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• # Linear Force from 3HP motor

Discussion in 'Calculations' started by JoeS, Nov 18, 2014.

1. ### JoeSNew Member

Joined:
Nov 2014
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Hello All,

I just found this website and thank goodness it exists.

So I have a motor turning an eccentric arm that drives a rod pushing a plate back and forth (imagine an old locomotive).

The motor is running at 3 HP at 1725 RPM. The stoke is 1.75" (so the radius of the eccentric arm assuming it is a disk is .875"). It moves a plate that weighs 75 lbm.

I need to find the tonnage force the plate pushes.

I'm thinking I need to get torque and translate that to the linear motion in one direction (the push stroke vs the pull stroke), but it seems like I keep getting too low of a force....thoughts?

2.
3. ### Auto EngineerActive Member

Joined:
May 2012
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What happened ?

Which came first the chicken or the egg?

A bit more detail given in this thread to work with?

4. ### bdeuellMember

Joined:
Mar 2014
Posts:
21
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torque output will vary with the speed of the motor, the actual speed under load not the rated speed.

there is a good image of the torque speed curve for induction motors here (if thats what you have):
http://www.engineeringtoolbox.com/electrical-motors-torques-d_651.html

Im sure you have done the same calculation already but i calculated your motors torque output at rated speed and power. I got 9 ft-lb which would yield a minimum linear force of 125 lb assuming no loss in your mechanism.

hope this helps.

5. ### PierArgWell-Known Member

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Mar 2013
Posts:
100
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Hello JoeS and welcome to the forum!

The problem is rather simple: if you know the mechanism geometry it is simple to transform the motor torque in linear force.

6. ### DanaWell-Known Member

Joined:
Sep 2010
Posts:
422
3
HP = torque * rpm / 5252, so your 3HP at 1725 rpm gives 9.13 lb-ft or 110 lb-in torque. At mid stroke (when it's moving fastest) with a 0.875" lever arm, that's 125lb force. At the ends of the stroke when it's stopping and reversing, the theoretical force is infinite.

Whether that force is sufficient to accelerate and decelerate your 75lb plate at 1725 cycles/sec is left as an exercise for the student...