charge pressure dropping under load

[sterlclan]

On my 630 I just changed the hydraulic pump, the bronze filter, hydralic filter ,and all the hydraulic oil my charge pressure is around 100-150 cold or with no load add a load and some heat and it will drop to less that 30 as soon as you let off the handles it comes right back up bucket functions don't affect it much replenishing valves? bad hystat?or would the rotating groups do this to the pressure? ..............Jeff

 

[Tazza]

Is the machine running bad or its just something you noticed? If it works ok, i'd leave it alone.
As far as i was aware, the charge oil is supplied to replenish the oil that by-passes worn motors. I'm not sure how the charge oil enters the the pump, i assumed the whole inside of the pump was at charge pressure so any losses from the rotating groups would not actually be lost. If anything i would think it would be leaking from the motors. They are made to leak internally anyway, just as they wear it gets a little higher.
If your charge pressure is pretty well constant when not under load i would suspect that the replenishing valves would be ok. It wouldn't hurt to check them if you are really worried, its just not a fun job gaining access to them.
I really should setup a gauge on mine to check pressures at different speeds and load.

 

[skidsteer.ca]

Is the machine running bad or its just something you noticed? If it works ok, i'd leave it alone.
As far as i was aware, the charge oil is supplied to replenish the oil that by-passes worn motors. I'm not sure how the charge oil enters the the pump, i assumed the whole inside of the pump was at charge pressure so any losses from the rotating groups would not actually be lost. If anything i would think it would be leaking from the motors. They are made to leak internally anyway, just as they wear it gets a little higher.
If your charge pressure is pretty well constant when not under load i would suspect that the replenishing valves would be ok. It wouldn't hurt to check them if you are really worried, its just not a fun job gaining access to them.
I really should setup a gauge on mine to check pressures at different speeds and load.

I know on my 553 the charge pressures was fairly constant with just the drives, regardless of speed.
but fluctuated widely with use of the loader arms or bucket cylinders. especially if you forced the main relief open by bottoming the cylinder.
Mayby remove the case drain hoses off the wheel motors and see what volume of flow is coming through them. Compare side to side.
Ken

 

[sterlclan]

I know on my 553 the charge pressures was fairly constant with just the drives, regardless of speed.
but fluctuated widely with use of the loader arms or bucket cylinders. especially if you forced the main relief open by bottoming the cylinder.
Mayby remove the case drain hoses off the wheel motors and see what volume of flow is coming through them. Compare side to side.
Ken

yup I was thinking motors myself wont the fluid run out of the fittings that i take the drains from?and tazza it gets all groany "sometimes" other times it works ok for a 28 year old machine ill keep you posted..........Jeff

 

[Fishfiles]

In case you didn't know a 630 s/n 12999 down the charge pressure is 80-90 psi , s/n 13000 up is 120-200 , ---------------when you checked the bronze case drain filter was there much junk in it , have you changed the hydraulic filter lately , not that it would help your problem but to cut it open and to see whats in it could give and idea what's up , I have just learned from the Ace at the local dealer than charge pressure can be lost in a bad drive pump ( sunstrand) , but the rotating group usually will put golden colored particles in the system when it wears , from what you say about heating up and dropping pressure sounds like a charge pump senirio

 

[Tazza]

In case you didn't know a 630 s/n 12999 down the charge pressure is 80-90 psi , s/n 13000 up is 120-200 , ---------------when you checked the bronze case drain filter was there much junk in it , have you changed the hydraulic filter lately , not that it would help your problem but to cut it open and to see whats in it could give and idea what's up , I have just learned from the Ace at the local dealer than charge pressure can be lost in a bad drive pump ( sunstrand) , but the rotating group usually will put golden colored particles in the system when it wears , from what you say about heating up and dropping pressure sounds like a charge pump senirio

The gold particles are bronze from the swash plates that the rotating group runs on.
The groan you speak of, does increasing the RPM fix this? i found at low RMP my 731 would make quite a bit of noise under load but kick the revs up to almost full and it would hardly make a noise (if you could hear over the racket the engine made).
If you crack a case drain line you will get a fair amount of fluid out of the hose. You would really need to plug the line.
The only thing i can really recommend is to watch the pump temperature. If it starts cavetating from a blocked filter it will groan and squeel and the pump will heat up, and it will heat up FAST. When my suction line filter was blocked (and blocked it was!!!!) the pump was un-touchable within minutes. My filter looked like it was never changed or cleaned, there were a few alloy particles and a few bits of silicone stuck in there.
As you said, its almost 30 years old. Its done pretty well, just as mind did. Its still running strong and its new owner absolutley loves it.
If it still runs strong and doesn't heat up i'd leave it all alone. Just watch your pump temperature and you should be pretty well rite.
As Fishfiles said, checking the insides of the filters is a pretty good idea. It will let you know if there is a part failing or not. I have done it a few times before and was happy that there were no visible metal particles.

