how do cushion valves work

[jonsstihl]

I need a crash course on cushion valves.
I have a pronovost pxpl 80 snow blower with a blade on the back.

at idle it takes 8 seconds to lower the blade on the extension stroke of the cylinder. It takes 6 seconds to lift the blade on the retract stroke.

the problem I have is when I go to PTO rpm it takes 3 seconds to extend the cylinder and 8 seconds to retract. It should be taking about 2 seconds to retract the cylinder.

it is almost as if the cushion valve in the circuit is turning my spool valve into a regeneration valve.

I have an exploded view of the cushion valve in my owners manual but none of the parts are labeled

I did a few tests with my pressure gauge hooked into the remotes. I only get 1500 psi of pressure in one direction and when I switch the pressure gauge to the other side I get full system pressure approx 2500 psi until the blade hits the stops then it is like something shifts in the valve and then I only get 1500 psi on the same side that I got 2500 psi before hitting the stops.

I think the valve is not adjusted to the same setting on each side but I'm not even sure how to adjust the valve.

thanks

 

[jonsstihl]

tests were done at 1600 rpm but if I drop the rpm to idle the 1500 psi setting turns into 1000 psi. this is on a new to me grand L4740. it has three remotes one with float but the resulting slow retraction at PTO rpm is present all the time. It doesn't matter which valve it is hooked to.

 

[jonsstihl]

here are pics of the whole setup the valve and the only apparent adjustment on the valve

 

[Dave M7040]

You have a blower most of us only dream to own.

Your tractor is a very complicated one hydraulically. I have a WSM for the preceeding year tractor but not yours nor can I find one.

I suggest you do your pressure tests with the blower hoses disconnected so you only are checking the rear remotes to see what pressure the tractor relief valve is set to.

Plug the gauge into each side of a rear remote, engine at full rpm, move control to send pressure to port gauge is plugged into. Read gauge. Just do it for a few seconds no longer.

Consider making the manual for your loader available so we can read it. For example upload it to Jumpshare and then you can send people like me a link to download it.

I am going to send you a private message.

Dave M7040

 

[jonsstihl]

I have a WSM for the grand l 40 series. I didn't check each individual remote, but the system pressure is approx 2600 psi Which is close enough to the factory setting.

Thanks for the help.

I have to admit the float valve does some weird stuff when hooked to the blade circuit like reversing the direction of the cylinder movement when the valve is in the partially applied versus fully applied. I'm assuming that is because the valve floats in both directions not just the detent position but typing it out now doesn't seem to make sense.

 

[jonsstihl]

History on the tractor is not much it is a 2012 with 520 hrs. It has been well taken care of because it looks like a new one. I just got it in february so I haven't used it much.
I am stopping by the dealer that sold me the blower tonight. they did not set up the blower on my tractor. but they should still have adjusted the valves. so far I have only talked to the dealer and not directly to pronovost.

and on a side note, this blower is not exactly compatible with a speeco quick hitch. the top link pin is too high and so you have to install the top hook upside down and there is interference with the bottom hooks and the brackets so I had to take the cat 2-3 bushing out of the lower pins so that the hooks would seat. other than a little slop it does hook up with a few minor mods.

 

[jonsstihl]

The deflector on the chute has no valve so when I hook up my pressure gauge I should be getting full system pressure at the end of the retract and extend strokes.

the chute motor has it's own cushion valve that the sticker on the bracket says should be set to between 1800psi and 2000psi. I get a reading of 2300 psi when I test it at the end of its travel at 1600 engine rpm .

the blade circuit is where the voodoo is happening. there is a magic box thats seems to change the system pressures but now I am not even sure it is a cushion valve.

I am fairly certain there is nothing wrong with the tractor because both chute rotation and chute deflector circuits operate normally on any of the three remotes it is just the blade circuit that has this problem and or than a few extra symptoms when the float remote is used, the syptoms are the same no matter which remote it is hooked to.

 

[oldnslo]

jonsstihl,
By "cushion valve" are you referring to cross port relief valves or end of cylinder stroke cushions?

Cross port relief valves: The setting will vary with the return(tank) line back pressure. A float spool should not allow any pressure to build on either line while in float so that should not cause any problems.

Did provonost provide a hydraulic schematic for this blower? Curious if Provonost installed an orifice in the manifold to limit cylinder speed that could under some set of circumstances cause the functions that you are experiencing.

 

[Dave M7040]

I corresponded with Pronovost.
If you provide me with the serial number of your blower they will send me the manual.
Then, it might be easier to help you.

Dave M7040

 

[jonsstihl]

I talked with the dealer yesterday. the hoses come out of the valve and tee to the two cylinders to actuate the blade. it was suggested that the hoses may be switched so that the cylinders are fighting each other and somehow the valve is balancing them out similar to a regeneration valve.
it was also suggested that maybe the wrong end of the circuit (either retract or extend was going through the valve.

checked both out and hoses are not switched and the correct side of the circuit (retract) is going through the valve according to the parts diagram.

the orifice idea is a good one maybe it just isn't on the correct hose. Sounds like I need more info on cross port relief valves instead of cushion valves.

I also found out when looking at the parts list that this valve is not a cushion valve it says its a relief valve (rl-50)

one side of the circuit (retract) goes through the valve like we can see in the picture and the other circuit doesn't actually go through the valve but is tee-d into it. can't see the tee in the picture but it is on the bottom.

thanks for all the help so far. I going to call pronovost and see what they think.

 

[jonsstihl]

I also realize I was reading the sticker wrong. the pressure setting for the chute rotation should be 1000 psi not 1900 psi. some of their blowers have a rotating drum so you can bypass the chute.

is there any reason why I would be getting a reading of 2300 psi if the cushion valve is set for 1000psi. my gauge is tee-d in between the rear remote and the hose going to the hydraulic motor. does the cushion valve relief internally??

 

[oldnslo]

I also realize I was reading the sticker wrong. the pressure setting for the chute rotation should be 1000 psi not 1900 psi. some of their blowers have a rotating drum so you can bypass the chute.

is there any reason why I would be getting a reading of 2300 psi if the cushion valve is set for 1000psi. my gauge is tee-d in between the rear remote and the hose going to the hydraulic motor. does the cushion valve relief internally??

Is the cushion valve for the hydraulic motor a cross port relief of a counterbalance valve? If relief valve it should relieve pressure from the inlet of the motor to the outlet of the motor. ANY back pressure on the valve outlet (tank side) is additive to the relief setting.

Good luck on getting this resolved.

 

[jonsstihl]

to be honest I have no idea what the difference is between the two but the cushion valve on the chute rotation does have two adjustment screws because the inlet and the outlet alternate depending on which direction you rotate the chute. the cushion valve is mounted directly on the motor.

the dealer mentioned that maybe the valve is bypassing internally and that would explain why i'm getting a pressure reading higher than what the setting should be. I think that is bull but I know only enough about hydraulics to be dangerous. My thinking is if the cushion valve is opening to protect the motor the entire pressure side of the circuit will be at the pressure the valve is set to open at. Am I wrong.

 

[jonsstihl]

I don't have any back pressure in the chute rotation circuit because when I rotate the chute the other way my pressure gauge goes to zero. which should mean that their is no restriction in the return to tank circuit of that particular remote.

 

[oldnslo]

Counterbalance valves use external pilot pressure to open and are used on motors to prevent free wheeling and act like brakes. This pilot pressure is taken from the inlet of the motor to open the valve on the outlet port. I would question why they would be required for chute rotation

I suspect relief valves in this case to limit the motor torque for rotation as a safety to prevent breaking or bending something.

100% agree your pressure readings are puzzling