Hydraulic Valves for Motors: The 5 That Do 80% of the Work
13 min read
When I run training sessions on motor circuits, I put up one chart before anything else. It splits motor control into two parts: moving and stopping. In my experience, moving is about 10% of the problem. Stopping is the other 90%. Most enquiries we get about hydraulic motor valves start with speed, but most of the trouble on site starts when the operator lets go of the lever.
Here’s the short answer. Five valves do about 80% of the work on a motor circuit. A shuttle valve releases the brake. A reducing valve protects the brake. A counterbalance valve controls a load that wants to run away. A relief valve kills momentum. A check valve stops cavitation. Not every circuit needs all five, and 80% is my rule of thumb, not a law. The rest of this article is about which one goes where, with circuits from jobs we have built at Tucson.
Why motors are harder to stop than cylinders
A cylinder is forgiving. Put a good load holding valve on it, and it will sit there. A motor is more temperamental. Its pressure surfaces can’t take the same abuse, and every motor leaks internally. Oil slips past the gears, vanes, or pistons into the case and goes back to the tank through the case drain. So past a point, no valve can hold a motor still. The oil just leaks around it.
That’s why a motor needs a parking brake for rest. Most motor makers build a spring applied brake into the motor itself. Valves do the braking while the motor is moving. Think of a car, the foot brake works while you drive, the handbrake holds the car once it is parked. One isn’t better than the other, and you need both.
Which hydraulic motor valves does your load need?
Start with the load, not the valve. I split motor loads three ways.
A resistive load fights the motor all the time. Conveyors and drills are like this. Stop the oil and the load stops by itself. All you need to control is speed, whatever the load.
An overrunning load wants to run ahead of the motor. A winch letting a load down is the classic case. Gravity pulls the same way the load is moving, so the load drives the motor. Here you are braking against gravity.
An inertial load has no gravity problem, but it has a lot of mass. A slew on a crane or a rotating drum keeps going after you stop feeding it oil. Here you are braking against momentum.
Gravity and momentum need different hydraulic motor valves. Mix them up, and you get either a load that runs away or a motor that breaks.
Moving the motor: meter-in or meter-out?
People ask me which one is correct. It depends on the load. For a resistive load, meter-in is fine, because the load always pushes back on the motor and keeps it in check. For an overrunning load, control has to sit on the outlet side, because the load will pull the motor faster than the oil coming in. That is the counterbalance valve’s job, which comes up below.
Either way, if the speed must stay steady as the load changes, use a pressure-compensated flow control. Without compensation, flow through an orifice changes with the pressure across it. A compensated valve opens up when that pressure difference drops, so the same flow keeps passing. More on the types in [LINK: topic 5].
Holding it still: the brake shuttle and the reducing valve
The parking brake on most motors is spring applied and pressure released. So you need a way to send pressure to the brake whenever the motor is being driven, in either direction. That is the shuttle valve. It sits between the two motor lines and passes whichever side is higher to the brake. Drive the motor clockwise or anticlockwise and the brake releases. Centre the spool, both lines drain to tank, and the spring puts the brake back on. That means the spool has to drain both lines in neutral. In practice this is rarely a problem, because a brake shuttle is most often paired with counterbalance valves, and they need both lines drained anyway. More on shuttles in [LINK: topic 30].
The second valve on the brake line is the one people forget. Working pressure on a winch can be far higher than what the brake piston and its seals need to release. So we put a reducing valve after the shuttle. It caps the brake release pressure and protects the brake. On my training slides, I mark this one “don’t forget to explain this”, because it gets left off drawings so often.
Braking against gravity: winches
A winch lifting a load is easy. The motor drives the load all the way up, and when it stops, the parking brake takes over. Lowering is where it goes wrong. The load now drives the motor, and through the motor, it tries to drive the pump. The pump can’t supply the oil the motor wants, so the inlet side cavitates.
