T 300 hyrdrostatic pump keeps blowing HELP HELP

[excavatorman]

Hello,I have a 04 t-300 that i am on my 3rd hydrostaic pump reseal...Ive had a dealer out,hydraulic repair shop that built the pump and nobody knows why...The problem-The whole pump assembly is new and this is what happens...Everytime i get it all in and back together it will run for about 20 seconds and then blow out between the 2 halves....I can hear the motor start to lug and then it blows....I recently installed a bypass from 1 half to the tank and i am gun shy now...Dealer says it is all hooked up correctly but they stung me the first time so went to Hydraulic pump shop...Any ideas please getting very expensive.

 

[Leejohn]

Who makes your t-300?

 

[muddstopper]

If the pump is blowing under pressure just running, it has to have some sort of blockage between the pump and return to tank. Have you checked to see that the linkage to the pump is centered in neutral. It could be that the pump is partially engaged letting it build pressure without that pressure having a route back to tank. Another guess is did you clean the tank after the first pump blew. If your hydraulic system is a closed system, there is a good chance that the lines and valves could be full of metal shavings from the original pump that failed. This could cause all matters of stuck check valves. Simply draining the oil and changeing the filter wont get the system clean. This could mean disassembly of the valve body and or possible replacement of the stuck checkvalve.

I have only worked on one hydrostatic pump and ended up replaceing it. It was just plain worn out. As in wallowed out beyond repaired. But having worked with many hydraulic systems, if the pump is making enough pressure to blow apart, chances are somewhere in the system you have blockage, otherwise the pump should pump, but not build pressure.

 

[DaveOmak]

If there is a relief valve on the discharge side of the HST, it may be faulty or the wrong size.... a relief valve that is too small will allow pressure to build beyond the capacity of the HST....

 

[oldnslo]

Is this a Bobcat T-300 track type skid steer? Are you blowing the seals out of the pumps or the seal between the pumps? If yes it sounds like you have a case drain line issue. On the pump housing there should be line that drains the case directly to tank. Any excessive back pressure or blockage in this line will cause the seals to blow or the housing to crack, etc.

On most HST's used on skid loaders the charge pump flow is dumped into the pump housing to aide in cooling. This flow would fill the case and build pressure quickly if the case drain line is not connected or blocked.

 

[excavatorman]

Dave thanks for reply ill check this as well but where would this location be for this relief valve?Would any other relief valves cause this?

 

[excavatorman]

Thanks for your help...Ill check this out and re drain tank....Ive used new fluid 3 times so far..Was a new case drain filter when i started as well,ill check that again....Where are the check valves located?Is it possible a drive a motor could cause a problem?I did install a line from 1/2 of hydr drives to the tank,shop says to do that ...The 4 plugs in those halves just took 1 and screwed in fitting for hose....it runs but i only 6 sec and shut it off

 

[CJONE]

Dave thanks for reply ill check this as well but where would this location be for this relief valve?Would any other relief valves cause this?

Like oldnslo stated, you have a problem in your case drail line from the stat to the tank. That would include the oil cooler. I do hydrostats all the time and a plugged drain line does exactly what you are describing.

 

[DaveOmak]

Dave thanks for reply ill check this as well but where would this location be for this relief valve?Would any other relief valves cause this?

I don't know where they are...... I know just enough about hydraulics to make me dangerous..... I do know, if your hydraulic system is pumping 5 gal/min (as an example) the rest of the system must be capable of handling 5+ gal/min or your have trouble... If everything is off in the system, that means the relief valve must be able to handle 5+ gal/min, or the return line.....
Dave

 

[CJONE]

Thanks for your help...Ill check this out and re drain tank....Ive used new fluid 3 times so far..Was a new case drain filter when i started as well,ill check that again....Where are the check valves located?Is it possible a drive a motor could cause a problem?I did install a line from 1/2 of hydr drives to the tank,shop says to do that ...The 4 plugs in those halves just took 1 and screwed in fitting for hose....it runs but i only 6 sec and shut it off

Case drain filter?? should not have one. Looking at this you should have 2 t-stats piggybacked, is this what you have?