Here, 20 ÷ 10 = 2 = diameter of wheel at the pitch circle. The circumference of 2 inches being 6.28 inches we have, as the degrees of angle of the axes of the shafts are to 45°, so is 6.28 inches (the circumference of the wheels, to the pitch sought).
Here, 6.28 inches × 45° ÷ 25° = 11.3 inches, which is the required pitch for the spiral.
Fig. 190.
When the axes of the shafts are neither parallel nor meeting, motion from one shaft to another may be transmitted by means of a double gear. Thus (taking rolling cones of the diameters of the respective pitch circles as representing the wheels) in [Fig. 190], let a be the shaft of gear h, and b b that of wheel e. Then a double gear-wheel having teeth on f, g may be placed as shown, and the face f will gear with e, while face g will gear with h, the cone surfaces meeting in a point as at c and d respectively, hence the velocity will be equal.
Fig. 191.
When the axial line of the shafts for two gear-wheels are nearly in line one with the other, motion may be transmitted by gearing the wheels as in [Fig. 191]. This is a very strong method of gearing, because there are a large number of teeth in contact, hence the strain is distributed by a larger number of teeth and the wear is diminished.