in a dam-type coupling the lubrication is ensured by the... lubricant To offset this marginal lubrication, many high-performance couplings have nitrided teeth The high centrifugal forces present in a coupling do not affect the oil as long as it is a homogeneous fluid; however, the oil not only has components which are added for the purpose of enhancing its properties, but also has unwanted impurities Under centrifugal forces, some of the additives and most of the . Lubr. Eng., 32, 166, 197 5. 3. Ku, P. M., Gear failure modes — importance of lubrication and mechanics, ASLE Trans., T 19, 23 9, 197 6. 4. Wedeven, L. D., What is EHD?, Lubr. Eng., 31, 29 1, 197 5. 5. Lipp,. Eng., 32, 8, 197 6. 8. Standard AGMA 25 0.04, Lubrication of Industrial Enclosed Gear Drives, American Gear Manufacturers Association, Arlington, Va., 197 4. 9. Standard AGMA 25 1. 02, Lubrication of. at 40°C for optimum performance. p = ρ ω 2 (R 1 2 − R 2 2 ) / 2 574 CRC Handbook of Lubrication 565-5 79 4/10/06 5:07 PM Page 574 Copyright © 198 3 CRC Press LLC Especially demanding conditions