Hydraulic systems are not very efficient. A typical machine turns between a third and two thirds of the power it takes from the supply into useful work. The rest becomes heat, and nearly all of that heat ends up in the oil. Whether the oil runs at a comfortable 50 °C or a damaging 85 °C depends on two things you can explore above: how much power is wasted, and how well the system gets rid of it.
In simple terms
Follow the band in the diagram from left to right. It starts as the electric power the motor takes. The motor wastes a little, which warms the air around it. Then each part of the hydraulic system takes its share:
- The pump loses some power to friction and internal leakage.
- The relief valve, if oil is going over it, turns that oil’s whole pressure into heat.
- Valves and pipes need some pressure just to push the oil through them, and that pressure is lost as heat too.
Whatever is left at the end is the useful work: lifting, turning, pressing.
Now try “Held at end of stroke”. The machine is doing nothing, but the motor is still working hard, because all the pump’s oil is being forced over the relief valve. Every kilowatt goes into the oil. That is the most common reason a machine overheats.
The oil warms up until the heat leaving it equals the heat going in. The tank walls get rid of a little. A cooler, with a fan blowing air through fins, gets rid of much more. When the air around the machine is 45 or 50 °C, as on a UAE summer afternoon, the cooler has less temperature difference to work with, so everything settles hotter. Dust packing the fins makes it worse again.
The engineer's explanation, the deep dive, key numbers and the quiz open with a free account.
For engineers
Power in a hydraulic line: P (kW) = p (bar) × Q (L/min) / 600. Any pressure drop at a flow is a power loss at that point: Δp × Q / 600 kW becomes heat.
Default case in the animation: 60 L/min, load needs 120 bar, about 10 bar lost in valves and lines.
| Item | Calculation | kW |
|---|---|---|
| Hydraulic power from the pump | 130 × 60 / 600 | 13.0 |
| Pump losses (overall efficiency about 0.85, plus churning) | 3.2 | |
| Valve and line losses | 10.4 × 60 / 600 | 1.0 |
| Useful work | 120 × 60 / 600 | 12.0 |
| Heat into the oil | 3.2 + 1.0 | 4.2 |
Held at end of stroke at 180 bar, the same pump puts 18 kW over the relief plus about 4 kW of pump loss: 22 kW of heat, five times as much, for no work at all.
Heat balance. The oil settles where heat in equals heat out:
T(oil) = T(air) + Q(heat) / (h·A + C)
- h·A is the tank: roughly 10 to 15 W per m² per °C for a steel tank in still air. A 200 L tank has about 1.9 m² of surface, so only about 23 W per °C. At 30 °C above ambient that is under 1 kW.
- C is the cooler’s capacity, usually quoted as kW per °C of entry temperature difference (oil in minus air in). The animation uses 0.5 kW/°C for a clean air-blast cooler.
With 4.2 kW of heat, a clean cooler and 45 °C air, the oil settles near 53 °C. Held at end of stroke it heads for 87 °C. Without a cooler the tank alone could never shed 4 kW, which is why almost every working machine in the Gulf has one.
Warm-up rate. Oil holds about 1.6 kJ per litre per °C. A 200 L tank absorbing 4 kW with no cooling warms by roughly 0.8 °C a minute.
Deep dive
Where the losses hide on real machines:
- Relief valves cracking open below their setting, or set too close to the working pressure, bleed oil to tank during normal work. A relief valve body that is noticeably hotter than the pipe around it is a strong clue. Our lesson on relief valves shows how they crack and creep.
- Internal leakage in worn pumps, motors, valves and cylinders. Every leak from high pressure to tank is a small relief valve. A worn piston pump with a high case-drain flow can turn several kilowatts into heat.
- Fixed pumps on closed-centre or idle circuits. A fixed-displacement pump that can’t unload through the control valve or an unloading valve always pumps over the relief when nothing is moving. Load-sensing and pressure-compensated pumps (lesson 16) avoid most of this.
- Undersized lines, filters and coolers with high pressure drop. Return lines especially are often too small for the flow that comes out of the full-bore side of a cylinder.
- Throttling to control speed. Flow control valves and counterbalance valves turn the throttled pressure difference into heat by design. That is fine for short movements, and expensive for continuous ones.
Why heat matters. Mineral hydraulic oil oxidises roughly twice as fast for every 10 °C above about 60 °C, forming varnish and sludge. Thin hot oil leaks more, so the machine gets slower, which makes operators hold the lever longer, which makes more heat. Seals harden and crack. Most oil and seal suppliers recommend keeping the bulk oil between about 40 and 60 °C, with 80 °C as a hard upper limit for standard nitrile seals.
Cooling in UAE conditions. Air-blast coolers are sized on the difference between the oil and the air entering the cooler. A cooler sized in a 30 °C climate loses a third of its margin in 45 °C air. Sand and dust pack the fins quickly on deck and on construction plant, and salt deposits do the same at sea. Cleaning the fins (blowing them through from the clean side), checking the fan and its thermostat, and making sure hot air isn’t recirculating back into the cooler are simple and very effective.
Finding the heat source. Measure the temperature of each component with a contact or infrared thermometer while the machine works. The component that is much hotter than the oil around it is where the power is being lost. Our overheating guide walks through the checks in order.
Key numbers
| What | Typical value |
|---|---|
| Heat from a pressure drop | Δp × Q / 600 kW (bar, L/min) |
| Oil temperature to aim for | 40 to 60 °C |
| Upper limit for standard nitrile seals | about 80 °C |
| Steel tank heat loss | 10 to 15 W per m² per °C |
| Heat capacity of mineral oil | about 1.6 kJ per litre per °C |
| Oil life above about 60 °C | roughly halves for every extra 10 °C |
