DRY AND BOUNDARY LUBRICATION BEARINGS

DRY AND BOUNDARY LUBRICATION
BEARINGS

Whenever the load on the bearing is light and the shaft speed is low, wear is not a critical problem and a sleeve bearing or plane-slider lubricated by a very thin layer of oil (boundary lubrication) can be adequate.

Sintered bronzes with additives of other elements are widely used as bearing materials. Liquid or
solid lubricants are often inserted into the porosity of the material and make it self-lubricated. However, in heavy-duty machinery—namely, bearings operating for long periods of time under heavy load relative to the contact area and at high speeds—better bearing types should be selected to prevent excessive wear rates and to achieve acceptable bearing life.

In most applications, the sliding surfaces of the bearing are lubricated.

However, bearings with dry surfaces are used in unique situations where lubrication is not desirable. Examples are in the food and pharmaceutical industries, where the risk of contamination by the lubricant forbids its application. The sliding speed, V, and the average pressure in the bearing, P, limit the use of dry or boundary lubrication. For plastic and sintered bearing materials, a widely
accepted limit criterion is the product PV for each bearing material and lubrication condition. This product is proportional to the amount of frictionenergy loss that is dissipated in the bearing as heat. This is in addition to limits on the maximum sliding velocity and average pressure.

 For example, a self lubricated sintered bronze bearing has the following limits:

Surface velocity limit, V, is 6m=s, or 1180 ft=min

Average surface-pressure limit, P, is 14 MPa, or 2000 psi

PV limit is 110,000 psi-ft=min, or 3:85 106 Pa-m=s

In comparison, bearings made of plastics have much lower PV limit. This is
because the plastics have a low melting point; in addition, the plastics are not
good conductors of heat, in comparison to metals. For these reasons, the PV limit
is kept at relatively low values, in order to prevent bearing failure by overheating.

For example, Nylon 6, which is widely used as a bearing material, has the following limits as a bearing material:

Surface velocity limit, V, is 5m=s

Average surface-pressure limit, P, is 6.9 MPa

PV limit is 105 103 Pa-m=s

Remark.

 In hydrodynamic lubrication, the symbol for surface velocity of a rotating shaft is U, but for the PV product, sliding velocity V is traditionally used.

Conversion to SI Units.

1 lbf=in:2 ðpsiÞ ¼ 6895 N=m2 ðPaÞ

1 ft=min ¼ 0:0051 m=s

1 psi-ft=min ¼ 6895 0:0051 ¼ 35 Pa-m=s ¼ 35 10 6 MPa-m=s

An example for calculation of the PV value in various cases is included at the end of this chapter. The PV limit is much lower than that obtained by multiplying the maximum speed and maximum average pressure due to the load capacity.

The reason is that the maximum PV is determined from considerations of heat dissipation in the bearing, while the average pressure and maximum speed can be individually of higher value, as long as the product is not too high. If the maximum PV is exceeded, it would usually result in a faster-than-acceptable wear rate.