*Project Gutenberg*. Public domain.

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. The passing hollow *E*
is located and defined by drawing the radial line
*B c* from the center *B* through the intersection
of radial line *A i* with the dotted arc *b*,
which represents the pitch circle of the safety roller.
We will name this intersection the point *l*.
Now the end of the guard point *C* terminates
at the point *l*, and the passing hollow *E*
extends on *b* sixteen degrees on each side of
the radial line *B c*.

[Illustration: Fig. 82]

The roller action is supposed to continue through
thirty degrees of angular motion of the balance staff,
and is embraced on the circle *a* between the
radial line *B k* and *B o*. To delineate
the inner face of the horn *p* of the fork *F*
we draw the short arc *g*, from *A* as a
center, and on said arc locate at two degrees from
the center at *B* the point *f*. We
will designate the upper angle of the outer face of
the jewel pin *D* as the point *s* and,
from *A* as a center, sweep through this point
*s* the short arc *n n*. Parallel with
the line *A i* and at the distance of half the
diameter of the jewel pin *D*, we draw the short
lines *t t’*, which define the inner faces
of the fork.

The intersection of the short line *t* with the
arc *n* we will designate the point *r*.
With our dividers set to embrace the space between
the point *r* and the point *f*, we sweep
the arc which defines the inner face of the prong
of the fork. The space we just made use of is
practically the same as the radius of the circle *a*,
and consequently of the same curvature. Practically,
the length of the guard point *C’* is made
as long as will, with certainty, clear the safety
roller *B* in all positions. While we set
the point *f* at two degrees from the center
*B*, still, in a well-constructed escapement,
one and a half degrees should be sufficient, but the
extra half degree will do no harm. If the roller
*B’* is accurately made and the guard point
*C’* properly fitted, the fork will not
have half a degree of play.