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.
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.
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.
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.
variations of the crown shaving method will produce a gear to compensate for
off-lead or misalignment conditions.
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
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
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.
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.
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.
is the owner and CEO of Forest City Gear Co. in Roscoe, Illinois.