Engineers are often tasked with increasing the productivity of a machine while reducing costs. An important requirement of any machine design is to achieve the fastest possible cycle times. When the machine design utilises pneumatics, this often leads to cylinders and actuators being run as fast as possible without introducing excessive shock loads into the system.
One of the main functions of a pneumatic cylinder is to decelerate the piston of the cylinder at the end of the stroke. Manufacturers have developed various types of cushioning both internally to the cylinder and also externally.
Why Use Cushioning On A Pneumatic Cylinder?
The general design of a pneumatic cylinder consists of a cylinder body, piston rod, piston end caps. The piston can be constructed of either a disc or cylinder when compressed air is applied to the cylinder port; the piston transfers this force into a linear movement.
Insert pneumatic cylinder video/ image
Piston end caps are generally constructed with metal end caps; piston deceleration is required as the piston reaches its end of the stroke to avoid the high impact resulting from the metal piston hitting the metal end cap. A pneumatic cylinder with no cushioning would result in:
- Increased wear on the contact surface.
- Vibration or bouncing- caused by the bouncing of the piston and end cap impact, any bouncing at the end of the stroke means that the piston hasn’t stopped.
- Noise can be a result of the impact of the piston and end cap, noise can be unpleasant for operators working nearby, in severe cases noise could be a health and safety hazard.
What Types Of Cushioning Are Available?
Manufacturers have developed various methods to reduce the impact of the piston and end cap impact. Dependent upon the application and type of cylinder being used there may be some different types of cushioning available. The most common types of cushioning include external cushioning, mechanical cushioning, adjustable pneumatic cushioning and the relatively new type of self-adjusting cushioning.
Any mechanism that is applied external to the cylinder body is known as external cushioning. The most common form of external cushioning is a shock absorber; the shock absorber is used to decelerate the load. External cushioning can an effective form of cushioning. However, it does increase the footprint of the cylinder and adds additional weight to the design. As the shock absorber is also a moving part, it also introduces an additional hazard to the system.
In their most basic form, shock absorbers convert the kinetic energy of a load into heat that is dissipated into the atmosphere. Shock absorbers stop moving loads with no rebound and without transmitting potentially damaging shocks and vibration to the equipment.
Designs of shock absorbers do vary, however, in general, they consist of a cylinder body, a piston and a means for returning the piston to its initial position such as a spring. The mechanical spring can be integrated into the design of the cylinder body or mounted externally.
Shock absorbers are available with fixed cushioning, adjustable cushioning and self-adjusting cushioning. Fixed cushioning is the most economical form of cushioning, designed for predetermined loads and speed and cannot be changed to meet changing conditions. Adjustable shock absorbers can accommodate varying loads and are adjusted via an adjusting screw on the outside. Self-adjusting cushioning take the benefits of easy setup and installation of a shock absorber with the performance of an adjustable shock absorber.
Selecting a shock absorber requires the design engineer to consider some factors; the most important factor is the type of load that is to be stopped. Load types include; pure inertial, free-falling, rotating and loads with propelling force such as a pneumatic cylinder.
Once the load has been established the load weight, and velocity is the next factors. For some applications the number of impacts per day, ambient conditions and the potential shock to equipment.
Mechanical bumpers or mechanical cushioning use a flexible material often an elastomer to reduce the impact load. The elastomer material can be incorporated into part of the piston or the end cap of the cylinder.
Mechanical cushioning is effective at reducing impact noise and small amounts of energy. Mechanical cushioning cylinders are limited for applications with slow speeds, small loads or applications with short stroke lengths.
Adjustable Pneumatic Cushioning
To overcome the limitations of mechanical cushioning pneumatic suppliers introduced adjustable air cushioning. Adjustable air cushioning limits the volume of air released at the very end of the stroke. Adjustable cushioning construction consists of a variable orifice, a ‘spud’ on either side of the cylinder piston closes off the air flow to the main piston chamber, this traps air in the cylinder end cap which is bleed off through a small passage controlled by a needle valve.
