an Engineering & Service Bulletin
Technical Topics
J.H. WRIGHT & ASSOCIATES
MIXER GEAR DRIVES? THE REAL BEST CHOICE...
Often, mixer drive specifications read: "All helical or spiral bevel gearing, no worm gear drives are acceptable". When questioned as to the reason for this worm gear prohibition, the answer is usually that they are inefficient and generate more heat. The true source, however, of this specification probably has more to do with sample specifications passed out over the years by mixer vendors who were only offering helical/spiral bevel gearing in their boxes. Today, however, there are manufacturers who combine a worm gear set with a helical set to achieve far less heat generation and high efficiencies for mixer service.
Typical gear box efficiencies for helical/spiral bevel boxes run from 95% to 98%. By the same token, helical/worm gear boxes run from 94% down to 90%. Your efficiency "loss" using helical/worm drive is, therefore, in the 4 to 5% range. If you are specifying a mixer drive to handle 150 horsepower, efficiency means something. However, if your mixer is in the 2 to 10 horsepower range you had better take a long look at initial cost.
There are a couple of facts that manufacturers of only spiral bevel boxes "forget" to mention:
- They are among the most difficult to manufacture to high tolerances and finishes.
- They are the toughest to mount and remount accurately.
Because of these two failings, the gears usually must be purchased as a "matched and lapped" set and require several shimpacks to properly locate and set for back lash. If you don't do all these things just right, that 5% efficiency advantage can disappear quickly.
A trip out through the mining and ore processing plants in the western states is highly instructive as to mixer gear box choice. Where the mixers normally operate under high shock and overload conditions, worm gear drives are the norm.
As far as heat generation is concerned, all mixer gear boxes should be specified to have a thermal rating that exceeds its mechanical rating at operating temperatures in your location. If you do this, you'll get an efficient gear box with enough lubricant to operate without a cooling fan regardless of the type gearing.
But, I've saved the best for last, and you'll have to pay attention to follow me through this reasoning.
With helical/worm gear sets, it's easy and economical to get a high gear reduction, for example; a helical/worm gear set giving an output speed of 56 RPM might well be the price equal of a helical/spiral bevel box with an output speed of 100 RPM.
But in mixing, torque is everything, it's torque that does the mixing--not horsepower. What I'm saying is that 5 HP at 56 RPM is the mixing equivalent of 9 HP at 100 RPM. Each produces the same torque( 475 ft/#) and each will do the same blending job.
So would you rather pay the power bill on a 5 HP/56 RPM helical/worm gear mixer at 92% efficiency or a 9 HP/100 RPM helical/spiral bevel mixer at 97% efficiency?
Maybe that prohibition of worm gear sets on mixers doesn't make as much sense as it might appear.
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