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# Physics/Physics - Mass in Motion

Question

Part Support
Steve;
I recently sent a question about upward forces seen by a 4000 lb tube set in motion at 20 RPM and stopped abruptly.
I am attaching a PDF of the tube, the rollers it sits on and the top support.  The tube is being honed by a 24" diameter tool driven by a 40 HP motor.  The rollers under the part only support the part.  The tube rotation is normally stopped by a 1" diameter rope wrapped 3 to 5 times around the part and kept in tension.  The rope enters and exits the part from underneath and provides little upward motion resistance.  The rope is periodically loosened for a 1/4 second to 1/2 secondor so to let the part start to rotate.  the rope then locks up to stop rotation.  this lockup is presently very rapid (around .1 to .2 seconds to stop).  The tool weighs 2000lb and moves from end to end of the tube.  a typical part is 10 feet long.  The part is allowed to rotate to keep from honing the bottom of the part more than the top.

Hello John,

Thank you for including the image, I think I have a fairly good picture of the equipment now. I assume that when the system decides to stop the tube, that the drive that made the tube rotate has been disconnected or disabled. I interpret your question about an upward force to mean that there is an unexpected and undesirable upward jump when the rope tightens to stop the rotation.

What I think is happening is explained by the principle of conservation of momentum. That rotating tube would have considerable momentum. If the tube is rotating clockwise from the view in the image when the brakes are applied, it would try to make the apparatus that the rope is tied to, plus the tube, continue rotating clockwise with the same angular momentum. Try to look at the image you sent me as I do, not knowing what all the pieces are for. Let me exaggerate the situation to make what I'm saying more pronounced and I hope more clear. I picture the rope brake is somehow part of the "tool". If the tool holding the tube is sitting on the floor (without being fastened down) when the brakes are applied I think you can picture that the tool might start to rotate and raise up on the right side (as viewed in the image). Imagine that a green member of your team grabbed hold of the spinning tube trying to make it stop. It would carry that person around with it.

I expect that you do have the apparatus tied to the floor. Once the structure of the building is involved in the sharing of momentum, the affect would be unnoticeable because of the larger mass of the building. But I imagine there is some play in some components of the apparatus. It is possible, that before the building is included in the mass that the principle of conservation of momentum is being applied to, that less massive parts with some play are being forced to rotate noticeably to obey the principle of conservation of momentum. And I suspect that the rotation of those parts with some play makes the upward jerk that you observe. There is much guesswork here. I haven't seen the equipment or the problem. If you understand what I am suggesting and can say "that's not it", then I don't know. But I'm willing to consider it further if you want to send a followup.

If that does seem to be a possible explanation, what adjustments can you make? Eliminate free play available before being limited by the attachment to the floor. Make the stop more gradual.

I hope this helps,
Steve

Physics

Volunteer

#### Steve Johnson

##### Expertise

I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.

##### Experience

I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

Education/Credentials
BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University