A wheel that is driven by a revolving screw, or worm as it is termed, is called a worm-wheel, the arrangement of a worm and worm-wheel being shown in [Fig. 8]. The screw or worm is sometimes also called an endless screw, because its action upon the wheel does not come to an end as it does when it is revolved in one continuous direction and actuates a nut. So also, since the worm is tangent to the wheel, the arrangement is sometimes called a wheel and tangent screw.
The diameter of a gear-wheel is always taken at the pitch circle, unless otherwise specially stated as “diameter over all,” “diameter of addendum,” or “diameter at root of teeth,” &c., &c.
When the teeth of wheels engage to the proper distance, which is when the pitch circles meet, they are said to be in gear, or geared together. It is obvious that if two wheels are to be geared together their teeth must be the same distance apart, or the same pitch, as it is called.
Fig. 9.
The designations of the various parts or surfaces of a tooth of a gear-wheel are represented in [Fig. 9], in which the surface a is the face of the tooth, while the dimension f is the width of face of the wheel, when its size is referred to. b is the flank or distance from the pitch line to the root of the tooth, and c the point. h is the space, or the distance from the side of one tooth to the nearest side of the next tooth, the width of space being measured on the pitch circle p p. e is the depth of the tooth, and g its thickness, the latter also being measured on the pitch circle p p. When spoken of with reference to a tooth, p p is called the pitch line, but when the whole wheel is referred to it becomes the pitch circle.
The points c and the surface h are true to the wheel axis.
The teeth are designated for measurement by the pitch; the height or depth above and below pitch line; and the thickness.