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Ezrider

anyone know much about hydrostatic drives

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iv got a old small skid steer. its a gehl 2500 long story short i was using it the other day broke the coupling that conects the two hydrostatic drives together and damaged the oil feed line to the left side pump. didn't look like the hose was leaking fixed the coupling and went to finnish what i was doing 20 mins later left side drive quits working. then as i was trying to limp it back twards the shop the right hand drive quits. both sides just freewheel. so i figured the hose that i thought was not leaking must be sucking air and the hydrostatic drives filled with air. replace the hose screwed around with it a while, with all 4 tires off the ground . spin the tires by hand till it biult up pressure and couldn't spin them anymore. and the right side came back to life after a couple trys. after it came back to life it was slow at first after running it for a few mins it was normal. the left side however i can get it to run slow for a couple mins and then it goes back to freewheeling. 

 

i pressurized the hydrolic tank with air to just a couple psi to check for any leaks i found a slight leak where the linkage comes threw the case on the left drive but non of the hoses are leaking and i think its leaked there for a while. or at least its always been damp around there. 

 

basicly it seems like there is eaither air traped in the pump that doesn't want to work its way out or its introducing new air somewhere. it seems like once you get it moving though it should move the fluid threw and purge any air rater quickly like the other side did. but maybe there is a trick to getting the air purged out?

 

im at a bit of a loss i know hydrostatic drives are basicly just a hydro pump that pumps into another pump with a varaiable vane that controls the ratio to make drive power and that is about as far as my knowlage of them goes. never had one apart. guess im looking for idea's from what i can find parts are hard to come by for these drives couldn't find much found some reports of people getting them rebiult at a few different places for $3k witch im not spending that kind of money on this thing. 

 

its too small for a lot of things i need to do but its a hell of a lot better than nothing. esp once the snow starts flying witch isnt far away now. i guess if the drive is shot im not going to spend 3k to have it rebiult id rather put that money twards buying a different skid steer. esp considering the engine burns about a quart of oil per hour. if its a take it apart and replace a seal that can be found at a normal hydrolic shop that is doable. 

hr.jpg

Edited by Ezrider

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I would try pulling a vacuum on the air bound side and you  should be able to get the air out. Ive done that with the  power steer system on my scoopmobile that was air bound.

Edited by 41chevy

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might be a challenge to try to pull a vacuum on the return side its probably a 1.5-2 inch hose that comes off the gear box. hose is kinda old and stiff would be hard to get it off without damaging it. and then would have to figure out a way to get from a 2 inch hose to a vacuum pump. wet dry vac mabye....lol if i can get the hose off without damaging it might be easyer to try to plug the tank side of the return line and pressurize the tank. 

im thinking the pump is probably damaged from running dry. or sucking air from where the control arm comes out of the case. seems like hydrostatic pumps should move quite a bit of fluid seems like once it started moving it should push all the air out rather quickly unless i am missing something. 

 

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sounds air locked. Pumps will go bad if ran dry but you likely would have had to starve it for a long time.  as soon as it runs dry you cant move it so unless you let it idle for a long time after that I think it would be ok?   only way to remove air is to bleed it out like on a car but most times the system will self prime as you work the control valves. Can you look inside the hyd tank when its running? if you can look for air bubbles and that is a good sign that it needs to be primed again. if the tank is full try and move all the functions back and forth with a line cracked open a little to see if air or fluid is escaping.

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2 hours ago, 41chevy said:

sounds pretty close to what i have tried to do run it in nurtal and each direction with the tires off the ground and then try to drive it a few feet back and forth the problem i am having is after i get it blead out enough to move on its own it gets air locked again

 

2 hours ago, Lmackattack said:

sounds air locked. Pumps will go bad if ran dry but you likely would have had to starve it for a long time.  as soon as it runs dry you cant move it so unless you let it idle for a long time after that I think it would be ok?   only way to remove air is to bleed it out like on a car but most times the system will self prime as you work the control valves. Can you look inside the hyd tank when its running? if you can look for air bubbles and that is a good sign that it needs to be primed again. if the tank is full try and move all the functions back and forth with a line cracked open a little to see if air or fluid is escaping.

