Plain bearings sleeve bearings
Plain bearings are the simplest type of support, consisting solely of a contact surface without rolling elements. They are characterized by low cost, high load capacity, compactness, and low weight. They can operate in a temperature range from -200 °C / -328 °F up to 300 °C / 572 °F and withstand pressures up to 100 MPa / 14.5 ksi in metallic materials. The coefficient of friction with adequate lubrication ranges between 0.01 and 0.15, while dry it reaches values of 0.1 to 1.0 depending on the materials.
Types of plain bearings
Section titled “Types of plain bearings”By the motion they allow:
- Radial (journal) bearing: the most common, a shaft rotates inside a hole or bushing. Also called a friction bushing or sleeve bearing.
- Linear bearing: allows rectilinear displacement, either by a cylindrical bushing on a shaft or two flat surfaces (guides).
- Axial (thrust) bearing: supports loads in the direction of the shaft; typical in propellers or thrust plates.
By their construction design:
- Solid bushing: one-piece sleeve, available in plain cylindrical and flanged versions.
- Split bushing: has a longitudinal cut to facilitate mounting on shafts with shoulders.
- Clinch bushing: similar to split but with a fold that joins the ends of the cut, improving retention.
Materials
Section titled “Materials”The most commonly used materials and their indicative properties are shown in the following table.
| Material | Maximum PV (MPa·m/s / psi·ft/min) | Maximum temperature (°C / °F) | Coefficient of friction (dry / lubricated) | Typical applications |
|---|---|---|---|---|
| Babbitt (tin/lead) | 10 / 285 000 | 130 / 266 | 0.02 – 0.10 | Turbines, compressors, high load at low speed |
| Bronze SAE 660 | 2.6 / 75 000 | 230 / 446 | 0.10 – 0.20 / 0.05 – 0.15 | General machinery, mining equipment |
| Sintered bronze (Oilite) | 1.75 / 50 000 | 170 / 338 | 0.08 – 0.15 | Small electric motors, appliances |
| Gray iron | 2.0 / 57 000 | 260 / 500 | 0.15 – 0.25 / 0.07 – 0.15 | Linear guides, furnace supports |
| PTFE (Teflon) | 0.05 / 1 400 | 260 / 500 | 0.04 – 0.10 | Food equipment, corrosive environments without lubricant |
| Nylon (polyamide) | 0.10 / 2 800 | 100 / 212 | 0.10 – 0.40 | Quiet bushings, lightweight parts with marginal lubrication |
| Carbon graphite | 0.15 / 4 300 | 350 (in inert atm.) / 662 | 0.10 – 0.25 | Furnaces, hot fluid pumps, aggressive atmospheres |
Standard dimensions
Section titled “Standard dimensions”ISO 4379 establishes dimensions for cylindrical friction bushings. Inner diameters and wall thicknesses are standardized. Below is a selection of common commercial sizes.
| Nominal shaft diameter (mm / in) | Bushing inner diameter H7 (mm / in) | Outer diameter (mm / in) | Length (mm / in) |
|---|---|---|---|
| 10 / 0.394 | 10.015 / 0.3943 | 13 / 0.512 | 10 / 0.394 |
| 15 / 0.591 | 15.018 / 0.5913 | 18 / 0.709 | 12 / 0.472 |
| 20 / 0.787 | 20.021 / 0.7882 | 25 / 0.984 | 20 / 0.787 |
| 30 / 1.181 | 30.025 / 1.1821 | 38 / 1.496 | 30 / 1.181 |
| 50 / 1.969 | 50.030 / 1.9697 | 60 / 2.362 | 50 / 1.969 |
| 80 / 3.150 | 80.035 / 3.1510 | 95 / 3.740 | 80 / 3.150 |
Typical housing tolerances are H7; for the shaft, f7 (normal clearance) or e7 (greater clearance for expansion) is recommended.
Load capacities
Section titled “Load capacities”The capacity of a plain bearing is evaluated by the maximum allowable pressure, the limiting speed, and the PV factor. The following table provides indicative values for continuous service life with adequate lubrication.
| Bushing material | Maximum pressure (MPa / psi) | Maximum speed (m/s / ft/min) | Maximum lubricated PV (MPa·m/s / psi·ft/min) |
|---|---|---|---|
| Babbitt | 15 / 2175 | 10 / 1968 | 10 / 285 000 |
| Bronze SAE 660 | 30 / 4350 | 5 / 984 | 2.6 / 75 000 |
| Sintered bronze | 20 / 2900 | 2.5 / 492 | 1.75 / 50 000 |
| Gray iron | 10 / 1450 | 3 / 591 | 2.0 / 57 000 |
| PTFE | 3 / 435 | 0.5 / 98 | 0.05 / 1 400 |
| Nylon | 7 / 1015 | 1 / 197 | 0.10 / 2 800 |
For dry operation, PV values are reduced by 40% to 80%.