During each cycle, the cylinder traps a fixed volume of air, however as air is highly compressible, the load, velocity and air pressure contained in the cylinder all impact upon the effectiveness of the air cushion performance. For any application, there is usually only a small window of adjustment that will result in optimal cushioning performance. Once the needle-valve is set, any change in the parameters; change in weight, pressure or velocity affects the cushion reaction, requiring the cylinder to be readjusted.
What is the problem with improperly adjusted air cushioning?
Cylinders that have been incorrectly adjusted or not adjusted at all can have detrimental effects on the lifetime of both the cylinder and workpieces.
Cylinders that have not been adjusted enough (under damping) result in high-end position impacts, the trapped air within the cushioning does not create enough back pressure to act effectively. Under-damping increases the wear on the cylinder due to the increased impacts and cause vibration that could affect other components. Cylinders with under-damping also contribute to increased noise for operators.
For cylinders have too much adjustment (over-damping) result in a high initial reaction at the end of the stroke which slows the load too quickly. Over-damping can sometimes lead to end of stroke bounce, where the piston changes direction multiple times in the deceleration time, resulting in vibration and increased machine cycle times.
Self-adjusting cushioning is a relatively new design of cushioning that takes all the advantages of adjustable pneumatic cushioning but eliminates the need for manual adjustment. The operation of self-adjusting cushioning is the same as adjustable cushioning. However, the cushioning chamber is not exhausted via an adjustable cross-section but via notches in the cushioning piston.
Slots running the length of the cushion piston make it possible to exhaust the air cushion independent of the cushioning length, the cleverness of this design is due to the geometry of these notches. The patented air channels provide phased venting of the cushioning air; the system automatically adapts its characteristics to suit the cylinder's load and speed to provide optimum end-position cushioning.
As the exhaust flow of the cushioning chamber changes over the damping stroke, self-adjusting cushioning works with the most common speed and mass combinations making it suitable for most applications.
Benefits Of Self-Adjusting Cushioning
The innovative self-adjusting cushioning has many advantages over traditional methods of pneumatic cylinder cushioning; without any need for manual adjustment self-adjusting cushioning ensures the optimum cushioning action every time, even if the pressure and friction affecting the cylinder change.
Self-adjusting cushioning also reduces acceleration forces acting on the cylinder and workpieces, this minimises the vibration and extends the life of the cylinder.
Traditionally adjusting the cushioning on a pneumatic cylinder requires two people; one to adjust the dampening characteristics and one to operate the valve controlling the cylinder. Based on tests carried out self-adjusting cushioning will save engineers five mins installation and set-up time per cylinder, a typical package sorting system with sixty driven cylinders would save five hours of installation/commissioning time.
With no settings to change self-adjusting cushioning is truly a fit and forget solution, it is also tamper-proof!
|Accessibility||For cylinders that are located in inaccessible locations, once installed would not have to be accessed, constant adjustment of adjusting cushioning is sometimes a problem.|
|Consistent Cushioning Quality (Safety)||PPS Self-adjusting cushioning always deliver the same optimum performance, this cannot be said for adjustable cushioning as different people can adjust the cylinders or cylinders are not adjusted at all.|
|Tamper Proof||Operators cannot change the cushioning action that could lead to malfunctions.|
|Long Service Life||Not only for the cylinder but the entire installation.|
|Optimum Cushioning||PPS Self-adjusting cushioning provides optimum cushioning even with varying mass/pressure within the permissible range of the cylinder.|
Self-adjusting cushioning cannot be used on applications where the cylinder is being operated at its limits; heavy loads or cylinders with very high or very low speeds.
What Cylinders Are Available With Self-Adjusting Cushioning?
The innovative self-adjusting cushioning is a unique design to Festo and not offered by any other supplier. Self-adjusting cushioning is identified as PPS cushioning in the Festo type codes and is available on a number of cylinders currently, with more ranges being added.