Heat and Hydraulics: Keeping it Cool

(An explanation of why it is important to run your hydraulic system as cool as possible and near the recommended temperature range)

Hydraulic oils are intended to lubricate and do the work at elevated temperatures. It is the sealing and rubber components that canít withstand excess heat.

Because of heat, seals become hard and brittle over long periods of time thus elevated oil temperatures shorten the seal life. In the hydraulic ram applications, if the seal is not pliable it will not wipe contaminants from the ram shaft and abrasive dirt is ingested into the hydraulic system causing rapid wear on all hydraulic components.

The most commonly used hydraulic hoses today have a maximum operating temperature of 215 to 250 F. When these temperatures are exceeded the hose wall starts to deteriorate and flake off with the movement of oil through the hose. This does not necessarily occur the first time the oil is overheated, but with repeated temperatures in excess of hose manufacturerís recommendations. Once the hose has become ìcookedî it is ruined and system problems will develop as the hose walls deteriorate. This, of course, will cause damage to the rest of the hydraulic system. Keep in mind, we are talking about all of the hoses on the system, not just one single hose.

Having in mind the cost of down-time and costs of replacing failed hydraulic components is sufficient reason to investigate whether your system is running at elevated temperatures and could possibly benefit from additional cooling of the oil. Lower oil temperatures will result in extended seal and component life.
Suggested oil operating temperatures are*:
Automatic Transmission/
Torque Converter Oil 200-??? F.
Engine Oil 180-200 F.
Hydraulic Oil 120-180 F.
Hydrostatic Oil
(Closed loop systems) 160-180 F.
* Dunham-Bush Fluid-to-Air Heat Exchangers for mobile and industrial applications catalog

Checking Your System for Elevated Temperatures

Two methods will work quite easily. One of these is really simple and accurate: Install a combination reservoir-fluid-level-and- temperature gauge. It may be purchased for less than $20.00. Two important conditions are being monitored with one indicator! The other method, and probably the best, is a temperature gauge mounted in the instrumental panel where it will be under constant surveillance and give adequate warning of excessive temperature.

Having determined that your hydraulic system is operating near maximum or above suggested oil temperature levels, what to do next? You should consider installation of an supplementary hydraulic oil cooling system. Assuming that most of our readers are using mobile equipment hydraulics, stationary hydraulics, which have other considerations, will not be included in this article.

In order to accurately select a heat exchanger, several bits of information need to be considered:
Maximum ambient air temperature (degrees F.)
Fluid temperature desired (degrees F.)
Fluid flow rate, GPM (gallons per minute)
Air flow-rate available at engine cooling fan in FPM or CFM.
Space/ area available for mounting of heat exchanger.
Heat load in BTU/hr. BTU/hr may be calculated using the following formula:
Horsepower input
x .3(30% is lost to heat) x 2545 = BTU/hr.
(Example: 100 HP x .3 x 2545 = 76,350 BTU/hr)

If the heat exchanger is to be mounted in front of or behind the radiator, then you need to supply the velocity or cubic feet per minute of air the engine fan is moving. If a separate cooling system is called for, then Item 4 is engineered into the system when mounting a hydraulic or electric driven fan.

Armed with the information above, it is now possible to determine the proper size and type heat exchanger needed for your application.