Worm Gear Gearbox

[Paul Copioli]

Here are some facts about worm gear efficiency:


Efficiency = Tan(Lead)*(1-f*Tan(Lead))/(f + Tan(Lead))

Lead = Worm Lead Angle
f = Coefficient of Friction (Mu)
For a bronze gear and a steel worm f = 0.05
Easy to purchase worm gears have lead angles ranging from 4.5 degrees to 22 degrees. Here is the bounding range on efficiencies

Lead angle of 4.5 degrees has an efficiency of about 61%

Lead angle of 22 degrees has an efficiency of about 87%

This assumes everything is lined up!! Tolerances are very critical for a worm gear set-up.

The lead angle of 22 degrees corresponds to worms with 4 thread starts and the lead angle of 4.5 corresponds to worms with 1 thread start. The worm with one thread start has 4 times the gear ratio for the same package size than the worm with 4 starts. A worm gear box has several advantages for FIRST robots. The main two are package size and weight. We have used worm gears on our robots for three years, but only once on our drive transmission (see white papers under CCT).