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View Full Version : Proper Idler pulley design



RB211
09-16-2010, 09:44 PM
I am designing a belt tensioner Idler pulley, using a swing link.
This idler could see as much as 800lbs of force in a worst case scenario.

By placing the idler and swing link perpendicular to the centerline of the Driven and Driver pulleys, allows the idler and its mounting bracket to absorb the most force. In doing this however, the system is in its max extension and will not allow for any further tensioning of the belt.

Another way I could do this is with the idler being mounted to a linear slide, allowing for proper geometry and additional extension.

What are your guys thoughts on this?
http://www.flightschoolreview.net/images/Idler.jpg

J Tiers
09-16-2010, 10:04 PM
I don't know anything about your design constraints....... so this may not apply.

But most idlers are put on the slack side, which takes off most of the load. if your system can impose a load of 800 lb, it tends to sound as if this is on the tension side, or that the rotation reverses. Reversing would mean there IS no strictly "tension" side.

It strikes me that to put the idler pivot as you suggest may require you to know quite a bit about the belt size, and the probable belt stretch, etc, over the life of the belt. You may be in the position of guaranteeing things you can't control.

My suspicion is that despite spring issues, you'd be better off to allow more compliance. Unless, of course, the belt is a steel band belt, or the like.

RB211
09-16-2010, 10:12 PM
The idler is on the slack side, using a Gates Power Grip GT2 belt, 8mm pitch. This is an airplane drive so my safety factors are quite a bit higher than for most applications. The belts are 85mm wide. Talked with Gates and have been using their software for designing the pulley layout.
Thinking about it now, there really is no way this could even see close to 800lbs.
The Driver and Driven CAN see 800lbs of pull towards each other, but that is not the slack side.

RB211
09-16-2010, 10:52 PM
http://www.flightschoolreview.net/images/LinearIdler.jpg

Perhaps this is better...

J Tiers
09-16-2010, 11:07 PM
Thinking about it now, there really is no way this could even see close to 800lbs.
The Driver and Driven CAN see 800lbs of pull towards each other, but that is not the slack side.

OK, by the look of it the max force on the belt will be a LOT less....... Even on the tight side, if the angle was fairly shallow, the idler force might be easily a quarter or less of the belt tension.

I think I like the swinging idler.......less chance of twisting out-of-plane, less friction, much better defined bearings and so forth..... I REALLY do not like the slot, even with the sliding "frame". Not to say it can't work, but the swing arm is simpler and more direct. Slider is better 'damped", might not be an unadulterated blessing, though.

darryl
09-17-2010, 12:00 AM
I would use a swing arm as well. The idler should go on the slack side, I agree with that. On first thought, I'd be placing the pivot point such that the arm sits parallel to a line the slack side of the belt would take if it went straight from pulley to pulley. But you can alter this either way, depending on how you wish to control the slack side. As you've noted, the arm could well end up pivoted in such a way that it 'runs out' of adjustment range. If it's mounted sort of between this and parallel, and on the side where the direction of the belt tends to swing the arm towards perpendicular, it seems to me it would be about right. I would want the running direction of the belt to help slightly towards pivoting the arm in the direction of the belt tightening. I would say if the arm parallel to the belt is 0, and perpendicular is 90, then I'd want it at about 35 or so. That leaves lots of motion to compensate for belt stretch without approaching too closely to the point where the adjustment range is gone.

Note that I'm giving these ideas based on the slack side of the belt being mostly straight between the pulleys, which it won't be because of the slack that's being taken up.

That's another design issue to resolve- how much slack is there going to be. Does the belt wrap say 90 degrees around the idler, or more like 20 degrees or so. How much leeway do you have to set that general angle. Again my opinion only, but I'd be aiming for a wrap of about 30 degrees or so, just to keep the motion of the swinging arm neither too little or too much.

Bear in mind these are my gut feelings- I'm not a power train engineer.

oldtiffie
09-17-2010, 12:27 AM
The Gates web site and belt details are here:
http://www.gates.com/sync/index.cfm?drive=powergrip&location_id=743

It has a design manual listed on it here:
http://www.gates.com/request.cfm?requesting=ptcatalog

The Google listing is comprehensive - here:
http://www.google.com.au/#hl=en&&sa=X&ei=s-mSTMfbLIKivgPJg9WZDQ&ved=0CBYQBSgA&q=Gates+Powergrip+GT2+belt&spell=1&fp=62a3b4c4a0e828f3

Why are we second-guessing either or both Gates design manual and their technical support? Are they inadequate, deficient or non-co-operative?

The general principles of belt design and installation are pretty well known, documented and reasonably accessible.

It basics of it are covered at "Belts and Pulleys" at pages 2391>2419 in my Machinery's Handbook 27.

I went back and re-read the second post by the OP:

The idler is on the slack side, using a Gates Power Grip GT2 belt, 8mm pitch. This is an airplane drive so my safety factors are quite a bit higher than for most applications. The belts are 85mm wide. Talked with Gates and have been using their software for designing the pulley layout.
Thinking about it now, there really is no way this could even see close to 800lbs.
The Driver and Driven CAN see 800lbs of pull towards each other, but that is not the slack side.

My question here is that if it is for "airplane" use would not be the "Aircraft" Regulatory authority have mandatory procedures and limits - as well as the design or approval of it by a qualified Professional air-craft Engineer?

What does or has the Engineer have or had to say about this?

If there is not an Engineer on the job, why isn't there?

RB211
09-17-2010, 02:49 AM
The Gates web site and belt details are here:
http://www.gates.com/sync/index.cfm?drive=powergrip&location_id=743

It has a design manual listed on it here:
http://www.gates.com/request.cfm?requesting=ptcatalog

The Google listing is comprehensive - here:
http://www.google.com.au/#hl=en&&sa=X&ei=s-mSTMfbLIKivgPJg9WZDQ&ved=0CBYQBSgA&q=Gates+Powergrip+GT2+belt&spell=1&fp=62a3b4c4a0e828f3

Why are we second-guessing either or both Gates design manual and their technical support? Are they inadequate, deficient or non-co-operative?

The general principles of belt design and installation are pretty well known, documented and reasonably accessible.

It basics of it are covered at "Belts and Pulleys" at pages 2391>2419 in my Machinery's Handbook 27.

I went back and re-read the second post by the OP:


My question here is that if it is for "airplane" use would not be the "Aircraft" Regulatory authority have mandatory procedures and limits - as well as the design or approval of it by a qualified Professional air-craft Engineer?

What does or has the Engineer have or had to say about this?

If there is not an Engineer on the job, why isn't there?

I used all of Gates advice, idler no less than 3" diameter, used 3.5", placed within 1/3rd of the span near the driver on slack side ,etc, etc, etc. My question was in regards to swing link vs slider, not the finer points of the Gates belt, which we took the advice of the engineers on.