We shall next give the details of
drawing the double roller, but before doing so we deemed it important to
explain the action of such guard points more fully than has been done
heretofore.
HOW TO DESIGN A DOUBLE-ROLLER ESCAPEMENT.
We have already given very desirable forms for the parts of a
double-roller escapement, consequently we shall now deal chiefly with
acting principles as regards the rollers, but will give, at Fig. 82, a
very well proportioned and practical form of fork. The pitch circle of
the jewel pin is indicated by the dotted circle _a_, and the jewel pin
of the usual cylindrical form, with two-fifths cut away. The safety
roller is three-fifths of the diameter of the pitch diameter of the
jewel-pin action, as indicated by the dotted circle _a_.
The safety roller is shown in full outline at _B'_, and the passing
hollow at _E_. It will be seen that the arc of intersection embraced
between the radial lines _B c_ and _B d_ is about sixty-one and a half
degrees for the roller, but the angular extent of the passing hollow is
only a little over thirty-two degrees.
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