What are the possible causes for overheating oil in hydraulic system?

There are two ways to solve overheating problems in hydraulic systems: decrease heat load or increase heat dissipation. 

Hydraulic systems dissipate heat through the reservoir. Therefore, check the reservoir fluid level and if low, fill to the correct level

There are two main sources for unusual heat 

1. Leakage 

2. Problem in pump and oil flow. Check that there are no obstructions to airflow around the reservoir, such as a buildup of dirt or debris.

Heating of hydraulic fluid in operation is caused by inefficiencies. Inefficiencies result in losses of input power, which are converted to heat. A hydraulic system’s heat load is equal to the total power lost (PL) through inefficiencies and can be expressed as: 

PLtotal = PLpump + PLvalves + PLplumbing + PLactuators 

If the total input power lost to heat is greater than the heat dissipated, the hydraulic system will eventually overheat. Installed cooling capacity typically ranges between 25 and 40 percent of input power, depending on the type of hydraulic system.