This section will provide a short introduction to pressure and flow
control valves and their operation
the relief valve is generally used in every hydraulic system it is normally
closed and is connected between the pump outlet and the reservoir it is used to
limit the pressure in the system to a preset maximum pressure this is done by
diverting some or the entire pump output to the tank when the pressure setting is
reached here you will see an example of how a relief valve functions the
configuration and working method will vary depending upon the manufacturer
port a is connected to the outlet side of the pump and ports B and Y are
connected to the tank when the valve piston is closed the pressure is the
same at both sides of the valve and it is held down by the spring in the lower
chamber closing port B this means there is no flow through the valve when the
pressure on both the top and below the valve piston reaches the opening
pressure of the small valve it will open then there will be a pressure drop over
the nozzle in the valve piston and it will lift so that oil can flow from A to
B pressure reducing valves on normally open valves they are used to maintain
reduced pressure in certain parts of the system they are actuated by pressure and
tend to close as the pressure reaches the valve setting in this valve the
direction of flow is from B to a when the small valve is closed the pressure
is the same on both sides of the valve piston the spring will hold down the
valve piston and the valve is open when the pressure reaches the opening
pressure of the small valve it will open then there will be a pressure drop over
the nozzle the valve piston will lift and flow from B to a will be reduced if
the pressure in Port a becomes higher than the pressure setting of the valve
the valve piston will lift and close the port B port Y is connected to the tank
the unloading valve is often used in systems with two or more pumps it can
unload one pump when the required system pressure is achieved
normally the valve is closed but it opens when the pilot pressure is equal
or higher than pre-set opening pressure of the valve
this figure shows a system with two pumps when the pressure is six mega
Pascal's it will unload the large pump and only the small pump will deliver oil
to the system flow Direction is from A to B when the valve is closed the oil
pressure is the same on both sides of the piston the piston is held down by
the spring when the pressure on top of the piston is equal to the preset
opening pressure of the small ball valve it will open oil will flow through the
nozzle and there will be a difference in pressure on the piston the piston will
lift and oil can flow from A to B when the ball valve has opened it will remain
open until pressure in port X is lower than the opening pressure of the ball
valve normally the valve is closed in one direction and open in the other
direction to open the valve in the normally closed direction one of these
conditions must be present pressure at the entry must be equal to the preset
opening pressure sufficient pilot pressure pressure at entry plus pilot
pressure is equal to the preset opening pressure the valve is normally closed
from A to B and open from B to a through the check valve the spring presses the
valve against the valve seat and the oil pressure in Port a presses on the piston
when this pressure is high enough it will lift the piston and oil will flow
from A to B the valve will also open if it is piloted in port X then the small
piston will lift the big piston and oil will flow from A to B the counterbalance
valve is used to prevent creeping of heavy loads prevent overload prevent
over-speeding and runaway loads prevent accidents if pipes or hoses break if a
heavy load is placed on top of a long rod it will move from side to side to
prevent the load from running away in either direction a system of to
counterbalance valves is put in place the function is to control the oil flow
and thus the speed of the actuator the valve is normally fitted after
the direction control valve in order to prevent power loss flow control valves
are essentially designed to control flow in one direction only and are therefore
provided with a bypassing check valve the direction of flow is from P to a by
moving the piston downwards it will choke the oil flow if the oil flows in
the opposite direction the oil will press the piston upwards and flow
unrestricted from a 2p