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add or adjust backlash

add or adjust backlash

add or adjust backlash

(OP)
I'm looking at the feasibility of a lab set-up that would replicate two different systems to help develop our controls. The main issue I have is backlash between the motor under test and the dyne. It has a significant impact on our controls and must be accounted for in the lab rigs. Ideally it would also be adjustable. (Reflected inertia is also important, but something I should be able to easily deal with.)

system 1: Backlash is <10°. electric motor under test - 1200hp, 1500rpm
system 2: Backlash is <50°. electric motor under test - 400hp, 1500rpm

Ideas:
- My leading idea is to use multiple stages of planetary gearsets, but that isn't adjustable nor cheap.
- Use a 3-jaw coupling and take the rubber isolator out, but that isn't going to last very long.
- Use a chain drive and adjust the tensioner, but worried that the chordal action will introduce an oscillation.
- Create a custom gearbox where the gear centers can be adjusted to adjust the backlash. A lot of work for a small range of adjustment.

Any other ideas? ISZ

RE: add or adjust backlash

A Thomas disk-pack shaft coupling has negligible torsional deflection.

Walt

RE: add or adjust backlash

50 degrees of backlash is going to be interesting. I'd be thinking about a very coarse spline with lots of angular clearance, basically your 3 jaw coupling idea. More or less the same would be a massive steel flywheel with steps on the inner diameter of the rim, mating to a 3 toothed inner, with rubber pads of appropriate size to adjust the clearance.

Your mounting bolts are going to have an exciting life.

I don't suppose you can eliminate the problem by fitting a fluid coupling? I'd sleep easier at night with that in the system.

Cheers

Greg Locock


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RE: add or adjust backlash

I am sorry, because I thought OP was trying to eliminate backlash and not create it! Backlash is only active for dynamic torque, so I would consider a soft elastomer coupling and tune a torsional resonance to create angular deflection.


Walt

RE: add or adjust backlash

I'd use the 3-jaw coupling, especially the kind where the 'spider' wraps around the coupling instead of being trapped inside it, and fabricate a bunch of spiders from multiple materials with varying amounts of clearance between the spider's teeth and the jaws' teeth.

Mike Halloran
Pembroke Pines, FL, USA

RE: add or adjust backlash

Mike' suggestion is fine, but considerable torsional vibrations (dynamic torque) is needed for angular motion.

Here is another concept that uses a single universal joint. The motor and dyno shafts can have angular misalignment that causes dynamic torque. Note that the frequency is at 2x shaft speed, that may not be suitable for your application.
http://www.sdp-si.com/catalogs/D757-Couplings-Univ...

Walt

RE: add or adjust backlash

I don't understand fully what is trying to be achieved here but how about a sprag or roller clutch? That will let your load coast when it needs to.

RE: add or adjust backlash

(OP)
TugboatEng "I don't understand fully what is trying to be achieved here...."

The machine system being developed has 40-50° of backlash which can not be reasonably reduced (3 planetary stages, 1 big spur gear stage), and the system constantly changes speed and direction when in use by the customer. I need to create a replica in a lab to assist with controls development which requires that I accurately duplicate the backlash, reflected inertia and load seen at the motor shaft.

Load is easy, just need a properly sized dyne and I have quite a range already available. Inertia shouldn't be an issue either, add mass as necessary. The big problem is getting that much backlash. As others have said generally the goal is usually to minimize backlash - not increase it!

ISZ

RE: add or adjust backlash

I'm wondering if anyone has ever built a dynamometer that could reproduce the various loads' dynamics directly, so all you would need to do is direct couple the motor to the magic dyno.

It's the sort of thing you could probably do at the ~1HP level with a disc coupling, a 3 phase generator, a very fine pitch encoder, and some really fancy power electronics.

At the 1200HP level, the fancy electronics would have to be a lot bigger, and development of same would involve a lot of very expensive smoke.

Mike Halloran
Pembroke Pines, FL, USA

RE: add or adjust backlash

In that case you could use two roller clutches back to back, one to drive each direction. The phasing of the clutches can be changed to adjust backlash.

RE: add or adjust backlash

A three jaw coupling with the jaws machined should allow 10 deg (with spider installed for some cushioning). If you cant get 50 deg with the same approach, perhaps 25 deg and add two couplings in series.

je suis charlie

RE: add or adjust backlash

Quote (TugboatEng)

In that case you could use two roller clutches back to back, one to drive each direction. The phasing of the clutches can be changed to adjust backlash.

This looks to me to be the slickest solution thus far

There is one issue that will be difficult to simulate; whether or not its a problem depends on the actual accuracy required of your test.

In the field, your 40-500 of backlash is really a stack up of 40-50 10 steps of backlash- which means the inertia of the load is actually a function of angle relative to start position.

The only way to simulate this would be some moving masses very accurately controlled relative to shaft rotation, or to use the same number and size of backlash 'steps' as the actual system.

Which means you'd need to know the backlash and before/after inertia of every part of the system, not just the sums.

RE: add or adjust backlash

(OP)
I hadn't thought of the roller clutch idea, think that has some merit. I see some built to handle the requisite power level but they don't list the backlash, have to call the vendor. Also have to ask about the shock load capability. ISZ

RE: add or adjust backlash

This sounds like a custom coupling with an adjustable torsion spring across the center member assembly. Won't be cheap but it would allow you the flexibility to adjust the backlash based on the desired level of torque you are trying to transmit. Increase the spring pre-load to decrease backlash, decrease the spring pre-load to increase backlash. My only concern is, what is the actual spring constant from the system you are trying replicate.

When it comes to couplings we are always here to help.
WWW.PSCCOUPLINGS.COM

RE: add or adjust backlash

I assume the OP wants zero torque transmission between the backlash limits ie no springs.

OP if you need a near step increase in (static) torque at the backlash limits, there must necessarily be a large spike in torque under dynamic conditions and therefore limited life of the impacted faces. Perhaps design your own 3 jaw coupling without the usual cantilevered sections. Think of a drum with 3 slots engaged by an internal 3 legged star wheel. Something like a clutch centre (Link) with the springs removed.

je suis charlie

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