My Homemade Skid Steer Machines

[justinlovestobuildstuff]

Main Question: Could the 2 cu in. motor difference between the two machines be the issue?

Hello all, this forum helped me out in the past (thank you!) and I again need a little help. I've built from scratch two skid steer machines with various interchangeable attachments, not sure what to call them but they dig, drill, lift and can move about 500 lbs of materials at once. I recently did some major upgrades to my first machine, the green one (Its been hacked up and redesigned a few times). New engine, pump, cylinders, chain drive, wheels, arms, valve body, flow control valve. They both are low pressure systems (2500 psi max). Try not to laugh at them too much, they work well and cost only a couple thousand to make.

My issue now is I cannot crawl up a steep 50 degree incline as I am with my other silver machine. The system pressure will not rise above 2000psi when attempting to crawl up said incline. The motor wont stall out just bog down and sit at 2000psi. The hydraulic fluid heats up pretty quick in this situation.

The machines now are now identical in powertrain terms other than the capable silver one has two 17 cu in. drive Gerotor motors and the green one has two 19 cu in Gerotor drive motors. Could the 2 cu in. difference between the two machines be the issue? The silver machine has tons of power and torque. It almost seems like fluid is getting bypassed yet I've maxed out all pressure relief valves and even bypassed the flow control valve. I understand the GPM of this system is incredibly low to what the motors call for, but I'm fine with 5mph top speed, torque is what I need.

Other general info:
16 hp gas engine
Tried both a .45 cu in. and .74 cu in gear pump, no difference
Green machine is about 300 lbs heavier then the silver machine
Green machine has much smaller fluid tank, 2 gal vs 8 gal on silver machine
These are direct drive hydraulic motor to driveshaft setups. No gear reduction. A chain links front and rear wheels together.
Silver machine has two 1/2 hoses feeding valve body, Green machine just has one 1/2 hose.
Fixed displacement pump, open center system.

 

[Dave M7040]

Post the brand and spec's for your motors.
You need to have an idea of the torque requirements of your drive system. A quick and dirty way would be to remove the motor, rig up a way to connect a simple torque wrench to the drive and see how many foot pounds is required to rotate the wheels.
Without that number the rest of what you are trying to do is a waste of time and effort.
Some motors are described as high torque low speed which strike me as describing your application but in the one case I looked at the motor is turning 1,000 rpm which seems far too fast for your direct drive system.

There are others on the forum with far more hydraulic knowledge than I have. I am just looking at basic engineering principles in providing my comments.
Great projects and I admire your ingenuity and creativity
Dave M7040

 

[justinlovestobuildstuff]

Thanks Dave. Here at the only specs I have, the current pump is putting out roughly 10 GPM-and that feeds the two 19cu in motors.
What's strange is that the other machine which works great is running roughly 6 GPM feeding two 17cu in. motors. Something isnt right...

 

[ptsg]

Does the silver machine also have the flow control valve after the pump? Could it be that the relief valve on the flow control valve is limiting your pressure for the travel circuit?

By the way, nice machines. I'm all for homemade stuff. Might not look pretty, but just the joy making and using it, it's great in my opinion.

 

[justinlovestobuildstuff]

Thank you, yes both machines have the flow control. I thought that might have been the issue and bypassed it completely and made no difference.

 

[oldnslo]

What component gets hot the quickest? That would indicate your leak point or pressure loss limiting the system to 2000 PSI. 500 PSI is 20% of your available pressure to produce torque.

 

[Dave M7040]

Thanks Dave. Here at the only specs I have, the current pump is putting out roughly 10 GPM-and that feeds the two 19cu in motors.
What's strange is that the other machine which works great is running roughly 6 GPM feeding two 17cu in. motors. Something isnt right...

We need the spec's for the other motor also.
You also need to do the torque wrench test I suggested
Dave

 

[oldnslo]

As a point of reference here are the theoretical torque produced by the motors at 100 % efficient

17 CIR at 2,000 PSI = 451 ft-lbs
17 CIR at 2,500 PSI = 563 ft-lbs

19 CIR at 2,000 PSI = 504 ft-lbs
19 CIR at 2,500 PSI = 629 ft-lbs

CIR = cubic inch rev

Another variable is tire diameter. Larger diameter will reduce torque capabilities but provide higher travel speed.

 

[Vigo327]

Great projects!

Out of what has been said so far, the comment about finding where the heat is being generated is the quickest ticket to what's going on.

I would like to see better pics of the valve sets on the machines, but my guess right now is that there is a leak between valve sections in one of the valve sets that just isn't becoming an external leak.

 

[justinlovestobuildstuff]

Left side fine metering spools, 1st aux, 2nd left side motor, 3rd right side motor- through power beyond fitting to lift,tilt, rear blade, aux. I wonder if power beyond fitting is leaking.
No problem hitting high psi and stalling engine with cylinder dead head.