Control of hydrulic motors

[gromit]

I have an old Hustler mower, 6' cut, that is before the era of the hydrostatic drives. It has belts and pulleys for the drive to the mower deck, which simplicity personified, but the collection of rotating c**p to provide forward and reverse with adjustable ground speed defies description. I want to replace the 2 cylinder Wisconsin, with a VW diesel that I have, and get rid of all the squealing belts (6) and pulleys (14) involved in the ground drive.

I have 50 years machining experience machining, and 40 years welding, and have a shop more than adequately equipped to do both. I have 2 gear type pumps, and 2 gear type hydraulic motors- I have no idea what the GPM of either are, except that the pumps (identical) are half again larger than the motors (again identical). I am thinking that less than 1 HP at each wheel will be adequate, and I can play with the ground speed by varying sprocket sizes that go to the wheel reduction boxes. I know that the ideal is hydrostatic drive, but I would have to buy all that. As much as possible, I want to keep the cost down, as this is a backup machine for another later Hustler (with Hydrostatic). Even hoses aren't a problem, because I have the hose, end fittings and a crimper.

My question is- can I use two four way valves to get independent drive to the two wheels with forward and reverse of either at any time (to steer), and can I "throttle" the valves to temporally slow down around trees and bushes?

Is there another type of valve that would give better control- at a realistic price-considering that the machine itself is of very limited value- especially in its present (stock) form?

Was going to do this conversion with a generator, and electric motors (old starter-generators from lawnmowers), but decided that the motors weren't capable of enough watts output for continuous use--hence the re-think.

Thanks in advance!

Gromit

 

[aczlan]

Most zero turns use two variable displacement pumps for propulsion. Then they don't need a separate valve.

The problem I see with a valve is that the fluid will get very hot from bypassing if you aren't running full bore all of the time.

Aaron Z

 

[gromit]

But most of the time it will be running full speed--


Gromit

 

[gromit]

Doing a search and spending a few hours reading here, it would seem that what I propose is possible by using 4 way open center valves, which, as I understand it, would offer limited oil heating and power loss (power loss not a priority problem- would only affect fuel consumption) as opposed to closed center valves- Is this correct?

Any input J J?

 

[J_J]

There are some variable speed pump at Surplus hydraulics. I don't know if the volume is satisfactory for your hyd motors. As far as using open center valves, and not motor spool valves, is that if you release the valve quickly, you stop the flow real quick, and you had better have some relief valves across the hyd motors.

If you should go to neutral suddenly, the hyd motors will try and pump fluid if there is any fwd or rev momentum . I don't think the fwd and reverse will be as smooth as when using the VSP pumps.

If someone would plug the components into the hyd simulation software, It might show some problems that should be avoided.

https://www.surpluscenter.com/item.asp?item=9-7887&catname=hydraulic

 

[gromit]

Thanks J J !

Surplus has"motor control" valves, I guess that that is the way to go.

Gromit

 

[J_J]

The motor valves are designed to free wheel or coast in neutral so the hyd motor can come to a slow stop . You would need some brakes if using those valves.

 

[J_J]

You have these motor spool valves, and the the one way speed control valve.

https://www.surpluscenter.com/item.asp?item=9-6760&catname=hydraulic

https://www.surpluscenter.com/item.asp?item=9-064-50&catname=hydraulic

 

[AKKAMAAN]

would offer limited oil heating and power loss (power loss not a priority problem- would only affect fuel consumption) as opposed to closed center valves- Is this correct?

Hydraulic power loss will ALWAYS affect system temperature.....a power loss is, if power not turned into mechanical work, a loss into HEAT! That can easily become a PRIORITY PROBLEM! In fact, power loss and heat is probably the most common and serious problem in hydraulic system.
You can not get away the heat problem by throttling a constant flow for speed.

 

[gromit]

Thanks J J and AKKAMANN

I've rebuilt a fair share of hydraulic pumps and cylinders, but this is the first that I have tried to cobble a system together, and it shows. Braking is not a problem as land is virtually flat, and machine has rudimentary brakes--wouldn't trust them on a steep slope- but that's not in the operating picture. I like the idea that the motors will freewheel, as the later machine that I have locks both drive wheels if the pumps aren't running, and a couple times I have had to drag the thing back from the far reaches of the property with our Porsche tractor and the mower wheels locked.

OK, I can understand that throttling causes restriction, and restriction causes heat, but I can control ground speed by throttling the engine and keep the deck speed up by driving the deck with the same pair of vari-speed pulleys that now control the ground speed, if the deck speed works out too low when creeping. Thanks to both of you for the information that you have shared!