3. Introduction to Pressure Valves
Pressure valves are used to limit the pressure in a particular line of the hydraulic
circuit.
Generally, they are used as safety valves to protect the system against excessive
pressure.
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4. Types of Pressure Valves
Safety pressure valves :- The safety pressure valves are not operated frequently and therefore
they are designed to be oil tight when closed.
Bypass pressure valves :- The bypass pressure valves operate almost continuously and therefore
the design requirement is not oil tightness but higher wear resistance of seals and packings.
1) On the basis of functional requirements
It is important to note that the basic design is identical for both.
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5. 2) On the basis of performance
The ball / poppet is pressed against the opening by a
spring, whose force can be regulated by the means of a
threaded sleeve.
When the pressure of oil coming through port 1 exceeds
the spring pressure, the ball is raised and the oil is
drained back into the reservoir through the ports 2 and
3.
This type of valve can only be used as a safety valve
and not suitable for the bypass valves due to pressure
pulsations and vibrations.
A) Ball or poppet valve (simplest type) :-
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6. B) Spool type pressure valve (better performance than ball type) :-
Ports 1 and 2 of the valve are connected to the pressure line, (port
1 directly and port 2 through a constricted passage) port 3 is
connected to the reservoir.
At equilibrium,
where,
P = force acting at the head end of the valve
F = friction force
Ps = spring force
W = weight of the spool
When due to increase in the pressure, force (P + F) exceeds (Ps +
W), the spool gets displaced upwards and port 1 gets directly
connected to port 3, thus allowing excessive oil to be drained back
to the reservoir, resulting in a pressure fall.
P + F = Ps + W
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7. C) Compound relief pressure valve (can absorb minor pressure variations) :-
Pressure line is connected directly to the pilot end and the lower face of piston by ports 1 &
2 respectively, it is also connected to the piston end by port 3 through a constricted passage,
port 5 is connected to the reservoir.
At equilibrium,
where, P0 = force at pilot valve end
P1 = force at lower face of piston
F = friction force
P2 = force on piston end
W = weight of piston & pilot
Fs1 = force on spring 1
Po + P1 + F = P2 + W +
Ps1
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8. As long as resultant force Pr < Ps2 of the ball valve, piston remains stationary.
However when Pr > Ps2, the ball valve opens, pressure at the piston end drops.
Piston along with pilot moves upwards & gets directly connected to the draining
port 5 and hence excess of oil is drained back to the reservoir.
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9. Conclusion :-
Pressure valves are used to control high pressure to avoid damages like cavitation, etc.
Ball type pressure valve is generally used only as safety valves.
Spool & compound relief type pressure valves are used mostly as bypass valves.
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