MISCELLANEOUS MECHANISMS 409 
" eaee” be arranged to be in tension when in action, as shown in 
Fig. 664. 
__ If friction is neglected, the reaction of a tooth on the pawl acting on 
it will be perpendicular to the face of the tooth, and in order that the 
pawl may not slip out of gear the line of action of the reaction must 
evidently pass between the axis of the wheel and the axis of the pin 
which carries the pawl when the pawl acts with a thrust, as in 
Figs. 662 and 663 ; but when the pawl acts with a pull, as in Fig. 664, 
then the above-mentioned line of reaction must lie beyond the axis of the 
pawl pin away from the axis of the wheel. 
3 The limiting position of the line of the reaction of the tooth on the 
pawl, when friction is considered, is shown in Fig. 665 for a pushing 
Fia. 665. Fig. 666. 
” 
ratchet, and in Fig. 666 for a pulling ratchet. LN is the normal to the 
face of the tooth. When slipping is about to take place between the 
tooth and the pawl, LR, the line of action of the reaction of the tooth on 
the pawl, will make with LN an angle NLR equal to ¢, the friction angle, 
and when this force is just about to rotate the pawl on its pin, LR will 
touch the friction circle F, as shown. 
* In order that there may be no lost motion of the lever C in Figs. 662 
and 664, the angle through which it swings must be an exact multiple of 
the angle @ subtended by one tooth of the wheel at its centre, and the 
amount of possible lost motion or back lash will be slightly less than @ 
when the angle of swing of the lever is 
not an exact multiple of 6. The possible ffVP"—dllr 
back lash is therefore smaller the smaller Lo <<~ 
the pitch of the teeth of the wheel. 
Reducing the pitch of the teeth reduces 
their strength, and in order to reduce the 
possible back lash without reducing the 
pitch of the teeth, two or more pawls are 
generally used in the manner shown in 
Fig. 667, which represents the ratchet 
mechanism of the Williams universal 
ratchet drill. In this example the ratchet 
wheel has twelve teeth, and there are five 
pawls, but only one pawl at a time can Fig. 667. 
with the wheel. The maximum 
possible back lash is in this case just under one-sixtieth of a revolution, 
or just under 6°. 
Reversible ratchets are used when it is desired to drive the ratchet 
wheel in either direction. Fig. 668 shows a form of reversible ratchet 
DMSWW 
