This will
be understood by inspecting Fig. 107, where _A A'_ shows a lever fork as
if in contact with both banks, and the jewel pin, represented at _B
B''_, just passes the angle _a c'_ of the fork. The circle described by
the jewel pin _B_ is indicated by the arc _e_. It is well to put a
slight friction under the balance rim, in order that we can try the
freedom of the guard pin. As a rule, all the guard pin needs is to be
free and not touch the roller. The entire point, as far as setting the
fork and bankings is concerned, is to have the fork and roller action
sound. For all ordinary lever escapements the angular motion of the
lever banked in as just described should be _about_ ten degrees. As
explained in former examples, if the fork action is entirely sound and
the lever only vibrates through an arc of nine degrees, it is quite as
well to make the pallets conform to this arc as to make the jewel pin
carry the fork through full ten degrees. Again, if the lever vibrates
through eleven degrees, it is as well to make the pallets conform to
this arc.
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