How do I spec a Hydraulic motor

[woodlandfarms]

OK, I just don't get it. I think I get it, then I don't...

I have an application, I want a motor to go so fast and I have so many gallons and so many PSI. I go to the Burden and when I think I start looking at what I think I need to be looking at it no longer makes any sense.

JJ and others, you have recommended some books and I promise, once I have read the 3 foot high stack of scripts by my bed I will get on this issue.

So, here are the specifics and my confusion. I want 500 RPM with 18GPM and 3000 PSI. JJ has recommended a cool motor that works but, but, it has a tapered shaft. So I start looking in the Cubic inch section (8.6 cubic inches) and there are other motors, albiet more expensive at this point, but they look like they might work. But then I look at the details and they have different PSI ratings or different RPMs. How is this possible if they are all the same cubic inches?

Sorry I am so dense on this...

Oh,and is there some sort of code to motor numbers? I am looking on Ebay and see all sorts of numbers and just wondering if there is a way not to google each one.

Carl

 

[SnowRidge]

OK, I just don't get it. I think I get it, then I don't...

I have an application, I want a motor to go so fast and I have so many gallons and so many PSI. I go to the Burden and when I think I start looking at what I think I need to be looking at it no longer makes any sense.

JJ and others, you have recommended some books and I promise, once I have read the 3 foot high stack of scripts by my bed I will get on this issue.

So, here are the specifics and my confusion. I want 500 RPM with 18GPM and 3000 PSI. JJ has recommended a cool motor that works but, but, it has a tapered shaft. So I start looking in the Cubic inch section (8.6 cubic inches) and there are other motors, albiet more expensive at this point, but they look like they might work. But then I look at the details and they have different PSI ratings or different RPMs. How is this possible if they are all the same cubic inches?

Sorry I am so dense on this...

Oh,and is there some sort of code to motor numbers? I am looking on Ebay and see all sorts of numbers and just wondering if there is a way not to google each one.

Carl

Each manufacturer uses their own codes. Most of the big names have their catalogs on-line in the form of PDF files.

You can use the wheel motor with the tapered shaft for applications other than wheels. Power Trac now uses one on the 48 brush cutters. You just have to match the hub or coupling or whatever to the shaft. The taper will be some standard, probably SAE. The catalog page for the motor will list it.

A non-tapered shaft will probably be easier to deal with, but you may save a bunch of money with a surplus wheel motor. Then again, you may not.

Your specification is incomplete. At a minimum, you need to also specify torque at rated pressure and flow. Take a look at the catalogs to find out all the variables that go into the specs.

 

[woodlandfarms]

Your specification is incomplete. At a minimum, you need to specify torque at rated pressure and flow.

Huh? Sorry... are you asking if I know how much torque I need?

 

[SnowRidge]

Huh? Sorry... are you asking if I know how much torque I need?

Yes. You are going to do work with the motor. The motor will have to deliver a certain amount of torque to accomplish the job. If the motor can't deliver enough torque to do the job, it will stall.

 

[woodlandfarms]

So where would I find the torque required to operate a PTO driven Chipper? Never seen a note like that in a specs sheet, but never thought to look at it. Was just looking at the HP needed to operate it.

 

[dynasim]

I would disagree with the necessity of specifying the torque. The speed, flow, and the psi also specify the motor(they all have nearly the same efficiency). Most people want to maximize torque within a given set of constraints(available hydraulics and speed).

Motors have different pressure capabilities, and you need to ensure that the pressure capability of the motor is greater than the pressure of the hydraulic supply.

I would also note that Hydraulic flow X Pressure Drop X Efficiency X Conversion Factor = Power Produced. If you specify those things, you specify the horsepower avaiable on the motor shaft.

Chris

 

[SnowRidge]

So where would I find the torque required to operate a PTO driven Chipper? Never seen a note like that in a specs sheet, but never thought to look at it. Was just looking at the HP needed to operate it.

Use PTO horsepower and calculate.

Example: a particular commercial post hole digger requires 30 PTO horsepower. You would like to know how much torque would be required to drive an auger that fits the commercial unit directly with your homemade hydraulic PHD.

Torque = 30*5252/540 = 292 foot pounds at the PTO.

Assuming the PHD has a 3:1 reduction gearbox, the torque required at the auger is around 875 foot pounds, ignoring PTO shaft and gearbox losses.

This site has some useful formulas, including torque of a hydraulic motor.

I mainly brought up torque because you have indicated you also want to drive a home made hydraulic PHD.

 

[SPYDERLK]

OK, I just don't get it. I think I get it, then I don't...

I have an application, I want a motor to go so fast and I have so many gallons and so many PSI. I go to the Burden and when I think I start looking at what I think I need to be looking at it no longer makes any sense.

JJ and others, you have recommended some books and I promise, once I have read the 3 foot high stack of scripts by my bed I will get on this issue.

So, here are the specifics and my confusion. I want 500 RPM with 18GPM and 3000 PSI. JJ has recommended a cool motor that works but, but, it has a tapered shaft. So I start looking in the Cubic inch section (8.6 cubic inches) and there are other motors, albiet more expensive at this point, but they look like they might work. But then I look at the details and they have different PSI ratings or different RPMs. How is this possible if they are all the same cubic inches?

Sorry I am so dense on this...

Oh,and is there some sort of code to motor numbers? I am looking on Ebay and see all sorts of numbers and just wondering if there is a way not to google each one.

Carl

Motors are speced in cubic inches per revolution. A Gallon is 231 cubic inches. So you have 18 x 231 = 4158 cu inches per minute delivery. So the motor displacement that will use all this flow at 500 rpm is 4158/500= 8.3 cu inch per revolution. This will use all your hydraulic HP [about 30] at 500 rpm. You dont have to stay right at this number tho for a chipper. A little faster might be good. That would mean a slightly smaller motor. Surplus Center sells 3 motors with ready made PTO splined output shafts. I think the Item #9-7368-125 would be a good match. It is 7.63 cu inch. You could go to 540 rpm at your max delivery or slow your engine to get 500 if you really need to.
larry

 

[woodlandfarms]

Motors are speced in cubic inches per revolution. A Gallon is 231 cubic inches. So you have 18 x 231 = 4158 cu inches per minute delivery. So the motor displacement that will use all this flow at 500 rpm is 4158/500= 8.3 cu inch per revolution. This will use all your hydraulic HP [about 30] at 500 rpm. You dont have to stay right at this number tho for a chipper. A little faster might be good. That would mean a slightly smaller motor. Surplus Center sells 3 motors with ready made PTO splined output shafts. I think the Item #9-7368-125 would be a good match. It is 7.63 cu inch. You could go to 540 rpm at your max delivery or slow your engine to get 500 if you really need to.
larry

Larry! Thank you for a great explanation! And I am looking at those motors in a moment.