Differential Cylinders – Lifting and Retracting at Different Speeds

A differential cylinder is a hydraulic linear motor. Unlike synchronous cylinders, for example, a differential cylinder only has one piston surface available as an application point for hydraulic pressure. The second application point required for retraction is the rear side of the piston. However, this is reduced by the cross-section of the extending piston rod. Linear motors with extendable piston rods are therefore usually designed as differential cylinders.

Characteristics of Differential Cylinders

Because the rear side of the piston has a reduced effective area due to the piston rod, the force and speed change depending on the direction of travel, even with constant pressure. The performance of the linear motor is maximal when the cylinder extends, and it is weaker and slower when the cylinder retracts. This must be taken into account when designing the hydraulic system.

Options for Compensating Differential Loss

It is usually in the designer’s interest for the linear speeds of the hydraulic motor to remain constant. The faster the cylinder retracts, the sooner it can extend again. The performance and cycle time therefore depend not only on the extension speed but also significantly on the retraction speed.

The designer has the following options to synchronize the retraction speed of the cylinder with the extension speed, despite the reduced effective area on the piston:

• Increase pump capacity
• Increase volume flow through valve control
• External load on the piston rod
• Throttle extension speed

Increasing the pump capacity would be triggered electromechanically by passing the limit switch. The problem with this solution is that it changes the volume flow and internal pressure throughout the hydraulic system. All actuators would change speed when the cylinder retracts. This solution is therefore primarily suitable for small systems that do not operate any actuators other than the differential cylinder.

Instead of changing the volume flow throughout the entire system, it is sufficient to ensure a greater inflow of hydraulic fluid only at specific points. Lines with a larger cross-section and throttle valves are suitable for this.

If the piston rod is loaded during retraction, the additional pressure can increase the retraction speed of the differential cylinder. Excavators, for example, use this solution.

If the goal is only synchronization and not an increase in performance, a throttle valve on the extension side can be a solution. However, this reduces the system to the speed of the retracting cylinder. This solution is therefore always associated with a loss of performance.

Maintenance of Differential Cylinders

Differential cylinders hardly differ in design from other hydraulic cylinders. They consist of a piston rod, inlet valves, seals, and the cylinder liner. Primary wear occurs mainly on the seals. These can be easily replaced. Provided that the piston and piston rod are undamaged, the differential cylinder will regain full performance after repair.