 

[skidsteer.ca]

The gold particles are bronze from the swash plates that the rotating group runs on.
The groan you speak of, does increasing the RPM fix this? i found at low RMP my 731 would make quite a bit of noise under load but kick the revs up to almost full and it would hardly make a noise (if you could hear over the racket the engine made).
If you crack a case drain line you will get a fair amount of fluid out of the hose. You would really need to plug the line.
The only thing i can really recommend is to watch the pump temperature. If it starts cavetating from a blocked filter it will groan and squeel and the pump will heat up, and it will heat up FAST. When my suction line filter was blocked (and blocked it was!!!!) the pump was un-touchable within minutes. My filter looked like it was never changed or cleaned, there were a few alloy particles and a few bits of silicone stuck in there.
As you said, its almost 30 years old. Its done pretty well, just as mind did. Its still running strong and its new owner absolutley loves it.
If it still runs strong and doesn't heat up i'd leave it all alone. Just watch your pump temperature and you should be pretty well rite.
As Fishfiles said, checking the insides of the filters is a pretty good idea. It will let you know if there is a part failing or not. I have done it a few times before and was happy that there were no visible metal particles.

You would want to extend your case drains back to the reservoir and run the unit up on blocks. Watch the end of the lines and compare the flow from side to side, If its too much your charge pump can't supply the volume loss from the drain. As it hase been said there will be some flow, just check to see if its excessive
Ken

 

[sterlclan]

You would want to extend your case drains back to the reservoir and run the unit up on blocks. Watch the end of the lines and compare the flow from side to side, If its too much your charge pump can't supply the volume loss from the drain. As it hase been said there will be some flow, just check to see if its excessive
Ken

fishfiles yup I hacked the filter and it was clean Its got a new hydraulic pump/filter/fluid and the bronze one is new too Tazza itll groan and do nothing much throttle doesn't help but after first starting it works new not a groan in it sometimes for 10 min sometimes for 35 but once it starts it doesn't stop till it cools off Ken how much is too much?it looked like plenty to me but im a body man/truck driver/operator/part time mechanic.excessive loss makes sense to me as the pump cant replace it fast enough perhaps that is why if I drop the lift arms fast it helps extra fluid =plenty charge I haven't tried that with the gauge to see if it helps with pressure but it makes it move better. how hard is it to repair the drive motors? sorry about the one long paragraph firefox and slooooooow connection=no patience. thanks guys..............Jeff

 

[Tazza]

fishfiles yup I hacked the filter and it was clean Its got a new hydraulic pump/filter/fluid and the bronze one is new too Tazza itll groan and do nothing much throttle doesn't help but after first starting it works new not a groan in it sometimes for 10 min sometimes for 35 but once it starts it doesn't stop till it cools off Ken how much is too much?it looked like plenty to me but im a body man/truck driver/operator/part time mechanic.excessive loss makes sense to me as the pump cant replace it fast enough perhaps that is why if I drop the lift arms fast it helps extra fluid =plenty charge I haven't tried that with the gauge to see if it helps with pressure but it makes it move better. how hard is it to repair the drive motors? sorry about the one long paragraph firefox and slooooooow connection=no patience. thanks guys..............Jeff

Drive motors are pretty simple, you just need to be careful with aligning the valve plate. I have never done it myself, but i have a book on it and another member of the forum has done it and he said it was very easy. Just keep it all clean and use lots of oil for re-assembly.
I don't know just what seals are inside the motors, i do know bobcat want 2 arms and a leg for the kit. You can get the seals from your local hydraulic seal shop much cheaper though.
I'm not too sure if it will be a seal problem or just wear between the outer housing and the geroler. I still don't fully understand how this setup works, but it works well.
The symptoms you describe still sound like a blocked bronze filter to me, even though it has been replaced. Did it work properly when it was changed? Mine was working well then i pulled the motor and pump out to paint the machine. When it was all back together it was doing the same thing as you mention. It turns out the filter needed another clean. Cleaned it up and it was good to go.
One thing you can try to see if the pump is cavetating is to remove the pressure switch and install a valve with clear tubing running back to the hydro oil tank. There should be no bubbles when the machine is running. If there are bubbles you have a restriction somewhere causing cavetation or low charge pressure. This is what bobcat sent me a fax detailing this for removing air from the system too.
I don't know enough about the system to know how much case drain is acceptable. I know my 743 must have a fair bit as on a hill it will creep quite a bit without the brake on. One machine i sold a few months back did not move any where near as much.

