Cylinder cushioning: the three most common methods

Pneumatic cylinders are set in motion by means of air pressure, they accelerate, but at some point they also have to slow down and come to a stop. Depending on the application, different types of damping are used for this: a method or element to completely absorb the kinetic energy that has built up, causing the cylinder to stop in a controlled and safe manner. In this first part of a series of articles on pneumatic cylinders, we explain the three most common types of cushioning and their properties. What are the advantages, the limitations and when do you actually use mechanical, pneumatic or hydraulic end cushioning?

Why end cushioning?

Cost pressures and increasing product demand make it necessary to increase production speeds in automated manufacturing processes. This reduces the cycle time of the machine and thus the throughput time, resulting in more products per unit time.

In order to achieve these higher speeds, it is necessary for the various components in the production facilities to move faster. And with pneumatic cylinders, this leads to greater amounts of energy being released when the fast-moving cylinder has to stop again. In order to manage this in a proper, safe and controlled way, proper damping of the end position is necessary. This absorbs the kinetic or kinetic energy as efficiently as possible, minimising wear and tear and shocks.

Three types of end cushioning

There are roughly three methods for damping specific pneumatic cylinders.

  • Mechanical and elastic damping: With this operating principle, a material is used that deforms elastically when the cylinder collides with it at a specific speed. Elastic means that the material automatically returns to its original shape after the cylinder rod has been retracted and is therefore ready for the next impact.
  • Pneumatic and servo-pneumatic damping. With (servo)pneumatic damping, the cylinder is slowed down by absorbing the energy using compressed air. This can be a fixed amount but also a regulated back pressure which depends on the cylinder speed at the moment of damping.
  • Hydraulic damping. Finally, cylinders can be slowed down by storing the kinetic energy in viscous fluids such as oil.

Elastic damping

Within the installations in which pneumatic cylinders are used, elastic and pneumatic damping are the most frequently applied forms of damping. This is partly due to the good price/performance ratio and partly to the amount of energy that must be absorbed. Hydraulic damping, for example, is more common when damping very large forces that do not actually occur in a pneumatic cylinder.

We will now describe the most important properties, limitations and possible applications for each type of damping.

Elastic damping (P)

Elastic damping refers to the elastic element on the piston or in the end cap of the cylinder. This element can absorb a small amount of kinetic energy by elastically deforming or, in other words, compressing it. Due to the elastic nature of the material, it then returns to its original state. Depending on the speed (inertia) at which the element is compressed (the impact: mass x speed), the element lasts longer or less time.

Elastic damping is suitable for low-speed applications. This implies: a low load or a short stroke length. The hardness and thus the compressibility of the elastic material differs per type and manufacture of cylinder and largely determines the effect and the ability to absorb energy.

Adjustable pneumatic damping (PPV)

When dynamic forces increase, additional damping is required to prevent overloading of the pneumatic cylinder and the system. With the adjustable air cushioning (recognisable by the abbreviation PPV), a specific volume of air is trapped in the end chamber of the cylinder. The air output - and thus the degree of damping - can be manually adjusted with an adjustment screw.

The required setting of the exhaust air cushioning depends on the mass of the cylinder, the speed at which it is moving at the moment of damping, the acceleration (or own deceleration): speed at which the cylinder should come to a stop, the working pressure and the resistance in the cylinder. A certain amount of experience is required to make the correct setting. When this is lacking, a trial and error process, starting with a relatively high damping, is a solution to reach the optimal setting. During the service life of the cylinder, optimal damping can only be maintained if the setting is regularly checked and adjusted if necessary.

Self-adjusting pneumatic buffering (PPS)

A special form of damping is the "self-adjusting pneumatic cushioning". Here, air is also trapped in the end position of the cylinder to achieve a braking effect. In contrast to PPV, however, manual adjustment is not possible and also not necessary. The escape of the buffer air depends on the cushioning boss attached to the piston. PPS therefore adapts automatically to the conditions and minimises the effect of shocks in the machine. The pleasant result is a constant quality of damping; even under varying loads. In addition, it is a time-saving solution because the user or machine builder does not have to make time- and labour-intensive settings. Finally, it is a 'clean design', as the absence of the adjusting screw greatly reduces the risk of dirt accumulation.

Just like the elastic damping, the self-adjusting pneumatic cushioning is only suitable for applications with not too high loads and speeds.

July 2022