Air cylinder deceleration and noise reduction IMPROVING AIR CYLINDER DECELERATION

Transcription

1 Air cylinder deceleration and noise reduction IMPROVING AIR CYLINDER DECELERATION

2 Cylinder Deceleration, Dampening, and Cushioning: How it s achieved and the benefits Introduction Pneumatic Cylinders perform a vast array of functions in many industries. From electronics to automotive to packaging, the performance of a cylinder is an integral component of a cost effective, efficient manufacturing facility. While all cylinders function in a similar manner, there are ways in which cylinders can do their jobs quicker, more efficiently, and quieter. This paper discusses the importance of air cylinder deceleration, and how utilizing better deceleration designs can impact a cylinder s performance, and how this can affect the work place environment and the bottom line. Most cylinder manufacturers have developed a variety of methods to enhance the operation of their cylinder. Primary targets for improvement have been in the areas of deceleration, reduction of vibration (caused by metal to metal bounce) and the reduction of noise caused by vibration. End of Stroke Cushioning One of the primary objectives of cylinder operations is to slow down (decelerate) a cylinder s piston rod as it enters its end of stroke. There are two distinct camps when it comes to deceleration. The first involves a mechanism external to the body of the cylinder known as external cushioning. External cushioning incorporates a shock absorber type mechanism that helps to decelerate the load. The drawback to external cushioning is that it increases the footprint of the cylinder, adds weight, and involves more moving parts. The second mechanism is known as internal cushioning. It incorporates a design that is internal to the cylinder, hence operates within the cylinder footprint, and tends to be simpler in function. We will be discussing internal cushioning in this paper. Before going into how cylinder deceleration is best is achieved it is necessary to understand why we need to decelerate the piston, and the results that take place when the piston is not slowed sufficiently at its end of stroke. The Importance of Cylinder Deceleration Conventionally designed cylinders have a metal piston and metal end caps. Piston deceleration is required as the piston enters end-of-stroke in order to avoid hard metal to metal contact with the end caps. Metal to metal contact manifests itself in two ways: 1) vibration or bouncing caused by the bouncing of the surfaces against each other, and 2) noise, the product of vibration. Vibration, or bouncing, at end of stroke means the piston isn t really stopped, it is still moving. Depending on the severity of contact a piston may bounce (vibrate) 3 to 4 times. Noise is the result of this vibration. In the work place this noise can be at best irritating, at its worst, noise can create an unpleasant and dangerous environment especially if it exceeds OSHA requirements. Conventional Deceleration: The Air Cushion As the piston nears its end of stroke it must be decelerated before reaching end of travel. The most accepted method for deceleration is air cushioning. Air cushioning is the result of the piston rod entering the cylinder s end cap, the air in the cap is squeezed or compressed resulting in a negative force acting upon the piston. Conventional air cylinders rely 100% on the non-linear air cushion to decelerate the load by compressing the gas in the cylinder s controlled volume at the end of stoke. As an air cylinder is traversing through the stroke, a constant velocity is achieved by controlling the exhaust flow out of the cylinder. This exhaust is allowed to flow freely out of the cylinder through the center of the end cap downstream to the flow control. As the cylinder nears the end of stroke, the cushion spear of sleeve enters the cushion seal eliminating the free flowing exhaust path. As the controlled volume decreases without an exhaust path, the gas compresses. The exhaust is then metered out through the cushion needle for desired cushion performance. 1

3 Deceleration Formulas To Decelerate a cylinder, you must generate a negative force. F friction = ma The objective is to decelerate the cylinder to a very low velocity before end of stroke metal to metal impact or to absorb the cylinder s momentum before dissipating the resultant energy with end of stroke metal to metal impact. A typical air cushion provides an elevated backpressure resulting in a negative force component used to decelerate the load. Typically before the cylinder reaches the air cushion, the actuator achieves a constant velocity by achieving equilibrium between the net driving force and the frictional forces. F friction = ma = 0 When the cylinder enters the cushion an elevated backpressure is achieved by compressing the controlled volume of air in that chamber. This elevated backpressure provides a negative force component to decelerate the load. (-F) friction = ma In our lives we decelerate everyday to avoid hard contact, and we also meter our deceleration. Imagine pulling your vehicle into a parking space. As you pull up to the curb you decelerate to avoid hard contact. You use your foot brake to meter how dramatically you slow down. If you don t slow down enough hard contact is experienced. If your deceleration is slow it takes too long to bring your vehicle to a stop. It s a trade off. Not enough deceleration means a hard stop; too much deceleration means it takes too long to stop. Adjustable Air Cushion To control, or meter, the volume of air being released during the compression phase at end of stroke an adjustable cushion is used. This typically consists of a threaded needle screw that adjusts into an orifice in the cylinder end cap. By adjusting the screw further into the orifice you lessen the amount of air that can escape in a given time. Slowing the exhaust of air creates back pressure which slows the piston as it enters the end cap. The slower the air exhausts the more dramatic the deceleration.»the piston is moving from right to left.»exhaust air is venting through the center of the end cap.»the cushion sleeve engages the cushion seal blocking the free flowing exhaust air.»the exhausting air can only escape past the cushion adjustment screw. the exhaust air quickly compressed providing the pressure to decelerate the load.»the screw is adjusted to the application to bring the load to a gentle stop. Illustrations courtesy of Norgren, Inc. The objective of cylinder deceleration is 1) to reduce vibration, and 2) to reduce noise. With the adjustable cushion we can accomplish both objectives...to a point. You may be inclined to say Just increase the time it takes to slow down the piston to the extent that we avoid hard contact, thus eliminating bounce, and bring in the piston so slowly we don t hear any contact. Yes, that will work. But how long does this take? By taking longer to decelerate and stop the piston, you have drastically increased cycle time. And remember: high speed = high cycle rate = $. 2