 

can't look inside the tank, like normal for a skid steer 10 lbs of crap cramed in a 5 lb bag although this one really isnt as bad as a lot. still no way to see in the hydro tank other than see whats on the dip stick. but the hydro line suctions from the tank the line goes uphill to a filter then t's post filter and to each hydrostatic pump. back down hill witch is kinda a weird setup to me but its how its always been and how the factory diagram shows it being plumbed. i have thought of trying to change the filter as well but at this point i hate to chance introducing more air into the system. from the hydrostatic pump it then returns into the gear case where it fills the gear case about half full and then returns to the tank threw a large hose by gravity. there is a cover on the very top of the gear case that i might be able to remove and look for bubbles. i do know i have air in the pump. there is some plugs on the front of the case where a optional preasure relife valve can hook up i have been able to get air to purge out cracking the top plug and then will get movement of the drive again for a short period of time. however it will still be weak.

 

it seems like it really should not have this much trouble pushing the air out. especially considering the return is quite a bit higher than the suction line. of course i suppose the pump tolerances are probably not as tight as they should be as this skid steer was biult in the 70's

hydraulic tank is full and bucket controls work flawlessly however they do run on there own pump

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If the Tank is low and the filter and pump is high I suspect its sucked air in and you will need to add oil to the filter and try and get it to prime again. To me it sounds just like a fuel pump on a Mack where you need to fill the fuel filters when you install new ones or you likely louse the prime on the system. you should know when you pull the filter if it lost prime by how much fluid is in the filter. another suggestion is to intorduce some air pressure into the tank to try and force fluid up into the top part of the system.  I have primed a few trucks the same way

  I recently replaced both hoses on a Track motor in a excavator. The owner ran it out of hyd oil trying to crawl it up to the road and out of the mud. I was able to replace both hoses Refill the tank as it had ran dry. and within 10 seconds the track motor bleed itself and started to move again. Mind you this is a completly different machine but It did self bleed. the only difference is that the tank sat higher  or level with the the pump and the air in side the track motors were lower than the pump. This worked to my advantage.

 

Good Luck with it.

 

Trent

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the hose that leaked was post filter. so the filter shouldn't have gone dry. and the right side drive is working properly the left is still giving me fits. i am amusing everything at the filter and below is fine do to the right side drive still operating properly. i have thought of changing the filter because maybe if its partially plugged up might be making it harder for a possibly weak pump to work. most systems iv worked with always had the filter on the return side rather than the suction side as pumps like to push better than pull. so i am thinking a restriction at the filter might be adding to my problem. however at the same time i hate to possibly introduce more air into the system.

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Any chance the supply hose is collapsing internally? Or partially plugged? bad supply valve?

Sounds to me like you are blaming the hose when you don't really know for sure if that caused the problem. Same as any other trouble shooting problem; start at the beginning and check everything without focusing on what you think is the problem.

If you have air in the system one of two things are happening; air is being introduced where it shouldn't be or you are not supplying oil at a fast enough rate to supply the system. You stated that you can manually bleed the system to the point where it will operate and then the problem returns. That pretty much rules out an air lock and sounds like a supply problem to me. 

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i think i will try to replace the filter and go from there. im pretty sure no hoses are collapsing both sides use the same feed prior to the filter and the side that is still having issues is the one with the new hose. 

 

preasureizing the tank with a few psi of air revealed no leaks prior to the pump, but i agree it seems like it is still introducing air somewhere. 

Untitled.jpg

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i e-mailed all state ag parts and asked them if they would have a hydrostatic drive motor for my skid steer, they e-mailed me back and said they reviewed there inventory and would likely have one and wanted me to call them. so there might be a option there if they don't want a arm and a leg for one.  i really planned on getting a different skid steer this year winter but it just might not be in the cards have not had the best year this year. but i still need something to push snow with. don't want to put a ton of money into it its probably only worth a couple grand at best with everything working properly on it. and the engine is pretty tired. 

 

plan for today i think will include changing the filter then i will unhook the hose from the hydrostatic drive pressurize the tank to push the fluid out of the tank threw the filter and out the end of the supply hose to make sure i have no air trapped prior to the drive. try priming it again and see what happens.

