OF FACE GEARS.

189

Chap. 6.]

Fig. 39, is a spur and face wheel, or wallower, whosepitch circles should always meet exactly.

The rule for describing the sides of the cogs, so asnearly to approach the figure of an epicycloid, is as fol-lows; namely: Describe a circle a little inside of thepitch circle, for the point of your compasses to be set in,so as to describe the sides of the cogs, (as the four cogs atA, Plate V. fig. 38—39,) as near as you can to the curveof the epicycloid that is formed by the little wheelmoving round the great one; the greater the differencebetween the great and small wheels, the greater dis-tance must this circle be within the pitch circle: in do-ing this properly, much will depend upon the judgmentof the workman.*

article 80.

OF FACE GEARS.

The principle of face gears, is that of two cylindersrolling with the side of one on the end of the other,their axes being at right angles. Here, the greater the

* The following is Mr. Charles Taylor’s rule for ascertaining the true cycloidi-cal or epicycloidical form for the point of cogs:—

Make a segment of the pitch circle of each wheel, which gear into eachother; fasten one to a plain surface, and roll the other round it as shown, PlateV. fig. 37, and, with a point in the moveable segment, describe the epicy-cloid o b c; set off at the end o one-fourth part of the pitch for the length ofthe cog outside of the pitch circle. Then fix the compasses at such an opening, that with one leg thereof, in a certain point, (to be found by repeatedtrials,) the other leg will trace the epicycloid from the pitch circle to the end ofthe cog: preserve tire set of the compasses, and through the point where thefixed leg stood, sweep a circle from the centre of the wheel, in which set onepoint of the compasses to describe the point of all the cogs of that wheel whosesegment was made fast to the plane.

If the wheels be bevel gear, this rule may be used to find the true form ofboth the outer and inner ends of the cogs, especially if the cogs be long, as theepicycloid is different in different circles. In making cast-iron wheels, it is abso-lutely necessary to attend to forming the cogs to the true epicycloidical figure,without which they will grind and wear rapidly.

The same rule serves for ascertaining the cycloidical form of a right line ofcogs, such as those of a saw-mill carriage, 8tc., or of cogs set inside of a circle orhollow cone. Where a wheel works within a wheel, the cogs require a very dif-ferent shape.