an Engineering & Service Bulletin
Technical Topics
J.H. WRIGHT & ASSOCIATES
WILL THE REAL GEARBOX RATING PLEASE STAND UP ?
Figures don't lie, but liars do a lot of figuring. All too true when you try to get a straight answer from some mixer salesmen. They play a "numbers game" that's hard to follow unless you know some of the ground rules.
The American Gear Manufacturers Association (AGMA) is an association of the large gear manufacturers in the United States, very similar to ASME in that they promulgate standards under which their members manufacture gearing. They have "Founder" members who are the manufacturers themselves and associate members who buy gearing from the manufacturers and assemble it into boxes of their own construction. For the record, only one mixer manufacturer in the United States is in the "Founder" category.
This association has developed a series of calculations and formulae which can be applied to any gearbox to arrive at its AGMA Rated Mechanical Horsepower.
They have also developed a table of recommended minimum service factors to be used with these horsepower ratings.
For example, a mixer operated under uniform load 24 hrs/day would require a 1.25 service factor if driven by an electric motor, 1.5 if driven by a diesel engine because of the pulsations. Only operated 10 hours per day or less, the service factor would be 1.0 with the electric motor.
You use these service factors by taking the horsepower of your driver and multiplying by the service factor. Thus, a 15 HP electric motor mixer operated 24 hr/day needs a gearbox with an AGMA rating of at least 18.75 HP.
O.K. - Now for the Numbers Games:
GAME #1: Instead of using the driver nameplate horsepower rating, I use the applied load of the mixer; that is, what I size the mixer blades to draw. Since this is normally 80% of drive horsepower, I now only need a gearbox AGMA rated for 15HP. or put differently, I tell you I've got a 1.25 service factor on the box, really I've got 1.0 based on the motor size.
GAME #2: You'll notice that the service factors are lower for a gearbox only used 10 hrs. or less per day. So, if I'm pretty sure you'll never run your mixer but one shift, I just quote you a 1.25 service factor, but based on the one shift operation, after a year or so, my parts business may be great!
GAME #3: Those of you who purchase gearboxes made in Germany will recognize DIN (Deutsches Institut Fur Norming) as the equivalent of AGMA. It's also, for all practical purposes, the same as ISO. DIN and ISO have a system much the same as AGMA for rating gearing and gearboxes. The problem is that DIN has far higher allowable stress loads than AGMA.
COMPARISON OF ALLOWABLE LOADS
If you think that a gearbox rated at 15 HP by DIN standards is anywhere near as "beefy" as one rated at 15 by AGMA standard, I've got a bridge in Arizona I'd like to sell you.
GAME #4: In addition to the calculations and formulae from AGMA, to arrive at a mechanical rating of the gearbox, they also specify a thermal rating for each box. This is the horsepower you can get out of the box without overheating the lubricant and requiring an additional source of cooling such as a fan or heat exchanger. Anybody in our area of the South who buys a gearbox whose thermal rating does not exceed its mechanical rating will find that it comes equipped with a cooling fan whose horsepower is deducted from the horsepower available to do your mixing. Good Luck!ONE FINAL THOUGHT - Service factors tend to be just numbers but consider this- A roller bearing designed for 50,000 hrs. at a 1.25 S.F. will only have 23,760 hrs. life at 1.0 S.F. but the life jumps to 91,810 hrs. at 1.5 S.F.!
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