Hydraulic Cooling and Heat Balance
Every hydraulic system turns part of the drive power into heat. If that loss heat stays in the oil, temperature rises, the oil ages faster and viscosity drops below the safe range. This guide shows how to set up the heat balance and match tank, cooler and oil temperature cleanly to each other.
View hydraulics guideHow do I set up the heat balance?
The heat balance compares heat added and heat removed. Added is the power loss from pump, valves, throttles and lines; removed is via tank surface, lines and a cooler. As a rule of thumb, 20 to 30 percent of the installed drive power ends up as heat in the oil. If removal is smaller than input, temperature rises until it settles at an equilibrium, often at too high a level.
Which oil temperature is still safe?
An ISO VG 46 mineral oil works best between 50 and 60 °C. In this window viscosity sits in the pump's design range, the lubricating film carries load and ageing stays slow. Above 60 °C oxidation speeds up noticeably, and from about 80 °C in continuous duty the oil ages fast, seals harden and service life drops.
- Rule of thumb: every 10 °C above 60 °C roughly halves oil life.
- Mind cold start: oil that is too cold and thick can cause pump cavitation.
- Fit a temperature sensor in the tank and define a warning threshold.
Tank or cooler - what suits my system?
Small systems often reject heat through the tank surface alone. When that is not enough, an active cooler is added. Air-oil coolers are simple and self-contained but need space and cool ambient air. Water-oil coolers are compact and powerful but require a cooling-water circuit. A tank volume of 3 to 5 times the flow per minute remains the basis in either case.
A fan cools the oil against ambient air, self-contained and low maintenance.
Coolers in the guideFrequently asked questions
How large must the hydraulic tank be?
As a guide the tank holds 3 to 5 times the pump flow per minute. That gives the oil dwell time to de-aerate, settle and cool. With high power loss or tight space, an active cooler supplements the tank surface.
Why does my hydraulic oil get too hot?
Usually more heat is generated than removed. Common causes are constant bypassing at the pressure relief valve, undersized cooling, fouled cooling fins or a tank that is too small. Check the continuous duty point and the heat balance.
Which oil temperature is optimal?
For common ISO VG 46 mineral oil the ideal range is 50 to 60 °C. There the viscosity is right and the oil ages slowly. The system should not run continuously above 80 °C, or service life drops sharply.
Size cooling and heat balance cleanly
Our hydraulics specialists help with power loss, tank size and the choice between air and water coolers.
Practical sizing
Heat balance from the real continuous duty point, not peak load.
Right temperature
Oil in the ideal window for long life and stable viscosity.
Safety reserve
Cooler sized with reserve for ambient and fouling.
Expert advice
Hydraulics specialists support tank and cooler choice.