 

[skidsteer.ca]

Drive motors are pretty simple, you just need to be careful with aligning the valve plate. I have never done it myself, but i have a book on it and another member of the forum has done it and he said it was very easy. Just keep it all clean and use lots of oil for re-assembly.
I don't know just what seals are inside the motors, i do know bobcat want 2 arms and a leg for the kit. You can get the seals from your local hydraulic seal shop much cheaper though.
I'm not too sure if it will be a seal problem or just wear between the outer housing and the geroler. I still don't fully understand how this setup works, but it works well.
The symptoms you describe still sound like a blocked bronze filter to me, even though it has been replaced. Did it work properly when it was changed? Mine was working well then i pulled the motor and pump out to paint the machine. When it was all back together it was doing the same thing as you mention. It turns out the filter needed another clean. Cleaned it up and it was good to go.
One thing you can try to see if the pump is cavetating is to remove the pressure switch and install a valve with clear tubing running back to the hydro oil tank. There should be no bubbles when the machine is running. If there are bubbles you have a restriction somewhere causing cavetation or low charge pressure. This is what bobcat sent me a fax detailing this for removing air from the system too.
I don't know enough about the system to know how much case drain is acceptable. I know my 743 must have a fair bit as on a hill it will creep quite a bit without the brake on. One machine i sold a few months back did not move any where near as much.

The case drain returns to reservoir whatever oil is bypassing within the motor and not returning to the hydrostat pump and being pumped back to the motor again. The charge pressure has to replace the oil coming out the case drain. Whats excessive now, I don't know but it should be no more flow then that tiny line can easily handle, and should be compareable side to side.
And yes dropping the boom quickly on my 553 sent charge pressure sky high, like adding a second charge pump
Ken

 

[sterlclan]

Drive motors are pretty simple, you just need to be careful with aligning the valve plate. I have never done it myself, but i have a book on it and another member of the forum has done it and he said it was very easy. Just keep it all clean and use lots of oil for re-assembly.
I don't know just what seals are inside the motors, i do know bobcat want 2 arms and a leg for the kit. You can get the seals from your local hydraulic seal shop much cheaper though.
I'm not too sure if it will be a seal problem or just wear between the outer housing and the geroler. I still don't fully understand how this setup works, but it works well.
The symptoms you describe still sound like a blocked bronze filter to me, even though it has been replaced. Did it work properly when it was changed? Mine was working well then i pulled the motor and pump out to paint the machine. When it was all back together it was doing the same thing as you mention. It turns out the filter needed another clean. Cleaned it up and it was good to go.
One thing you can try to see if the pump is cavetating is to remove the pressure switch and install a valve with clear tubing running back to the hydro oil tank. There should be no bubbles when the machine is running. If there are bubbles you have a restriction somewhere causing cavetation or low charge pressure. This is what bobcat sent me a fax detailing this for removing air from the system too.
I don't know enough about the system to know how much case drain is acceptable. I know my 743 must have a fair bit as on a hill it will creep quite a bit without the brake on. One machine i sold a few months back did not move any where near as much.

I tried it without any filter same thing so i put the new one in again i didnt see any bubbles but a lot of fluid if the system is "closed" drastic loss in the drives may be more than 11 gallons a min. can the hystat pump more gpm than the hydralic pump? If so that would create suction and loss of pressure wouldnt it? Jeff

 

[skidsteer.ca]

I tried it without any filter same thing so i put the new one in again i didnt see any bubbles but a lot of fluid if the system is "closed" drastic loss in the drives may be more than 11 gallons a min. can the hystat pump more gpm than the hydralic pump? If so that would create suction and loss of pressure wouldnt it? Jeff

The hydrostats can move likely 300% the oil the hyd pump does, but most of it is pumped around and around and a percentage is let out the case drains to help cool the system.
If case drain flow is exceeding even 5 gpm I'd guess that to be alot. If the flow is under that then maybe the replentishing valve is sticking and the charge oil is not being replaced.
My 553 would bounce around wildly at various times if the boom was moved quickly or a cylinder was bottomed and the relief opened. But usually it was quite constant + or - 5 to 10 psi (95% of the time)
wheel spend had little efect on the pressure.
Ken

 

[sterlclan]

The hydrostats can move likely 300% the oil the hyd pump does, but most of it is pumped around and around and a percentage is let out the case drains to help cool the system.
If case drain flow is exceeding even 5 gpm I'd guess that to be alot. If the flow is under that then maybe the replentishing valve is sticking and the charge oil is not being replaced.
My 553 would bounce around wildly at various times if the boom was moved quickly or a cylinder was bottomed and the relief opened. But usually it was quite constant + or - 5 to 10 psi (95% of the time)
wheel spend had little efect on the pressure.
Ken

Have you ever seen the movie BACKDRAFT? the scene in which the fire hose flops around?it kinda looked like that...Im looking into parts for one motor now Ive got one brand new one cant afford another as soon as I rip into it Ill let you know what I find thanks again .............Jeff

 

[Tazza]

Have you ever seen the movie BACKDRAFT? the scene in which the fire hose flops around?it kinda looked like that...Im looking into parts for one motor now Ive got one brand new one cant afford another as soon as I rip into it Ill let you know what I find thanks again .............Jeff

I would like to know the ideal leakage for these motors, i may look into it when i get time, it can't be any more than a few percent, otherwise so much energy would be wasted.
Remember the motors aren't even under load. When they are under load you will get increased flow.

