|Figure 1 - Load ratings of three common bronzes. Temperatures should not exceed 300°F with most lubricants.|
|Figure 2 - Maximum PV (psi X fpm) of three common bronzes.|
|Figure 3 - Maximum load-speed ratings for two common zinc alloys and SAE 660 bronze.|
In a plain bearing operating under hydrodynamic or full-film lubrication, a film of lubricant completely separates the shaft and bearing. It would therefore seem logical that any bearing material of required strength could be used because there is no metal-to-metal contact. However, because most applications exhibit less than full-film lubrication at least occasionally, a bearing of proper material and design must be selected to ensure satisfactory operation.
The type of lubrication may also make the difference between a successful application and one in which the bearing fails prematurely. For reference, lubrication principles are discussed in the PT Accessories Product Department of this handbook. For this discussion, however, a few terms are defined:
- Boundary lubrication - bearing and shaft surfaces rub together with only a thin film of lubricant separating them. Grease-lubricated bearings generally operate with a boundary film.
- Mixed-film lubrication - bearings support part of the load on a boundary film where the shaft is closest to the bearing. The remainder of the load is supported by hydrodynamic, or full-film, lubrication.
- Full-film or hydrodynamic - the shaft is separated from the bearing by a continuous film of self-pressurized lubricant with no metal-to-metal contact. This fluid film is generally about 0.001 in. thick, but films as thin as 0.0005 in. are sufficient if shaft surface finish is held within 10-min. RMS and bearing inner surface is held to 30-min. RMS maximum.
In many situations, the bearing itself contains or acts as the lubricant. Such prelubricated or self-lubricating bearings are discussed later in this bearing department.
A bearing's load capacity is often determined through experience and generally is expressed as pounds per square inch (psi) of projected bearing area. A rule of thumb: maximum load capacity for static or very-low-speed applications is 1/3 the bearing material's compressive limit. Compressive limit is that which results in permanent deformation of 0.2%.
Rarely are industrial bearings loaded over 3,000 psi. In fact, most carry loads under 400 psi. A bearing's load capacity varies widely with its size and type of material. Figure 1 shows load capacities for three types of bronze, a material commonly used for plain bearings.
Another method of determining a bearing's load capacity is through maximum PV factor. This is the value of pressure on the bearing, in psi, times the shaft speed, in feet per minute. As with pressure, PV factors should be used only as a guide because other conditions also affect load capacity. Figure 2 shows maximum PV factors for three common types of bronze.
Although PV factor serves as a useful guide in determining bearing capacity, the factor can be misleading in some situations. For example, Figure 2 shows that lubricated sintered bronze accommodates a PV factor of 50,000. However, a load of 15,000 psi operating at 2 fpm would be unacceptable because the load exceeds the compressive strength of the material. Similarly, an application may have an acceptable PV factor, though speed, rather than load, exceeds limitations. Maximum permissible speed is a function of lubrication, alignment, shaft surface finish, and hardness. Also, temperatures must stay within bearing and lubricant limits.
Though it must be understood that neither P nor V for a given material can be exceeded, the magnitude of heat generated for a high P in combination with a low V will be far less than a low P, high V, situation. Velocity, more than pressure, influences temperature due to sliding for the same product of P and V.
Bearings operating with full-film lubrication typically exhibit a coefficient of friction between 0.001 and 0.020, depending on mating surfaces, lubricant, clearances, and speed. For a mixed-film bearing, the coefficient ranges between 0.02 and 0.08, and for boundary-lubricated bearings, between 0.08 and 0.14.
The coefficient of friction in a bearing application is important because the higher the coefficient of friction, the higher the heat generation. Excessive heat reduces life of the bearing. Excessive heat may also cause expansion of the shaft, housing, or bearing, or any combination of these. This expansion reduces the clearance between the shaft and bearing, further increasing operating temperature, resulting eventually in premature bearing failure.
