Suppressing resonance as vibrator coasts to a stop 1

[dougholmes]

I am designing a machine who's primary function is to vibrate. A shaker screen. This kind of device is commonly supported on steel coil springs. The one I am working on will have a sprung weight of approx. 15000 lb, with 8 coil springs in parallel, each compressed 1 inch under normal load. Normal running speed is in the order of 800-900 RPM. When power to the machine is turned off, it coasts to a stop. It coasts down through the natural frequency of the springs (188 RPM) slowly enough that resonance causes too much spring deflection, allowing the machine to move with much too great an amplitude. Worst case would be the machine hopping off its springs. I have not actually seen that happen, but have heard of machines like this doing that kind of thing if everything was just "wrong" enough.

I would like to know a way to keep the resonant stroke buildup from happening. The internals of the machine will not allow motor braking as the way to accomplish this.

Thanks for whatever help can be supplied.

 

[IRstuff]

What are you using for damping? Otherwise, the only thing dissipating the energy are your springs.

TTFN

 

[dougholmes]

TRstuff,

That is the whole point of my question. Right now, there is no damping, so the unit does go through this resonant amplification stage as it slows to a stop. I want to surpress this, but have not seen (on the market) anything which looks like it would help.

The question is, "does anyone know of anything or process or 'trick' I could use for damping?"

Doug

 

[GregLocock]

Most expensive way first - get an electrically controlled automotive shock absorber and switch it to high resistance when the motor is switched off.

pneumatic or hydraulic cylinder and close a throttle valve .

I'm assuming that this thing is only switched off infrequently so that you cannot afford to fit a permanaently engaged damping element - eg a rubber sleeve over the springs.

Fit chains or travel limiters to prevent it falling off the springs, these could also add a non linearity to the spring arte which will damp the resonance.

A neat trick would be to decouple one of the springs as you approach the resonance, and then re-couple (?) it once you are through the resonance.

Obvious one - use the back emf of your motor and fit a big resistor. Oh sorry , that's motor braking.







Cheers

Greg Locock

 

[dougholmes]

Greg,

The screen would typically be running continuously for 8 to 10 hours a day with a stroke amplitude of approximately .75" Whatever I attempted to do would have to be able to survive in an abrasive dust environment for years of this mistreatment.

We have tried travel limiters, but they did not have much elasticity, and when the mass of the screen was hauled to a screeching stop (so to speak) hitting the limiter, it appears to have tended to create stress cracks in new and unusual locations on the machine. Management (and especially warranty cost conscious management) does not think that attempt was a very good way of doing it.

Some of our units can allow motor braking, and I am lobbying to have that applied whenever possible. It would really be nice to find something which would allow 3/4 to 1 inch of free play, but would start sucking energy out of the system when the stroke increases, and would draw more energy (perhaps on a square of the distance function) as the stroke amplitude increased... Somehow, I can't help think I am looking for "magic" to happen here.

Doug

 

[GregLocock]

Coo, you aren't asking for much are you? How about some really big rubber bump stops, with enough clearance between them for normal use? They won't come into play until the system resonates, so wear should be OK, and the units you see on a truck's axles should have the right sort of load rating. Typically rubber has a 2 - 10 degree loss angle if you want to work out the equivalent damping. Stiffer rubber has more damping.





Cheers

Greg Locock

 

[dougholmes]

Greg,

Interesting idea (the truck bumpers). There are some reasons I thought I couldn't use a bumper like you described, but on re-considering it, there just might be something useful I can do with the concept.

Doug

 

[sms]

I suppose the other approach is upon shutdown you engage stiffeners that push the resonance above 900 cpm, but only during shutdown. I am not sure of your arrangement, but some sort of rod at each spring that is free to move during normal operation, that gets clamped down for shutdown? Just a thought....

 

[butelja]

I think the rubber bump stops placed just outside the operating amplitude are a good idea. The non-linearity of the gap and the rubber bumpers themselves should limit amplitude very well at resonance. It sounds cheaper than motor braking too. This is similar to a seismic snubber that is used on inertia base mounted rotating equipment.