Good end-position cushioning dramatically decreases the impact forces at the end of a pneumatic cylinder’s stroke, playing a crucial role in defining the minimum travel time and maximum speed of pneumatic cylinders.
Due in part to the compressibility of air, travel speeds and energy tend to reach their peak at near the end positions of pneumatic drives. Cushioning is therefore needed to minimise the impact of the piston on the end cap, reducing wear, noise and transmitted shock and vibration to other parts of the machine and process.
Elastic buffer cushioning
There are numerous ways of cushioning pneumatic drives; elastomeric buffer cushioning is the most commonly used form in automation as it offers basic cushioning performance at lowest cost.
However, only a small amount of kinetic energy can be absorbed, so elastomeric buffer cushioning is really only suitable for lower operating speeds, smaller loads and applications that typically involve short working strokes.
The hardness of elastomeric materials varies and s the performance of elastomeric buffer cushioning will ultimately depend on the manufacturer’s choice of materials. Moreover, these materials’ characteristics will changes over time, with repeated impacts and compression.
Adjustable pneumatic cushioning
An alternative method – particularly when the dynamic forces and loads are greater or when additional cushioning may be required in order to prevent the drive and system from being overloaded - is adjustable pneumatic cushioning.
Air cushioning operates by adding an extended boss to the piston that ducts exhausting air over approximately the last 20mm of stroke through a variable restriction, generating a throttle and cushioning effect. The exhaust air flow can be controlled using an adjusting screw, allowing manual setting and adjustment of the braking effect.
If adjustable end-position cushioning is installed, optimum cushioning can be achieved for applications requiring higher energy absorption. With experience, operators are able to adjust the cushioning effect for an optimum combination of load and speed, as required by the application. Adjustment is critical; if the system is insufficiently cushioned, end of stroke impacts can be excessive and will quickly damage the actuator or machine.
On the other hand, too much cushioning extends the cycle time of the motion and can give rise to jerky, inconsistent movements as the piston literally bounces back off the cushioned air before it slowly escapes. In more modern drives, it is customary to combine adjustable pneumatic cushioning with elastomeric cushioning materials in order to improve cycle times and reduce impact shocks and noise.
Self-adjusting pneumatic cushioning
An alternative method, which is based on a Festo patented principle, is a self-adjusting cushioning technology called PPS. This system works by allowing the exhaust air to escape through an arrangement of slots in the cushioning boss that are of various lengths and profiles. The cross-sectional area of this exhaust channel changes through the cushioning stroke, allowing the end-position cushioning PPS to adjust automatically to the different energy levels generated by any variability in load and speed.
PPS cushioning even reacts to changes in system dynamics such as friction and working pressure, and ensures optimum cushioning without the need for manual adjustment. And as there is no need for manual adjustment, PPS saves the system operator time during drives installation and maintenance, while ensuring process reliability and protection against faulty settings.
Whilst the correct setting of cylinder cushioning is relatively straightforward when conducted within an clean and orderly original equipment manufacturing environment, it is often far more difficult to set correctly within the busy, often cramped and well guarded confines of a production site. In addition, eliminating a cushioning adjustment screw not only reduces production costs but also removes a potential dirt trap – particularly useful when cylinders are deployed in clean areas such as food processing lines.
The single biggest advantage that PPS has over other types of end-position cushioning technology is that it provides optimum cushioning results for most speed/load combinations in approximately 80 percent of all industrial applications.
For applications that involve extremely high travel speeds, it may be necessary to consider a hybrid method of end-position cushioning, combining all the features of adjustable pneumatic cushioning with self-adjusting pneumatic cushioning, plus an additional adjustable cushioning air bypass to improve overall performance.
Festo also offers pressure-sensitive pneumatic cushioning that can cope with changes in a system’s load/speed combination or pressure fluctuations in the working air. Like other pneumatic end-position cushioning systems, this patented system generates its braking effect by compressing a trapped volume of air, automatically adapting to changes in load and offering an even greater application range.
The exhaust air flow rate required to dissipate energy is regulated by a specially shaped, sprung loaded ball escapement system. The cross-section of the cushioning duct is adjusted depending on the buffer pressure. This increased performance comes at a cost, however, as it needs to be adapted to suit individual customer applications.
Servo-pneumatic cushioning soft stop
If you are seeking a ‘top-of-the-range’ system, servo-pneumatic cushioning can regulate the cushioning process directly. The approach to the end positions of the pneumatic cylinder is automatically controlled using a combination of displacement encoder, proportional valve and electronic controller. Counter air flow is channelled in at specific, pre-defined positions to slow the movement by regulating the air flow rate through the proportional valve.
This method produces excellent cushioning characteristics. As a closed loop system the end-position controller automatically compensates for fluctuations in the supply air pressure and dynamic loads. Servo-pneumatic end-position cushioning is very effective, but its relative complexity makes it more expensive, and is therefore best reserved for those applications where other methods, like those described above, are less able to cope – applications involving high cycle rates, strokes of more than 300mm and/or loads greater than 10kg, for example.
As has already been stated, Festo’s PPS system reliably meets the needs of more than 80 percent of industrial applications and has become something of a de-facto standard for the company’s pneumatic actuator product ranges. There are more sophisticated alternatives, as described in this article, but these should only be considered for the most challenging of applications.
Steve Sands is with Festo in the UK