Machsupport Forum
Tangent Corner => Tangent Corner => Topic started by: fer_mayrl on February 24, 2009, 02:39:09 PM
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Hello Guys,
I was wondering, when there is a specification for backlash in planetary gearboxes, they always say:
Backlash 5 arc-min for example.
Is it 5 arc-min at the input or at the output? anyone knows?
THanks!
Fernando
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If it wasn't for the output I'd be upset. ;D But, if your thinking of buying one, better to ask the seller and be safe.
Brett
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Bret,
I was thinking it was the other way around, i´d rather have 5 arcmin backlash in the input, and if the ratio is 5:1 then you'd have 1 arcmin at the output, which would translate into les linear backlash.
I might be wrong!
Regards
Fernando
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No Fernando, your exactly right. I have my head up my butt. :P I was thinking of overdriveing. I have a 3000 RPM motor I would like to turn a spindle 10,000 with if I can get the right bearings and balanceing.
You got any good links to multi speed (2 or 3 ranges), high tolerance gear boxes a man can afford? For the spindle they wouldn't have to be very high tolerance really.
Brett
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Hey Brett,
Not really, i have never come to use any gearboxes for stepping up, and certainly not to 10,000 rpm!
at 10,000 rpm, bearings alone might cost a lot. (just guessing)
I might suggest using a motorized high frequency spindle with a VFD,
or trying to do it with a pulley system and a vfd on an induction motor.
If you get a 3000 rpm motor you might get away with runing it at 120Hz which will get you 6,000 rpm, then a 2:1 pulley system might do the job, just check belt ratings to see if they can take your HP at those rpm.
Fernando
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No Fernando, your exactly right. I have my head up my butt. :P
Brett
So just another normal day then ;D
Hood
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So just another normal day then
Hood
Better than average. ;D
Brett
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I'd say it would be measured at the input of the gear train....IE: the motor shaft. Assuming the load (or ball screw) were held stationary, the measurement would be the angle of rotation of the input device.
RC
Gearbox backlash is the angular rotation in degrees, minutes, and seconds of the amount of play in a shaft. What that means is; if you were rotating a shaft in one direction, pushing a load forward, then you reversed the rotation, the amount of free play before the load starts to move in the opposite direction is the backlash. It is measured in degrees, or parts of a degree (there are 60 minutes in a degree, and 60 seconds in a minute). The best explanation I ever heard for arc-minutes, or arc-seconds, is this.... If an airplane left New York City heading in a straight line for Los Angeles and was off by ONE-ARC-SECOND, it would miss the center of the runway in L.A. by 12 feet. So over a distance of 3000 miles, you would be off by 12 feet. If you were off by 1 ARC-MINUTE, you would miss the runway by 60 times 12, or 720 feet. (about 2 football fields).
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Ok then, a 5 arc minute 10:1 planetary gearbox, would have .5 arc minute backlash on the output shaft...
If you add a 1" diameter pinion on that shaft... then...
S=r*angle .......... S=.5*0.000145...........angle in radians... that translates to 0.000072" of linear backlash??
That seems too low... i guess i'll ask some manufacturer.
Best regards
Fernando
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Ok, read a little before going to bed.
According to Neugart, the backlash stated in all catalogs is measured on the low speed side, so on a common application of reducing the rpm output of a servo, the backlash is measured in the output.
So there you go
Regards
Fernando
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Nice write up Fernando...thanks.
But as far as Mach and a machine tool is concerned, any backlash compensation applied would be in reference to the input of a planetary reducer.
For instance, if the reducer was 60:1, and the output backlash was 1 arc min., then the input backlash would be 1 arc degree which is what would be used to configure the compensation.
Like your PDF said, Industry standard for servo heads is to measure at the output so you would need to do the math or better yet, set it up at the machine.
Thanks again for the tutorial,
RC
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Well yea... for compensation in mach you would have to translate it to the input.
But actually i was looking for that info for gearbox selection :)
Best regards
Fernando
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Backlash is still calculated in real world units. It does not matter where it occurs as it still looks like it occurs at the moving stage. How many steps per inch/mm/furlong, how much backlash in inches/mms/furlongs
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Interesting write up. Attached is a conversion table i keep around for reference. If you are checking the backlash on a rotary and have a dial indicator the conversions come in handy.
RICH