Marine hydraulicsLesson 20 of 20
How a hydraulic crane circuit works: slew, luff and hoist from one pump
One pump feeds a valve bank with a section for each movement. Run two at once and the oil goes to whichever needs the least pressure, so the crane's behaviour depends as much on the circuit as on the load.
8 minutes · For signed-up learners, free
A ship’s provision crane brings together almost everything in the earlier lessons: a pump and relief valve, directional valves, a cylinder held by a counterbalance valve, a winch motor with a brake, and a filter keeping the oil clean. In the animation, move two or three levers at once and watch where the oil goes. Then load the hook past its safe working load, and try the faults.
In simple terms
A deck crane has three movements. Slewing turns the whole crane round on its pedestal. Luffing raises and lowers the boom. Hoisting winds the wire in or out to lift and lower the hook. Each one has its own section in the valve bank, worked by its own lever.
All three are fed by one pump. That is fine when only one lever is moved: that movement gets all the oil it can use. When two are moved together, the oil does what water does: it takes the easiest path. The movement that needs the least pressure gets its oil first, and the harder one waits. In the animation, hoist and luff together with a few tonnes on the hook: one of them slows down or stops. Operators know this as “the crane won’t hoist while I slew”.
The heavier the load and the further out the boom, the more pressure luffing needs. If it needs more than the relief valve allows, the boom simply won’t go up. That’s not a safe overload system (modern cranes have proper load limiters), but it shows why a crane that has “lost power” usually has a pressure problem.
Two things hold the load when the levers are in the middle. A counterbalance valve on the luff cylinder traps the oil under the piston, so the boom can’t fall. A spring-applied brake on the winch holds the hook, and only releases when oil pressure pushes it off. If either leaks or slips, the boom or the load creeps down.
The engineer's explanation, the deep dive, key numbers and the quiz open with a free sign-up.
- Play with it: Slew, luff and hoist at once, change the hook load and pump flow, and watch one pump share its oil.
- Break it: a leaking counterbalance valve, a worn hoist motor, a slipping winch brake and a clogged return filter.
Continue this lesson free
The rest of this lesson, the interactive Provision crane: slew, luff and hoist on one pump and the quiz are open to signed-up learners. It's free, takes a minute, and opens every lesson in all 14 courses.
Free awareness training. Al Safwan Marine is not an approved training provider, so no certificate is given, and these courses are not a qualification. They don't replace STCW training, manufacturer training or the certification technicians need for this work. The manufacturer's manual and your flag's rules always come first.
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This lesson is also published by our sister site hydraulicsuae.com.
