Does your cold excavator or loader work great, but after 30 minutes, its movements become sluggish and lifting a full bucket requires high RPMs? This is one of the most common symptoms of a hydraulic system failure. This phenomenon often confuses many mechanics. The experts at BRhydraulic explain why rising oil temperature mercilessly exposes component wear and exactly where to look for the cause of power loss.
1. The magic of "cold" vs. "hot" oil
The key to understanding this problem is the viscosity of the hydraulic fluid. Cold hydraulic oil is thick. This density allows it to act as a natural sealant, temporarily "patching" microscopic clearances in worn system components.
As the machine operates and the temperature rises (optimal operating temperature is usually 40-60°C), the oil drastically changes its consistency, becoming as thin as water. Thin oil offers no resistance and always seeks the easiest path of escape instead of pushing the actuator piston.
2. The prime suspect: A worn hydraulic pump
In most cases, the pump (usually a gear pump) is responsible for the loss of power once the system heats up.
-
Internal clearances: Over the years, the gears and internal thrust plates (so-called "pressure plates") become worn.
-
Oil churning: When the oil becomes thin, instead of being pumped at high pressure, it squeezes through the worn gaps back to the suction side.
-
Vicious cycle: The pump begins to circulate the oil within itself. System pressure drops, and the friction caused by the oil forcing its way through the gaps leads to further, extreme overheating.
3. Hidden internal leaks
If the pump is new and fully functional, hot oil will reveal faults in other parts of the system. We call these internal leaks (because not a drop of leakage is visible outside the machine):
-
Leaking actuators: The seals on the piston inside the cylinder may be worn. Thin, heated oil flows from the piston-side chamber to the rod-side chamber – the actuator lacks the power to lift the load even though the pump is generating pressure.
-
Worn control valve: If the spool surface of the control valve is scratched (as we wrote about in our previous article), hot oil escapes through the gaps directly to the return (T-line) to the tank.
4. The worst mistake: adding thicker oil
A common "garage" practice intended to save a worn machine is changing the oil to a thicker grade (e.g., switching from viscosity 46 to 68) or using thickening additives. This is a death sentence for the system. Oil that is too thick causes massive suction resistance in a cold engine, which immediately triggers destructive cavitation and ultimately "finishes off" the pump.
Summary
A loss of power when the oil is hot is a clear sign that the physical components of your system have worn down. Instead of experimenting with oils, use a pressure gauge to check the pressure both cold and hot.
Need replacement parts? Contact the specialists at BRhydraulic.eu.