A flow control valve sets a cylinder’s speed by forcing the oil through a small adjustable orifice, because speed equals flow divided by piston area. To move an 80 mm ram at 10 mm/s needs 3.0 L/min, which takes an orifice of about 0.96 mm diameter with 5 MPa across it and wastes 251 W as heat. Put the valve in the outlet line when the load can pull the piston along, and watch the rod side, where pressure can rise.
Part of Hydraulic Systems
A flow control valve is an adjustable restriction in the oil line that limits how much oil reaches or leaves a cylinder and so how fast the cylinder moves.
Speed Is Flow Over Area
The ram’s speed is the volume flow into it divided by its area.
Worked example: an 80 mm ram at 10 mm/s.
- Area = 5.027 x 10⁻³ m²
- Flow = 5.027 x 10⁻³ x 0.01 = 5.03 x 10⁻⁵ m³/s = 3.0 L/min
If the pump delivers more than this, the surplus must go somewhere, normally through the relief valve at full pressure (Pressure Relief Valves), where it becomes heat.
An Orifice Passes Flow in Proportion to the Square Root of the Pressure Drop
The standard orifice equation (recalled from fluid texts) is Q = Cd x A x √(2 x Δp / ρ), with Cd the discharge coefficient (about 0.65 for a sharp orifice, recalled) and ρ the oil density (about 870 kg/m³, recalled). To pass 5.03 x 10⁻⁵ m³/s with a drop of 5 MPa:
- √(2 x 5,000,000 / 870) = 107.2 m/s
- Area = 5.03 x 10⁻⁵ / (0.65 x 107.2) = 7.2 x 10⁻⁷ m² = 0.72 mm², a hole 0.96 mm across
- Heat in the valve = Δp x Q = 5,000,000 x 5.03 x 10⁻⁵ = 251 W
The valve works by wasting energy. If the load drops the pressure, the speed changes: since Q goes as √Δp, a fall in the pressure across the orifice from 5 MPa to 1.25 MPa halves the flow and so the speed. A pressure-compensated valve holds the drop constant by a spring-loaded plunger, and gives a steady speed, but is harder to build.
Meter-In or Meter-Out
- Meter-in: the valve sits in the line going into the cylinder. It controls speed when the load resists the motion (a press pushing down on work). It cannot stop a load that pulls the piston along (a ram lowering a weight), because the cylinder then pulls oil through faster than the valve admits it and cavitates.
- Meter-out: the valve sits in the return line. The cylinder is held between the pump pressure and the restriction, so it cannot run away when a weight lowers it. This is the choice for a jack lowering a load.
Meter-out raises rod-side pressure. On a cylinder with a smaller rod-side area the throttled return oil sees a higher pressure than the pump side by the area ratio. For an 80 mm bore with a 40 mm rod, the ratio is 50.27 / 37.70 = 1.33, so if the bore side is at 20 MPa and there is no external load, the restriction on the rod side holds that side at up to 26.7 MPa (Directional Control Valves). Check the rod-side ports and hose against that figure.
Method: Build a Needle Valve
Step 1: Turn a needle with a long taper (a shallow cone) and a threaded stem.
Step 2: Make a matching seat in the valve body so that as the needle backs out the gap opens gradually.
Step 3: Seal the stem with an O-ring or packing and fit a locknut.
Step 4: Calibrate by timing the ram over a measured distance at each setting with the same load, and mark the stem. A tapered needle gives fine control at small openings.
Common Mistakes
- A fixed orifice used where the load varies. The speed follows the load.
- Meter-in with an overrunning load. The load runs away.
- Ignoring the heat. The valve converts every watt of Δp x Q to heat.
- Dirt in a tiny orifice. A 0.96 mm hole clogs; keep the oil filtered.
Safety
A throttle can trap and multiply pressure
A closed or nearly closed throttle in the return line holds oil on the rod side at a pressure above the pump’s, and a throttle that clogs suddenly stops a moving load that has momentum. Never search for a leak with a hand: a fine jet at a throttle injects oil through skin. Use card or wood, keep clear of the ram’s line, and relieve pressure before opening any valve.
What’s Next
- Learn next: Hydraulic Fluids — oil that keeps the orifice clear and the valve working
- Combine with: Hydraulic Jack — a controlled lowering valve