Overview

Consider the cylinder that slams to a stop at the end of every stroke. That cylinder is wearing out over time. What’s happening at the end of the stroke is causing maintenance issues for plant workers. Often, plant workers find that the end of the stroke is causing issues for the cylinder seals. The end of the stroke can also cause stress on the mounting bolts and end caps of a cylinder.

Why the End of Stroke is Important

When a cylinder is extending or retracting, the energy is released once the cylinder reaches the end of its stroke. If the end of the stroke is not designed to gradually slow and stop the cylinder, the remaining energy is released as a shock load. With repeated shock loads, the mounts and the connections for the cylinder wear out. This can also cause stress on the seals. Shock loads are especially hard on the mounting components of a cylinder when the load is extended over a long period of time. This is normally the case with equipment that has a long and continuous operation cycle time.

How Slow-up Works

To slow the piston down, a restriction in the form of a tapered plug or a cushion sleeve is used. This restriction obstructs the fluid’s path of exit and, as a result, increases back pressure, which in turn slows the piston down. The fluid is allowed to flow only through a small orifice. A check valve allows fluid to flow only in the direction of the piston’s stem. For full-stroke operation, the valve allows fluid to flow in the other direction. A constant and fixed restriction is used in some designs, while other designs allow an operator to adjust the restriction with a needle valve. Designs that incorporate a check valve in the piston allow for better control and varaibility over the cushioning effect. Piston-type designs tend to be more effective at lower flow rates and less bulky compared to other designs.

Fixed versus Adjustable Cushioning Designs

The main advantage of fixed cushioning is its simplicity. It is an appropriate choice for designs where the load and speed do not vary significantly from cycle to cycle. On the other hand, if there is a significant variation in the load and/or speed of the system, then the deceleration rate can be set by the system’s maintenance personnel. It is then the responsibility of the design engineer to provide a means for the maintenance personnel to adjust the deceleration rate.

Worn Cushioning Systems

Cushioning systems can be considered worn out when they no longer adequately absorb the impacts experienced by a system. This can be evidenced by an increase in both the level and the type of vibration experienced by a system, as well as undue load knock experienced by the system. Worn cushioning systems can also lead to fatigue failures of the system’s structural components. The failure of cushioning systems is usually progressive and therefore can be detected by routine inspection and/or testing. Routine inspections and testing are typically performed during system maintenance.

Working with an Established Cylinder Manufacturing Company

Understanding the application and manufacturing experience with cylinder internals, is what helps construct the best cushioning design. Teutonic Engineering based in Coimbatore, India, produces hydraulic cylinders and can customize cushioning designs based on the requirements of the application. When interaction with the supplier integrates manufacturing and application know-how, specifying new equipment becomes less guesswork for the end user.

Conclusion

Protecting the end-of-stroke cushioning extends the life of seals, hardware and the machine, and is good design practice. In various types of heavy machinery, strokes that are fast and heavily loaded are the most damaging. In these cases, the hydraulic cylinder cushioning provides the first and best defense against fatigue and shock failures. For these types of machines, an appropriate design of a cushioning system is essential.