Hydrostatic transmission problem 3

[dwaldron]

I'm not a mechanical engineer so bear with me. I hired a guy (also not an engineer) who claimed to have a lot of experience with hydrostatic drives to build a hydrostatic drive system for a mobile machine. I now have a problem that I need to fix (designer of system is long gone).

System description - hydraulic gear pump driving two gear motors.

Problem - when machine is going down hill it "runs away/freewheels." Not safe! Plus, could cause fluid cavitation in the motors...I think because the system provides no positive back pressure to keep fluid in the motors. I think the fix is to "check" the fluid flow through the motors...am I right?

Question - If I'm right, is there available on the market a type of pressure drop sensing control valve that I can plumb into each "in and out" port of the motors to restrict the fluid flow and thus create a braking effect for downhill operation? I need a valve that will only restrict the flow when the machine is going downhill, yet does not restrict the flow when the machine is on flat ground. An operator controlled valve as opposed to an automatic valve would also work. I’m also looking to not spend an “arm and a leg” to fix the problem. Does anyone have a solution for me? Thanks. Dan

 

[budt]

Dan;

What is controlling direction and speed?

There are Over Center Valves that could keep the motors from running away without affecting their power at other times. Several companies make them. Some suppliers call them Internal and External Piloted Counter Balance Valves.


Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[dwaldron]

Hi Bud,

Thanks for your response.

It is a track (crawler) machine. I have a Cross double-spool valve with two operator-control sticks attached to the Cross valve for controlling direction and speed (amount of pump flow to the motors and how much flow to the left and right motor for direction control - forward, backward, and turning).

I'm not familiar with Over Center Valves. Are they the same as Counterbalance valves? I will look them up on the web. Parker has some Counterbalance valves that look like they may work.

Dan

 

[Deaks]

Dan,
Whoever told you this was a hydrostatic transmission should be shot.
Running away downhill is an obvious consequence of using an open loop circuit without motion control valves (aka counterbalance valves).
HST would normally be associated with a closed loop system where dynamic braking is an inherant feature.
Although you can get an open loop system to work, the machine doesn't need to be too big before you run into lack of power and heat problems.
My advice would be to get a suitably qualified/experienced person to do some calcs for you, because how do you know the motor/pump/engine sizing is enough to retard max weight down max hill? Perhaps someone like Bud?
If you do nothing else, make sure you have a secondary braking system to avoid L&L costs (laundary and litigation) when it all falls in a heap.

 

[budt]

Dan;

Mobile calls them Over Center because one of their main functions is on a Man Lift where an arm may be resistive for half its arc and running away the other half of travel.

The external pilot opens the valve from pressure required to raise the load while the internal pilot stops the load on the running away side unless hydraulic force is applied to make it go that way.

From your first post the machine must run reasonably well except for the unsafe condition of running away downhill. A local company uses Walden vehichles foor handling material in a bucket and they use the same setup except the operator controls foorward and reverse with a foot valve and steering is handled by cylinders that bend the unit in the middle from a steering wheel. These units are quite jerky and pull wheelies regularly when a cowboy is driving.

Deaks is right about theis not being what most consider to be a Hydra-Static Drive. Also about there may be an overheating problem if this machine is used for over an hour or so at reduced speed.

Deaks is also right about the secondary braking unit. A steep downhill, one you can't climb, could open the over center valves and allow full speed ahead though with some retarding. A simple fix could be a way to stop or retard flow in the motor lines to either stop movement or retard to some reasonable speed.

The Cessna company makes small hydra-static pumps in single and dual configuration. They are not cheap but would be a good way to make your machine function smoothly, reliably and run cool. I couldn't find their web site but this company could give you a quote on a doublle pump unit.

http://www.precisionfluidpower.com/quote.htm

Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[dwaldron]

Deaks, Bud,

Guys, thanks for your help. Ya Deaks...you know what they say, "buyer beware", I guess the guy I hired knew just enough to be dangerous!! Bud, you are right, the machine does a great job on flat ground. Deaks, I did install a hydraulic disc braking system. I don't use the machine on hills that much, and with the brakes to supplement, it may work fine to use the counter-balance valve approach. It is a small machine...gross weight 1 ton...max load - 1 ton. Steepest working slope I will encounter is a 20 degree grade with a run of less than 100 yards at a time. Bud, I will also check out the Cessna pump idea. One issue I have is all the hydraulic drive system is really "shoe-horned" into a small space that leaves not room!! I'm wondering if I can fit the counter-balance valves to the motors due to tight space. I will check out both options, get some dimensions and prices and see what works...wish me luck on this one guys...I'll keep you posted.

Dan

 

[kcj]

I would put the POCB valves at the motors anyway, to safeguard against loss of braking control if a hose broke.

Sun and many others make split flange or pad mount valve blocks for most common types of drive motors with the ports on the same side next to each other. You did say gear motor, so maybe its a low end one with o-ring ports on opposite sides. The valves would have to be adaptor mounted then.

kcj