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Q & A: stabilizing industrial lettuce spinner

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Most recent answer: 05/10/2015
Q:
I am building an industrial lettuce drier. It's basically a giant lettuce spinner-- visualize a washing machine on spin cycle. The lettuce is inherently out of balance-- it's just the nature of the beast. We are currently making the material decisions for machining the basket that will spin and hold the lettuce. We can, through the machining process, make sure that the basket is perfectly balanced. Is it worth the enormous extra cost to get the basket machined out of very thick stainless in order to increase the mass of the basket? Will this increased mass help to stabilize the inherently out of balance lettuce? A conceptualized version of the question would be:Take two discs of the same size, however one is twice as heavy as the other. Now place a 5 gram weight at the exact same distance from the center of each disc. Now spin the discs at the same RPM. Will the heavier disc exhibit less vibration than the lighter disc? Is there any form of gyroscopic stabilization occurring?Any other suggestions for helping to stabilize the system once the inherently out of balance lettuce is introduced?
- Adam (age 27)
Howard, CO, USA
A:

Great question, and very clear!

Using a heavier metal basket, keeping the basic dimensions the same, won't reduce either the force or the torque that has to be exerted on the unbalanced load. Look at it this way. For a balanced basket, the net momentum is zero, so there's no change in momentum and no force needed other than whatever is needed to counteract gravity. There is an angular momentum as the basket spins, but it doesn't change so no torque is needed other than a bit to cancel the frictional torque. Now add some unbalanced lettuce. The changing force and torque needed to keep it spinning is the same regardless of the weight of the basket. 

I've assumed here that the basket spins with very little wobble. If it's allowed to wobble off-axis a lot, then those wobbles would involve changing its momentum and its angular momentum, so they do involve changes in the force and the torque. That would make a harder physics problem to solve.

Mike W.


(published on 05/07/2015)

Follow-Up #1: force and displacement

Q:
Thank you so much for replying so quickly! I am a bit confused by the answer, would you mind clarifying some things? If I am not mistaken, it sounds like you are saying that a heavier basket will not decrease the torque necessary to spin an off balance load vs a lighter basket. We have plenty of torque to spin whatever load we want as we have up-sized our motor dramatically. We are more concerned with trying to dampen vibrations as a result of the uneven distribution of lettuce within the basket. Think of trying to dampen the vibrations of a washing machine that's out of balance on spin cycle. It is our hope that using a heavier basket might accomplish this. Let's take the situation with the two discs, one being heavier, both having a 5 gram weight on the edge. Assuming that the disc is well balanced, the force causing the vibration can be fully attributed to the 5g weight on the edge. Using the calculation for centripetal force (F=m*v^2/r) isn't the force exerted by the 5g weight the same in both the light disc and the heavy disc? If so, when the equivalent force acts on a heavier basket, it should accelerate it less so than the lighter basket right (F=m*a)? Assuming this is right, wouldn't the heavier basket experience a lower magnitude of vibration as compared to the lighter basket, thus dampening the vibration of the system? Let's say the heavy disc is twice as heavy as the lighter one, wouldn't that 5 gram weight cause the same frequency of vibration, but half the magnitude? Once again, thanks so much for taking the time to help us with this conundrum!
- Adam (age 27)
Howard, CO, USA
A:

Yes, we're in agreement. I considered the limit where the mounting held things very nearly in place. The mounting then has to supply changing forces and torques to do so. We agree that those changing forces and torques are the same regardless of the mass of the basket. 

If the restoring forces are weak, allowing the basket to wobble a lot, then your argument is correct. A heavy basket will wobble a smaller distance to compensate for the unbalanced load, keeping the net momentum constant. 

What sort of suspension will you use? Is the position allowed to wobble much? What about the angle of the axis?

Mike W.


(published on 05/09/2015)

Follow-Up #2: industrial lettuce spinner

Q:
I very much appreciate you getting back with us on this. We suspected the heavier basket would help, now we feel more comfortable spending the extra dough. The basket is spun vertically on a 1.5" shaft that is mounted through two bearings. The shaft is ~15" long, with one bearing at the bottom and one at the top. Even with this setup, there is still some small room for wobble within the bearing assembly when the load is off balance. Furthermore, the forces are such that it actually wobbles the entire machine when the load is way off balance. In retrospect, we wish we had extended the shaft below the bottom bearing and included a heavy flywheel to counteract the basket up top-- but the machine is already far along and now we're just looking for thoughts on how to stabilize what we've got. Any other suggestions other than making the basket heavier?
- Adam (age 27)
Howard, CO, USA`
A:

Hi Adam- I'm no expert in this stuff, but I don't think the heavier basket will help if there's just a small room for wobble. The reason is that if the wobble is mainly constrained by forces and torques, with only a little of the free wobble left, the magnitude of those forces and torques is not sensitive to the basket weight. It's only if you go toward the other limit of unconstrained wobble that the distances can be reduced significantly by using a heavy basket.

Here's a thought. If the motor is rigidly mounted to the spinning basket, you could let the whole combination wobble around held loosely in place by some springy supports. You can see right away that that would greatly reduce the changing forces exerted by the bearings on the axle. Using a heavy basket would then reduce the physical distances that the whole assembly would wobble. I guess if the spring supports also let the whole assembly (motor+axle+basket) twist around some, that the internal torques exerted by the bearings would also be reduced. I'm focusing on the forces and torques exerted by the bearings because that's where you'd expect damaging wear to occur.

Mike W.

p.s. DId you guys consider just using the spin cycle of a commercial washing machine? Does that question reval my extreme ignorance?


(published on 05/10/2015)

Follow-up on this answer.