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Implement Alley Discussion Forum

How Do You Calculate Pull From HP?

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Bill VA

10-09-2004 14:31:13




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I need to figure out how to calculate pull in punds from drawbar horsepower.

Here is how I think I need to do it - is this correct?

HP=(pounds x distance)/(seconds)x(550)

If I solve for pounds

pounds=(HP)x(seconds)x(550)/distance

What I am trying to do is calculate the amount of force a tractor would develop in it's forward motion, assuming that the tractor would be restrained by a cable and start spinning.

Soooo, I would make these assumptions:
HP = 25
Seconds = 1 (actually I think it should be more like .3, but I use 1 as a safety factor)
Distance = 1/2 of the rear tire diameter, therefore 6 feet/2=3 feet

At 25 hp, the force (in pounds) generated by the tractor would be 4583.3 pounds. I would need a cable that would at least handle that amount of load, or it would break.

How does this sound?

FYI, I'm not sizing a cable, but designing a custom bumper for a fellows tractor. Apparently, he wants a bumper that extends across his front tires and will bring the bumper to a dead stop if he hits something vs bends the bumper!

Thanks!
Bill

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Rick Kr

10-11-2004 08:00:38




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 Re: How Do You Calculate Pull From HP? in reply to Bill VA, 10-09-2004 14:31:13  
Bill,
As stated by others, I would concern myself more with the momentum or the impulse of the tractor.
Momentum (mass x velocity) or Impulse (mass X velocity^2). If you are trying to prevent tractor damage, energy needs to be absorbed somewhere. If the bumper does not bend, and the object being hit does not, something else will, like the tractor. Then you still need to worry about the operator on the tractor, even with the lap belt, your face will have a permanent imprint of the steering wheel. I can understand a nice bumper, but it should be more like a brush guard, it will take a little abuse but it should crumple before the tractor.
Take a 7000lb tractor at 12mph, and try to stop it dead, something will break!
Just my opinion (and unfortunately experience hitting a tree in my orchard)
Rick

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Bill VA

10-10-2004 10:39:59




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 Re: Pull From HP... more thoughts in reply to Bill VA, 10-09-2004 14:31:13  
Very interestering responses - thanks!

HP is a ultimately a measure of torque or force x distance with a time component.

I think that if you use the drawbar HP, you have accounted for all losses from the engine through the contact patch of the rear wheel to the extent it slips and one looses draw bar hp. Having said that, if you limit your time, you limit the build-up of hp beyond the tractor's rating.

Sooooo , the force x distance, it would seem to me is the radius of the rear wheel x some force. That force is derived from the given HP, in this case 25, and the amount of time, 1 second.

What are your thoughts on this?

BTW, I agree on impact too. HP is just a starting point. Further, I assume that time is really 1/3rd second and with a safety factor of 3, I arrive at 1 second.

Thanks!
Bill

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Paul Janke

10-10-2004 07:24:05




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 Re: How Do You Calculate Pull From HP? in reply to Bill VA, 10-09-2004 14:31:13  
I agree with what was said below about the horsepower being immaterial, it is momentum which counts. It might be easier to make impact absorbing mounts than to make a bumper to hold up at whatever speed you are talking about. The mounts to the tractor need to be done in such a way that the tractor won't be broken. It occurred to me while doing the first part of this that you might be talking about pushing instead of bumping. If so, it might be easier to use the weight on the drive tires times the coefficient of friction on whatever surface you will be using it. I think the best you can hope for (trying to remember from 25 years ago) is about .8, or 80% of the weight of the back of the tractor. Somebody please correct me if I'm wrong (again).

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Paul Janke

10-10-2004 07:23:21




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 Re: How Do You Calculate Pull From HP? in reply to Bill VA, 10-09-2004 14:31:13  
I agree with what was said below about the horsepower being immaterial, it is momentum which counts. It might be easier to make impact absorbing mounts than to make a bumper to hold up at whatever speed you are talking about. The mounts to the tractor need to be done in such a way that the tractor won't be broken. It occurred to me while doing the first part of this that you might be talking about pushing instead of bumping. If so, it might be easier to use the weight on the drive tires times the coefficient of friction on whatever surface you will be using it. I think the best you can hope for (trying to remember from 25 years ago) is about .8, or 80% of the weight of the back of the tractor. Somebody please correct me if I'm wrong (again).

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Allan in NE

10-10-2004 04:39:05




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 Re: How Do You Calculate Pull From HP? in reply to Bill VA, 10-09-2004 14:31:13  
Mornin' Bill,

Not tryin' to be a smart a$$ or anything, but wouldn't it be easier to just show this fella how to run a tractor? :>)

Allan



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Indydirtfarmer

10-10-2004 10:27:26




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 Re: How Do You Calculate Pull From HP? in reply to Allan in NE, 10-10-2004 04:39:05  
Not that you really have to TRY to be a smarta$$.....

Pure natural talent!

LOL. John



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2x4

10-09-2004 22:29:17




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 Re: How Do You Calculate Pull From HP? in reply to Bill VA, 10-09-2004 14:31:13  
1hp = power it takes to raise 550 lb.weight straight up for a distance of 1 foot in one second.



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Jerry/MT

10-09-2004 21:27:05




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 Re: How Do You Calculate Pull From HP? in reply to Bill VA, 10-09-2004 14:31:13  
If you know the HP at a given rpm, you can calculate the torque at the engine shaft at that rpm by Teng = Constant X HP/rpm. (The constant is 550 x 60 = 33000)If you then know the speed reduction through the transmissision and the rear axle, you can calculate the torque at the rear axle by,

Rear Axle Torque = Teng X rpm engine/rpm rear axle. (This assumes no power losses in the transmission and rear axle. If you know the efficiency of the gear train you can multiply the result above by the eficiency.)
Having the rear axle torque, you can divide that by the diameter of the tire and get the force applied to the tires. That will be the maximum you can have. the tires usually have some slip so not all of that gets applied to the ground.
Hope this helps.

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Jerry/MT Correction......

10-09-2004 21:30:29




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 Re: How Do You Calculate Pull From HP? in reply to Jerry/MT, 10-09-2004 21:27:05  
You divide the Rear axle torque by the tire RADIUS (not diameter) to get the force. Sorry about that.



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TomH

10-09-2004 20:01:34




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 Re: How Do You Calculate Pull From HP? in reply to Bill VA, 10-09-2004 14:31:13  
I think it has more to do with momentum than HP.

Does it matter if the tractor was accelerated to 5mph by a big motor or a little motor? It's still going 5mph either way.



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steveormary

10-10-2004 07:54:26




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 Re: How Do You Calculate Pull From HP? in reply to TomH, 10-09-2004 20:01:34  
And I thought this was going to be about how many plow bottoms you could pull. I have a formula for that somewhere.

steveormary



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