Help with choosing a hydraulic motor for a project

[cstamm81]

I am looking to tinker with an old Ariens single 28 inch blade mowing deck, converting into a FEL / backhoe mounted trimmer. I need help figuring out what type of hydraulic motor to use, and the specs. I would like to power the motor with either of my tractors, so we are looking at 7 to 10gpm at about 2500psi. In my mind, the blade would turn at about the same speed as a standard finish mower would at working tractor rpm. My thought was to use a love joy coupling from the motor to the spindle. I believe I will need a crossover valve as well, to keep from having issues with the abrupt stop of the hydraulics.

 

[MossRoad]

You can't say 7 to 10 and get an accurate speed.
10 is 3 more than 7.
3 is about 42% of 7.

So if you are pushing it at 10, you're gonna get about 42% more flow through it. The speed will be a huge increase. So be aware of that.

 

[cstamm81]

You can't say 7 to 10 and get an accurate speed.
10 is 3 more than 7.
3 is about 42% of 7.

So if you are pushing it at 10, you're gonna get about 42% more flow through it. The speed will be a huge increase. So be aware of that.

Makes sense, thanks. I’ll assume I will only run it on my L5740. I could fathom using it hanging off the front of the loader, and on the backhoe. That said, the machine specs at a flow of 9.8gpm. Am I safe to assume there is a true 9.8gpm at the loader and the hoe? I would most likely be using diverter valves, rated at about 13gpm so their shouldn’t be any restrictions there.

 

[Dave M7040]

A blade tip speed of less than 19,000 feet per minute should be your goal for safety reasons.

Your precise tip speed can be determined by using the following equation: FPM = RPM x . 262 x Blade Diameter (inches).

The above equation yields an rpm of 2,500.

You claim to have 7 gpm @ 2,500 psi.

HHP=P*Q/1714, where P stands for pressure in pounds per square in., Q stands for flow rate in gallons per minute, and 1714 is a conversion factor necessary to yield HHP in terms of horsepower.

Hydraulic Horsepower = 2,500 psi * 7 gpm/1714

So you would have 10 horsepower available to drive a 28" lawnmower blade which is insufficient. Even if you say you have 10 gpm, it would not make much of a difference.

A hydraulic motor rotatating a blade at speed should be controlled with a motor spool hydraulic valve to avoid the abrupt stop you are concerned about.

Unlike a valve for hydraulic cylinders, a motor Spool Valve does not hold in its neutral position allowing the motor and rotating blade to coast to a stop.

Many tractor owners come up with ideas for utilizing their machine's hydraulic system.

In almost every instance the tractor does not have the capacity to power motors.

For the tractors ( large) that can drive hydraulic motors, many suffer from overheating of the hydraulic system as minimal to no cooling is incorporated into their design.

Companies like Landpride make pto driven hydraulic power packs equipped with a cooling fan. These units require 52 pto HP.

HRS3 Series Hydraulic Reservoir Systems | Land Pride

The Landpride system needs 52 pto horsepower and only manages to output 43 usable horsepower. It produces 25 gpm @3,000 psi

I will do the same Hydraulic Horse power calculation using Landpride's spec's

HHP=25 gpm*3,000 psi/1714

HHP=43 horsepower

I am sorry to pour cold water on your plans but best you know the reality before going too far and ending up completely frustrated.

Dave M7040

 

[MossRoad]

I'm spinning three 20" blades for a 60" cut with 8gpm at 2500psi with no problems. I think 8gpm@2500psi comes out to less than 12 HP.

He would have no problems spinning one 28" blade.

 

[cstamm81]

I actually have a 20gpm 3000psi Erskine PTO power pack. My thought was to use the tractor hydraulics so I could mount the cutter on the loader or the hoe. That said, I believe I can accomplish what I want with using just the loader, with a cylinder to angle the cutter. As the power pack is designed to power things like a snowblower, I’m assuming when you shut off the PTO, the motor will coast to a stop with the PTO winding down? This would eliminate the need for a motor valve or crossover.