You can treat this two ways. The first is to treat the symptom. Put an anti-cavitation check valve on the motor so it can draw oil and doesn’t starve. This works when the load can’t hurt anything by coming down fast. The catch is that the load accelerates freely, and when the brake finally bites, it bites at high speed. Brake pads and discs wear out much faster.
The second is to treat the cause and stop the load from free-falling at all. Most loads that need a hydraulic winch are not featherweights, so this is the route I’d take in most cases. The easiest way is a counterbalance valve, also called an overcenter valve. It holds back pressure, but pressure from the motor’s inlet side pilots it open. When the motor is driving the load, pilot pressure is high, and the valve opens up. When the load starts driving the motor, the inlet starves, pilot pressure drops, and the valve partly closes keeping load under control. For this to work, the spool valve has to drain the service lines in neutral, so the pilot falls away and the valve shuts when not in use.
Keep one thing straight. A counterbalance valve is a pressure control, not a flow control. It decides how much pressure the load has to overcome. The oil then takes the path of least resistance. The basics are in [LINK: topic 15].
Pilot ratio and jerky movement
If you’re unsure about pilot ratio, my answer is 4:1. It is the best all-round choice for motor work. The two things I see behind jerky movement most often are the pressure setting and a pilot ratio that is too high. With a high ratio, a few bar of pilot can throw the valve open, and the load lurches.
The quickest fix on site is to raise the counterbalance setting. That forces the system to work at a higher pressure, and the pilot line stops swinging around so much. Be clear, though. This is the easiest fix, not the best one. There are better ways to calm a jerky circuit, covered in [LINK: topic 20] and [LINK: topic 17].
Counterbalance plus shuttle: the pedal and the handbrake
Most winch blocks we build put the two together. A dual counterbalance valve controls the load in both directions, and a shuttle in the same block feeds the brake. [LINK: product page – counterbalance valve]
I explain it with a car. The counterbalance valve is your foot on the pedal. That is control while you are moving. The brake shuttle is the handbrake. That is stopping, once you have stopped. You wouldn’t drive a car with only one of them. Winches and swing motors get their own article in [LINK: topic 23].
Braking against momentum: slews and cross-port relief
A slew has no gravity problem. Its motor turns both ways and is loaded the same both ways. What it has is a load far bigger than the motor turning it. Excavators, mobile cranes, derrick cranes, reclaimers and windmills all have this. When the operator centers the spool, that mass keeps turning and drives the motor like a pump. What happens next depends on the spool. With an open centre or a drained centre spool, the oil has somewhere to go, so the slew drifts on past where the operator wanted it. With a closed centre spool, the oil is trapped, and the pressure can damage the motor internals or even the shaft.
The answer is a cross-port relief valve with a closed centre spool. The closed centre cuts the motor off from the pump. The relief valves let oil cross from one motor line to the other at a set pressure. The momentum gets used up pushing oil over the relief, and it goes off as heat and noise.
The setting is a trade-off. A higher setting stops the slew quicker with less sweep, but it is harder on the motor. A lower setting stops it gradually, with more sweep and more drift. I set it the same as, or a little above, the system relief, and always above the highest pressure the load itself needs. Set it lower, and the cross-port relief opens while you are working, and the motor loses torque. Between drift and damage, the machine builder has to choose where to sit. [LINK: product page – cross port relief valve]
Pressure spikes in mobile circuits get their own article in [LINK: topic 35].
Why the motor cavitates when it stops
A cross-port relief has a weak spot. Once the spool is closed, the only oil available to feed one side of the motor is the oil coming out of the other side. But motors leak, and some of that oil leaves through the case drain. With those losses, there isn’t enough oil to go round, and the motor cavitates. More on spotting it in [LINK: topic 36].
So we add make-up check valves. These are large flow checks with light springs. When one motor line drops below return pressure, its check opens and draws oil in to make up the loss. For motors with high case drain, we drain the relief valves straight to the tank and let the make-up checks pull oil back in. The motor stays full, and the reliefs protect the spool valve.
