A modified gear tooth profile improves the performance of servo motors

The demands of today’s servo motor market are very demanding. To combine quiet operation, high speeds and high torques they really need one type of gear design, which conventional gear geometry does not allow.

To meet the challenge, US company Bayside developed a totally new type of helical gear, called HeliCrown, that was incorporated into its Stealth Planetary Gearhead. HeliCrown combines the power of helical gears with the smooth, quiet engagement of tooth crowning.

A major cause of gear noise is the instantaneous engagement/disengagement of the gear teeth, so any modification in this area should minimise the effect.

As Figure 1 shows, there are three methods (A, B and C) of modifying the gear teeth. Each removes a band of metal from the gear tooth, decreasing the contact area of the tooth, resulting in reduced load-carrying capabilities. Reducing noise in this way, however, is accomplished at the expense of reducing gear strength.

But by using a computer-generated gear model on the Helicrown design, it was possible to determine the optimum location and amount of gear tooth material to be removed, without diminishing tooth strength, (Figure 1, D).

Since most noise occurs at the entry and exit points of the gear tooth, only removing metal in these areas leaves most of the tooth intact. The resulting profile provides a balance between speed and torque, producing a quieter gear tooth with 30% to 40% more torque than conventionally modified gears.

To protect against wear and pitting, a heat treatment was needed to provide an extremely hard gear surface (65Rc) with a ductile core, all without sacrificing gear precision. Plasma-assisted case-hardening, or nitriding, was the answer.

Plasma nitriding is a computer controlled process in which the chemical composition of the material can be precisely controlled. By diffusing elements through the use of plasma, the metal surface can be treated to the required hardness.

In addition to providing a quiet and compact servo motor, the company claims that by adopting an all-helical planetary gearhead, the new design sports lower backlash as well as higher stiffness.

The Stealth servo motors are now available in the UK from Harmonic Drive Systems.

{{Harmonic Drive SystemsTel: Chichester (01243) 545400Enter 430}}