Bobcat 773F

[JLDBobcat]

I have a 773 F series and it keeps blowing out the right-side drive motor case seal. It then dumps all the hydraulic fluid into the chain case. We rebuilt the motor, and everything looked good, after running it for a while it blew out the case seal again. The other thing to note is that we replaced the main hydraulic control valve that the motor case drain line goes back to before this problem showed up. It was a used control valve brought online. My questions are what causes pressure to build and blow out this seal? Why does the case drain on the right side go to this control valve when the left side goes right back to the pump? Is it possible that we are building pressure in the control valve and sending it back through the case drain line? We checked the shuttle valve in the motor when we rebuilt the motor and it was good. This does not happen immediately, and I don't know how long it takes as you don't see any performance issues until it runs out of fluid. The machine drives well, as good power, and tracks straight. Any advice will be greatly appreciated.

 

[JLDBobcat]

We also replaced the case drain filters before blowing the 2nd case seal. The old one was not plugged but we changed it as a precautionary step.

 

[Tazza]

We also replaced the case drain filters before blowing the 2nd case seal. The old one was not plugged but we changed it as a precautionary step.

I wonder what could be causing the pressure to blow the seal out, this circuit should have pretty well no pressure there, it just flows back to the tank. I can't remember if this line was tied into the charge pressure system, which is about 120PSI
When you replaced the motor, did you give the motor carrier shaft a wiggle to be sure there was no play? i wonder if a bearing is going out, causing movement that is damaging the seal.
Another option you have is to T into the line and install a gauge to see what the pressure is.

 

[JLDBobcat]

I wonder what could be causing the pressure to blow the seal out, this circuit should have pretty well no pressure there, it just flows back to the tank. I can't remember if this line was tied into the charge pressure system, which is about 120PSI
When you replaced the motor, did you give the motor carrier shaft a wiggle to be sure there was no play? i wonder if a bearing is going out, causing movement that is damaging the seal.
Another option you have is to T into the line and install a gauge to see what the pressure is.

Tazza, Thanks for the reply. We didn't notice any bearing play. What do you mean by the charge pressure system? We put a gauge on the line coming from the main valve body going to the case drain to see if we were building back pressure to the motor and had none. Now I need to pick up a tee so that I can see the pressure coming from the motor itself. Do you know how much flow goes through the case drain line?

 

[farmshop]

Tazza, Thanks for the reply. We didn't notice any bearing play. What do you mean by the charge pressure system? We put a gauge on the line coming from the main valve body going to the case drain to see if we were building back pressure to the motor and had none. Now I need to pick up a tee so that I can see the pressure coming from the motor itself. Do you know how much flow goes through the case drain line?

Charge pressure is the gear pump that feeds the high pressure hydraulic pump

 

[Tazza]

Charge pressure is the gear pump that feeds the high pressure hydraulic pump

If it's showing no pressure, that's a good thing. Charge pressure is generated by restricting the returning oil from the pumps, through the oil filter and cooler to give about 120PSI. This pressure is used to charge the main drive pump, it replaces the fluid that is lost due to internal leakage in the pump and motors. If you aren't getting pressure building up on the small line coming from the drive motors, i don't know why fluid is getting past the seal.
The thing is, when running the motors without load, there may not be much fluid getting out via this small hose, when there is a load is when there should be some oil coming out.
If the seal goes again, grab the shaft that the seal runs on, see if you can feel any movement.
If thecase drain line is not blocked and not getting high, there is no other reason for the seal to blow out on you.