A hydraulic circuit needs three kinds of valve: a directional valve to send oil to one side of the cylinder or the other, a relief valve to cap the pressure, and a flow valve to set the speed. The oil that the pump delivers must always have somewhere to go, and the pump must be sized for the power the work needs: moving an 80 mm ram at 16.6 mm/s against 100.5 kN takes 5 L/min of flow and 1.67 kW at the pump.
Part of Hydraulic Systems
Control valves steer, limit and meter the oil between the pump and the cylinder. Everything that moves, holds or stops a hydraulic machine goes through one of them.
Three Jobs, Three Valves
- Direction: a spool valve routes pump oil to the cap end or the rod end of a cylinder and lets the other side drain; see Directional Control Valves.
- Pressure: a spring-loaded relief valve opens when pressure reaches its setting and dumps oil back to the reservoir; see Pressure Relief Valves.
- Flow: a needle valve or orifice sets how fast the oil moves and so how fast the cylinder travels; see Flow Control Valves.
A basic circuit reads: reservoir, pump, then the relief valve (teed off the pump outlet), then the directional valve, then the cylinder, with every return line going back to the reservoir through a filter.
The Pump’s Oil Must Always Have a Path
A pump delivers flow, and if the flow has nowhere to go the pressure climbs until something gives. With the cylinder at the end of its stroke or the directional valve closed, the relief valve is the only path and it must be able to pass the whole pump flow. A directional valve that is open in its centre position lets the pump’s oil go to the reservoir at low pressure whenever the lever is neutral, which saves both heat and strain.
Worked Example: Flow and Power for a Press Stroke
An 80 mm bore ram (5.027 x 10⁻³ m²) pushing 100.5 kN at 20 MPa, moving at 16.6 mm/s:
- Flow needed = area x speed = 5.027 x 10⁻³ x 0.0166 = 8.34 x 10⁻⁵ m³/s = 5.0 L/min
- Hydraulic power = pressure x flow = 20,000,000 x 8.34 x 10⁻⁵ = 1,668 W
- Check from the output side: force x speed = 100,531 x 0.0166 = 1,669 W
The pump’s shaft must supply at least 1.67 kW, more once pump and valve losses are counted (no efficiency figure is sourced here). That power has to come from somewhere: the corpus gives one person cranking about 50-75 W and a treadle 60-100 W (Chain Pump), and a 20 L/s flow over a 5 m fall on an 80%-efficient wheel about 785 W (Water Wheel Types). A press run at that rate by hand or by that wheel will not keep pace; slow the ram, which cuts power in proportion.
Valves Are Where Contamination and Heat Show Up
Valve spools and seats have fine clearances and stick when the oil carries grit; see Hydraulic Fluids. Every pressure drop across a valve turns into heat, so a circuit that spends most of its time throttling or on relief warms its oil.
Common Mistakes
- No path for the pump’s oil. A dead-headed pump with a stuck or missing relief valve bursts a line.
- Putting a valve between the pump and its relief valve.
- Sizing the relief valve smaller than the pump flow.
- Ignoring where the power comes from. A circuit that needs 1.67 kW cannot run from a 75 W crank.
Safety
Valves control energy and a stuck one releases it
A spool valve that sticks can drive a cylinder when no one touches it, and a relief valve that sticks shut lets pressure climb until a line or fitting bursts. A burst or pinholed line injects oil through skin; never search for a leak with a hand, use card or wood. Keep clear of the line of the cylinder, support loads mechanically, and relieve all pressure before servicing.
What’s Next
- Learn next: Directional Control Valves — extending and retracting a cylinder
- Combine with: Pressure Relief Valves — the valve that protects the rest