Choosing a self-lubricating bushing is a balance of three variables: the load it carries, the speed at which the shaft slides against it, and the frictional heat that combination generates. Engineers capture that balance in three quantities - bearing pressure (P), sliding velocity (V) and their product (PV). Working through them in order is the single most reliable way to specify a sintered bearing that lasts.
Bearing pressure, P
Plain bearings are rated on projected area, not on the true curved contact area. The projected bearing pressure is the radial load divided by the projected area of the bore:
P = W / (d x L)
where W is the radial load in newtons, d is the bore (shaft) diameter and L is the bearing length, both in millimetres, giving P in N/mm2 (MPa). A short bush on a large shaft spreads the load over little area and sees high pressure; lengthening the bearing or increasing the diameter lowers it. That relationship is exactly why the length-to-diameter (L/d) ratio matters so much in sizing.
Sliding velocity, V
Sliding velocity is the surface speed at the bore:
V = pi x d x n / 60000
with d in millimetres and n in rev/min, giving V in metres per second. For oscillating or reciprocating motion the mean sliding speed is well below the peak, which is one reason self-lubricating bushes tolerate oscillating duty so well: the same shaft diameter that would be fast in continuous rotation produces only a modest mean V when it merely rocks back and forth.
The PV product and why it governs life
Friction at the interface converts mechanical work into heat at a rate proportional to load times speed - hence the PV product (N/mm2 x m/s), effectively a measure of frictional heat input per unit area. Every self-lubricating material has a PV ceiling above which the bearing runs too hot, the oil film thins, and wear accelerates. Crucially, two bearings can share the same PV yet behave very differently: a slow, heavily loaded pivot (high P, low V) and a light, fast-spinning bush (low P, high V) sit at opposite corners of the same envelope. Each also has an independent ceiling on P alone and on V alone, and all three limits must be respected together.
| Indicative class characteristic | Sintered bronze (BP25 class) | Sintered iron (FP20 class) |
|---|---|---|
| Maximum continuous sliding speed | of the order of 6 m/s | of the order of 2 m/s |
| Continuous PV ceiling | of the order of 1.8 N/mm2 x m/s | lower than bronze |
| Unit-load capacity at very low speed | good | higher than bronze |
| Best-suited duty | continuous rotation, higher speed | low speed, high load, oscillating |
These figures are indicative of the material class and are offered only as a starting point for sizing - they are not certified ratings for a particular reference. Confirm the permissible P, V and PV for the exact size, geometry and duty through a technical review before finalising a design.
A practical sizing sequence
- Fix the shaft diameter and the maximum radial load, then compute P from the projected area.
- Establish the maximum shaft speed, then compute V.
- Multiply for PV and compare against the material-class envelope above.
- If PV or P is marginal, lengthen the bush to raise L, step up the diameter, or move from a heavier-load choice to one with more speed capability.
- Confirm the operating temperature and select the impregnation oil to match.
Geometry: L/d ratio, wall and flange
An L/d ratio in the region of 0.5 to 1.5 covers most rotating applications. Longer bearings reduce pressure and help maintain alignment, but they demand a straighter, better-supported shaft; very short bearings edge-load easily. Sintered bushes have a defined wall thickness for their size band, so pressure is normally managed through length and diameter rather than wall. Where an axial or thrust component exists, a flanged bush is preferable: Bushingo's flanged BP25 bronze (C03-C60) and flanged FP20 iron references carry an integral collar that locates the bearing and reacts light thrust - for example a bore-20 flanged bronze bush with a 26 mm outside diameter, 20 mm length, 32 mm collar and 3 mm flange. Pure thrust or flat sliding is better handled by a friction plate than by a bush flange.
Matching the Bushingo range to duty
For continuous rotation and higher speeds, cylindrical BP25 bronze covers bores from 2 mm to 125 mm across three size bands. For low-speed, heavily loaded or oscillating duty, FP20 sintered iron spans bores from about 3 mm to 100 mm at lower cost. For flat and thrust sliding - slideways, wear pads and thrust faces - Bushingo's Metagliss self-lubricating friction plates are supplied as 375 x 375 mm plates in thicknesses from 4 mm to 20 mm and in three grades (M10, M60 and M90). Finally, temperature closes the loop on selection: standard TURBO T100 impregnation suits ordinary duty to around 100 degC, high-temperature PE1116 extends the usable range from -40 degC to +150 degC, and food-grade PE1152 serves hygienic equipment.