Design Decisions: Fixing Noisy Gear Trains

Noiseassociated with gear trains has been a common problem for gear designers for along time. With the demands for smaller gearboxes transmitting more power athigher rpm, coupled with incumbent demands for greater efficiency, gearengineers are always searching for new ways to reduce vibration and limit noisewithout increasing costs.

Somepopular solutions to the noisy gear problem include enlarging the pinion toreduce undercut, using Phenolic, Delrin or other noise-absorbing products orchanging to a helical gear train. Other methods include tighteningspecifications to ensure greater gear quality or redesigning the acousticalabsorption characteristics of the gearbox. Occasionally, experimentation withgear ratios can limit harmonic frequency amplification, which otherwise cancause a gearbox to amplify noise like a finely tuned stereo system.

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

A secondbenefit of the crowning approach to gear cutting is the minimization ofmisalignment problems caused by inaccurate machining of the casting, housing,shafting, gearboxes or bearing journals. Crowning can also reduce lead problemsin the gears themselves, which causes the gears to wear unevenly and bindbecause of eccentricities and position errors.

Twovariations of the crown shaving method will produce a gear to compensate foroff-lead or misalignment conditions.

Oneapproach produces a crown by rocking the table during the reciprocation of workand cutter. The degree of crown is readily changed by this method. The otherapproach is plunge feeding, which requires dressing the shaving cutter to thedesired crown.

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

The crownform can be produced on gear teeth in several other ways. One method is toshape the gear by use of a crown cam in the shaper back-off mechanism. Theproper radius of the gear is calculated by using the amount of crown on theflank and the pressure angle of the gear. Unfortunately, the blocks, while notcomplex, can be expensive.

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

Primecandidates for use of the crowning technique are the small fractionalhorsepower motor manufacturers or anyone dealing with spur or helical pinionsthat are susceptible to noise or misalignment. Because crowning on foreign gearhobbing equipment has been available for a greater length of time, this methodhas been developed to a greater extent in Europe.

Fred Youngis the owner and CEO of Forest City Gear Co. in Roscoe, Illinois.
http://wwwforestcitygear.com