Reducing, sequence, unloading & counterbalance valves
The relief valve caps the maximum. Four more pressure valves use the same spring-and-pilot idea for different jobs: a lower pressure for one branch, actions triggered in order, resting the pump, and holding a load up.
Source: Rabie, Fluid Power Engineering, Ch. 5.
Before you start
What you need first
- The relief valve — normally closed, spring-set, senses its inlet, dumps to tank (Topic 23).
- The box + arrow + spring + pilot way of reading a pressure-valve symbol (Topic 23).
- Dumped flow wastes power \(N = pQ\) (Topics 4, 23).
What you'll be able to do
- Say what a reducing, sequence, unloading and counterbalance valve each does.
- Tell which is normally open vs closed, what each senses, and where its outlet goes.
- Pick the right pressure valve for a given job.
Start here · the map
One idea, five jobs
Every pressure valve is a box held by a spring that senses a pressure through a pilot line and opens when that pressure reaches the spring setting. What changes from valve to valve is three things: is it normally open or closed, which pressure does it sense, and where does its outlet go.
| Valve | Normally | Outlet goes to | Its job |
|---|---|---|---|
| Relief (Topic 23) | Closed | Tank | Limit the maximum pressure |
| Reducing | Open | Sub-circuit | Hold a lower pressure for one branch |
| Sequence | Closed | A second working line | Start the next action in order |
| Unloading | Closed | Tank | Rest the pump at low pressure (save power) |
| Counterbalance | Closed | (back to tank line) | Hold a load up; stop it running away |
Pressure-reducing valve — a lower pressure for one branch
Sometimes one part of a machine — say a clamp — must press at a lower pressure than the rest of the system. A pressure-reducing valve sits in the branch line feeding that part and holds its outlet (downstream) pressure at a set, lower value.
It is normally open: oil flows straight through until the downstream pressure reaches the setting, then the valve throttles (partly closes) to stop it rising further. So it controls the pressure after itself, sensing its own outlet through the pilot line — the mirror image of the relief valve.
| Relief valve | Reducing valve | |
|---|---|---|
| Normally | Closed | Open |
| Senses | Its inlet | Its outlet |
| Sets | The maximum pressure | A lower outlet pressure |
| Outlet | To tank (dumps oil) | To the sub-circuit (small drain only) |
Sequence valve — do things in the right order
A sequence valve stays shut until the pressure reaches a set value, then opens to let the next action begin. It looks and works like a relief valve (normally closed, spring-set, senses its inlet) — but its outlet feeds a second working line instead of the tank.
Example — clamp, then drill:
- Oil first extends the clamp cylinder.
- When the clamp is tight, it stops moving, so the pressure rises to the sequence-valve setting.
- The valve opens → oil now feeds the drill cylinder.
The clamp always finishes before the drill starts — the order is enforced by pressure, with no electrical sensors needed.
Unloading valve — rest the pump to save power
When the machine needs no flow for a while, an unloading valve dumps the whole pump flow to tank at low pressure. Recall the cost of dumping, \(N = pQ\): at full flow but near-zero pressure, the wasted power — and the heat — is tiny.
It is often paired with an accumulator (Topic 30): the accumulator stores oil under pressure to do the work, and while it is charged the unloading valve lets the pump rest. When the store runs low, the pump cuts back in.
Counterbalance valve — hold a load up
A counterbalance valve keeps a set back-pressure under a raised load — like a press ram or a lift — so the load cannot drop on its own under its own weight. It is a normally-closed pressure valve placed on the cylinder's outlet (return) side: the load can only descend when the supply pressure is high enough to keep the valve cracked open, so the motion stays controlled instead of running away.
✏️ Try it yourself
- A clamp must finish closing before a drill starts. Which valve enforces this, and what triggers the change-over?
- One branch of a 200 bar system must run at only 60 bar. Which valve, placed where, and is it normally open or closed?
- The machine sits idle for half its cycle. Which valve lets the pump rest, and why does that save power (use \(N=pQ\))?
Common mistakes to avoid
| Mistake | Fix |
|---|---|
| Calling a reducing valve "normally closed" | It is normally open, and it senses its outlet (not its inlet). |
| Confusing sequence with relief | Same symbol, but sequence feeds the next working line; relief dumps to tank. |
| Thinking unloading "switches the pump off" | The pump keeps running; the valve just gives its flow an easy path to tank at low pressure. |
| Forgetting why counterbalance is needed | Gravity (or a load) would let the cylinder run away; the back-pressure holds it. |
Recap — the whole topic on one screen
| Valve | Normally | Outlet | Use it to… |
|---|---|---|---|
| Reducing | Open (senses outlet) | Sub-circuit | give a branch a lower, steady pressure |
| Sequence | Closed | Next working line | start actions in a fixed order |
| Unloading | Closed | Tank (low pressure) | rest the pump and save power |
| Counterbalance | Closed | Return line | hold a load up; stop it running away |