Bobcat Loader Drifting

[Bobcat N Mtns]

I have a Bobcat B200 loader/backhoe. The front loader drifts down whether loaded or not and it starts drifting immediately, there is no obvious leaks. Am I correct to assume this is probably the control valve. Is this something for a dealer or could a homeowner repair. I am not a mechanic but well equipped with tools.
PS, I am a new member and looking forward to reading and chatting with you guys.

 

[kw22614]

:welcome:
Maybe try your question on the Bobcat forum - should be more experience there. KW

 

[J_J]

Your cyl seals are probably leaking.

Could be the valve, but I doubt it.

Switch curl hoses with lift hoses and see if it still drifts.

 

[SSdoxie]

Here is an operation test that could help locate problem area before removing or swapping hoses.
Roll bucket out so cutting edge is vertical and power down with the lift to raise front wheels off the ground. Release lift control back to neutral, and shut engine off.
If front axles start to drift down (loader drifting up), gently feather the joystick forward and back to see if there is a point where the drift stops. If there is this point, most likely problem is spool not centering... Check spool centering spring, float detent for binding, and joystick itself.
If it drifts steady then follow JJ's advise to swap both lift hoses to bucket circuit and bucket hoses to lift.

If drift stays in the loader arm, the cylinders are by-passing. If it moves to the bucket function, control valve spool is by-passing.
There is no circuit/port relief in the loader arm circuit that would allow it to drift down without it going past the seals or the valve spool.

 

[IceStationZebra]

Your cyl seals are probably leaking.

Could be the valve, but I doubt it.

Switch curl hoses with lift hoses and see if it still drifts.

IF the valve doesn't leak it is IMPOSSIBLE for a cylinder to drift very far. (unless it is a double rod design which aren't very common) You could even take out the piston seals completely and it won't drift down all the way (assuming all air is bled and valve is tight of course). The cylinder will only drift far enough to equalize the forces.

Lets say the rod is being forced in. As the rod goes into the cylinder it is trying to displace oil, but if it has nowhere to go, the force on the piston will go up, and eventually the forces will equalize. In this case the effective area would be the rod diameter since the pressure is equal on both sides of the piston. If the rod is being pulled out it is the opposite and the pressure in the cylinder will go down and may even create a vacuum.

My guess is that the load-check in the control valve is leaking. I doubt it realies soley on the spool valve because they always leak a little.

Brian.

 

[J_J]

If there is a load on the cyl with leaky seals, they will leak down.

 

[SSdoxie]

IF the valve doesn't leak it is IMPOSSIBLE for a cylinder to drift very far. (unless it is a double rod design which aren't very common) You could even take out the piston seals completely and it won't drift down all the way (assuming all air is bled and valve is tight of course). The cylinder will only drift far enough to equalize the forces.

Lets say the rod is being forced in. As the rod goes into the cylinder it is trying to displace oil, but if it has nowhere to go, the force on the piston will go up, and eventually the forces will equalize. In this case the effective area would be the rod diameter since the pressure is equal on both sides of the piston. If the rod is being pulled out it is the opposite and the pressure in the cylinder will go down and may even create a vacuum.

My guess is that the load-check in the control valve is leaking. I doubt it realies soley on the spool valve because they always leak a little.

Brian.

There is a rated amount of leakage around the spool. How much depends tolerances (OD of the spool to ID of valve case), this thin clearance is what keeps the spool lubricated to move. Only time it is IMPOSSIBLE for a cylinder to not drift with blown seals or no seals is if cylinder has load lock installed between cylinder and the valve.
Load check in the control valve is located at the incoming oil from pump side of the valve spool, so the spool would have to be by-passing for the oil to get back to the load check.

 

[Bobcat N Mtns]

To SSDoxie
I performed the test you recommended, I curled the bucked down and raised wheels off the ground, cut the engine off and it started to lower the wheels slowly but if jerk the control lever all the way back it will stop lowering the wheels (drifting). If you pull the controls back slowly it will just drop but quickly all the way back and it quits. Do you still think it is a spool problem. Don't laugh but where is the spool and spool spring. Thanks to all for your replies