Hydrostatic pump issue

[ssande]

I have a custom vehicle which utilizes a hydrostatic pump for motion. I am experiencing noise at the pump and drive system has started to free wheel down an incline. Since this is a similar set up to a zero turn I thought perhaps someone could offer some advice. The forums for hydraulics on the internet are fairly sparse. Since this is not an off the shelf vehicle I will list some of the details and have posted a video (with sound) of what I am experiencing. In addition, when the bypass valve is open the sound is about the same (connects the high and low pressure sides creating an open circuit). My thought is air entering the oil suction line or a high pressure valve in the pump misbehaving. My hope is that someone with some experience would recognize what is happening and give me some starting points. Many thanks!

Component details:
1. Oil tank - 3 gallon with interior baffle, vent, 100 mesh strainer
2. Engine 31hp vanguard
3. Oil filter - rated 20 gpm
4. Hydrostatic pump Samhydraulik axial piston pump MD10V-18
Displacement 18 cm3,
Charge pump 5.4 cm3
Main pressure relief valves 3,625 psi
Charge pressure relief valve 290 psi
Input rpm 700-3,600 rpm
5. Solenoid Valve
6. Motors(2) Samhydraulik
200 cm3,
Cont 3,045 psi
Int 3,770 psi
Peak 4,495 psi
Max flow cont 19.8 gpm

IMG 2418 - YouTube

 

[oldnslo]

Noise & free wheeling point towards a loss of charge pressure and or air getting drawn into the charge circuit

I presume oil level is correct?

I recommend double checking all of the inlet connections to the charge pump for air leaks. I.e. cracked hoses, lose fittings, etc.

Is the inlet strainer clean?

Check your charge pump pressure at different engine RPM痴. Does it vary a lot with RPM or stay pretty constant

 

[3Lfarms]

I don’t know about you problem, but I do know you need to take it to Speed Week at the Bonneville salt flats.

 

[ssande]

Thanks for your reply. The oil level is adequate. I will start with the strainer and suction lines.
One additional observation is that if you let it free wheel down an incline when you get to the bottom it takes some time to "recharge" before the pressure builds again to move forward. We use the hydraulic brakes to prevent this from happening. My initial thought was the drive motors may be pumping the oil back to the pump going downhill and overrunning the high pressure valves and emptying out the closed loop of oil.