Many metallic, nonmetallic, and compound materials are available to designers. Bronze has probably been the most familiar plain bearing material because a variety of characteristics can be imparted to it by adding other metals. In general, softer materials are designed for lighter loads and higher speeds; harder materials for higher loads and lower speeds.
Metallic - The softest metallic bearing materials are babbitts. Both tin and lead-based babbitts have been widely used as bearing materials for years. They are much softer than bronze and are able to embed foreign particles, which helps prevent shaft scoring or wearing. Babbitt bearings offer excellent resistance to shaft scoring and seizing in boundary lubrication conditions. Because they are so soft, these materials usually serve as linings, with stronger material for support.
Copper-lead is also soft, though it approaches some of the softer bronzes in hardness. Steel backing is usually needed in copper-based bearings to raise strength. Another design has a thin babbitt bearing surface and steel backing, with copper-lead sandwiched between.
In terms of increasing hardness, the next material family is bronze alloys. They serve from very high-speed, light-load uses to very light-load, high-speed uses.
Leaded bronzes are widely used when start/stop cycles are high. But because these materials are soft, they are limited by low load-carrying and operating-temperature capacities. High lead content helps these bronzes resist seizing or scoring of the shaft. Maximum operating temperature of leaded bronzes runs typically from 400 to 450°F. Decreasing the lead concentration increases strength and hardness of the material, but decreases its conformability, scoring resistance, and ability to embed foreign particles.
Tin bronze contains much less lead than leaded bronze. This makes it more suitable for high loads at lower speeds. But lubrication is more important because tin bronze has less protection against seizing and scoring.
Manganese bronze is an alloy consisting mostly of copper and zinc. Addition of aluminum, iron, and manganese increases the material's hardness and strength. Thus, manganese bronzes can carry much heavier loads than the softer bronzes, but again, adequate lubrication must be provided. Also, higher quality shaft finish is required.
Aluminum bronze is a copper-based alloy containing up to 14% aluminum and various other metals. Aluminum bronze has become popular over the last few decades due mainly to its resistance to creep, corrosion, wear, and oxidation at high temperature, as well as its high strength.
Another popular bronze material is sintered bronze. It is made from powdered bronze which, when subjected to high pressure and temperature, forms a porous material. The finished material contains oil impregnated in the pores.
Sintered iron bearings, made similarly, have become a popular cost-effective alternative, especially in high-volume uses such as fhp motors. They also behave similarly in that either sintered bronze or sintered iron can operate in boundary (thin-film) or hydrodynamic (full-film) lubrication mode, depending on application parameters. In theory, the major differences between a porous and a nonporous bearing, presuming steady state and an adequate oil supply, are:
- In the "pressure wedge," oil escapes into the porous bearing's pores and reduces the hydrodynamic oil pressure available for load support.
- In the region of reduced pressure (the unloaded part of bearing clearance), oil is drawn from the pores and oil-film cavitation is reduced.
The two effects set up oil circulation in the pores.
Zinc-aluminum alloys have emerged in recent years as a cost-effective alternative to bronze alloys, However, successful application of zinc-aluminum alloys is restricted primarily to high-load, low-speed applications. Figure 3 shows the maximum load-speed-curves for two zinc-aluminum alloys plotted against SAE 660 bronze. The tests were conducted under the sponsorship of the International Lead Zinc Research Organization, by Battelle Institute, Columbus, Ohio.
Though the graph clearly shows higher load capacity of the zinc-aluminum alloys (which could also be interpreted as longer life under equal load), it should be pointed out that even though zinc-aluminum alloys cost much less than bronze, they are more limited by temperature than bronze, having maximum operating temperatures below 300°F. Zinc-aluminum alloys should therefore be limited to low-speed, low-temperature applications.
Because of high cost, other materials such as cadmium and silver are in only limited use. Cadmium can serve in high temperatures where no other material is satisfactory. Suspected toxicity of cadmium in some uses should be considered. Silver has good resistance to seizing and shaft scoring, and is usually electroplated onto a steel backing. When low cost is a prime concern, cast iron or steel bearings can be used at light loads. Flake graphite in the cast iron glazes the bearing surface, which is useful at speeds to about 130 fpm and loads to 150 psi.
