Most of us are aware that prolonged exposure to the sun—especially in hot weather—can wreak havoc on rubber, polymers, and other synthetics, rendering them weak and brittle. But prolonged exposure to the sun can also cause problems inside a hydraulic system.
Hydraulic component failures from pressure overload have been reported in agricultural equipment, which often is left exposed to the sun for long periods when it is not being used. These pressure overloads can be above and beyond system relief valve settings, and they occur while equipment is shut down, rather than when it is operating. These seemingly improbable events result from thermal radiation absorptivity from prolonged exposure to the sun.
Many designers believe that system relief valves will prevent such heat-related high pressures from developing or that expansion of hydraulic hoses will absorb any pressure increases. They will, but many system designers do not specify relief valves in the implement because they rely on relief valves in the tractor’s hydraulic system.
Obviously, the problem is more complex. Through laboratory stress analysis of a failed component, the pressure that caused the failure can be computed. Working backward from that number, an analysis of the situation can be made.
A Typical Scenario
Let’s begin with this scenario: Assume the failed component came from an agricultural implement. The implement had been disconnected from the tractor on a fall day. It was a heavy implement that could trap 2,500 to 3,000 psi in its hydraulic system. Further assume that the ambient temperature was between 20° and 30°F on that fall day. If we know what the failure pressure is, and subtract the trapped pressure, what remains is the amount of pressure that would have to be generated by external sources to cause failure.
For this thermal-expansion problem, the bulk modulus of the fluid is a key factor. The bulk modulus for typical hydraulic oils ranges from 240,000 to 250,000 psi and varies with temperature. For this example, we’ll use an average value to get a useful indication of the possibility of thermally generated pressure increases.