I am fairly new to the hydraulic business and need to create a hydraulic lift that has warm up loop that also raises the lift at the speed i choose based on the power given to the pump. and lowers slowly with a regulator.

i made a schematic of the way i think it is supposed to be done.

First off the flow control will be better between the direcional valve and the cylinder.

Secondly, are you sure about the transitional flow paths of the directional valve. I am not sure that there are valves that block all ports as the move from the normal position to the working position.

Because I am lazy and because it will help you to understand I suggest you research the following...

1) Counterbalance valves
2) Overcentre valves
3) Pilot operated check valves

All of the aboove will hold the load while the directional valve moves. The spool in the directional valve will allow oil from the cylinder to escape to tank as the it moves and the cylinder will drop slightly and there will be a noticeable bang as the valve moves. Any of the above valves will do what you need, but with varying degrees of success.

As you have drawn the system, any pressure generated at the flow control will be seen at the directional valve and the leakage across the spool could mean that the cylinder extends during the warm up.

The directional valve would be better with the P line blocked and A & B lines connected to T in the normal position. This will keep stop pressure build up in the cylinder during the warm up.

The pump could then be run against a releif valve or orifice to warm the oil. The size of the orifce can be calculated as...(metric units)

Flow(LPM)/Area(mm^2)x 1.44 = square of the pressure drop(BAR)

The answer squared is the pressure drop in BAR.

The temperature differential(deg's C) across the valve/orifice is equal to...

(5.7 x Pressure Drop)/100

It is worth noting that warm up cycles only warm the reservoir oil. The oil in the line and the cylinder will remain cold until it reaches the reservoir. When the warm oil is pumped out to the cylinder it will cool rapidly unless the the oil is worked hard to keep the temperature up.

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