Calculating torque requirements

[bobdxcool]

I am making a motorized cord reel. I am using a stepper motor with a gear with a gear ratio of 8:1. I need to run my application at 100 RPM. To retract the cord reel at 100 rpm at the output of the gear shaft , I need to run the motor at 800 rpm since gear ratio is 8:1. The torque at the motor at 800 rpm is 35kgcm, and at the gear shaft output is 35 x 8 x 0.9 (efficiency of 0.9)= 252 kgcm. The weight of the cord is 12 kgs and the diameter of the cord drum is around 140mm. I only need to pull the cord. So, I was thinking what would be the torque required to pull this cord. Can anyone help me with this ?

 

[racookpe1978]

Too much is unknown of the friction of the twisted, round, curled-up, bouncing cord across a ??? floor. The floor may have debris or sawdust or dirt or mud on it. Or may be clean and smooth too.

Unroll the cord six times across what you "think" will be a typical shop (or doctor's office!) floor.
With a light spring balance, weight the tension with the spring balance. A postal scale may work, it is likely much less than 1/2 pound (1/4 kilo).
As the cord is retracted your mass inside on the roll increases (inertia increases) but the drag force across the floor decreases (cord gets shorter, weighs less.)

 

[3DDave]

Set up the reel without the motor and use a indicating torque wrench (not a click wrench)to measure the torque required. This will include the effect of the increasing radius of the drum as cord is wound onto it, the decreasing drag as cord is pulled in, and the work done on the cord to bend it around corners and roll it onto the drum.

Other than that you have selected all the values you can and there are no calculations left to do.

You might consider what happens if the cord becomes caught with the motor switched on. Does the motor have a thermal overload cutout? Will it tear the reel off the wall? Is the drum strong enough to take the compression load of the wrapped cord?

 

[tygerdawg]

The analytical approach requires you to do a load-torque analysis on your system. You must design your system for Peak Torque. Peak Torque includes:
[ul]
[li]torque needed to accelerate the rotational inertia of the drum & motor & any other rotating mass (like your motor's rotor, couplings, etc.)[/li]
[li]torque needed to pull that cord's mass and accelerate it at the worst case of the drum coil-up dimension[/li]
[li]torque needed to accelerate any translational masses, if any[/li]
[li]torque to overcome any friction and all the snags & things listed by others[/li]
[/ul]

Once you have peak torque, then I usually apply efficiencies and safety factors to upsize it. Since you already have a motor, you can just check to see if your motor has sufficient torque > Peak Torque.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[mfgenggear]

100 rpm sounds to fast for that application.

 

[IRstuff]

Just watch out for the other end of the cord as it whips towards its finish. ;-) 3 ft/s ain't so bad, is it?

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[MikeHalloran]

What was wrong with the advice you got in the other discussion you started about the same device?


Calculating maximum speed and torque requirement of stepper motor
thread237-407347


More than one discussion on the same subject will not get you better advice.
It will annoy the regular respondents, who freely give their time, but hate to see it wasted.

Frankly, Bob, you seem to be shopping for an answer that you'd like to hear.



Mike Halloran
Pembroke Pines, FL, USA