Cylinder cushioning slows the piston down as it gets close to the cylinder end cap. This prevents a sudden impact at the end of the stroke, enabling the cylinder to operate at higher speed without excessive stress, vibration or noise. Increased operating speed translates to improved machine output and greater productivity, while a reduction in stress and vibration means a greater service life and reduced unplanned maintenance.
A number of cylinder manufacturers are now featuring innovative cushioning technologies in their latest cylinder designs. These include both pneumatic and hydraulic cylinders. Some are using variable or adaptive cushioning technologies, while others have opted for fixed linear cushioning.
Ideally, cylinder cushioning will enable the cylinder to operate at full speed for as much of its stroke as possible, while also preventing forces from exceeding safe working limits and preventing any impact loading at the end of the stroke. In practice, this means linear acceleration and deceleration at each end of the stroke, with a constant full operating speed in the middle of the stroke. The linear acceleration should go all the way down to zero, so there is no impact at the end of the stroke.
The maximum force that a cylinder can safely be subjected to may be regarded as a fixed parameter for the cylinder, or it may be a function of the required duty cycles. Newton’s laws of motion state that the force depends on both acceleration and mass (f=ma). The actual ideal velocity profile for a cylinder, therefore, depends on its operating parameters: maximum velocity, mass and possibly number of duty cycles.
TWO BASIC TYPES
Cylinder cushioning may be divided into two basic types: those based on some form of mechanical spring or rubber bumper, and those based on restricting the flow of fluid as the cylinder approaches the end of its stroke.
Flow restriction is typically used for pneumatic cylinders where the resulting back pressure creates an air-cushion, which acts as a spring. Good results can be achieved by fitting throttling non-return valves into the end ports of a pneumatic cylinder. Non-return valves do not restrict the inlet flow but enable control of the outlet pressure. Adjustable needle valves can enable these cylinders to be tuned to provide optimum cushioning for the particular operating configuration of pressure, speed and load. Manual adjustment requires experienced maintenance technicians to adjust these air-cushion valves. If the operating parameters are changed, the air cushions must be readjusted to ensure efficient machine operation.
“Manually adjusting the air cushion is not only time consuming, but it can put pressure on the technicians to complete the task as quickly and efficiently as possible, as well as incurring unnecessary production costs,” says Lee Hargrave, group marketing manager at Camozzi Automation.
Camozzi have introduced a new type of air cushion in its Series 23 pneumatic cylinders (pictured) that automatically adapts to changing conditions, eliminating the need for manual adjustment. The ‘auto-cushioning’ system restricts flow at the end of the cycle, causing back pressure and an air-cushion, just like many conventional cushions. The key difference is that instead of using a needle valve to restrict the exhaust flow, the air is exhausted through shaped sleeves containing a number of holes, accurately positioned with precisely set dimensions.
As the cylinder gets closer to the end of its stroke, more holes are closed off, leaving fewer holes for air to exhaust through and causing increased restriction to flow. At first large holes are available which will only cause a noticeable restriction at high cylinder velocities. The holes decrease in size. This elegant solution means that when cylinders are operating at high speeds, cushioning begins earlier in the stroke, and at low speeds cushioning is only noticeable right at the end of the stroke. The self-adjustment means that the cushioning is always appropriate for the combination of speed and applied mass, no adjustment is required, and impacts are avoided. This should result in a long, maintenance-free service life.
“The headache of manual adjustments is eliminated and productivity is increased, ultimately reducing maintenance costs. The self-adjusting cushion is a ‘fit-and-forget’ feature. Eliminating the need to regulate settings, the Series 23 pneumatic cylinders are ultimately tamper proof. Reducing the acceleration forces acting on components and workpieces, the appearance of wear and tear is reduced and time-consuming vibration is minimised,” Hargrave adds.
Festo uses a very similar principle to achieve automatic cushioning in its PPS cylinders. Rather than using holes of different sizes, they include grooves of different lengths to achieve the same effect, a change in the resistance to air flow as the cylinder nears the end of its stroke. Festo has estimated that self-adjustment will typically save five minutes per cylinder during installation and set-up. For installations involving large numbers of cylinders this could quickly add up to considerable savings.
Hydraulic cylinders are also moving towards non-adjustable systems to prevent end-of-stroke impacts. When AHP Merkle opened a new technical centre at Gottenheim in Germany, its engineers began investigating ways of improving performance of its range of hydraulic cylinders. This work has resulted in a new damping system that has been incorporated in a new range of tie rod cylinders.
Previously, its cylinders used an adjustable damping system that was set with an adjusting screw. However, if the damping was set incorrectly, it could result in cylinders being damaged. To avoid this risk, the new linear damping system is non-adjustable. Due to the newly revised damping geometry, an almost linear deceleration can be realized which is characterized by a low load on the cylinder. These cylinders are capable of braking 900kg at a rate of 1 m/s.
“Due to the high power density of the hydraulic system, a cylinder can move large masses at a high speed without problems. But what happens when reaching the stroke end position? The energy at the stroke end can become very high so that cylinder components may be damaged or destroyed. The braking action is decisive. For this reason, we have developed our new linear damping,” says Brian Taylor, a director at AHP Merkle UK and Ireland distributor CyTec Systems UK.
There is an inherent trade-off between self-adjusting and adjustable cushioning, or damping, systems. The ideal velocity profile for a cylinder depends on its maximum velocity, mass and possibly number of duty cycles. It is not possible for a self-adjusting system to achieve an ideal velocity profile for any given combination of these parameters.
It may, however, achieve a perfectly acceptable one. With an adjustable system there is a risk that parameters will change or a cylinder will be set incorrectly, leading to cushioning that is much further from the ideal profile and results in damage. The choice is, therefore, between a failsafe, low-maintenance option or something that can be fully optimised