410 APPLIED MECHANICS 
used on a screw-jack. A common example of the reversible ratchet is to 
be found in the feed 
motions of planing and 
shaping machines. 
In the various forms of 
ratchet which have been 
illustrated, the ratchet is 
kept. in contact with the 
wheel either by the weight 
of the ratchet or by the 
action of a spring, and 
when the pawl is moving 
backwards over the teeth 
it drops from one tooth 
Fra. 668. 
on to the next with a clicking noise. . To avoid this clicking several forms 
of silent ratchet have been designed. In the form shown in Fig. 669, 
the pawl B has attached to 
it an arm C, at the lower 
end of which there is a 
recess containing a plug D 
pressed outwards by a spring 
against a facing E on the 
ratchet wheel A. When the 
relative motion between the 
wheel and the pawl would 
cause the latter to rise and 
fall on the teeth of the 
wheel, the friction between 
HH 
Fig. 669. 
E and D causes the arm C and pawl B to swing round until B comes in 
contact with the stop 8, and the pawl remains clear of the teeth on A. 
When the relative motion between the wheel and 
the pawl is in the opposite direction, the friction 
between E and D causes C and B to swing back 
until B engages with a tooth on A. Another 
method of operating the arm C is shown in Fig. 
670. An extension of the rim of the ratchet wheel 
has a groove cut in it into which is sprung a ring 
Fia@. 670. 
H, between the ends of which there isa gap to receive the lower end of the 
arm C. When C is pressing against one end of the ring H, the pawl is 
pressed against the stop, and is out of gear, and when C is in contact with 
the other end of H, the 
pawl is in gear with a 
tooth on the wheel. The 
friction between the ring 
H and the bottom of the 
groove into which it fits is 
sufficient to operate C when 
there is relative motion be- 
tween the wheel and the 
pawl. 
Fia. 671. 
Friction ratchets have been very successful, their application to 
