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When ordering special (made-to-order) worms, the pitch, pitch diameter, pressure angle, number of threads and hand should always be specified, as should the physical dimensions illustrated in 4.7.
Note: Sometimes a pinhole through the hub is required (rather than a keyway). If this is the case, be sure to specify the pin dimensions and location.

As noted in our discussion of spur gears, gear dimensions are important because they provide valuable information when determining how best to meet a customer’s needs. Here are definitions you need to know in order to determine the basic dimensions of worm gears. (See Figure 4.8)
• Pitch Diameter – the diameter of the pitch circle (which, you will remember, is the “imaginary” circle on which the worm and worm gear mesh.
• Working Depth – the maximum distance the worm thread extends into the tooth space of the gear.
• Throat Diameter – the diameter of the throat circle at the center line of the worm gear face (the lowest point on the tooth face).
Outside Diameter – the largest diameter of the worm gear teeth. It is equal to the diameter of the outside circle.
• Root Diameter – the smallest diameter of the worm gear. It is equal to the diameter of the root circle.