What is Hydraulic charge pressure?

[JRP]

I bought a used T190. The lower left corner has a bar graph gauge for what I think is hydraulic charge pressure, which always looks low, about four bars are black at 2800 RPM. The error report shows several faults on low hydraulic charge pressure.

What is hydraulic charge pressure?

How high should it be when normal?

What is good/bad ?

At what pressure should I worry about it?

What makes it low? I understand the sensor (sender) is a common problem.

Thanks for your help

 

[dfkrug]

Your charge pump is a trochoid (gear) hyd pump that "charges" the
inlet to your hydrostatic drive pump. It also provides make-up oil for
the HST, since oil is used (lost) in lubricating the piston shoes.

I would expect that your charge pressure should be 100-300 psi. If it
is too low, your HST will not run right. The pump could be shot, or
the lines could be compromised. I assume the oil is full and clean,
and filters are not clogged.

Sounds like an interesting project.

 

[SSdoxie]

Your charge pump is a trochoid (gear) hyd pump that "charges" the
inlet to your hydrostatic drive pump. It also provides make-up oil for
the HST, since oil is used (lost) in lubricating the piston shoes.

I would expect that your charge pressure should be 100-300 psi. If it
is too low, your HST will not run right. The pump could be shot, or
the lines could be compromised. I assume the oil is full and clean,
and filters are not clogged.

Sounds like an interesting project.

Charge pressure on T190 is rated at 360 PSI with engine at full throttle. In addition to suppling replenishing oil to the hydrostatic pump, the charge pressure is used to release brakes in the drive motors. The brake piston requires approximately 180 PSI to release the brakes.
The onboard controller monitors the charge pressure compared to engine RPM. There are different low pressure parameters vs engine RPM that could trigger the warning. For instance working hydrostatic drive extremely hard with engine at a low RPM could cause it.

Most common is sendor getting weak and sending incorrect voltage signal to the controller.

 

[dfkrug]

The onboard controller monitors the charge pressure compared to engine RPM.

Having these on-board diagnostics is a good thing, but no substitute for
an old-fashioned pressure test at the test port. Esp when the presssure
sending units are known to be a weak point.

What kind of brakes are used in the drive motors?

 

[JRP]

I don't know about the brake. It is pressure to release.

 

[SSdoxie]

There are multiple discs which applied by spring pressure when machine is off or park brake set. Charge pressure pushes on a large piston to compress the sring and release brakes. A leak around one to brake psiton seals can also cause some loss of charge pressure.

The onboard monitoring is there to give you an early warning of a pending problem, after that it is standard hydrostatic diagnostics to find out why. Just like any oil pressure sensor, fuel guage sendor, etc, these can fail but don't just assume it is the sendor check actual pressure with a guage then go from there.

 

[JRP]

SSdoxie,

Thanks for your comments. I found the "vitals" section of the display. It shows charge pressure to be about 340 psi at full throttle. I compared to a T300 yesterday which was about 440 psi.

With 2200 hours on it I think my T190 is worn, but not worn out. I may change the hyd filter and fluid and see if that helps. It seems like that would probably be a good idea anyhow.

Is using the "Bobcat fluid" brand very important? I use a John Deere brand universal hydraulic fluid in my Kubota and was wondering if that would work OK..

 

[SSdoxie]

Charge pressure also serves as back pressure on the drive motor to help prevent it freewheeling down hill. A T300 is much heavier machine so would need higher charge pressure to hold machine at desired speed. Too high of charge pressure for the machine can cause loss of push power and excess heat, so don't try to set your T190 up to match the T300.
Filters should be changed every 500 hrs. At 2000+ hrs. your due for fluid change as well.
You're T190 is also due for oil change in the outer bearing hubs of the drive motors. This is a special synthetic fluid available only through Bobcat dealer. 1 bottle will have enough oil to do both motors. On the T190 you need to remove the tracks and sprockets, drain oil out bottom of the motor and then put 1/2 the bottle in through the top plug.

Bobcat fluid?, JD?, Kubota? I have no idea what each fluid consists of and for me I don't care. I can't put a value on piece of mind knowing if I use the fluids and filters designed for that machine and do have a problem, it is probably due to something else not because I substituted oil or filter.

I your particular case, the intermittent low charge pressure warning may be from incorrect hydraulic fluid.