Bevel gear set . | Page 2
OK here's my take of milled bevel gears Vs generated as a pplied to a larging VBM drive: The quality level acceptable for one class of gearing may be unsuited for another for a varity of reasons. Cost, difficulty of manufacture, efficiency, noise, accuracy, etc all are interdependent.
Gearing made of pear wood suited for 16 th Century windnills that's lasted for centuries that can be readily repaired by a skilled mller in a moment would be entirely unsuited for a helicopter - an admittedly absurd stretch but germane in this instance. Bevel gears whose tooth spaces were made via formed cutters are suited for many applications for a onsey-twosey application but are costly to produce in quantity compared to those having involute flanks generated by a bevel gear shaper like a Gleason #3 of the same era.
A true bevel gear tooth profile has involute driving flanks proportioned over its face width that taperto the pitch cone apex in form and in proportional with that projected on its back cone. Such a tooth profile if perfectly mounted would bear its full length and if perfectly rigid would transmit power smoothly without pitch to pitch error, indexing error, profile, or iany of a dozen other error modes possible in production gearing.
This is not a perfect world. Gears deflect in service in proportion to load and even whopper gears on VBM table drives are subject to material elasticity and part deflection. Which is why straight bevel gears are made with profiles intended to bear as "central toe" ellpises - so heavy loading and subsequent deflection enlarges the contact bearing not quite to the back cone and - destructive edge bearing. Such gear pairs operate smoothly and reliably throughout their load and RPM range
If a VBM table drive is to perform acceptably, it has to drive smoothly over its entire range of load conditions for its entire working life. This is a tall order requiring gearing of superior quality and housing, shafts and bearings of superior rigidity and accuracy. Note I used a qualitive term: "superior". There are AGMA classifications that quantify acceptable errors for their respective classes of fits. Since there are 15 classes the allowable errors overlap considerably and they range from as-cast gearing intended for exposed service in construction site cement mixers up to gearing to be found in the most refined instrumentation and mechanically driven apparatus.
As speed, power density, and connected inertia increase so increases the need for profile accuracy and generated involutes. A milled gear is at best be a compromise because it is a formed gear The cutter can be accurate only for a specificnumber of teeth and was intended for and only on gear of a cylindrical pitch surface. Tooth countsa to either side of the niminl count have lower profile accuracies but are still suited for lower class applications.
The small tenths really count when it comes to gear noise and profile accuracy. Gear materials are very rigid and their accommodating deformations are necessarily very small. The smallest bump or eccenctricity affect gear noise and since every little sould represents a departure from smooth running, gear noise directly indicates load variations as products of profile inaccuracy. Think of a rubber tire Vs a wagon wheel. The rubber accommodates the surface irregularities over which it rolls and the steel tired wagon wheel does not - which is why wild west buckboard wagons rumble over dirt roads and why plastic and phenolic gears run quieter than metal.
Bevel gears cut with formed gear milling cutters are an approximate involute only at the back cone surface. As an involute tooth tapers to the apex, its profile has to narrow in precise proportion to its distance from the pitch cone. In a milled bevel gear, the formed involute peters out in a small fraction of the face width into a crafted compromise whose quality varies with the method used and the skill of the operator. The best that can be accomplished with a formed cutter is to follow a system of mutliple cuts and hand filing that produces a tooth profile that approximates an involute and thus an "approximate" tooth bearing. The gear will still run but be noisy and fall far short of the designed-for load HP.
Countless gear pairs have been milled over the years and used successfully on low powered, low speed applications like a manual milling machine knee elevating screw drive. People with a well developed sensitivity to mechanism performance would consider such a gear drive a success capable of scaling up to a powered application - which may bear out in actual service. However success on a small scale is not always satisfactoriy on a larger scale.
A milled pinion driving a cleaned-up but once heavily rusted VBM table bevel gear is certain to be unsatisfactory by reason of noise, load capacity, tooth count telegraphing onto the work surface, service life, and possibly other problems. The drive will certainly turn the table and work to speed but only to about 10% of its actual load capacity. I'd suggest trying it if the scale was smaller but I would reccommend against milling the teeth on.that expensive blank. There is poor probability of ucccess.
Such a course of action would, I think, be irresponsible unless the limitations of this expedient and its poor likely outcome had been thoroughy discussed and understood by the machine's owner and the people in his enploy most affected by the decision. They have to commit to an "one your head be it" decision absolving the gear cutter of all responsibility for poor gear performance - preferably in writing.
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bevel gears? is it possible?
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