LS remains at high pressure standby. 1

[grego1982]

CASE MX135 had a powershift service performed and after assembly, there was a problem with the hydraulics. The pump remains at high standby pressure (100-150bar) and adjusting the compensation does not help. The steering system works well, but the rest is not good, as if all the oil was directed to the steering pump. I disconnected the MFD, PTO, DIFF, FWD, Powershift lines, etc. to rule out leakage - no change. I disconnected the LS lines outside the steering system - no change. But disconnecting the LS steering line improves the operation of the pump and other systems. The priority valve has been checked, the PFC pump also looks good. Everything indicates that the steering system forces pressure on the pump, but the steering pump has been checked and is good... I have no idea what it could be and what else can be checked...

 

[zeusfaber]

Not familiar with this machine and haven't seen a drawing. Does the system have any orifice plugs in it? Are any of them blocked? Were any of them (or any necessary blanking plugs) left out during reassembly?

 

[grego1982]

Below is a diagram of the LS system in this machine, as you can see there are at least two orifaces affecting the LS lines, one in the priority valve (checked) and one in the steering pump. I didn't disassemble it, I just took it to the service center, where they said it works fine, but maybe they didn't check everything?. The assembly looks correct, it's difficult to screw the wires in any other way.

 

[FluidPowerUser]

Some questions...

1) Did you ask the service centre what they did to check the steering unit? Just because oil comes out, it doesn't mean that the LS signal can decay when the steering wheel is not moving.

2) Does the pump go to high standby pressure immediately after cranking the engine or does it go up after steering is operated and not settle back?

3) What is the normal standby pressure?

4) Does the pressure stay above 100 BAR all of the time, or does it decay after a while?

5) Is item 25 the steering unit? I assume yes. It looks like it has dynamic load sensing and that needs be checked out. There is constant flow from the pump pressure that needs to get back to tank through the orifice in item 25. If the orifice is blocked, the pump will always keep the load sense pressure high.

Servicing the transmission shouldn't affect the steering. But if they have disturbed the system and some dirt has got in, then it will get trapped at an orifice and it might be the orifice on item 25 that is blocked.

 

[grego1982]

1) The service only said that the steering pump was working properly, I don't know what exactly they checked, although they had a description of the problem. They didn't disassemble anything.

2), 3), 4) The pressure increases automatically approximately 5 seconds after starting the engine. And remains constant if no system is used, but if the steering is used the pressure increases and decreases.
Normal standby pressure should be 27-41bar. I can set them with a compensator, but then nothing works. The steering system begins to function between 100-150bar.

5)Yes exactly, I will quote a fragment from the manual:
"A small flow enters the signal system through the dynamic orifice (10e) wchich is located in the sterring priotity spool and fills the signal system, however a portion of this small flow is vented to tankt hrough an orifice (25a) in the stering control unit (25)and limits the pressure in the signal circut (SL) to 13bar (200 psi)"

Orifice 10e was checked, but 25a not I also measured this pressure and it is 25bar
Does this mean disassembling the steering pump?

 

[FluidPowerUser]

Your description of the pressure rise does suggest that the dynamic orifice in the steering unit is blocked.

When the engine starts, the flow builds and eventually saturates the system and the pressure builds. The high standby pressure is created the level of restriction offered by the blocked orifice. If the orifice was completely blocked the pump pressure would go to max.

If you were to remove the LS hose from the steering unit and put it into a bucket, I would expect that the pump pressure will remain at the lower standby pressure (25 BAR).

In testing the steering unit, the service centre would just turn the wheel and see that flow comes out of the ports. If they have a load sense drain valve in the their circuit, it will allow the load sense pressure to drop off and everything would appear to be OK. They may not even run the steering on a load sensing system, it could be just a simple fixed pump providing the oil flow.

You are going to have to find where the dynamic load sense orifice is in the steering unit and get it out and see if anything is in there. My bet is that something is in there.

 

[grego1982]

Yes, after removing the LS hose from the steering unit, the pressure returns to normal and the remaining systems work properly.

 

[FluidPowerUser]

It's looking more and more likely that you have a blocked orifice in the steering unit.

Do you happen to know the manufacturer of the steering unit? Is it a Danfoss unit? Or perhaps Eaton? If you can find the model number, we can help you locate the orifice that is blocked.

 

[grego1982]

Danfos with a dual gerotor, I will provide the unit number later. Thanks for your help!

 

[grego1982]

Steering unit: Danfoss OSPD 60/185 LSR 150G8106

 

[FluidPowerUser]

The OSPD steering units with LSR have dynamic load sensing and have reaction steering.

The actual schematic drawing for the steering unit looks like this:

In neutral, the load sense signal should drain back through the spool and into the tank line of the steering unit, then back to tank.

The is a special note in the service manual to make sure that the spool and sleeve are aligned properly:

The spool in the sleeve has really small clearance and the reset springs are not very stiff. If there something in the spool and sleeve assembly that is keeping the spool and sleeve offset and not in the neutral position, that will block the load sense drain and cause higher standby pressure.

If you are able, you can get the spool and sleeve out to check the assembly. Or you can buy a replacement or get the steering unit serviced. It's not hard to get the spool and sleeve out of the steering unit and there are step by step instructions on the Danfoss website.

If you move the steering wheel from side to side very slightly, you might be able to see the stand by pressure at the pump respond. It won't move the steering cylinders, but will just adjust the dynamic load sense pressure that is causing the pump to come off min displacement.

Good luck!!!

 

[grego1982]

Many thanks!

 

[grego1982]

Hello again, I dismantled the steering unit and the first thing I saw was that one of the springs was deformed. Could this be the cause?

 

[FluidPowerUser]

Yes, that could cause a slight offset off the rotor and sleeve.

Just to add

The post above shows the marks on the sleeve and rotor that need to be aligned for the LS pressure to drain to tank.

 

[grego1982]

I have bent the springs..., everything has been cleaned and reassembled and it works!!! The pump starts working at 30 bar. Once again, many thanks for your help!!!

 

[FluidPowerUser]

Great to hear. No problem, happy to help.

You fixed your tractor and learned something on the way too. All good.

Cheers