pneumatic Accesories

[ ALLABOUTAIR] Thomas W. Kreher | Applied Pneumatic Controls, Inc. :FPJS/O06///VOL.13///ISS.8 PNEUMATIC APPLICATION ACCESSORIES VERY FEW PNEUMATIC APPLICATIONS ARE READY TO PERFORM AS DESIRED WITHOUT ACCESSORY ITEMS TO FINE TUNE, AUGMENT AND PROTECT COMPONENTS. Tubing I am a fan of Nylon or Polyurethane plastic tubing and “Push-IN” or “One-Touch” fittings to connect the valves and actuators, etc. The tubing is available in about a dozen colors so that lines may be color coded. Each line may be cut to fit with a tube cutter, knife, scissors, etc. With a little effort to keep the cut ends fairly square, just push the tube firmly into the fitting and you are connected for modest vacuum to approximately 150 PSIG in most cases. The fittings seal on the OD of the tubing with an O-ring that may be replaced. They attach over the tubing and do not restrict the full flow as do bibs, barbs and fittings that insert into the tubing. The tubing is flexible with a modest bend radius and may be bundled, tied, held in tube, etc. for neatness and function. The push-in fitting has no loose parts, is re-usable and is easily disconnected. This is a blessing when the cylinder lines need to be reversed. Some applications require copper or stainless steel tubing. For these I suggest you get out the tube bender and do the best you can. Other applications may a lot of sense with hoses require specific materials for considered when selecting and flexible lines. Safety chemical compatibility. All a hose size. Hoses come in glasses wouldn’t stay on I can suggest is a positive natural rubber, synthetics, long with several pounds of attitude and caution when wire reinforced…you name it. hose whipping everything male metal fittings are Air Line Fuse within reach. The air fuse is screwed into a female plastic A device that will shut off flow designed to slow or close off port. The taper of NPT threads if an air line breaks makes the source of fluid energy may create hoop stress in the softer plastic. ISO G threads with Couplers an O-ring face seal The ubiquitous hose is often attached with a plug that mates a would be better. coupler located in the air supply manifold or air distribution line If the thread form for that area. When disconnected, the plug allows compressed air is compatible, within the hose to vent to atmosphere. Couplers typically shut off try fittings in an when disconnected. As you know, the smallest hole or passage acceptable plastic in the air line controls the total flow. SWAG—the entire industry of material with a North America would be 10% more productive if all coupler and plug male thread. assemblies, including hoses, were replaced with one size larger. If compressed air “don’t get no respect,” the size of the coupler and Hose plug that control the amount of compressed air available are seldom (If you drag the even considered. air line on the floor, try hose.) For air tools and devices that are moved frequently, rubber or synthetic hose is tough, flexible and handles abrasion quite well. Nylon tubing is specified by the OD size; hose size is designated by the Inside Diameter (ID). The fittings will have an insert inside the hose that reduces the flow, which should be (Warning! The following paragraph contains adult content.) A couple years ago the marketing consultant hired by our coupler supply company asked about ideas to increase sales. This particular brand arguably lasts longer and leaks less. I suggested that to glamorize his product, he might make either prophylactics or balloons with their company name, logo, etc. available. A salesperson could slip one over each coupler they see in any given plant or ask maintenance to unplug hoses and slip the prophylactic over each coupler on Friday night. By Monday morning, the couplers that leaked would either have an enlarged balloon or it may be broken or have blown off. This would identify the couplers that should be replaced to stop the air loss. In the message sent to the consultant I suggested this would identify couplers that should be replaced, have a little fun and raise the status of the lowly coupler from a utilitarian device to sex object. I gave my self mental pats on the back for unusual inspiration. The marketing Guru who had asked for ideas stopped all communication and I never heard from him again. I offer this idea to you. Go have a little fun. 6 | September/October 2006 www.ifps.org | www.fluidpowerjournal.com FPJSO06_01_20.indd 6 9/1/06 5:51:14 PM [ ALLABOUTAIR] pressure, especially in larger lines, consider a “swing check” that may be mounted to permit gravity to close it. when the force of excess flow drives the sensing mechanism forward to close. Check Valve One way, stop valve and many other colorful names are used to designate this humble device that allows air to flow one way but not return. The check valve is the fluid flow equivalent of an electrical diode that allows free flow of current in one direction only. This handy device is used in dozens of different ways. For example, a check valve used with a reservoir will allow free flow into the tank. During times when the pressure is low, the check valve keeps the higher pressure within the tank from bleeding back into the system. The most common pneumatic check valves have a spring with light force to close the valve. This establishes a “cracking pressure” to open. Usually the manufacturers of the check valves strive to keep the cracking pressure low (one to two PSIG). Like a relief valve, the cracking pressure will cause an equivalent pressure drop. With 100 PSIG into the check valve with a 2 PSIG cracking pressure, the resultant pressure down stream or past the check valve will be 98 PSIG or slightly less. This is interesting and useful and some manufacturers offer various levels of cracking pressure. If we put a check valve with a 10 PSIG cracking pressure in one outlet of a Tee or Wye and supply the third port with 100 PSIG, our two outlet lines will show approximately 100 PSIG and 90 PSIG respectively. A constant Delta P of 10 PSIG. You may also do this with an inline relief valve and readily adjust the differential pressure. For lower cracking Shuttle Valve For various purposes, more than one input or supply may be required or desired. The simple shuttle valve is a selector/isolator and provides a logical OR function. Pressure applied to one port OR the other OR both will produce output pressure or flow. The inlet flow paths are approximately equal. If two inputs of different pressure are applied simultaneously, the higher pressure will drive the shuttle to close off the lower pressure and flow through. With a shuttle valve, I may latch a valve on. One signal may pilot the valve. When operated, the output from the valve itself can be connected to the pilot shuttle valve to hold it on. To reset, the pressure must be removed from both the pilot signal and the valve that is operated. A shuttle valve typically shifts the shuttle away from the inlet pressure to block the other inlet and flow through. If the pressure is exhausted, it will follow the inlet path back to exhaust at the source. Circle 104 Quick Exhaust Valve In forward flow, from say 100 PSIG through the valve to extend a cylinder or fill a volume, the air is forced from a constant source. When the exhaust path is opened to atmosphere, the 100 PSIG initial pressure in the volume decays quickly and 63% of the initial pressure in the volume is lost in the first time constant. The volume will not be entirely vented until after five time constants. Obviously Continued on page 9 Circle 154 September/October 2006 | 7 FPJSO06_01_20.indd 7 9/1/06 5:51:24 PM