Page 213 - Tribology in Machine Design

198 Tribology in machine design

Figure 5.17

and eccentricity, the load capacities p and P are doubled. The zero capacity
of the bearing in case (c) represents a typical situation for the crankpin
bearings of four-stroke-cycle engines. The same is true in the case of the
bushing of an idler gear and the shaft that supports it, if they turn with
opposite but equal magnitude velocities relative to a non-rotating load on
the gear. The analyses discussed give some ideas on relative capacities that
can be attained and indicate the care that must be taken in determining n'
for substitution in the load number equation. However, it should be noted
that the load numbers and actual film capacities are not a function of n'
alone.
The diameters d and lengths / of the two films may be different, giving
2
different values to p = P/ld and to (d/l) in the load number, but they may be
adjusted to give the same load number. Also, a load rotating with the shaft,
case (b), appears to give the bearing the same capacity as the bearing
illustrated by case (a). However, unless oil can be fed through the shaft to a
hole opposite the load, it will probably be necessary to feed oil by a central
annular groove in the bearing so that oil is always fed to a space at low
pressure. With pressure dropping to the oil-feed value at the groove in the
converging half, the bearing is essentially divided into two bearings of
approximately half the l/d ratio. Since d/l is squared in the load-number
equation, each half of the bearing has one-fourth and the whole one-half the
capacity of the bearing in case (a).
Another way to deal with the problem of the rotating load vector is
shown in Fig. 5.18. Letcoj andco 2 be the angular velocities of the shaft or the