Nonmetallic - Nonmetallic or self-lubricating bearings often require no liquid lubricant. Self-lubricating bearings are most effective in applications where relative motion is not sufficient to circulate oil or grease required for metallic bearings. Self-lubricating bearings are also used for temperatures beyond the scope of conventional lubricants. These temperatures may range from 400 to 750°F or higher. Self-lubricating bearings are especially well suited for corrosive environments.
Friction, coupled with rapid wear, limits the application of self-lubricating bearings. The coefficient of friction of self-lubricating bearings running completely dry generally ranges from 0.1 to 0.4. The mechanical energy lost in the bearing is converted to heat, which must be dissipated. The materials generally are poor conductors of heat, so it is important to provide a means of dissipating heat from the bearing. Typically, about half the heat flows radially outward to the support housing, while the other half transfers to the shaft and flows axially away from the bearing.
The most common self-lubricating materials include polytetrafluor-oethylene (PTFE), graphite, and molybdenum disulfide (MoS2). PTFE is a soft, waxy solid, which is usually compounded with reinforcing materials such as composite fabrics with epoxy resins. It is also compounded with metal or ceramic powders to build strength and improve thermal conductivity, or is supported on a porous bronze substrate or stainless steel or bronze screen.
Graphite is too weak for use by itself. Tiny graphite flakes are generally bonded with carbon or thermosetting resins. Suppliers of carbon bearings offer scores of individual grades tailored to requirements of specific applications.
MoS2 crystals are generally bonded with resins or metal. In many cases MoS2 is incorporated into plastic bearings, such as nylon, to improve bearing life. Other plastics often used for bearings include Acetal and Polyimide.
Both wear rate and friction are greatly reduced when self-lubricating bearings run submerged in any liquid because the liquid cools the bearing. In fact, even poor lubricants - ammonia, propane, and water - form enough of a hydrodynamic film to carry part of the load. The self-lubricating benefits provide an ÒartificialÓ lubricant film for startups, shock loads, and other transients, while the fluid provides full-film lubrication once adequate speed is reached.
Self-lubricating bearings can improve performance even in fully lubricated applications. For example, with hydrodynamic lubrication, self-lubricating bearings have friction coefficients similar to those of lubricated metals. Yet, self-lubricating materials can provide longer life because they resist wear at startup when lubricants are not fully effective. Another advantage is lower startup torque, which reduces the system's power requirements.
A prelubricated bearing is made of a nonmetallic material with grease pockets. With this type of bearing, an initial supply of lubricant is applied at startup, and is gradually released as the bearing wears.
Grooving for lubrication
Often, a metallic bearing's axial length must be large to carry the required load. To provide lubricant throughout the entire length of the bearing, the ID often contains oil grooves. Most short bearings have no grooves. However, most have an oil hole centrally located in the unloaded area of the bearing. In general, oil will flow unaided by grooves approximately 1/2 in. axially to each side of the oil hole. If the bearing has an axial length greater than 1 in. (not including the oil hole diameter) a groove is usually necessary. A groove may also be required to produce an oil film when the bearing length-to-diameter ratio is greater than 1:1. With a grease film, the ratio may approach 1.5:1 without a groove. To provide a complete and continuous film in a grease-lubricated bearing, grease must be pumped into the bearing continuously.
Groove depth is generally 1/16 in. or about 1/3 the wall thickness. Oil grooves are usually 1/8 in. wide. Grease grooves may be 1-1/2 times the width of an oil groove. The groove should come no closer to the end of the bearing than 0.05 times the length of the bearing, or a minimum of 1/8 in. The groove can penetrate one or both ends of the bearing if oil is introduced at these points.
Grease-lubricate sintered bronze bearings only if they are grooved. Ordinary soap-based grease should not impregnate the pores because the soap will clog the pores. Grease should be introduced through a hole drilled in the unloaded portion of the bearing.