Bobcat 763 Help

[Hilbilly]

I've been in the process of converting my 3pth snow blower to hydraulic so I can run it on the Bobcat. I got the pump mounted on the blower with direct drive and put the blower in the SS snow bucket to see if it would work. It did but not well. So I did some checking and I think I have a pressure relief valve issue with the SS. I put a hose between the auxiliary out and in ports with a pressure gauge on the line. I'm getting about 600 psi in that line at full throttle. According to the service manual it should be 3000 psi. Also according the service manual the Main Relieve valve should be adjusted to increase the pressure. I found a picture showing the location of the main relief valve and tried adjusting it but there is no change. The relief valve has a cap over the adjusting screw and a locking nut. I have removed the cap, loosened the locking nut and used an Allen key to turn the adjuster in. So far I have got a total of 5 partial turns (maybe 1/8th turn each) clockwise and there has been no change. Am I doing something wrong or is the valve defective?

 

[oldnslo]

By connecting the aux ports you have created a flow path so you will not read system pressure. Just plug you gauge into the aux pressure port and activate that function. You will now read full system pressure.

FYI: If the blower ran to slow that is flow issue not a pressure issue. In hydraulics flow equals speed and pressure is force or torque.

 

[Hilbilly]

Thanks oldnslo.

I got it working and have a cross over relief valve between the SS and the blower motor. The relief is set at 2900 psi. The motor is rated for max continuous pressure of 2973. At full throttle the blower works but does not throw the snow very far and bogs down quite easily. I think the problem a lack of flow, like you say. I thought about putting sprockets on the motor and the blower gear box to increase the blower fan speed but that would reduce the torque and then I'm back to the blower bogging down. I talked to the hyd motor dealer and asked about using a larger displacement motor but they are rated to run at even slower speeds. I guess I'm stuck with this setup, unless you have any ideas to improve on it.

 

[oldnslo]

What make & model of motor did you use? Did it have a flow Vs RPM chart?

Also be nice if the chart included torque vs differential pressure

What size hose did you have on the loop when you where initially trying to check pressure. The 600 PSI is all back pressure and reduces the amount of power to run the motor.

 

[Xfaxman]

This is how I setup my pressure gauge:

I can put it in line to see if I am going too fast in thick grass or heavy brush:

The low-flow brush hog uses 3/4" hose.


.

 

[Hilbilly]

What make & model of motor did you use? Did it have a flow Vs RPM chart?

Also be nice if the chart included torque vs differential pressure

What size hose did you have on the loop when you where initially trying to check pressure. The 600 PSI is all back pressure and reduces the amount of power to run the motor.

The pump is a house brand from Princess Auto and here is a copy of the table in the owners manual.

I used 1/2" hose. Which I think is too small but the steel lines to the Bobcat aux connections are only 1/2".

This is how I setup my pressure gauge:

View attachment 541819 View attachment 541821


I can put it in line to see if I am going too fast in thick grass or heavy brush:

View attachment 541820



The low-flow brush hog uses 3/4" hose.


.

View attachment motor.pdf

I like that setup and thanks for the tip on the 3/4" lines. I think that might be part of the problem but I think there is a bigger one producing that 600 psi back pressure.

 

[mikester]

You should consider picking up one of those cheap laser tachs at PA they were on sale again recently for like $20. Disconnect the motor from the blower and confirm your hydraulic motor RPM vs the skidsteer engine RPM. I'd suspect you are only getting something like 12-13 GPM on your flow which is keeping your PTO RPM's down.

On my setup I used the 5.9cu.in motor and my tractor was pushing at best 12 GPM and 2500 psi but I was able to throw the lighter snow a good 40 feet. Then again maybe your snowblower gearbox might have a lower gear ratio to the secondary impeller which is why it doesn't throw as far. Heavy wet snow meant I had to go slower and it didn't throw as far.

 

[Hilbilly]

Good timing; I picked up one of those laser tach's yesterday when I was in the City.

This blower has never thrown snow very far. The tractor has a claimed 38 hp pto and running it at pto speed I might get 25' out of it when the snow isn't heavy. The smaller motor might be an idea. The tech at PA even suggested I bring this one back if I wanted to try different one. Right now I think the bigger concern is to resolve the heat and back pressure issue. These could kill the motor and the SS. By the way did you hook up the case drain on your motor. I did not.

edit, Xfaxman; do you think the back pressure could be caused by plugged case drain filters. I've never checked them in the 200 hours or so that I have on the machine. It appears the hydraulic fluid needs to be drained from the system to check them. Is that right?

 

[Xfaxman]

--------------------------

edit, Xfaxman; do you think the back pressure could be caused by plugged case drain filters. I've never checked them in the 200 hours or so that I have on the machine. It appears the hydraulic fluid needs to be drained from the system to check them. Is that right?

No, they were not in the the loop with your pressure gauge.

It appears the hydraulic fluid needs to be drained from the system to check them. Is that right?

No experience with case drain filters, so can't answer that question.

 

[mikester]

Good timing; I picked up one of those laser tach's yesterday when I was in the City.

This blower has never thrown snow very far. The tractor has a claimed 38 hp pto and running it at pto speed I might get 25' out of it when the snow isn't heavy. The smaller motor might be an idea. The tech at PA even suggested I bring this one back if I wanted to try different one. Right now I think the bigger concern is to resolve the heat and back pressure issue. These could kill the motor and the SS. By the way did you hook up the case drain on your motor. I did not.

edit, Xfaxman; do you think the back pressure could be caused by plugged case drain filters. I've never checked them in the 200 hours or so that I have on the machine. It appears the hydraulic fluid needs to be drained from the system to check them. Is that right?

If it never threw snow very far then its the gearbox and its out of your control. I'm curious to see what your motor RPM is with your existing setup