 

[Tazza]

I would like to know the ideal leakage for these motors, i may look into it when i get time, it can't be any more than a few percent, otherwise so much energy would be wasted.
Remember the motors aren't even under load. When they are under load you will get increased flow.

If you have a new motor see if you can compare the difference in case drain flow. This would give you a good idea on the problem.

 

[Fishfiles]

If you have a new motor see if you can compare the difference in case drain flow. This would give you a good idea on the problem.

I don't know what the psi max acceptable specs are for Bobcat's travel motors case drain but most of the hydraulic motors that I have worked on in vibratory plate compactors and what not are between 50-100 psi , and it is important as anything more than that will blow out the seal in the end of the motor and put oil into the planetary , chain case or what not , some times I have had to run a seperate line back to the tank from the drive motor to relieve excessive back pressure on the drive motors of attachments as the OEM plumbing would cause the back pressure to go over what the seals could handle --------also know that when the internal seals blow inside a Bobcat travel motor that a hesitation will start , when you push the travel lever to move it will take a second or two before the machine moves , probally because the oil has to build up pressure and with an internal leak it takes a couple of seconds longer for the pressure to build up ------- the travel motor is pretty easy to reseal , about an hour to take it out and put it back in and another hour to reaseal it , take care to mark the sections and put it all back to together the same way it came apart ------I really don't think that the travel motor is your problem but I am pretty confused trying to deciffer the post on the problem , you say you have a new hyd pump , do you mean the gear pump on the end of the sunstrand (transmission pump) which works the boom , tilt and aux or do you have a new complete package which woud be the sunstrand and the gear pump ----even if it's new doesn't mean it's ok , could have been damaged on start up by trash still in the system from the previous failure

 

[sterlclan]

I don't know what the psi max acceptable specs are for Bobcat's travel motors case drain but most of the hydraulic motors that I have worked on in vibratory plate compactors and what not are between 50-100 psi , and it is important as anything more than that will blow out the seal in the end of the motor and put oil into the planetary , chain case or what not , some times I have had to run a seperate line back to the tank from the drive motor to relieve excessive back pressure on the drive motors of attachments as the OEM plumbing would cause the back pressure to go over what the seals could handle --------also know that when the internal seals blow inside a Bobcat travel motor that a hesitation will start , when you push the travel lever to move it will take a second or two before the machine moves , probally because the oil has to build up pressure and with an internal leak it takes a couple of seconds longer for the pressure to build up ------- the travel motor is pretty easy to reseal , about an hour to take it out and put it back in and another hour to reaseal it , take care to mark the sections and put it all back to together the same way it came apart ------I really don't think that the travel motor is your problem but I am pretty confused trying to deciffer the post on the problem , you say you have a new hyd pump , do you mean the gear pump on the end of the sunstrand (transmission pump) which works the boom , tilt and aux or do you have a new complete package which woud be the sunstrand and the gear pump ----even if it's new doesn't mean it's ok , could have been damaged on start up by trash still in the system from the previous failure

nope I replaced just the vane pump on the end of the hystat pump the oil has been changed three or four times it comes out clean if the charge pressure was going into the transmission pump wouldnt you see leakage? the inside of the first drive was all gouged up and the other side has a bad shaft I dont think mixing and matching parts is a good idea so I have to wait for the dough to get another drive anyway

 

[sterlclan]

nope I replaced just the vane pump on the end of the hystat pump the oil has been changed three or four times it comes out clean if the charge pressure was going into the transmission pump wouldnt you see leakage? the inside of the first drive was all gouged up and the other side has a bad shaft I dont think mixing and matching parts is a good idea so I have to wait for the dough to get another drive anyway

well after pricing both parts and a new pump I went mix and match, some new seals,some old parts and the shaft from the funky motor cleaned everything good soaked with oil and put it back together. tomorrow Ill start to reinstall them Ill keep you posted ... Jeff

 

[sterlclan]

well after pricing both parts and a new pump I went mix and match, some new seals,some old parts and the shaft from the funky motor cleaned everything good soaked with oil and put it back together. tomorrow Ill start to reinstall them Ill keep you posted ... Jeff

oops meant to say new drive