An automotive company built an assembly plant on land that had a limestone base. Research found that if the limestone was removed, crushed and re-laid, the plant stability and harmonics would be improved to a point that would offset the cost of crushing the limestone.
They used a large rock crusher that had a fairly simple hydraulic circuit. Two Caterpillar front end loaders would dump large chunks of broken limestone into a chute from either side of the rock crusher. A mechanical vibrator would move the limestone down to a large rotating drum with 50 to 60 swinging hammers that would pulverize the limestone into stones no larger than a tennis ball. The crushed stone would then be laid back into the ground to build the base for the new plant.
A large pressure-compensated piston type motor powered the drum. If the rock pieces were large in size, the motor and drum speed would slow down, but the torque at 3000 psi would increase, thus giving it the sufficient crushing force it needed.
The problem they were experiencing was overheating of the hydraulic oil, which caused the over-temperature sensor to shut the unit down. It could be up to an hour before the system would cool down and reset the sensor.
We were asked to investigate and fix the problem. The customer was sure it needed a larger fan-cooled heat exchanger. We watched the operation for a couple of hours and noticed that the overheating shut-down occurred shortly after the crusher drum was stopped or allowed to free wheel for more than 20 to 30 minutes.
Any idea what could have been causing the problem? Can you think of a simple fix?
The overheating problem with the rock crusher machine was interesting in that it overheated when idling or coasting; but if loaded for hours, the cooler handled the heat exchange.
However, when idling or coasting, the large pressure compensated pump was fully compensated at 3000 psi, and the hot case drain oil was not cooled. The case drain was plumbed directly to the tank while the cooler didn’t see any system oil flow to cool.
We have found that providing the pump case drain with a set of check valves — one with a 5 psi setting plumbed to the tank, and the other a ½ to 1 psi plumbed to the main cooler — would have reduced idle heat buildup in the system. When the system is working, fluid flows from the actuators, and while idling, the only heat source from the pump case drain is cooled. This solved their problem.
If the operating system does not require a heat exchanger, but idles for long periods of time, a small air cooled heat exchanger can be installed on the case drain of pump.