Motor as Rotary Damper

We have an application where an operator needs to move an idler roll assembly up and down an incline of about 45^o. This will be a completely manual motion with the operator providing the energy to move the assembly using something similar to a handwheel. The assembly well be held in place with a ratchet and pawl assembly. The idler roll assembly will act as a lay-on or gap roll for a winding product roll so it needs to be able to move freely if it comes into contact with the building product roll.

Our concern is if the operator releases the pawl without having control of the assembly. At that point it will simply fall, which isn't so good. We looked at hydraulic rotary dampers but they are custom with 12 week delivery and are very expensive. So we thought we would use a PM motor to act as a rotary damper by connecting it to the handwheel shaft and shorting the motor leads. We can tweak the max speed by putting resistance into the armature circuit. We can make the damping action unidirectional by putting a diode into the armature circuit. A roll takes about 45 minutes to build so at worst the motor will see one cycle every 45 minutes. The assembly will contain about 200 lb-ft of potential energy and the majority of this will need to be eaten by the motor as heat as the assembly moves down. Based on the curent mechanical design we believe the motor will see about 75 lb-in of torque.

Now for the question. We are looking to cut costs out of this thing. So we are looking for a low cost permanent magnet motor to use in this application. Since the motor needs to move with the assembly the smaller the motor the better. I plan to size this thing small since the duty cycle and active duration are both so small. Does anyone have a good source for low cost PMDC motors?

Thanks,
Keith

Think I would use a screw to adjust position of idler. Granted this would result in manual raise and manual lower. The other option is to use motor and worm gear reduction to drive screw to move roll in both directions.

Dan Bentler

We tried to sell the customer on this but they wanted to stay with the "manual" system. So the physical system is what it is at this point.

The concern about the screw driven assembly is the contact case. With a screw the assembly will be held in place and the force between the idler roll and the product roll would increase very quickly. We can biuld in resilience but that is money. They can run this in either lay-on (intentional contact) mode or gap mode with a variable gap. So simple sensors aren't an option. I can give them a system that would do both of these automatically (we have done them before) but, again, they will balk at the cost.

Keith

Call your local bearing/pt guy.
There are numerous versions of mechanical one way clutches.
Several come to mind that will solve your problem.

gas, can you be more specific? I did talk to our local PT rep and all the brakes and clutches he mentioned were simple friction devices. So either my assembly wouldn't move at all or, once it started moving, it would pick up speed until it hit the bottom. He didn't have anything that he could guarantee would have a greater dynamic friction than static friction or, better yet, a velocity dependent resistance.

Keith

Would a hydraulic motor or pump be an option here? I'm guessing the motor speed will be between 30-60 RPM. Is that too slow for a hydraulic motor or pump to controllably pump fluid? A large flow control and loop of hydraulic hose would make for a pretty compact package.

Keith

You should be able to do this with a small motor and a VERY inexpensive motion controller by using only the derivative gain. You may be able to do well enough in a PLC.

The hydraulic motor solution would work. You could get a small pump that can pump in both directions and simply pump the oil through an adjustable orifice. It wouldn't be precise or linear but it would be simple.

Thanks Peter.

I came across an advertisement that pointed me to the DC motors on Automation Direct. They look pretty cheap. I'm going to try one of them. They use ceramic magnets so I shouldn't need to worry about demagnetizing the motor. Welding the commutator? That's another matter.

I'll keep the hydraulic motor in my back pocket. It's probably the better solution from a durability standpoint. If I destroy the DC motor I think hydraulic will be the next step.

Keith

Sorry, I didn't check this site 'till now.
There are some magnetic (permanent) back stops that I have used. Free movement in one direction and adjustable torque limiting in the reverse direction.
I'll call those guys tomorrow and PM you.
5 Years ago I think they were 3 around hundred bucks.
If I remember correctly 75 in/lb is in the range.

That sounds like that would do the trick, gas. Thanks for looking.

They do this kind of thing with lifts (elevators) to slow them down before applying a hard brake in an overspeed. In the old days it used to be a govenor (governor) attached to a brake.

http://en.wikipedia.org/wiki/Governor_(device)

Bryan

OK,
What we used was an eddy current brake. These are used to stop trains and roller coasters, also used to provide a load for gym equipment.
We pressed one way bearings into the bore of the brake, that gave us free running in one direction and controlled adjustable decent in the other.
I just did a quick search and found huge train brakes and small exercise equipment brakes but not what you are looking for. I'm sure they are out there. Maybe someone here will weigh in with a supplier.
We built 5 machines that used these in the early 90s and they are still going with no wear issues.

Thanks, gas. I forgot about eddy current brakes.

The quick search I did indicated slip RPM values well above where I would be direct drive. I am looking at 30-60 RPM so I will need to gear up the brake by a factor of 10-20 to get up in a reasonable area of the torque/slip curve. But, as you said, if I go with a one-way bearing where I connect to the rack that may not be such a bad thing. I just need to see what that does to my price point.

Thanks again for the info.
Keith

Quick Update

We finally got this thing put together and had a chance to test it. In the limited testing we have done it seems to work pretty well. It goes a little faster than I had assumed it would so either the weight is greater than we assumed, the motor resistance is a little higher than we assumed or the motor is nearing saturation and we are losing the linear relationship between current and torque.

We are currently running it with no additional resistance in the armature circuit. All we have is a diode between the two motor leads. We have been messing with it with a much higher duty cycle and shorter operational period than the assembly will see in actual production. So far nothing has cooked itself so we are hopeful.

Keith