64

PRINCIPLES OF MECHANISM.

that is to say, the 'pitch of the screiv on c d varies with the

number of teeth on its wheel a.

Let k and be the number of threads per inch on the cylindersC d and f E respectively, then

-r = t , and — t lf

and eq. (2) becomes —

Now, let there be an intermediate pinion and wheel, turningon the same axis, placed between A and b ; and let the pinion(acted upon by a) contain e, teeth, and the wheel e teeth; thenthe velocity ratio of the axis f e will be increased by the ratio

g

—, and hence eq. (3) becomes—e \

n, e

n e

Example .—Let n = 30, n l — 10, t l = ^ in.; required t.

Here by eq. (1), t = ^~ x £ = 1* in -

To produce a changing reciprocating rectilinear motion

by a combination of the camb and screw.

110. ef is a conical shaped camb, turning on the eccentricaxis A B, on which is cut the screw K B, working in the fixed nut

or hollow screw N ; nc a rod, rest-ing on the camb, constrained to movein the direction of its length, andto which the varying reciprocatingmotion is to be given. Here, whilstthe camb revolves, it has a con-

Fig. 81.

B tinuous motion in the direction ofthe axis A B, so that the lower ex-tremity, C, of the rod d c describes a

spiral or screw curve upon the cone whose pitch is equal to thepitch of the screw K B. The effect of this is to make c d reci-procate in its path in such a manner that the stroke in onedirection is shorter than that in the opposite direction.