Pneumatic cylinders normally have some form of cushioning to slow the piston down as it approaches the end of its stroke, so that it doesn’t impact on the end cap. This enables the cylinder to operate at higher speed without excessive stress, vibration or noise. Normally, valves are adjusted for cylinder speed and load but new cylinders are now available which adapt automatically, avoiding damage due to errors and reducing setup time.
To achieve the greatest productivity, or machine output, cylinders must be operated at the highest speed possible without causing faults. If a machine breaks down, then production is stopped while maintenance takes place, seriously impacting productivity. Cylinders are, therefore, required to operate at velocity profiles which minimize cycle time, while keeping stress and vibration within safe limits. This generally means that a cylinder should operate at its maximum velocity for as much of its stroke as possible, with constant acceleration and deceleration at each end of the stroke. The deceleration should ensure that the cylinder reaches zero velocity at the point of contact with the end cap, preventing any impact loading at the end of the stroke. The constant rate of acceleration should result in forces at the safe working load for the cylinder, according to Newton’s law (F=ma). This perfect acceleration profile depends on the safe working load for the cylinder, the maximum velocity achievable and the load conditions. The safe working load may be considered as a fixed parameter or as a function of the required duty cycles.
The simplest forms of cylinder cushioning are steel springs or rubber bumpers within the cylinder. These cannot be tuned to achieve an optimum velocity profile. A better approach is to restrict the flow of air as the cylinder approaches the end cap, effectively turning the cylinder into an air spring. This flow restriction is caused by the piston moving past ports in the cylinder. The standard approach is to port the air through an adjustable valve so that the degree of flow restriction can be adjusted to suit the operating conditions. Throttling non-return valves can be fitted into the end ports of a standard cylinder so that the inlet flow is unrestricted but the outlet pressure can be controlled. Each cylinder typically requires five minutes of adjustment during installation and setup. For machines with large number of cylinders this can add up to a significant amount of time. The use of adjustable valves is time consuming but perhaps more importantly it is prone to human error. If a mistake is made it could overload a machine, potentially resulting in unplanned maintenance. Each time the operating conditions change, the laborious and error prone adjustment must be repeated. If the conditions are constantly changing it becomes impossible to operate the cylinder at an optimum speed.
Camozzi’s Series 23 pneumatic cylinders use a new ‘auto-cushioning’ system which automatically adapts to changing conditions, without requiring any manual adjustment. It still uses outlet flow restriction to cause a back pressure which cushions the cylinder, however, instead of adjustable valves, the flow is restricted by a series of holes in sleeves which extend from the piston. In the middle of the cylinder’s stroke, these sleeves present no restriction to the exhaust flow. At the piston approaches the end cap, these cylinders slide into a hole in the end cap, forcing the air to flow through the holes in the cylinder. Initially, many large holes are available which will only cause a noticeable restriction at very high velocities. As the cylinder gets closer to the end of its stroke, the holes are closed off one-by-one, causing a progressively greater restriction to flow. Essentially the same method is used to achieve automatic cushioning in Festo’s PPS cylinders. Festo cylinders use grooves of different lengths instead of holes in different positions, but the principle is exactly the same.
What’s clever about this approach is that the faster the cylinder is moving, the earlier in the stroke cushioning begins. At very low speeds the cushioning occurs very close to the end cap. This simple automatic adjustment ensures that the correct cushioning is always applied with no need for manual adjustment and no possibility of human error overloading the cylinder. The result is increased service life and a reduction in unplanned maintenance.
“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 minimized.” Lee Hargrave, Camozzi Automation Ltd.
There is also a trend for hydraulic cylinders to use non-adjustable systems, primarily to avoid the risk of impacts due to cylinder cushioning being set incorrectly. Due to the very high forces that hydraulic cylinders can generate, this is of particular concern. Large masses can be quickly accelerated to high velocity meaning that impacts may cause considerable damage. For example, AHP Merkle have replaced adjustable valves with fixed damping which is able to achieve almost constant deceleration, or a linear velocity profile.
Adaptive cushioning has many advantages in terms of reduced setup and maintenance. However, it cannot achieve an ideal velocity profile for all operating conditions. The optimum cushioning depends on maximum velocity and mass. These systems primarily adapt to the cylinder velocity. There remains a compromise between a failsafe and low maintenance system and an adjustable cylinder which can be fully optimized.