CIRCULAR (LINEAR) PITCH
With a worm, circular (also referred to as linear) pitch is a distance measured along the pitch line of the gear. It can be determined by measuring – with an ordinary scale – the distance between any two corresponding points of adjacent threads parallel to the axis. (See Figure 4.1) With a worm gear, circular pitch is a distance measured along the pitch circle of the gear. It can be determined by measuring – with an ordinary scale – the distance between any two corresponding points of adjacent teeth. As noted above, this measurement should be taken on the pitch circle, which is approximately halfway down a tooth. (See Figure 4.2)
The dimensions of a worm thread are important because they provide valuable information when determining a customer’s needs.
As noted earlier, a worm thread is the part of the worm that wraps (spirals) around the cylindrical base of the worm, similar to the way the threads of a screw are configured. The following terms are used when describing the dimensions of a worm-thread.
• Addendum – the part of the thread from the pitch line of the worm to the outer edge of the thread. (See Figure 4.3A)
• Dedendum – the part of the thread from the pitch line of the worm to the bottom of the thread. The dedendum is equal to one addendum plus the working clearance (defined below). (See Figure 4.3A)
• Working Clearance – the distance from the working depth (defined below) to the bottom of the thread. (See Figure 4.3A)
• Working Depth – the space occupied by the mating worm gear tooth. It is equal to twice the addendum.(See Figure 4.3A)
• Whole Depth – the distance from the bottom of the thread to its outside diameter.
