Overview
Mobile and off-highway equipment contain hydraulic systems that stationary industrial machines rarely deal with. Excavators, wheel loaders and other equipment are exposed to sudden changes in load, vibration, long duty cycles and high temperatures and dust. Stable hydraulic pressure is a goal of engineering and is addressed with the use of hydraulic accumulators.
Understanding the Piston Design
An accumulator of the piston design utilizes a free floating piston within a cylinder to separate hydraulic fluid from a gas chamber that is pre-pressurized with mostly nitrogen. As fluid enters the accumulator, the piston moves with the fluid while gas is compressed and stored as potential energy. When pressure in the circuit decreases, the potential stored energy is expelled and the piston compresses the gas. The piston design is unique in that to separate the fluid path from the gas, an elastomeric separating element, commonly a bladder or diaphragm, is not utilized. This design of accumulator is better suited to handle high pressures and larger fluid volumes.
Mobile Circuit Working Principle
In mobile hydraulic circuits, a piston accumulator has a movable free piston that separates the gas and the hydraulic oil. If there is a demand for the flow of a mobile hydraulic circuit that is greater than what the pump is capable of producing, the accumulator is called upon to compensate for the difference. An example of this would be a fast extension of a cylinder or a sudden steering correction. Once the demand for the flow is less than what the pump can produce, the accumulator resets and charges itself for the next usage. In equipment that experiences rapid swings in pressure, the design of a piston accumulator is highly desirable.
Function of Off-Highway Equipment
Hydraulic circuits are frequently used in off-highway vehicles used in mining, logging, and other heavy-duty off-road applications. These circuits must adapt dynamically to difficult working circumstances. Rapid pressure spikes in such environments can damage system components. A piston type unit smoothes rapid pressure spikes and protects pumps and valves from cavitation while seals and hoses are not exposed to pressure shock. Additionally, pneumatic systems offer a comfortable ride and prevent structural fatigue.
Advantages for System Integrators
Piston type units are a preferred choice for systems that must operate in challenging application environments. Compared to rubber units, piston type units can operate in a wider temperature range and can perform more cycles without significant wear. Some units offer a position sensing feature, which supports a condition based maintenance strategy. Server units are constructed of metal and are therefore more compatible with a wider range of fluids, including some fire resistant hydraulic fluids, used in mining and forestry equipment.
Selection Considerations
The pressure rating and size of the accumulator is dependent on the specific duty cycle of the application. The performance is impacted by the maximum system pressure, the required volume, the operating temperature and the orientation of the unit. Gas charge is set to the minimum system pressure to avoid bottoming of the unit during operation, which significantly reduces seal life.
Maintenance and Service Life
Field inspection of seals, gas pre-charge, installation hardware, and a visual check for external leakage supports an accumulator’s extended service life. Off-highway equipment is regularly used in extreme configurations so it is common practice to check pressure in the accumulator as part of a preventive maintenance program. The replacement of seals in a proactive manner decreases unplanned equipment downtime for a construction or agricultural application, where equipment uptime is directly associated with productivity.
Supporting Reliable Hydraulic Performance
Teutonic Engineering partners with hydraulic system integrators and design engineers throughout the industrial and off-highway segments to provide a comprehensive range of hydraulic components that includes cylinders, power packs, manifolds and accumulators systems to satisfy the harshest application requirements. It is our practice to specify components based on the operating conditions of the equipment as opposed to providing a component that is a “best fit”.
Conclusion
Pressure control on mobile off-highway equipment is vital and Piston Accumulators are one of the most effective systems available for this purpose. Understandably, for engineers and integrators assessing hydraulic circuit components, it is important to understand how these systems perform in the real environment to select the most effective component for the equipment.