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Changed the filter filter was full of glitter pushed flud to the pump till I got no air then managed to get the return line off and plugged the side to the tank and pushed some fluid threw the pump started it worked but slower than the other side for a little while then stopped again. Exactly like before. The only leak that can be found in on the rod for the control. Mabye it is sucking air from there. I'm thinking it took some internal damage

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Looking at the diagram above I think maybe you have the supply and return lines confused 

The reason I say this is that I would have thought the supply lines come from separate supplies as in each valve for each side and on the ones you have marked they share a common supply 
So maybe your missing something on the supply side here as in the spool valves might be leaking the air in 
And plus a filter is normally on the return line otherwise the filter would have to handle a couple of thousand pounds of hydraulic pressure 

Anyway maybe some thoughts there and maybe I have no idea as I dont muck about with these so maybe best not to listen LOL

Paul 

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nope not reversed. return lines a large line that just gravity feeds down to the tank each hydro static drive has its own fluid pump inside the unit filter is under vacuum not pressure. fluid is sucked from the tank into the threw the filter to the drive unit where its pressurized in the unit and then converted into rotational force. there is a drive belt that runs down from the engine to the drive units. and spins a internal pump

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3 hours ago, Ezrider said:

Changed the filter filter was full of glitter pushed flud to the pump till I got no air then managed to get the return line off and plugged the side to the tank and pushed some fluid threw the pump started it worked but slower than the other side for a little while then stopped again. Exactly like before. The only leak that can be found in on the rod for the control. Mabye it is sucking air from there. I'm thinking it took some internal damage

Glitter is a failure somewhere, yes or no?

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Is there a by pass valve for each drive? Might be full of your glitter and stuck  partially open.

Edited by 41chevy
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well i see i`m not the only one fighting with a skid loader  have 763 bcat refusing to pick its boom up  it blew a lift arm hose lost quite bit of oil    replaced  both the hoses filled it  back up with oil went back to what i was doing finished that`  parked the machine . next time i tried to use it the boom would not lift  so i may have an air prob, to

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1 hour ago, carlotpilot said:

well i see i`m not the only one fighting with a skid loader  have 763 bcat refusing to pick its boom up  it blew a lift arm hose lost quite bit of oil    replaced  both the hoses filled it  back up with oil went back to what i was doing finished that`  parked the machine . next time i tried to use it the boom would not lift  so i may have an air prob, to

stupid question......could you have crossed the hoses between their correct ports and now one cylinder is trying to extend while the other side is trying to retract ?

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2 hours ago, Ezrider said:

swash plate piston pumps

 

 

Untitled3.jpg

EZ, a typical hydrostatic system is closed loop, that said the oil in one side will basically stay there except what is expelled thru the case drain lines. They may have inline filters in them to protect the rest of the system in case of pump or motor failure, With that said, you may have a plugged case drain that has caused an air locked motor thus resulting in a intermittent operation. if the machine only acts up in one direction it could be a stuck circuit relief valve. .........after looking at this and looking at your previous picture, what I just told you wont mean squat ! 

 Does the linkage "Feel the same" L vs. R    when actuated ? if not perhaps a roll pin failed on the swash plate control, had a new Holland do this once. does the book show you a test port for charge pump pressure? if so you could test that to see if it stays constant. if it does then im afraid the "glitter" is most likely the hydrostatic pump has failed. 

Edited by gearhead204
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when it does operate it will operate in both directions. but if under load (tires on the ground rather than in the air) there is a obvious cavitation noise. when applying the left drive. 

 

most of the time i have been trying to run it i have all 4 tires off the ground but once you get it blead out enough to drive the tires, it will continue to work in ether direction although at a reduced from normal speed till you put the controls to the neutral position for say 30 seconds or so then it will air lock again. 

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56 minutes ago, gearhead204 said:

stupid question......could you have crossed the hoses between their correct ports and now one cylinder is trying to extend while the other side is trying to retract ?

no   one hose is 2 feet longer  and it worked to finish loading some gravel after replacement  at present it does not even try to move

Edited by carlotpilot
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2 minutes ago, carlotpilot said:

no   one hose is 2 feet longer  and it worked to finish loading some gravel after replacement

I have a753 I think or 743 ( I can never remember unless im looking at it) Anyhow there is a red knob on the right side of the operators platform that is a dead machine boom lower by pass valve, could it be stuck in the bypass position? other than that you may have failed a cylinder piston packing or stuck port relief.

 I take it the bucket tilt function still works properly ?

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