Why is cylinder cushioning necessary?
Some form of cushioning is normally required to reduce the rate of travel of a cylinder before the piston strikes the end cover. Reducing the piston velocity at the end of its travel lowers the stresses on the cylinder while reducing vibration in the structure of which it is part. Efficient cushioning is usually necessary in applications in which precision is a must .Basically, this problem can be solved in one of three ways: by means of simple impact cushioning, by pneumatic cushioning or by fitting shock absorbers. The following discussion will deal mainly with pneumatic cushioning. This manual is intended to serve as an instruction on how to achieve ideal cushioning; in other words, how to optimize the degree of cushioning for a given mass. We hope that readers who use pneumatic cylinders on a practical level in their daily work, as well as those who design machines using them, will find the information useful.
Achieving ideal pneumatic cushioning
What is ideal pneumatic cushioning?
Ideal pneumatic cushioning means that the direction of travel of the piston is the same throughout the entire cushioning sequence and that its velocity is exactly zero when it reaches the end of its travel. The sound of end cover contact is negligible and the total cycle time is minimized. Thus, properly adjusted pneumatic cushioning can have positive effects on the working environment and on the total working cycle time.
Ideal pneumatic cushioning
Assuming that the operating pressure is 6.3 bar and that a cylinder with a specified piston diameter is to cushion a specified mass, the first step is to ensure that the piston velocity corresponds to that specified in the cushioning chart in the main catalogue or when using Bosch Rexroth calculation system. The piston velocity must be adjusted correctly in every case. The best results are obtained by installing throttling no return valves directly in the connection Ports in the cylinder end. This affords a free inlet flow while enabling the outlet pressure to be adjusted simply by altering the area of the exhaust port with an adjusting screw. Alternatively, directional control valves with integral restrictors may be used. Since the velocity of the piston in a cylinder may be difficult to detect by eye, an electronic aid which can be attached to the outside of the cylinder tube is now available and this enables the true velocity of the piston to be determined quickly and easily. It is also possible to measure the time for all sequences in a cylinder cycle. Ideal pneumatic cushioning can only be
DESIGN &CONTR OL PNEUMATIC SYSTEM CHAPTER 2
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achieved when maximum kinetic energy is used. Since Rexroth cylinders with variable cushioning are supplied with the adjusting screw open by only a half to one turn, the cylinder is over damped following adjustment of the piston velocity. Although the basic adjustment does prevent the piston from striking the end cover freely on its first stroke, eliminating the risk of damage, the cushioning is far from ideal. In addition, the extended cushioning sequence may make it difficult to maintain the cycle time. The following diagram illustrates the sequence of events which occurs when the adjusting screw is opened, all other parameters being constant.
Opening the adjusting screw a turn at a time moves the point to the left from the initial point (1), which corresponds to the initial adjuster setting. During the first 2–3 turns, the cushioning sequence is clearly seen to become progressively shorter. However, the end impact becomes greater. The usual reaction is to cease adjustment at this point and to return the adjusting screw towards the original setting to counteract the severe shocks. However, ideal cushioning is achieved by continuing to open the screw a further 1–2 turns, at which point – as the diagram shows – the end impact is minimal, with low noise and vibration levels. If the adjusting screw is now opened further, the end impact will become considerably greater without reducing the cycle time significantly. However, at the ideal pneumatic cushioning point, the total cycle time will be reduced substantially as a result of the much faster cushioning sequence (a 20–40% reduction is not (unusual).
Achieve ideal pneumatic cushioning
At present, the cylinder diameter is often specified on the basis of a specified thrust requirement, without allowing for the kinetic energy. Usually, this produces a cylinder cushioning capacity which is well in excess of that necessary for the application. In such cases, ideal pneumatic cushioning is either not required or is unachievable in every case. If the cushioning energy does not exceed 10% of the permissible value, it is possible to open the adjusting screw sufficiently for the piston to strike the end cover. The shock will be moderate due to the low load, while the impact time will be short and the braking effect of the pneumatic cushioning will be low due to the short time available for back pressure to develop. The following will apply if the cushioning energy is between 10% and 80% of the maximum permissible value: At the lower value, the piston can be cushioned partially with air, with impact cushioning accounting for the remainder of the energy absorption, although a certain shock will naturally occur. In this case, the adjusting screw will be almost completely open and the direction of piston travel will be unchanged. At the upper value, the adjusting screw will be almost completely closed.