Force Analysis & Motor Sizing

[kirannm]

Hi folks, 1st time long time,
I have what seems to be a simple problem; this is my 1st time sizing a motor to move a load.
Problem: I need to move a pallet on rails 36" forward and back. the rails are angle iron /\, the wheels are V grooved. The weight of the pallet is 20,000 lbs. Doing some statics and static rolling resistance calculations, Coef. ~.0018 -.002, I keep coming up with a number of ~40lbs-80lbs to break friction and start rolling. If I just look at a big square box that weighs 20k lbs, I have to overcome 15k lbs( Coef. .78 st on st). My numbers just seem off!?!? Then I have to size this motor to move this mass. Using motion simulators from Rockwell, Parker, Aerotech, I just don't have the experience i guess to get realistic motor sizes out of them. Any thoughts?

Thanks, -Kiran

 

[hydtools]

Now you need to define the dynamic problem. How fast over what distance, start and stop the load. Cycle time.
You know from your statics the minimum force to just start the load.

Ted

 

[kirannm]

That is a lot of weight. I am happy with taking 60 seconds to 90 seconds (or slower) to get to the final position, 36 inches.

Cycle time? Is this the total cycle from start to stop to return?

Do those forces seem correct to get this monster rolling? 40lbs of force? I think I have been confusing myself between the trees and the forest. Rolling static friction, I understand. Assuming no sliding the friction induces a moment about the axis of the wheel, point contact. Mark's give rolling coef. of frxn of .002 for steel on steel, other places close numbers.

Thanks, -Kiran

 

[GregLocock]

Well, do /you/ think you could reliably get a 10 ton wagon on a railway to move by leaning on it? And then consider that your rails will bind more than standard ones?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MintJulep]

Well, do /you/ think you could reliably get a 10 ton wagon on a railway to move by leaning on it?

Well actually, 4 guys can get a 40 ton subway car moving. Takes some grunting, but can be done.

This is of course with 34 inch diameter wheels and 6 inch diameter tapered roller bearings and real rail. With the little toy wheels and bearings implied by your sketch, on the back edge of an angle iron, I would expect greater resistance/ton.

So your numbers are off, although probably not by that much, all things considered. Doesn't seem that you have considered bearing friction however.

Don't forget that once you get this thing moving Newton's first law is in effect. How are you going to stop it?

 

[digger200]

Years ago one of the big brg manufacturers gave me an arbitary figure of 300LB/Ton to push something like a caravan, so on that basis you'ed need 2678LB

 

[dvd]

For an industrial dirty environment I might use a friction coefficient of 0.15, (this will take into account the dirt and misalignment that don't appear in your current calcs).

That gives 3000 lb x 3 ft/min = 9000 lb-ft/min. Divided by 33000 ft-lb/min/hp you end up with around 0.3 hp drive motor. Round up to 1/2 hp motor. Size your reducer to handle the drive load.

 

[kirannm]

thanks folks,

You have moved me in the right direction. I am very grateful for the advice !!

I hope I can return the favor.

-Kiran