Hi all ,great forum. I have a hydraulically driven hedge cutter that runs off the tractor power take off.It has two gear pumps that have separate intakes from a shared tank.The large pump drives the rotor and oil returns thru a filter to the tank.A secound small pump provides power to move the arms of the machine.It pumps oil thru a electric proportional valve block and the oil returns to the return line of the large pump.The machine works well from cold but when it becomes warm the arm function become slow but only when the tractor pto is driving it at its rated speed, when machine rpms are reduced to slighly above idle it works normally.It feels like there is too much pressure in the valve block as one ram that is single acting fails to contract under gravity.This is driving me mad.Any advice appreciated

What is your rated speed? How much is slightly above idle speed? How warm is your system(approximately)? Contract means lowering/rod retracting?

A circuit diagram will be helpful... especially around the cylinder that is having problem...

First, I assume this is what the equipment in question looks like.
rbiassets.com/GetImage.ashx/6781210814/fbg-300.jpg/medium

Second tractor PTO's normally come in two speeds, 540 or 1000 rpm @ rated engine speed. I have also seen on some European tractors an "overdrive" mode where you can throttle the engine down but still get the same shaft speeds.

SWL - Why don't you run the cylinders off of the aux circuits? My only guess is that there is some joystick involved to make operation more intuitive.

The single acting cylinder failing to contract makes me think that the back pressure is too high - BUT it should be highest with cold oil not warm/hot. Maybe there is too much leakage or something sticking when warm? I agree with maglub that a circuit diagram would be very helpful.

ISZ

Thanks for the replies.Tractor pto runs at 540 rpm,pump speed is 3-4 multiple of this.Original pump that powered the rams was 6-7 cc/rev and was replaced with larger 12-14 cc/rev.Could larger pump produce excessive pressure in valve block.I had also thought that if the main motor was worn oil pressure could build up in the return line when hot causing excessive back pressure in the valve block.website would not let me post picture of schematic

“Original pump that powered the rams was 6-7 cc/rev and was replaced with larger 12-14 cc/rev.”

If you double the flow rate of the pump without changing lines and valves it can cause problems, depending on how the system was originally designed. Fluid should move at a certain rate (m/s) depending on what part of the circuit is involved. If line is too small the fluid has to move faster and it can become turbulent and can create heat and other problems. The volume a line can carry varies as the square of the ID.

The following site gives the recommended rates for different lines:
http://www.hydraproducts.co.uk...or/Pipe-diameter.htm

Thanks for the interest.Operating machine again tonight.Main problem is with one large ram that is single acting.It lifts the first and secound arm of the hedger off the ground when joy stick is moved up .It does this normally quite quickly although sometimes not in which instance if joystick is returned to neutral and the action repeated it will lift arms rapidly.However ,when joy stick is moved to lower position, the ram frequently fails to retract under the considerable weight of the machine,on other occaisions it retracts in a jolty ,stop/start fashion, sometimes it will retract resonably fast and smooth but never as quickly as it is able to extend.One port of the ram is connected to the valve block(extension/lift).The other port is connected to the return pipe from the valve block at a t-junction.is there any way to adjust a proportional valve to increase speed of rams?

As NA said, the best way is to return the pump displacement to the original manufacturers values(6-7 cc/rev). Changing the pump displacement without changing the valves/lines will create more preesure drops(heat) on the system. Your proportional valve is just a single stage based on the original pump displacement(the way I analyze it) wherein the pots on the joystick controls the solenoids.

What is happening to your system is the result of exceeding the maximum flow to the proportional valve(bernoulli forces is too much for the solenoid to properly activate it).

As you noticed, the valve is operating better at little more than idle because your flow to the prop DCV is still within the maximum limit.

For gear pumps, average operating speed is 1800. Although they can operate at higher speeds, the inlet has to be big enough to accomodate suitable pump inlet flow(else cavitation will occur).

This message has been edited. Last edited by: maglub,

Try posting your JPG picture again, there usually isn't a problem.

You may have a partial air leak on the suction side, or possibly a vortex in the tank at high speed. At low speed, the pump doesn't draw any air, at higher speeds, the pump may be sucking air in,slowing functions down. If the single acting cylinder has a PO check, or load holding valve, the air introduced may prevent the holding valve from opening, or may jitter.

Can't really help without any more information.

Hi all operating machine for about three hours this afternoon and main ram stopped retracting .All the other rams continued working normally,they are all double acting.It also occurs to me that this ram will jam at full extension and the pressure must be relieved manually before it will retract.It feels as if the solenoid is sticking.Sometimes if the joystick is moved to the lower position the ram will not respond instantaneously but after delay of 1-3 seconds it will release. I have also noticed that when the ram will not release instead it will creep at about 20mm/min. These problems only manifest when the oil has got to about 60 degrees celius

Could there be a "hose broke valve", and after changing the pump, flow is faster, and the valve assumes that the hose is broken?

And when the PTO has lower rpms, the flow is smaller, and that valve works properly

only a quess...