Iron: Combine
low cost with good bearing qualities, widely used in automotive applications, toys, farm
equipment, and machine tools. Powdered-iron is frequently blended with up to 10% copper
for improved strength. These materials have a relatively low limiting value of PV (on the
V side), but have high oil-volume capacity because of high porosity. They have good
resistance to wear, but should be used with hardened-and-ground steel shafts.
Leaded-Iron: Provide improved speed capability, but are
still low-cost bearing materials.
Aluminum: In some applications they provide cooler
operation, greater tolerance for misalignment, lower weight, and longer oil life than
porous bronze or iron. The limiting PV value is 50,000, the same as for porous bronze and
porous iron.”
2.0 SIZING POROUS-METAL BEARINGS
The load-carrying capacity of porous-metal bearings can be measured by a friction/wear
criterion, which is a measure of the seat generated by the bearing. It is called the PV
factor. The PV factor, as its name implies, is the product of the bearing load, P,
expressed In pounds per square inch of projected bearing area, and the surface velocity of
the shaft expressed in feet per minute.
If d = inside bearing diameter (in.)
l = length of
bearing (in.)
F = bearing load (lbs.)
and N = shaft speed (RPM),
then P = F lbs/in2
(1)
ld
V = pdn
ft/mm.
(2)
12
and hence PV = ( F ) ( pdn )
ld
12
= pFN =
0.262 FN
(3)
12l l
Most engineering data
relating to the PV faotor lists an upper limit of the factor, i.e. a value which should
not be exceeded for satisfactory bearing operation. The working value of the PV factor,
however, is often less than this upper limit, for example if the sliding velocity is not
sufficiently high to maintain an adequate lubricating film. In addition, the PV limit is
affected by the static load-carrying capacity of the material, which should not be
exceeded. The latter is a function of environmental factors, bearing clearances and
geometry and the nature of the load (continuous, intermittent or shock loading). Detailed
information on these considerations is usually furnished by the metal manufacturer.
General guidelines are summarized in Table 1.
3.0 CLEARANCES
As in all bearings, satisfactory operation of porous-metal bearings require suitable
clearances between shaft and housing. While guidelines depend on the materials used and
the nature of the application, a representative chart showing recommended bearing
clearances for porous-bronze and porous-iron bearings is given in Figure 1: We carry a
full line of both thick and thin wall bushings. Please consult the Designer’s Data
section of this handbook for information on recommended shaft size and bore diameter to be
used with various bushing sizes.