Hydraulic winch torque questions

[John Vorberger]

I am designing a winch to put behind my tractor, and I am going to power it using a hydraulic motor. My tractor puts out about 1000 PSI of hydraulic pressure, and the hydraulic motor I want to use is 9.8 cubic inches per revolution. I calculated that at those specs, the hydraulic motor will produce 125 foot pounds of torque.

Now, my question is, is 125 foot pounds of torque enough for a winch? I was planning on attaching the winch drum directly to the hydraulic motor shaft, and the diameter of my winch drum is going to be 2 inches.

I am a little confused as to what the 125 foot pounds of torque for the hydraulic motor means. To me, a high school senior who took AP Physics, it means 1500 inch pounds, or 750 pounds of force for a 2 inch diameter winch drum. Most winches are 2,000 pounds or more, so 750 pounds of force is pretty weak.

Where am I going wrong? Can someone please help explain this to me?

Thank you,

John

 

[jrmichler]

A 2 inch diameter winch drum is 1 inch radius, so the pull force is 1500 inch-pounds / 1 inch radius = 1500 pounds. You use radius, not diameter, when calculating forces and torques.

Two inches is very small diameter if you are using steel cable. Winding on a drum that is too small both damages and weakens it. More than you ever wanted to know about wire rope, along with good information on winch drums: http://www.ushamartin.com/wp-content/uploads/2014/04/Wire-Rope-Handbook.pdf. A good source for wire rope and fittings is McMaster-Carr: www.mcmaster.com.

 

[John Vorberger]

jrmichler, thank you for the help. So I guess I should plan on using a larger diameter drum. Is a 6 inch drum more realistic? If that is the case, I will probably need to gear down the hydraulic motor using a chain and/or sprockets.

I am still confused as to why a small electric motor can pull 3,000+ pounds, but this big hydraulic motor is limited to 1500 pounds of winch capacity? Am I missing something?

 

[jrmichler]

That small electric motor is connected to the winch drum through a series of reduction gears. When the winch drum is groaning around at about 50 RPM, the electric motor might be turning (rough guess off the top of my head) 3000 RPM. That would be a reduction gear ratio of 3000:50, or 60:1. The noise you hear is normally the gears gearing, rather than the motor motoring. Keep in mind that, when the motor RPM is geared down 60:1, the torque is geared by the same ratio.

You might connect your motor to your tractor hydraulics and find out just how fast it spins with no load. It will spin almost the same speed at full load, so think carefully about how fast you want your winch to move.

If you want to use a roller chain to gear the hydraulic motor to the winch drum, here is a book with everything you need to know (and more) about roller chain: http://tsubaki.ca/pdf/library/the_Complete_guide_to_chain.pdf. Here's another one that's shorter and easier to use: https://www.diamondchain.com/wp-content/uploads/2017/03/TechnicalEngineering.pdf.

 

[Baluncore]

As jrmichler suggested, the first thing to do is find the flow rate.
When the tractor is running at 1200 RPM or at 2000 RPM, and producing 1000 psi, what is the available hydraulic oil flow rate?