The object of this design is to increase the bearing area, and thus, by causing the power transmitted to be spread over a larger area of contact, to diminish the wear. A mechanical means of cutting a worm to the required form for this arrangement is shown in [Fig. 83], which is extracted from “Willis’ Principles of Mechanism.” “a is a wheel driven by an endless screw or worm-wheel, b, c is a toothed wheel fixed to the axis of the endless screw b and in gear with another and equal toothed gear d, upon whose axis is mounted the smooth surfaced solid e, which it is desired to cut into Hindley’s[2] endless screw. For this purpose a cutting tooth f is clamped to the face of the wheel a. When the handle attached to the axis of b c is turned round, the wheel a and solid wheel e will revolve with the same relative velocity as a and b, and the tool f will trace upon the surface of the solid e a thread which will correspond to the conditions. For from the very mode of its formation the section of every thread through the axis will point to the centre of the wheel a. The axis of e lies considerably higher than that of b to enable the solid e to clear the wheel a.
[2] The inventor of this form of endless screw.
“The edges of the section of the solid e along its horizontal centre line exactly fit the segment of the toothed wheel, but if a section be made by a plane parallel to this the teeth will no longer be equally divided as they are in the common screw, and therefore this kind of screw can only be in contact with each tooth along a line corresponding to its middle section. So that the advantage of this form over the common one is not so great as appears at first sight.
Fig. 84.
Fig. 85.
“If the inclination of the thread of a screw be very great, one or more intermediate threads may be added, as in [Fig. 84], in which case the screw is said to be double or triple according to the number of separate spiral threads that are so placed upon its surface. As every one of these will pass its own wheel-tooth across the line of centres in each revolution of the screw, it follows that as many teeth of the wheel will pass that line during one revolution of the screw as there are threads to the screw. If we suppose the number of these threads to be considerable, for example, equal to those of the wheel teeth, then the screw and wheel may be made exactly alike, as in [Fig. 85]; which may serve as an example of the disguised forms which some common arrangements may assume.”