Hydraulic Accumulators: Pressure When You Need It

The pressure in a hydraulic system is primarily supplied by its pump. If the pump fails or is switched off, the hydraulic pressure quickly drops. To still be able to build up sufficient residual pressure at certain points in the system, hydraulic accumulators are used. These technical components do not serve to store hydraulic fluid, but rather to build up pressure. This pressure can then be released again via a valve.

Structure of a Hydraulic Accumulator

Hydraulic accumulators utilize the compressibility of gases. Compressed gases build up pressure and maintain it for as long as required. Hydraulic accumulators consist of a double-sided chamber. The system side of the chamber is filled with hydraulic fluid, and the pressure side contains air or a gas. When hydraulic fluid flows in, a separating element is pushed into the pressure side of the hydraulic accumulator, thereby compressing the gas within it. After the inlet valve is closed, the pressure is stored in the hydraulic accumulator. By opening the valve, the pressure is released again.

Types of Hydraulic Accumulators

Hydraulic accumulators are available in the following designs:

• Diaphragm Accumulators
  Diaphragm accumulators are cylindrical hollow bodies with an integrated rubber diaphragm. They have inlet valves on both sides, one for each medium. First, the gas side is pressurized and pre-tensioned. Subsequently, the hydraulic side is filled and connected to the hydraulic system. This allows any desired and technically permissible pressure to be built up in the diaphragm accumulator.
• Piston Accumulators
  Piston accumulators use a rigid piston instead of a flexible diaphragm. The advantages of piston accumulators are their lower susceptibility to malfunctions and their space-saving cross-section. However, they are longer than diaphragm accumulators with the same performance.
• Bellows and Bladder Accumulators
  Bellows accumulators use a metal bellows instead of a rubber diaphragm, making them very robust. They can be manufactured in particularly small designs. Bladder accumulators use a rubber balloon instead of a diaphragm fixed on all sides. They can absorb high pressures in a small installation space. Due to their large surface area, they absorb vibrations and pulsating shocks particularly gently.
• Spring Accumulators
  Spring accumulators use a pre-tensioned spring instead of compression gas. For technical reasons, they can only be manufactured as piston accumulators.

Applications of Hydraulic Accumulators

Hydraulic accumulators are used in hydraulic systems in various ways. One of the most well-known applications is the famous “hydropneumatics” developed by car manufacturer Citroën. This alternative system to conventional gas pressure shock absorbers provides a particularly low-vibration driving experience.

Further applications for hydraulic accumulators:

• Emergency and safety circuits
• Covering peak demands
• Compensation for minor leaks
• Pressure surge damping
• Pulsation damping

A modern use of hydraulic accumulators can be found in energy-saving excavators and material handling machines. The so-called “hybrid piston” uses the load of a full tool to build up pressure when lowering the boom. This pressure is then released again when lifting the unloaded boom. This allows for interesting energy-saving effects. A model that already successfully applies this method is the “SENNEBOGEN 860E Hybrid”.