4 Redefining the Cushion How do we avoid hard contact that results in bouncing, and noise, while at the same time making sure cycle times are reasonable? By redesigning the cushion seal and its attachment to the piston, deceleration problems can not only be remedied, the redesigned cushion seal can provide additional benefits to cylinder operation. Piston seals that extend beyond the face of the piston have proven very effective in cylinder deceleration and at the same time increase cylinder performance. When the piston nears end of stroke the rubber seal contacts the cylinder cap rather than the metal face of the piston. Hard metal to metal contact is avoided. These extended seals, usually comprised of nitrile based rubber pressed fit into a machined grove on the piston, absorb 80% of the energy as the cylinder completes its stroke. This reduces the pneumatic bounce. This large amplitude high frequency end of stroke impact vibration is not only destructive to the cylinder but also to the surrounding equipment. By eliminating this destructive vibration the life expectancy of equipment can also be extended. This photo shows an extended type piston seal. As the piston travels to the point of impact with the end caps, the compressive qualities of the seal provide the final decelerating force. This action compresses the seal and absorbs the remaining kinetic shock vibration and noise created by the impact. Reduced Vibration Photo of Norgren s EJ Ecology Seal. Photo courtesy of Norgren, Inc. The charts below compare different types of cushioning and illustrate the advantages the extended seals have in reducing vibration at cylinder end of stroke, and also decreasing cushioning time. Extended type cushion with Non-Adjustable Cushions 2" Bore Rod End Cushion Test Average deceleration force = 15 G's Time consumed during cushioning = sec. Number of bounces: 1 Pneumatic 1 Metallic Extended type cushion with Adjustable Cushions 2" Bore Rod End Cushion Test Average deceleration force = 20 G's Time consumed during cushioning = sec. Number of bounces: 1 /2 Pneumatic 0 Metallic Conventional cushion with Adjustable Cushions 2" Bore Rod End Cushion Test Average deceleration force = 78 G's Time consumed during cushioning = sec. Number of bounces: 2 Pneumatic 4 Metallic 2.5 lbs. 2" Bore Cap End Cushion Test Average deceleration force = 17.5 G's Time consumed during cushioning = sec. Number of bounces: 1 Pneumatic 1 Metallic 2" Bore Cap End Cushion Test Average deceleration force = 10 G's Time consumed during cushioning = sec. Number of bounces: 1 /2 Pneumatic 0 Metallic 2" Bore Cap End Cushion Test Average deceleration force = 60 G's Time consumed during cushioning = sec. Number of bounces: 3 Pneumatic 4 Metallic =.02 seconds 2.5 lbs. added to rod 3 Tests courtesy of the Milwaukee School of Engineering

5 Reduced Noise And what about the noise caused by the vibration? The high frequency metal to metal bounces demonstrated in the previous graphs highlight the vibration caused by a traditionally cushioned cylinder in a high speed application. We can analyze the vibration generated by the cylinder by measuring the noise level generated. End of stroke vibration generates extremely high noise levels in a typical work area. The table below summarizes sounds levels generated as by a conventional seal compared to an extended type seal. Sound levels are measured in Decibels (db) on a logarithmic scale, so intensities are actually reduced multiple times. Typical noise levels Rock concert 110dB Jackhammer 90 db Conversation 60dB OSHA Permissible Noise Exposures Duration per day Sound level dba (hours) slow response / / /4 or less 115 Summary of Sound Levels in Decibels PSI Air Sound Cushion type Pressure Conventional Extended Conventional Extended Level+ 5" x 6" 5" x 6" 2" x 6" 2" x 6" 95 End PSI+ Side End PSI+ Side Peak sound pressure is given in decibels (db) re:2 x 10 5 N/m 2. ++End position of mike was 3' on centerline from end of cylinder; side position of mike was 3' perpendicular to centerline abeam of end of cylinder. Note: At 5 feet, cylinder sound levels would be less by 9 db from side figure and 13 db from end figure. The total noise emitted will depend on the structure to which the cylinder is attached. If it is mounted on a thin flat plate of considerable area, the noise will be increased by a sounding board effect. Higher Cycle Rates In addition to reducing the noise caused by vibration there is further benefit: With the seal absorbing 80% of the energy the desired cushion orifice can be much larger thus allowing the cylinder to travel through the cushion in one-forth the time of a conventionally cushioned cylinder. This reduces in-cushion time by 75%. Add to this the fact that the extended type seal piston accelerates out of cushion at a higher rate. This is accomplished in two ways. First, the cushion orifice can be much larger for the desired end of stroke performance, and this larger orifice provides a larger bleed orifice to allow air into the cylinder at a faster rate while exiting the air cushion. Second, the seal acts as a compressed spring and provides an initial force equivalent to 80 psi to push or accelerate the cylinder into motion. Summary We have demonstrated the advantages of cylinders utilizing an extended type cushion. They can reduce wear, vibration, noise, and cycle times. If you answer YES to any of the question below, you will want to make sure the cylinders you specify on your next application are equipped with extended type seals. Are you concerned with vibration? Are you concerned with noise? Are you concerned with decelerating a load? Do you require a cushioned cylinder and do not want to sacrifice speed? 4

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