Return force for retracting hydraulic cylinders

The return force is the counterforce to the hydraulic pressure acting on the linear cylinder. When the cylinder is fully extended, this counterforce is needed to return it to its initial position. In hydraulics, there are various solutions for this. Return force can be distinguished between externally and internally acting forces.

Characteristics of External Return Force

External return forces can include:

• Weights
• Tension springs
• Second hydraulic cylinder
• Pneumatic pressure

The externally acting gravitational force is the simplest and most cost-effective solution to return the hydraulic cylinder to its home position. First, the hydraulic system is relieved, and then the vertically positioned piston is pushed back into the cylinder by an external weight. The displaced hydraulic fluid flows back into the tank. This solution is common, for example, in manual hydraulic devices such as engine hoists, car jacks, transmission jacks, or manual hydraulic presses.

Tension springs for piston retraction are rare, but they are occasionally used. They have the advantage that they can retract a cylinder even in a horizontal or even hanging position. However, they are not particularly efficient, as the spring force reduces the force of the extending piston.

A second external hydraulic cylinder is also rare. This solution is occasionally used for flap control. The cylinders are positioned opposite each other and can precisely and powerfully push a central element into a specific angle.

A pneumatic accumulator in the form of a piston or bladder accumulator can also serve to retract a piston externally.

Internal Return Force

Two design solutions are available for internal return force:

• Switching the volume flow as suction flow
• Switching the volume flow as pressure flow

The directed volume flow pushes the piston of a linear cylinder outwards. If the flow is reversed, it can be converted into a suction flow. This pulls the piston back into its home position. This solution sounds simple but is technically difficult to implement. In reality, the external air pressure pushes the piston back. The suction force only supports this. Therefore, this solution is only used at low pressures, such as in power steering. On a large technical scale with hundreds of bars of operating pressure, volume flow reversal has not proven effective for this purpose.

The internally acting force in double-acting cylinders is ensured by the changeover valve and the second hydraulic line. It is more elegant, precise, and reliable than externally acting solutions, but it is also more expensive. This technical implementation is recognizable by the second hydraulic line leading to the tip of the cylinder. If the piston is to be retracted, a changeover valve is actuated. This directs the volume flow from the end to the tip of the hydraulic cylinder and pushes the piston back into its home position.