Sleeve bearings are also known as journal bearings. It is a simple device for providing support and radial positioning while permitting rotation of a shaft. In the broad category of sleeve bearings can be included a great variety of materials, shapes, and sizes. Materials used include an infinite number of metallic alloys, sintered metals, plastics, wood, rubber, ceramic, solid lubricants, and composites. Types range from a simple hole in a cast-iron machine frame to some exceedingly complex gas-lubricated high-speed rotor bearings.
Sleeve bearings do have a number of advantages over rolling element bearings, as well as few disadvantages.
Advantages of using sleeve bearings:
- Inherently quiet operation because there are no moving parts.
- If sleeve bearings are properly selected and maintained, they do not fail suddenly.
- Wear is gradual, allowing scheduling of replacement.
- Well suited to oscillating movement of the shaft.
- With proper material selection, excessive moisture and submersion can be tolerated.
- With proper material selection, extreme temperatures can be accommodated.
Disadvantages of using sleeve bearings:
- High coefficient of friction.
- For the same boundary plan, much less load capacity.
- Life is not predictable except through experience.
In the application of sleeve bearings, the most important factor is the selection of the actual bearing material. The three most common industrial materials are babbitt, bronze, and cast iron. After these, there is an amazing variety of different bearing materials, often specialized for a particular application. In most cases, the details of selection are unique and assistance should be obtained from the manufacturer of the sleeve material.
Plain bearings made from babbitt are universally accepted as providing reasonable capacity and dependable service, often under adverse conditions. Babbitt is a relatively smooth bearing material, which minimizes the danger of scoring or damage to shafts or rotors. It often can be repaired quickly on the spot. Babbitt bearings are usually restricted to applications involving light to moderate loads and mild shock.
Bronze bearings are more suitable than babbitt for heavier loads bearings (75 to 200% higher), depending on specific conditions of load and speed. Bronze can withstand higher shock loads and permits higher speed operation. It is usually restricted to 300°F ambient temperatures if properly lubricated. Bronze is a harder material than babbitt and has a greater tendency to score or damage shafts in the event of a malfunction such as lack of relubrication. Field repair of bronze bearings generally requires removing shims and scraping or replacement of bushings. Bronze bushings are commonly available in both cast and sintered forms.
Cast-iron bearings are generally low in cost and suitable for many slow-moving shafts and oscillating or reciprocating arms supporting relatively light weight loads. The lubricating characteristics of cast iron are attributed to the free graphite flakes present in the material. With the use of cast-iron bearings, higher shaft clearance is usually utilized. Thus, any large wear particles or debris will not join or seize the beating. This material has been used to temperatures as high as 1000°F (where ordinary lubricants are ineffective), under light loads and slow speed intermittent operations.
Lubrication is just as important in sleeve bearings as it is in rolling element bearings. There are three basic conditions of lubrication for sleeve bearings: full film or hydrodynamic, boundary, and extreme boundary lubrication.
- In full film lubrication, the mating surfaces of the shaft and bearing material are completely separated by a relatively thick film of lubricant.
- Boundary lubrication occurs when the separating film becomes very thin.
- Extreme boundary lubrication occurs when mating surfaces are in direct contact at various high points. The first two categories give long bearing life, while the third results in wear and shorter life.
In a full film bearing, the coefficient of friction is in between 0.001 to 0.020, depending on the mating surfaces, clearances, lubricant type and viscosity, and speed. For a boundary lubricated bronze bearing, it is 0.08 to 0.14. Friction in a bearing design is important because temperature and wear are directly related to it. The lower the coefficient of friction, the longer the life of the bearing.
Either oil or grease lubricant can be used for lubrication as long as the temperature limitations for the grease or oil are not exceeded. Oil viscosity should be chosen between 100 and 200 SUS (Saybolt Universal Second) at the estimated operating temperature. Grease is the most common lubricant used for sleeve bearings, mainly due to lubricant retention. Grease lubricated bearings usually function with a boundary film. Many sleeve bearings use grooving to improve lubrication on long sleeves. If the sleeve length to diameter ratio is greater than 1.5 : 1, a groove should be choosed.
Under certain operating conditions, dry lubrication can be used successfully with sleeve bearings. Graphite cast bearings are inaccessible for relubrication. Typical operating bronze bearings are commonly used at elevated temperatures, in low speed or high load applications, or where the conditions for graphite bearings are 50 psi load with speeds to 30 sfm or a maximum Pressure Velocity (PV) factor of 1,500.
There are a number of factors that combine to determine the type of lubrication a bearing will have. Any of the following changes in the application would result in improved lubrication and longer bearing life:
- A greater supply of lubricant available at the bearing increased shaft speed, which gives increased oil film thickness
- Reducing the load, which will increase the oil film thickness
- Better bearing alignment
- Smoother surface finishes
- Use of a higher viscosity lubricant
The load carrying ability of a sleeve bearing is usually expressed in pounds per square inch (psi). This is calculated by dividing the applied load in pounds by the projected bearing area in square inches. Projected bearing area is found by multiplying the bearing bore diameter by the effective length of the sleeve. Few industrial bearings are loaded over 3,000 psi, and most are carrying loads under 400 psi. With cast-bronze sleeve bearings, 1,000 psi is acceptable. A usable figure for flat thrust washers is 100 psi. Below figure shows the maximum loads for sinstered, bronze, aluminum bronze and lead-tin bronzes.
Another way of finding load capacity is through its maximum Pressure Velocity (PV) factor. The PV factor is the bearing load pressure times the surface velocity of the shaft in feet per minute (sfm). For speeds higher than 200 sfm, use a PV factor of 20,000 for bronze sleeves and 10,000 for babbitt sleeves. Of course, there are maximum load limits and higher & lower speed limits that must also be kept in mind when working out with the PV factors.
Very careful shaft alignment of bearings is important during installation. Shaft journals must turn freely without binding in the bearing, otherwise, excessive heat and failure can happen. Sharp edges on the shaft or the bearing surface can act as scrapers to destroy lubricant films. Do not extend shaft keyways into bearing bores. Shafting should be of the proper size and finish. Shaft diameters for rigid sleeve bearing units are usually held to the regular commercial tolerances as shown in the below table. Standard shaft surface roughness of 32 Ra is acceptable for most applications. Graphited sleeves should have shaft roughness reduced to 12 Ra.
Recommended Shaft Tolerances for Sleeve Bearings
|1||Through 2″||Nominal to -0.003″.|
|2||2″ through 4″||Nominal to -0.004″.|
|3||4″ through 6″||Nominal to -0.005″.|
|4||6″ through 8″||Nominal to -0.006″.|