Selection criteria by application
Section titled “Selection criteria by application”| Application | Recommended bearing type | Material | Main reason |
|---|---|---|---|
| Main bearings of hydraulic turbines | Radial split bushing | Babbitt | High load capacity and easy replacement |
| Centrifugal pumps and mixers | Radial solid bushing | Bronze SAE 660 | Corrosion resistance and good thermal conductivity |
| Agricultural and construction tools | Radial solid bushing | Sintered bronze (self-lubricating) | Minimum maintenance, resists dirt |
| Hinges, pedals, and light linkages | Radial clinch or flanged bushing | Nylon or sintered bronze | Simple assembly and quiet operation |
| Furnaces and high-temperature equipment | Radial solid bushing | Carbon graphite | Withstands >300 °C without liquid lubricant |
| Food and pharmaceutical industry | Radial or linear | PTFE | Non-contaminating, resists chemicals, and self-lubricating |
| Machine tool guides | Linear (slide plate) | Gray iron or bronze | High rigidity and dimensional stability |
Mounting considerations
Section titled “Mounting considerations”- Recommended fits: housing H7, shaft f7 (typical clearance) or e7 (greater clearance). The following table provides indicative radial clearances cold.
| Shaft diameter (mm / in) | Recommended total radial clearance (mm / in) |
|---|---|
| 5 – 25 / 0.197 – 0.984 | 0.015 – 0.050 / 0.0006 – 0.0020 |
| 25 – 60 / 0.984 – 2.362 | 0.025 – 0.075 / 0.0010 – 0.0030 |
| 60 – 120 / 2.362 – 4.724 | 0.050 – 0.150 / 0.0020 – 0.0059 |
- Shaft surface finish: Ra 0.4 – 1.6 µm / 16 – 63 µin for metallic bushings; Ra 0.2 – 0.8 µm / 8 – 32 µin for polymer bushings.
- Shaft hardness: minimum 200 HB to reduce wear; hardened steel (>50 HRC) is recommended for contact with sintered bronze.
- Lubrication: metallic bushings require grease grooves and grease fittings; self-lubricated types (oil-impregnated sintered bronze or PTFE) do not need periodic external supply.
- Press-fit mounting: apply uniform force on the outer diameter; for split or clinch bushings, a mandrel is recommended to prevent the cut from closing.
- Initial break-in: for soft metals (babbitt), a break-in with gradually increasing load and speed is essential to seat the surfaces.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the main difference between a plain bearing and a rolling-element bearing?
Section titled “What is the main difference between a plain bearing and a rolling-element bearing?”A plain bearing supports the load through a lubricant layer or direct contact without rolling elements, whereas a rolling-element bearing uses balls or rollers. A plain bearing can withstand loads up to 100 MPa / 14.5 ksi in tighter spaces, but generates higher friction at low speeds, with a starting torque 3 to 10 times higher than an equivalent rolling-element bearing.
What PV factor is recommended for a self-lubricating sintered bronze bushing with SAE 20 oil?
Section titled “What PV factor is recommended for a self-lubricating sintered bronze bushing with SAE 20 oil?”The maximum safe PV factor is 1.75 MPa·m/s / 50 000 psi·ft/min for a service life of 10,000 hours at 20 °C / 68 °F. Above this value, oil loss and wear accelerate.
What is the maximum operating temperature of a pure PTFE bearing?
Section titled “What is the maximum operating temperature of a pure PTFE bearing?”PTFE can operate continuously up to 260 °C / 500 °F and withstand peaks of 300 °C / 572 °F without immediate degradation, maintaining a coefficient of friction of 0.04 to 0.10.
What radial clearance should I provide for a bronze bearing on a 25 mm / 1 in shaft?
Section titled “What radial clearance should I provide for a bronze bearing on a 25 mm / 1 in shaft?”A total diametral clearance of 0.050 to 0.075 mm / 0.002 to 0.003 in is recommended for a bronze SAE 660 bushing lubricated with grease, operating at less than 1 m/s / 197 ft/min and room temperature.
What surface finish does the shaft need in a dry polymer plain bearing?
Section titled “What surface finish does the shaft need in a dry polymer plain bearing?”The shaft roughness should be Ra 0.2 – 0.8 µm / 8 – 32 µin. A rougher finish increases friction and prematurely wears the polymer; a smoother finish prevents transfer of solid lubricant material.
How much is the load capacity reduced when going from continuous lubrication to dry operation?
Section titled “How much is the load capacity reduced when going from continuous lubrication to dry operation?”The load capacity (maximum pressure and PV) is reduced by 40% to 80%. For example, a bronze bushing that supports 30 MPa / 4350 psi with lubrication may be limited to 6-12 MPa / 870-1740 psi dry, depending on speed.
References
Section titled “References”- mcmaster.com: https://www.mcmaster.com/products/plain-bushings/