450: Air Cavitation in Hyd Filter

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Hoss54

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Feb 4, 2006
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I have a '96 450 skid steer, that I am having problems with the hydraulic filter, draining itself, back to the hydraulic tank. All the time..., I was thinking I had a sucton problem with the pump. Lift arms, and bucket jerky, like the pump was starving for oil... Pulled the hyd. tank, and checked for obstructions, and had a new suction hose made, in case of an unseen flap, or collapsed hose... Same thing....., and still foamy oil... The hyd. filter block made a slight squawl.., and has since I purchased it several months ago. Never owning one, I was thinking..., maybe it's the way it is.... Narrowed it down to air being trapped in the inverted filter. Called the local Bobcat shop foreman, and he said it SHOULD purge itself.... Went ahead and loosened the drain plug for the filter..., and poof!! A lot of air..!! Started it, and let it run for maybe 30 seconds..., and did it again... More air!! Refilled the oil tank, and did it again... More air..., but not as much... So figured I'd cycle it..., and work the rest out... Everything worked smooth as silk...., last night... After shutdown, kept hearing a gurgling sound, but could never trace it. Finally found it this morning!! The filter..., in it's inverted state, and probably 6-8 inches above the oil line in the resevoir, is draining back to the tank. When I started it up this morning..., same thing..., jerky on the lift arms, and bucket..., so air is trapped again... Seems a simple inline low pressure, high volume check valve would solve the problem.... If it were a rubber line, it may be an easy fix..., but it is a steel line... Still may try it... Did anyone ever own one of these, and have the same problem...?? Or was there ever a recall on these, to solve this issue...?? Do I have a faulty filter block, not allowing the system to purge the air from itself..., and allowing oil to drain back to the tank, once I did purge the system...?? By the time I found it, my local Bobcat dealer was closed.., and couldn't confer with the shop foreman. And trust me..., he's been good as gold to me this week!!! And he does know his stuff!! I'm just a poor dumb country boy.., but putting an inverted filter 6 to 8 inches above the full line in the tank, without a check valve..., is just asking for problems... It's pretty tough to push air down..., with a liquid..... Any ideas out here....?? Dave J.
 

Tazza

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I seriously doubt that is your problem.
Even if the filter is inverted NO air should be able to enter the system. Remember the hydraulic system holds back in the order of 2,500 PSI so a small vacuum will be no problem. Even the charge pressure side is about 120PSI.
I honestly think you may have a case drain filter blockage, this will make your hydraulic oil foamy. Talk to your dealer and see if there is a case drain filter on your machine, thats where i would start.
The hydraulic system is indeed self purging, run it long enough and it will remove all the air from the system.
Good luck
 

Hoss54

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Feb 4, 2006
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I seriously doubt that is your problem.
Even if the filter is inverted NO air should be able to enter the system. Remember the hydraulic system holds back in the order of 2,500 PSI so a small vacuum will be no problem. Even the charge pressure side is about 120PSI.
I honestly think you may have a case drain filter blockage, this will make your hydraulic oil foamy. Talk to your dealer and see if there is a case drain filter on your machine, thats where i would start.
The hydraulic system is indeed self purging, run it long enough and it will remove all the air from the system.
Good luck
Don't think there is a case drain filter. Checked the service manual, and no where does it say anything about one... You must realize..., this is the baby Bobcat... 16 hp Kohler..., and no fancy stuff on it. Not even an oil cooler.... Just two lines in, one from the hydrostatic pumps, and one from the control valve... one outlet.., directly to the tank. It goes into a T fitting, at the tank..., and the suction hose to the pump is also on this T. There are two poppet valves in this filter block. I assume to relieve pressure, so it won't blow the filter apart..., if the pressure flow is too great. One is in the return port to the tank, and the other goes no where. There is just a plug holding the pressure spring. It is factory.., shows it in the manual. They also use this block on the 453, which does have a cooler... I had it out, and apart, springs seem ok, and the valves seating properly... Let me ask you this.... There is a port, in the block just outside the poppet valve, from the top of the block. Is this where oil normally returns out of the filter, to the system?? It appears, that when the poppet valve is opened, it covers, or rather closes this port, protecting the filter. From this port.., to the hyd. tank is a direct shot. So when you shut it off, after a few minutes or less.. It's like the oil is gravitating to the oil tank, and air is coming back up the line... I heard it through a steel rod placed on the metal line... I've done it enough times..., I've checked them all, and it is the only place it does it... You can even feel it, and the tank gurgles when it does it... According to the Shop Foreman, the pressure in the filter block is only like 20lb. He checked to make sure..., when I told him I thought of trying to purge the filter block.... Once I purged that filter..., It worked like a charm... No foamy oil..., and I played with it for over an hour. Today..., it had the same problem. I tried to purge the system by cycling the lift, and dump..., while travelling.... Wouldn't do it..., and I worked it for almost 45 minutes... The port outside the poppet valve is what's got me wondering. What checks that oil flow, when it is shut down. The service manual does't show a cut away of the filter block. Just the removal and installation of the two poppets. That's why I'm wondering if I have a faulty filter block... Dave.
 

