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Design Decisions: Fixing Noisy Gear Trains

Noise associated with gear trains has been a common problem for gear designers for a long time. With the demands for smaller gearboxes transmitting more power at higher rpm, coupled with incumbent demands for greater efficiency, gear engineers are always searching for new ways to reduce vibration and limit noise without increasing costs.

Some popular solutions to the noisy gear problem include enlarging the pinion to reduce undercut, using Phenolic, Delrin or other noise-absorbing products or changing to a helical gear train. Other methods include tightening specifications to ensure greater gear quality or redesigning the acoustical absorption characteristics of the gearbox. Occasionally, experimentation with gear ratios can limit harmonic frequency amplification, which otherwise can cause a gearbox to amplify noise like a finely tuned stereo system.

Another approach to the gear noise problem that yields good results is ‘crowning’ or ‘barreling’ of the teeth. This technique involves changing the chordal thickness of the tooth along its axis. This modification eliminates end bearing by offering a contact bearing in the center of the gear.

 Design Decisions: Fixing Noisy Gear Trains
A second benefit of the crowning approach to gear cutting is the minimization of misalignment problems caused by inaccurate machining of the casting, housing, shafting, gearboxes or bearing journals. Crowning can also reduce lead problems in the gears themselves, which causes the gears to wear unevenly and bind because of eccentricities and position errors.

Two variations of the crown shaving method will produce a gear to compensate for off-lead or misalignment conditions.

One approach produces a crown by rocking the table during the reciprocation of work and cutter. The degree of crown is readily changed by this method. The other approach is plunge feeding, which requires dressing the shaving cutter to the desired crown.

Generally, it is faster to plunge feed, but the technique can subject the cutter to greater wear. Of course, it is more difficult to change the crown, provided one starts with good quality gears. Shaving improves the quality of profile and reduces error in the gear tooth, through the cutting and burnishing action of the cutters.

The crown form can be produced on gear teeth in several other ways. One method is to shape the gear by use of a crown cam in the shaper back-off mechanism. The proper radius of the gear is calculated by using the amount of crown on the flank and the pressure angle of the gear. Unfortunately, the blocks, while not complex, can be expensive.

An area ripe for the use of crowning is in the manufacturer of hydraulic wobble motors. Here, the application is strictly for misalignment problems rather than for noise reduction. An allied area involves heavily loaded pinions used in actuators for aircraft control surfaces. Generally speaking, it is more advantageous to crown the pinion because it makes more revolutions per minute and may generate more noise. In this case, it is of paramount importance to compensate for load deflection. Unfortunately, few companies in the United States have been applying this technology to commercial fine pitch gearing. However, the few manufacturers who have tried it are pleased with the results. Some users have reported a 5x to 10x reduction in noise, accompanied by less vibration, wear and power draw.

Prime candidates for use of the crowning technique are the small fractional horsepower motor manufacturers or anyone dealing with spur or helical pinions that are susceptible to noise or misalignment. Because crowning on foreign gear hobbing equipment has been available for a greater length of time, this method has been developed to a greater extent in Europe.

Fred Young is the owner and CEO of Forest City Gear Co. in Roscoe, Illinois.
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