Fluid Temperature

The following information will serve to assist those installing hydraulics in mobile applications for handheld tools. While many hydraulic circuits can run upwards to 200° F / 93° C, temperatures over 110° F / 43° C are uncomfortable to human touch. Our desire is to hold oil temperature to a maximum of 140° F / 43° C.

In almost any hydraulic tool circuit, oil cooling methods will be required except for very short periods of operation or in underwater and extreme cold environments. If you are involved in the design of a hydraulic tool circuit, use the following as guidelines.



  1. DON’T rely on a large reservoir to control oil heating. Large reservoirs, even with good air circulation, do not adequately dissipate heat.
  2. DON’T set relief pressure too low (open-center circuits) for percussion type tools (breakers, hammer drills, etc.). Pressure peaks may run up to 350 PSI over gauge pressure, popping the relief and causing heat as well as low tool performance.
  3. DON’T pump more oil than the tool should use and avoid flow controls if possible. Instead, size the pump for desired flow volume. Gear type flow dividers can be used to reduce flow more efficiently than valves, reducing heat.
  4. DON’T use heavy oils such as 30W or 10W30 engine oils. These will cause resistance in lines and add to backpressure and heat. Refer to Recommended Hydraulic Fluids for more details.
  5. DON’T run return oil through control valves or other circuit components, except coolers and return line filters.



  1. DO operate pumps at moderate speed — gear pumps usually generate less heat and are less prone to cavitation at speeds of 1,000-2,000 RPM.
  2. DO use generous line sizes — Especially on pump suction and return from tool to tank.
  3. DO use oils in 130-225 SSU at l00° F / 38° C range with high viscosity index.



Use an air-to-oil cooler of maximum size for space available. Use a shrouded, high capacity fan. Many vehicles do not cool well when parked with engine at low speed. Don't use "thermal" viscous-drive fan because these fans do not draw air unless the engine is hot. 


Read More:

Flow Controls

Quick Disconnects

Hose Types

Principle of Hydraulics

Open & Closed Centre Systems