Tazza

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Don't think there is a case drain filter. Checked the service manual, and no where does it say anything about one... You must realize..., this is the baby Bobcat... 16 hp Kohler..., and no fancy stuff on it. Not even an oil cooler.... Just two lines in, one from the hydrostatic pumps, and one from the control valve... one outlet.., directly to the tank. It goes into a T fitting, at the tank..., and the suction hose to the pump is also on this T. There are two poppet valves in this filter block. I assume to relieve pressure, so it won't blow the filter apart..., if the pressure flow is too great. One is in the return port to the tank, and the other goes no where. There is just a plug holding the pressure spring. It is factory.., shows it in the manual. They also use this block on the 453, which does have a cooler... I had it out, and apart, springs seem ok, and the valves seating properly... Let me ask you this.... There is a port, in the block just outside the poppet valve, from the top of the block. Is this where oil normally returns out of the filter, to the system?? It appears, that when the poppet valve is opened, it covers, or rather closes this port, protecting the filter. From this port.., to the hyd. tank is a direct shot. So when you shut it off, after a few minutes or less.. It's like the oil is gravitating to the oil tank, and air is coming back up the line... I heard it through a steel rod placed on the metal line... I've done it enough times..., I've checked them all, and it is the only place it does it... You can even feel it, and the tank gurgles when it does it... According to the Shop Foreman, the pressure in the filter block is only like 20lb. He checked to make sure..., when I told him I thought of trying to purge the filter block.... Once I purged that filter..., It worked like a charm... No foamy oil..., and I played with it for over an hour. Today..., it had the same problem. I tried to purge the system by cycling the lift, and dump..., while travelling.... Wouldn't do it..., and I worked it for almost 45 minutes... The port outside the poppet valve is what's got me wondering. What checks that oil flow, when it is shut down. The service manual does't show a cut away of the filter block. Just the removal and installation of the two poppets. That's why I'm wondering if I have a faulty filter block... Dave.
Well, it has be stuffed, the case drain filters are usually hidden in the port block.
I have no experience with these machines, maybee another person in here will know...
Now, does it do the same thing when you aren't moving too? as in park brake on and lift and tilt the bucket? if so, this won't be case drain as this is ALL hydraulics not hydrostatics which have no case drain. There are no oil leaks on this machine? as they could be the point where the air is coming on from?
 

OldMachinist

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May 24, 2006
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Did you ever figure out what was causing this problem? I have a 453F with the same problem and can't figure it out. Thanks Don
 

Hoss54

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Feb 4, 2006
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Any news?
Hi!! Come to find out..., it was the seals in the drive motors.. They would hold oil....., but let air back through themselves, and seek the highest point..., the filter... I finally gave up, and took it to the local Bobcat dealer... Shop foreman, and mechanics thought it was the pump.. For one thing..., the rental company I bought it from..., had the pressure jacked up to almost 4,000 psi...!! Had them replace it... He calld that afternoon...., and said it was doing the same thing...., with the new pump... But the old one was shot, at least I have a new pump... He told me the next morning he was going to have a phone conference with sevaral shop foremans, in the tri-state area... Two weren't sure...., and one told him it was the drive motor seals... He described what it would do..., to a "T".. Foreman called, and said they could have the seal kit within a day... Less than $60.00 I believe...., but don't quote me... But...., labor was $900.00...!! Had to pull everything out, to get to them... Told them to fix it... Been using it for 2 1/2 years..., and hasn't missed a beat... ( Knock on wood...!!) I've since built a tined manure bucket, and bale spear for it.... Handiest thing...., since sliced bread...., LOL!! Hope this helps someone else.... I must have missed the one posting about the 453...., geeze..., over a year ago..!! Sorry...!!
 

gost

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Sep 22, 2007
Messages
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Hi!! Come to find out..., it was the seals in the drive motors.. They would hold oil....., but let air back through themselves, and seek the highest point..., the filter... I finally gave up, and took it to the local Bobcat dealer... Shop foreman, and mechanics thought it was the pump.. For one thing..., the rental company I bought it from..., had the pressure jacked up to almost 4,000 psi...!! Had them replace it... He calld that afternoon...., and said it was doing the same thing...., with the new pump... But the old one was shot, at least I have a new pump... He told me the next morning he was going to have a phone conference with sevaral shop foremans, in the tri-state area... Two weren't sure...., and one told him it was the drive motor seals... He described what it would do..., to a "T".. Foreman called, and said they could have the seal kit within a day... Less than $60.00 I believe...., but don't quote me... But...., labor was $900.00...!! Had to pull everything out, to get to them... Told them to fix it... Been using it for 2 1/2 years..., and hasn't missed a beat... ( Knock on wood...!!) I've since built a tined manure bucket, and bale spear for it.... Handiest thing...., since sliced bread...., LOL!! Hope this helps someone else.... I must have missed the one posting about the 453...., geeze..., over a year ago..!! Sorry...!!
Ok, I hope that is not the problem on my Bobcat 453 -99. This is the tread http://www.skidsteerforum.com/DesktopDefault.aspx?tabid=54&frmView=ShowPost&PostID=8757 Thanks for the late update.
 

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