Understanding Common Hydraulic System Problems and Solutions

The important measures of hydraulic power are flow, measured as gallons per minute (gpm), and pressure, measured as pounds per square inch (psi). Consider this calculation regarding powering your hydraulic system:

Hydraulic Horsepower

hp = gpm x psi x .0007

• gpm = gallons per minute
• psi = pounds per square inch
• hp = horsepower
• .0007 = conversion constant
  
  

Noise, elevated temperatures and slow or erratic operation are all signs of problems with your system. The most common causes of poor hydraulic performance are particulate or water contamination, clogged filters, high fluid temperature and incorrect hydraulic fluids.

Water contamination can be problematic in hydraulic systems. Hydraulic fluid that has a milky appearance is a sign of water contamination. Water in hydraulic fluid can reduce fluid lubricity, cause pitting against metal surfaces, degrade additives and accelerate oil aging.

Mitigate the presence of water in hydraulic systems with these best practices:

• Be sure to follow proper system maintenance intervals for filters and fluid changes. To remove water from the system, conduct a complete drain of fluid in the system, clean the hydraulic reservoir of any contaminants and install clean fluid.
• Operating the system regularly will increase fluid temperature and help remove water from oil.
• To prevent water from entering the system, do not leave the system open or exposed to water entry and ensure proper vent cap installation.

Particulate contamination can be a problem. In normal operation, hydraulic fluids are circulating, and the filters are removing contamination. The filtration system is critical, so be sure to follow proper filter change intervals and use a filter with the correct ratings for the application. Use clean fluid when adding fluid to the system. In addition, proper service procedures and component installation is important for keeping contaminants out of the assembly.

There are two common reasons for knocking noises: aeration and cavitation.

When air bubbles contaminate hydraulic fluid, it results in banging or knocking noises due to the compression and decompression of the air as it moves through the system. Air contamination can decrease the efficiency of the system. Air in the system, called aeration, is caused by a leak on the inlet side of the pump. It’s important to maintain the proper hose connections and oil level in the system to prevent air contamination. Routine hose inspection and repair of any leaks help prevent aeration of the system.

When a vacuum bubble is created on the inlet side of the pump, it results in whining and banging noises due to the creation and collapse of the bubble. Vacuum bubbles created by a very low fluid pressure is called cavitation. The most common causes are fluid viscosity that is too high for the operating temperature or a fluid restriction on the inlet side of the pump.

To avoid aeration or cavitation, some issues to watch for and repair immediately are: a poorly designed inlet; a collapsed or otherwise restricted intake line; clogged or undersized reservoir breathers; loose intake-line clamps or fittings; porous intake lines; or a low reservoir oil level.

High fluid temperature can be caused by anything that either reduces the system’s capacity to dissipate heat or increases its heat load. This includes an excessive workload, a high duty cycle and/or a failed or worn component with internal leakage.

Hydraulic systems dissipate heat through the reservoir and heat exchanger if equipped, so the reservoir fluid level should be monitored. Also check for obstructions to airflow around the reservoir, such as a buildup of dirt or debris. Keep the reservoir clean for proper heat dissipation.

Inspect the heat exchanger to ensure the core is not blocked. The heat exchanger’s ability to dissipate heat depends on the flow rate of the hydraulic fluid and the cooling air passing through the exchanger. Check all cooling circuit components and heat-generating components and replace as necessary.
Installing a fluid temperature alarm in your hydraulic system can alert you to problems.

High fluid temperature can result when any component has abnormal internal leakage that increases the heat load on the system, such as when a cylinder leaks high-pressure fluid past its piston seal. A hand-held infrared thermometer or thermal camera can be used to identify parts that have internal leakage. While in operation, failed or worn components will show higher temperatures than others in the system. A common source of excessive heat due to internal leakage is an undersized or overused pressure relief valve.

In a hydraulic system, fluid flow determines actuator speed and response. A loss of speed indicates a loss of flow at the point of use.

Loss of flow in a hydraulic circuit can be caused by an external or internal leak. External leaks are often easy to find, such as a burst hose. Internal leaks are more difficult to spot, because they can occur in the pump, valves or actuators.

For slow operation when a load is applied, it’s always good to verify adjustable settings in the system. For systems with variable displacement pumps, it is common for a pump-mounted pressure compensator to reduce flow at a set pressure. Since the pressure compensator reduces flow at the set pressure, the set pressure should be verified. Pressure relief valves are another common component that can cause reduced flow. When relief valve set pressures are too close to system operating pressures, it can cause internal leakage and slow operation. For systems with both variable displacement pressure compensating pumps and pressure relief valves, the pressure relief valve should be set a few hundred psi higher than the pump pressure compensator to prevent excessive operation of the relief valve.

Because internal leakage influences heat load, the issues of high fluid temperature and slow operation often occur together. Fluid temperature increases result in less viscosity, which in turn causes internal leakage to increase. That causes the heat load — and fluid temperatures — to increase even more.

Maintaining the correct fluid level, using proper filtration and repairing any leaks are the first line of defense against excessive heat and contamination. Using the correct, high-quality fluid is also important.