Speed-control circuits
Where you put the throttle decides everything. Meter-in, meter-out and bleed-off are the three ways to set an actuator's speed — and the pressure-compensated valve is what finally holds it steady.
Source: Rabie, Fluid Power Engineering, Ch. 5.
Before you start
What you need first
- The throttle & orifice equation — flow \(\propto\sqrt{\Delta P}\), and a plain throttle drifts with load (Topic 27).
- Back-pressure on a cylinder, \(F = P_1 A_p - P_2 A_r\) (Topic 33).
What you'll be able to do
- Place a throttle for meter-in, meter-out or bleed-off.
- Choose the right one for a resisting vs a running (overhauling) load.
- Explain how a pressure-compensated valve keeps the speed steady.
Start here · the map
Three ways to control the speed
All three use a throttle (Topic 27) — they differ only in where it sits relative to the actuator:
- Meter-in — throttle the oil going into the actuator.
- Meter-out — throttle the oil coming out of the actuator.
- Bleed-off — divert some pump flow to tank before the actuator.
Meter-in control
The throttle sits in the inlet line, limiting the oil flowing into the cylinder. Less oil in → slower extend.
- Best when the load resists the motion (the cylinder pushes against the load) — then the metered inlet flow fully sets the speed.
- Cannot control a running (overhauling) load — a load that pulls the cylinder could draw it faster than the metered oil, leaving the cap side starved.
Meter-out control
The throttle sits in the outlet line, limiting the oil leaving the cylinder. Restricting the exit builds a back-pressure \(P_2\) on the rod side, which holds the load back.
- Needed for a running / overhauling load — the back-pressure stops the load running away, so it works even when the load tries to pull the cylinder.
- It is the safe choice whenever the load could reverse direction.
- The back-pressure costs a little force and makes a little more heat (recall \(F = P_1 A_p - P_2 A_r\), Topic 33).
Bleed-off control
A bleed-off throttle taps off some of the pump flow to tank before the actuator; the rest goes on to the cylinder. Open the bleed more → less reaches the cylinder → slower.
- Efficient — the surplus goes to tank at low pressure, not forced over the relief valve, so little power is wasted.
- Less precise — the cylinder gets "whatever is left", and that depends on the load, so the speed is not as steady.
Choosing the method
| Method | Throttle goes… | Best for | Note |
|---|---|---|---|
| Meter-in | Inlet line | Load resists motion | Simple, precise; fails on a running load |
| Meter-out | Outlet line | Running / reversible load | Holds back-pressure; a little more heat |
| Bleed-off | Tap to tank, before actuator | Efficiency | Less precise |
The fix for drift
Pressure-compensated flow control
A plain throttle's flow drifts because \(\Delta P\) across it changes with the load (Topic 27). The fix is a second spool, the compensator, in series with the throttle. Its one job: keep the pressure drop across the throttle constant (a small fixed value, about 4–10 bar).
Constant \(\Delta P\) across the hole → constant flow → steady speed, even as the load pressure swings — as long as there is at least that much pressure to spare.
| Type | How | Note |
|---|---|---|
| Two-way (series) | Throttle + compensator in series; surplus goes over the relief | Simple |
| Three-way (parallel) | Bypasses the surplus to tank, keeping pump pressure just above the load | More efficient |
✏️ Try it yourself
- Where does the throttle go for meter-in, and for meter-out?
- A crane lowers a load that tries to pull the cylinder down (a running load). Which method must you use, and why?
- A compensated FCV is set to 8 L/min; the load pressure jumps from 80 to 150 bar. What happens to the flow, and what did the compensator do?
Common mistakes to avoid
| Mistake | Fix |
|---|---|
| Using meter-in on a running load | The load can outrun the metered inlet; use meter-out for overhauling/reversible loads. |
| Thinking meter-out is free | Its back-pressure costs a little force (\(P_2 A_r\)) and makes extra heat. |
| Expecting a plain throttle to hold speed | Add a compensator to keep \(\Delta P\) — and so the flow — constant. |
| Calling bleed-off "precise" | It is efficient but less precise — the cylinder gets what's left after the bleed. |
Recap — the whole topic on one screen
| Idea | What you own now |
|---|---|
| Meter-in | Throttle the inlet; for loads that resist |
| Meter-out | Throttle the outlet; back-pressure for running loads |
| Bleed-off | Tap surplus to tank; efficient, less precise |
| Compensated FCV | Holds \(\Delta P\) constant → steady speed; three-way = efficient |