British engineer `cures` planetary gear backlash

A British mechanical engineer claims to have licked the problem of backlash in planetary gears. Sussex-based Tim Sweatman claims that his mechanism can reduce backlash down to two minutes of arc (0.0333°).

Backlash causes inaccurate gear motion and chatter, and can be expensive or impossible to eliminate. It results in gearbox wear, leading to heat, noise and vibration, and it makes the gearbox life difficult to predict.

The customary method of avoiding backlash is to use costly, high-precision gearboxes, but backlash can return as the gears wear. Another approach is to move the pitch centres of the gears closer together, but this can cause them to lock. Sweatman says he has eliminated these problems by altering the phasing between the gears. He argues that being able to control backlash could save both space and money.

"When faced with the problem of backlash resulting from, say, wear in a gearbox connected to a stepper motor, some engineers decide to do away with gears altogether and to use a larger motor," Sweatman explains. "This can increase the cost of a motor five to ten times and enlarge the dimensions of the drive system, which is clearly a disadvantage in a confined space like a robot arm."

Sweatman, who has been working on his designs for several years, first built a single-gear version and presented the concept at the Drives & Controls 1999 Conference. Some delegates at the conference said they would be interested in the mechanism if it could be adapted to work with an epicyclic (planetary) gearbox, which can handle more power.

Sweatman set himself the goal of building a planetary version which could be applied to any size of gearbox using standard components and tolerances.

He has now come up with a design which he has retrofitted to a three-planet API Positran gearbox, adding only 5mm to the box`s original 62mm length. He says that backlash can be removed dynamically by using encoder feedback and then adjusted manually by turning an outer collar.

On the prototype, one complete revolution of the collar removes 30 minutes of arc, but the mechanism could be modified to remove six minutes per revolution. Sweatman emphasizes that backlash is removed in the mechanism, not in the gears themselves.

He asserts that the mechanism offers adjustable, accurate and progressive control. There is no limit to backlash that can be removed, and the system provides full torque reversal, automatic wear compensation, and dynamic adjustment.

Sweatman is now looking for licensees the technology. He envisages applications in the automotive, machine tool, materials-handling, aircraft control and precision servo markets, among others.