There is one more valve that gets missed, and again my slide says “don’t forget to explain this”. Put a spring loaded check valve in the tank line. It holds a little back pressure on the return, so there is always oil in that line for the make-up checks to draw on. Without it, the checks may have nothing to pull. Check valves are covered in [LINK: topic 29].
Once the motor has stopped and neither line has pressure, the parking brake takes over.
A smaller block: the bridge circuit
Can we make it smaller and cheaper? Always. One way is a bridge circuit. Four check valves steer oil from whichever motor line is high, through a single relief valve, and back into the low side. One relief serves both directions, so there is only one setting to make, and the overrun is the same both ways.
Two motor circuits from our own jobs
The first is a cable reel on underground cable laying equipment. The reel has to keep the cable tight whether the machine is paying it out or pulling it back in. The cable is thick enough that paying out needs no help from the motor. The truck just drives forward, and the cable drags the reel round. The reel motor ends up pumping oil, and that oil goes over a low pressure relief, so the reel resists just enough to keep the cable taut. When the reel winds in, a higher relief caps the pull. On one job these were about 70 and 100 bar, but the settings change from customer to customer. Two settings, one for each direction, so the cable neither goes slack nor gets yanked.
The second is an oil boom reel, the kind that stores floating booms for oil spill work. It needs control, stopping, and coasting. The boom is pulled off the reel, so the reel has to turn freely without the motor fighting it. Getting it back in is the motor’s job. We built it with a dual counterbalance valve, a shuttle for the brake, and a solenoid valve across the two motor lines. Switch the solenoid on, and it connects both sides of the motor, so the reel coasts.
The brake on this reel is released through a small directional valve, piloted by the shuttle. When the shuttle passes pressure, it shifts the spool, and the spool connects the shuttle to the brake cylinder, so the brake disengages. When the shuttle pressure drops, the spring pushes the spool back and connects the brake to the tank, so the brake comes on again.
The five hydraulic motor valves at a glance
| Valve | Job on a motor circuit |
|---|---|
| Shuttle valve | Static control: releases the brake |
| Reducing valve | Caps pressure to protect the brake and cylinders |
| Counterbalance (overcentre) valve | Dynamic control of overrunning loads |
| Relief valve (cross-port) | Breaks momentum |
| Check valve (make-up) | Prevents cavitation |
This is my 80/20. It won’t cover every circuit, but if you understand these five, you can read most motor blocks you come across.
A longer version of my thinking on motor braking was published in FPSI magazine.
FAQ
Do I need a counterbalance valve on a hydraulic winch?
If the load can do damage when it comes down fast, yes. An anti-cavitation check alone stops the motor starving, but it lets the load accelerate and wears out the brake.
What pilot ratio should I use on a motor counterbalance valve?
If you’re unsure, 4:1. A very high ratio is one of the main causes of jerky movement.
How do I set a cross-port relief valve?
At or a little above the system relief, and always above the highest pressure the load needs. Higher stops the slew quicker but is harder on the motor. Lower is gentler but drifts more.
Why does my hydraulic motor cavitate when it stops?
Once the spool closes, the motor can only feed itself with its own outlet oil, and some of that leaks out through the case drain. Add make-up check valves and a back-pressure check in the tank line so they always have oil to draw.
Can valves hold a hydraulic motor still on their own?
Not for long. Every motor leaks internally, so at rest you need a parking brake. Valves do the braking while it moves.
Got a motor that won’t stop the way you want?
Send us the circuit, the load, the flow, and what the machine does through our technical enquiry form. We’ll tell you which valves we would fit, and why.
Raj Dabholkar is the Technical Director of Tucson Hydrocontrols, a hydraulic cartridge valve maker in Umbergaon, Gujarat, founded in 1987. A jet propulsion engineer by degree and a hydraulic design engineer by trade, he has spent over 20 years designing valves and solving field problems with OEMs in construction, defense, earthmoving, power generation, agricultural, and industrial equipment.
