Lack of pressure in Hydraulic Power Unit

[Kdowney50]

I have an old Vickers Dual Pump Power unit I'm using to pressurize the vertical ram of a laminating press. It has worked well for years (20+) but will no longer build up pressure when the Remote Control Pressure Relief Valve (C175) is cranked in.

Some details: This is a dual pump unit. It has a V460-10-LH Vane pump to fill the ram, and a two-stage V230-4G4 vane pump for pressure. The output of the V460 (large pump) passes through an unloading valve which is opened to tank when the V230 (small pump) builds pressure. The small pump discharge passes through a fixed pressure relief valve which sets output pressure. There is also a Remote Control Pressure Relief Valve which is used by the operator to adjust the pressure on the Laminating Press.

There are suction strainers on both pump intakes but no other filtration. We are using ISO 68 Hydraulic fluid. The fluid is not clean and I have found sludge in the bottom of the tank.

I have replaced the small pump (V230) with a known good pump. I have also replaced the fixed pressure relief valve and the remote control pressure relief valve. No change. I have disassembled and inspected the unloading valve and a check valve. They both "appear" to be OK.

Can someone suggest why this unit will no longer build pressure. What am I missing?

 

[hydromech]

Hey...

Either...

1) The pumps are not priming, can you check the pumps actually moving oil? Does the ram actually move but not build pressure..?

2) The directional valve is stuck? (Unlikely)

3)This is the most likely. The relief valve is stuck open.Have you checked the remote stage of the relief valve? The valve that the operator adjusts may be faulty or stuck because of contamination. If there is no pressure signal at the top of the main valve it cannot close and the system will not generate any pressure.

There will be a pipe coming out from the main relief valve to the remote control. If this line is vented(allowed to drain to low pressure) the main stage will always be open as it relies on the pilot signal to be able to stay closed.

Can you be sure that the new releif valves are intended for external piloting. It may be that the new valves need to be converted from internal to external. This is done by plugging up the vent port inside the valve.

Let us know if this makes any sense

Regards

Adrian

 

[budt]

Is it actually a Ram or a Double Acting cylinder? Seals in a Double Acting Cylinder can bypass enoug oil to keep pressure from building.

Either the Pump is not producing oil or there is a bypass somewhere in the circuit that is allowing oil to flow at the reduced pressure.

On a Trouble Shooting job, with the information given I would start by isolating the circuit to determine if it can make set pressure as close to the Pump Outlet as possible.

In other words, Isolate the components until you get to a point that pressure can no longer be obtained. A component between the last place you could build pressure and the next check where pressure is less than noramal is causing your problem.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[Kdowney50]

To answer your questions: I have a ram, not a double acting cylinder. The weight of the platens drives the ram down. The system develops enough pressure (flow?) to extend the ram and close the platens. Therefor, the pumps are priming. The directional valve is manual. I can extend and retract the ram so it works. I have replaced the pressure relief valve and the Remote control Pressure relief valve with known good components.

Attached is a simple schematic of the system.

I understand the concept of isolating the components and attempting to build pressure. I would have to do some major replumbing to accomplish this. There are no isolation valves or flow meters installed.

 

[Kdowney50]

I found an error in my attached schematic. Here is the corrected schematic.

 

[Kdowney50]

Sorry. I found more errors in my drawing. This "should" be the correct schematic.

 

[hydtools]

Flow is bypassing somewhere.
Perhaps the linkage on the directional valve has loosened and the valve is not getting full travel allowing flow to bypass to tank.
Is ram travel speed normal up to platen contact?
Can you observe the different returns to tank to see if oil is returning when it should not? Perhaps a stethoscope would help to hear where there is flow when the ram is stalled up.

Ted

 

[Kdowney50]

I agree that "flow is bypassing somewhere". I may be able to observe the returns to tank with an inspection mirror. I have a stethoscope and will give it a try. As for the directional valve being mis-adjusted . . .you may have something there. However, Ram speed appears normal up to platen contact. I will look into this and let you all know what I found. Thanks for all your help.

 

[budt]

Is the Remote Relief Valve actualy hooked up as your last schematic indicates?

A C-175 is rated at 3 GPM as a Relief Valve and could handle a small pump but is usually used to Remotely adjust a Pilot Operated Relief valve such as a Vickers C*-06.

In either case the "Pressure Relief Valve" on the left of the schematic must be set equal to or higher than the "Remote Control Pressure relief on the right. The remote control can only be set at a LOWER PRESSURE than the main relief. The main relief will always set maximum pressure.

When adjusting this setup I always set them at a low pressure, Adjustments screwed out. Start the pump and after I am satisfied there is flow I screw the Remote in all the way. If it is the correct part number that should be higher than maximum pressure I want the system to operate at.

Then adjust the Pilot operated Relief Valve to system pressure and lock it there

Now you are able to remotely adjust system pressure to anything lower than that set on the Pilot Operated Relief.

Most catalogs limit the length of the pilot line fro the Pilot Operated Relief to the Remote at 10-15 Ft.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[Kdowney50]

Yes, thats how its hooked up. I thought it was strange too. However, I have two other Vickers Power Units, smaller, but of exactly the same design. The Relief valve on the left of the schematic is indeed a CT-06-B10 (500-2000lb) Pilot Operated. I may be using the wrong symbol.

I will try setting the relief valve as you suggested. Stay tuned!

 

[budt]

"I may be using the wrong symbol."

You are using the Simplified Symbol that is commonly used since most understand how the valve actually works and the fact that the Relief Valve is Pilot Operated usually does not have any bearng on its function in the circuit.

The Complete Symbol shows the two relief valves, Direct Acting pressure adjusting section and the Pilot Operated main portion that actually allows excess flow to be diverted to tank in a circuit. They are close to each other and surrounded by the Chain Line Enclosure Box. I thought I showed that symbol in the Basic Book here,

http://www.hydraulicspneumatics.com/200/eBooks/

but it is in the Circuit Design book in Chapter 18 which won't be put up for a few months.

On a CT-06 Relief Valve the Remote Pilot Port is directly opposite the Adjustibg Knob or Screw and is Plugged for normal operation.

BTW,I have seen CT-06 Relief Valves with Broken, Wrong, Shimmed and any other possible screw up when it comes to the Spring in the Pilot Section. Took one apart one late night that had a spring so stiff that when you adjusted it in it went from no pressure to 2,500 PSI as soon as you felt resistance to the Adjusting Screw. Got the correct spring and the valve worked normally.

I think there is an attachment of the Cutaways from the Relief Valve Chapter of the Circuits Book. If so tou can see the Complete Symbol for a Pilot Operated Relief Valve on Pg.1 The facing page in the book gives an explanation of the symbols.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[Kdowney50]

Good info. I have used your e-book a lot on this project. There is a lot of good info in there. I have found it most useful. I especially like the way you break down the symbols to show how they work in a circuit. Placing them next to a cut-away of the actual component is probably the best part of the book. Thanks again.

I will get back on this Monday. I'll let you know what I find. Until then . . .