The lead angle is an important factor in determining the efficiency of a worm and worm gear set. The efficiency increases as the lead angle increases.
For a given pitch, the lead angle is controlled principally by two factors:
(1) the number of threads and
(2) the pitchdiameter of the worm.
The lead angle can be determined from the lead and pitch diameter by using a formula in concert with a table of cotangents (as follows).
(Pitch diameter of worm x 3.1416) ÷ lead = Cotangent of lead angle (See Figure 4.4)
Important: The mating worm and worm gear must have the same:
• Pitch
• Number of threads
• Tooth dimensions
• Hand
• Pressure angle
• Lead angle (See Figure 4.4)
WORMS–PRESSURE ANGLE
The pressure angle is the angle at which a force is transmitted from the worm thread to the worm gear tooth. It determines the relative thickness of the base and top of the thread. (See Figure 4.6)
