DC Motor - Torque required

[dalou]

Hi all,

I am new here as i want to ask something about an engineering project i am working on. I need a motor to rotate a 200 grams cylinder which will be hanging from the motor's shaft (vertically). I wanted less than 100 rpm so the motor I chose is a 2.1W 6V DC motor with a 50:1 reductor that leads to 60 rpm and 12.5Ncm. Will that be enough to keep that load rotating? I have no idea how much 12.5Ncm is and have nothing to compare it with. Moreover, what kind of coupling should I look for to fix the shaft of the cylinder to that of the motor? Will regular couplings be strong enough or will the cylinder fall down eventually? Thanks a lot for any responses, I really need your help here to make sure I won't buy anything wrong!!

 

[drawoh]

dalou,

Your primary problem rotating a 200g mass will be friction. You have not told us what will cause friction in your system.

Your next problem could be acceleration to speed, if you want to do it quickly, or if your software assumes it happens instantly. You need the moment of inertia of your rotating system. Again, we do not have enough information.

Couplings solve errors in shaft angle, centring and axial positioning. There are all sorts of couplings out there because there are all sorts of configurations out there that will cause you problems.

Do you have any mechanical engineers or designers in house? My experience has been that designing mechanical hardware and specifying the motors is mostly mechanical engineering. Electrically, motors are simple, at least until you get to the controller.

JHG

 

[dalou]

First of all, thank you for your response.
I do not mind about acceleration of the cylinder and how long it will take to reach the required speed. My only problem is if the torque provided will be enough to outdraw the friction of the system. Friction will be caused by rotating the cylinder (carbon surface) in an aqueous solution.

Regarding the coupling, I am using it to connect the two shafts together (so that the shaft of the cylinder will be rotating). I dont know if i m using the correct term here or not but from what I saw in a few catalogues these components are used to fix two shafts together; is that not right?. Problem is that I cannot support the cylinder shaft with anything else and therefore all the weight of the cylinder will be held by the motor shaft through the coupling.
Therefore, I am asking first if the motor shaft can stand that vertical load and secondly if the standard couplings using set screws will be able to hold the two shafts together or will the weight of the bottom shaft be too much and will not be able to be held at place by the screws. Hope I made things clear. Thanks a lot for help!

 

[drawoh]

dalou,

We are getting into mechanical engineering here. There are ways to work out the surface drag induced by your aqueous solution, if you know the density and viscosity, and the finish on your cylinder. If you have a cylinder and solution, you should be able to test for it. Again, is there a mechanical engineer at your site?

This all sounds small. Do you need to select an effient motor? If not, I suggest selecting a gearmotor capabable of supporting your cylinder on its shaft, and forgetting about the bearings and couplings. MicroMo has a couple of small gearmotors than can support 30lb.

If efficiency is not critical, it is a mistake to select a marginally powerful motor. If it turns out your power estimates were a little optimistic, your motor does not work!

JHG

 

[dalou]

drawoh thanks again for your response!

No there is no mechanical engineer here unfortunately, so I have to figure this out myself.. Also, I should make clear that I am not looking for a marginal solution and calculating the exact numbers would not be required.

So you think 12.5Ncm may not be enough then? What values of torque should I look for? Should I look for something like 100 Ncm to be on the safe side or is that too much?

About the couplings, I didnt really get what you mean.. The cylinder is fixed on a shaft other than that of the motor. What should I use to attach the two shafts together so that the cylinder would be rotating with the motor shaft and also held at place at the same time?

Thanks a lot for helping,
dalou

 

[dalou]

I just found a motor with 0.30Nm very similar to the one I had in mind.. Would that be plenty or is it marginal again?

 

[drawoh]

dalou,

You need to find the torque to turn your cylinder. Either provide someone with the data to do calculations, or test something. For testing, all you need is a cylinder, your aqueous solution, some string and a spring scale.

JHG

 

[dalou]

That would be good but I dont yet have the cylinders either. Anyway I think I ll just switch to a bigger motor to be sure. Thanks a lot for your help!!

 

[batter1979]

Torque of a rotating particle = I*alpha. moment of interia x angular acceleration. Make sure you use the right units. Just make sure you pick a motor with a higher breakdown torque than what is required at 100% duty cycle , and you will be home free. Not knowing exactly what you are dealing with, that is what I would do to be safe.

 

[batter1979]

You probably don't have a machine design text, so what I just told you wont do you any good, or mean anything to you. It's really not that complicated, but you definitely need an M.E., or a Shigley's, and a good learning ability. By the way... You work for a small company don't you? Asking you to do stuff out of your realm of knowledge. Since you are an engineer you must know it all... right?

 

[drawoh]

batter1979,

Your torque equation is for acceleration. The OP is not concerned with acceleration. He is concerned with friction.

JHG