FRICTION AND LUBRICATION 275 
and B the shaft. C is an eccentric clamped to the shaft. DD are end 
_ chambers connected by the passage E. F is a pump plunger, made from 
_ steel tubing forged on to a solid end. G are inlet holes in the plunger, 
which allow oil to pass to the inside of the pump when the plunger is at 
and near the top Of its stroke. The eccentric drives the pump plunger, 
the latter being kept up to the former by the spring H. I is a non- 
return ball valve, and J an outlet from the pump to the shaft. K is a 
sight feed plug supported by a cross pin beneath it. L is a screwed 
plug to drain off spent oil and dirt. MM are leather washers to prevent 
oil travelling along the shaft. N is a screwed plug giving access to the 
x oom E for cleaning purposes. As the plunger descends, the inlet 
es G are cut off by the casing, and oil is forced past the non-return 
valve and through the outlet J to the shaft, and thence to the end 
_ chambers DD. | 
Forced lubrication has been used with great success on high-speed 
steam-engines. The various bearings are connected by pipes and 
to an oil pump driven by the engine. The oil after being 
used passes through a filter back to the reservoir which supplies the 
| Pe Splash lubrication is common and simple, but, crude, and is used on 
high-speed vertical engines, especially on petrol engines. The engine is 
enclosed, and the crank case contains oil, into which the cranks splash as 
they rotate, throwing the oil over the various bearings. 
241. Friction of Sliding Keys.—In machines it is frequently neces- 
sary to move a piece longitudinally on a shaft, while there is a torque 
between the piece and the shaft. In such cases a sliding key may be 
fixed to the sliding piece and fit easily into a keyway in the shaft, or the 
key may be fixed to the shaft and fit easily into a keyway in the sliding 
piece, as shown in Fig. 419, where the looseness of fit between the piece 
_ A and the shaft B, and be- 
tween the key C and the 
keyway in A, is exaggerated. 
If the piece A is driven in 
the direction of the arrow 
D by a torque T, the forces 
which transmit this torque 
to the shaft are the equal 
forces P and Q at a distance 
_ from one another, so that 
Pr=T. Iftwo keys be used, 
as shown in Fig. 420, the equal forces P and Q will now be at a 
distance 2r from one another, and 2Pr=T. Hence the force causing 
the sliding friction in the second arrangement is only half what it is in 
the first arrangement. To get the full advantage of the two keys it is 
necessary that they be very accurately fitted, so that they transmit the 
whole of the torque without any pressure between the sliding piece and 
the shaft itself. 
. 242. Rotating Guides for a Sliding Piece.—It is well known that a 
piece mounted loosely on a shaft may be made to slide along the shaft by 
the application of a smaller force when the shaft is rotating than when 
the is at rest, and the greater the speed of the shaft, the smaller is 
‘Fig. 419. Fiq@. 420. 
