Fig. 184.
The forms of spur-gearing having their teeth at an angle to the axis, or formed in advancing steps shown in [Figs. 183] and [184], were designed by Dr. Hooke, and “were intended,” says the inventor, “first to make a piece of wheel work so that both the wheel and pinion, though of never so small a size, shall have as great a number of teeth as shall be desired, and yet neither weaken the wheels nor make the teeth so small as not to be practicable by any ordinary workman. Next that the motion shall be so equally communicated from the wheel to the pinion that the work being well made there can be no inequality of force or motion communicated.
“Thirdly, that the point of touching and bearing shall be always in the line that joins the two centres together.
“Fourthly, that it shall have no manner of rubbing, nor be more difficult to make than common wheel work.”
Fig. 185.
The objections to this form of wheel lies in the difficulty of making the pattern and of moulding it in the foundry, and as a result it is rarely employed at the present day. For racks, however, two or more separate racks are cast and bolted together to form the full width of rack as shown in [Fig. 185]. This arrangement permits of the adjustment of the width of step so as to take up the lost motion due to the wear of the tooth curves.
Another objection to the sloping of the teeth, as in [Fig. 183], is that it induces an end pressure tending to force the wheels apart laterally, and this causes end wear on the journals and bearings.