Marine hydraulicsLesson 5 of 20
Hydraulic oil viscosity: why temperature changes everything
Viscosity is how thick the oil is. Hot oil thins and leaks through every clearance; cold oil thickens and starves the pump. Choosing the grade means keeping the oil in its ideal band at your real working temperature.
6 minutes · For signed-up learners, free
A hydraulic machine that works perfectly at 7 in the morning and drifts, slows and loses pressure by 2 in the afternoon is very often an oil temperature problem. Oil viscosity, its thickness, changes enormously with temperature, and almost everything in a hydraulic system depends on it. Move the temperature slider and watch both the chart and the oil squeezing through a valve clearance.
In simple terms
Viscosity is how easily a liquid flows. Honey is thick, water is thin. Hydraulic oil sits in between, and it changes a lot with heat: a typical oil is about ten times thicker at 20 °C than at 80 °C.
The oil has two jobs that pull in opposite directions:
- It has to be thin enough to flow easily through the pump, hoses and valves, or the pump struggles and power is wasted.
- It has to be thick enough to seal the tiny gaps inside pumps, valves and cylinders, and to keep a film between moving metal parts.
Too hot, and the oil turns thin: it slips through clearances, cylinders drift, the pump delivers less, and metal starts touching metal. Too cold, and it’s like pumping syrup: the pump can’t fill properly, the machine is sluggish and a lot of power goes into heat.
The grade (ISO VG 32, 46, 68) is the oil’s thickness at 40 °C in centistokes. A thicker grade suits machines that run hot. The viscosity index (VI) says how much the oil thins with heat: a high-VI oil changes less, so it copes better with a cold start and a hot afternoon.
Tick “Cooler failed in a Gulf summer” to see what a UAE August does to an oil chosen for a cooler climate.
The engineer's explanation, the deep dive, key numbers and the quiz open with a free sign-up.
- Play with it: Oil temperature, ISO VG grade and viscosity index, with a live chart and a valve clearance leaking.
- Break it: a failed cooler in summer heat, and a top-up with the wrong grade.
Continue this lesson free
The rest of this lesson, the interactive Viscosity, temperature and leakage 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.
