Now that we’ve digested some basic pneumatic concepts we’ll put the jigsaw together and do some real work. How’s about pressing a pair of pants on an air operated legger? There are several different manufacturers of pressing equipment, and in our industry as many schemes which combine pneumatic components in different ways as there are manufacturers; probably more. As we get deeper into the enchanted forest of pneumatic choices; just like seeing photo of your brother-in-law on the post office wall, you’ll want to run home and tell your wife. You’ll be delighted as well, recognizing some of the methods and components you last remember seeing during a benighted attempt to repair your very own equipment.
Control The Flow Of Compressed Air Into
This is typically done with shut-off valve of some sort. Most common is the ubiquitous ball valve. If this valve is not ubiquitous it certainly should be (just messin’ with ya, look it up). Ball valves used in pneumatic applications are cheap, reliable and whether they are open or closed can be determined at a glance.
Condition The Incoming Air
On almost all pneumatically operated equipment a series of devices are used to clean and maintain the pressure of the air at a level compatible with equipment design. AIR FILTERS which remove particles of dirt and debris for the air stream and are also moderately effective in capturing condensed water and nasty oil from the incoming air stream. Normally, air enters the machine at a pressure at least 10 psig. higher than the pressure at which the machine is designed to operate. The job of reducing the incoming air pressure falls to a device called a PRESSURE REGULATOR. Important, since many of the “down stream” devices such as spool valves will operate properly only within a specified range of air pressure. Since the requirements for both an AIR FILTER and a PRESSURE REGULATOR are nearly universal, some smarty pants decided to build them both into a single unit surprisingly called a “FILTER REGULATOR” or if you want to sound hip while schmoozing with engineers an “F/R UNIT.”
Let’s Make This Puppy Do Something!
In the following example I’ll cobble together, “mixing and matching” methods and components used by different press manufacturers in order to present a wide variety of control schemes. The press head is open and the operator has placed one leg of the trousers on the buck of the press. In order to position the fabric correctly the operator steps on the vacuum pedal. The pedal is like a see-saw and the opposite end of the pedal rocks upward and presses on the STEM (a valve stem is the small movable projection that works a valve) of a THREE WAY VALVE (a three way valve switches compressed air into a cylinder, the ACTIVATION PATH or allows compressed air to leave the EXHAUST PATH, the cylinder. This SINGLE ACTING CYLINDER (a single acting cylinder has only one side which can be pressurized to move the piston and relies on some force other than compressed air to move the piston in the opposite direction) is directly connected to the vacuum valve on the buck of the press. Inside this cylinder is a spring pushing against a piston. This spring pressure keeps the vacuum valve closed. When the air enters the cylinder it pushes the piston back, compressing the spring opening the vacuum valve. When the operator steps off of the vacuum pedal the valve stem returns to the rest position closing the path for compressed air into the cylinder and opening a path which allows compressed air to leave, exhausting the cylinder. As the compressed air leaves the cylinder the spring is allowed to expand forcing the piston in the direction which closes the vacuum valve. Keep in mind when dealing with cylinders that it is just as important to provide for compressed air flow through an exhaust path out of the cylinder as it is to provide an activation path allowing air into the cylinder. On larger cylinders there is usually a muffler to reduce the explosive sound of the exhausting air.
Now that the trouser leg is properly dressed on the buck (instead of a mechanical we’ll now use an electrical/electronic control scheme) the operator presses the two hand switches on the press table in order to close the head (measure and compare, see last month’s article). These hand operated electrical switches send signals to the electronic control board which detects and evaluates the timing of the switch changes, determining whether the signals are valid (both of the operator’s hands are on the switches at the same time), (compute, see last month’s article), this is an OSHA required feature. If the signals are valid the control board activates the COIL of a 4 WAY SOLENOID VALVE (correct, see last month’s article).
A moment about coils and solenoid valves. A solenoid valve is a device which takes an electrical signal and converts that signal to some mechanical action. The coil of the solenoid valve is the electrical part that initiates the process, similar to a trigger on a gun. You’ll recall the three way type valve was used to switch compressed air into and allow air to exhaust a single acting cylinder. A four-way valve is used with a DOUBLE ACTING CYLINDER. Double acting cylinders are pressurized from either end depending on which end of the cylinder you want the piston to move towards (whether you want the cylinder to expand or contract). The four-way solenoid valve switches the activation and exhaust paths from one end of the cylinder to the other thus providing a power stroke in either direction of motion.
Ok, the four-way solenoid valve has been activated and compressed air path is switched to the lower end of the head closing cylinder, at the same time an exhaust path is switched to the upper end of the cylinder. The cylinder expands and starts to push the head closed. Now the head has some weight to it and it would be handy to be able to control the speed of that cylinders motion. This is easily accomplished by placing what is called a FLOW-CHECK VALVE in the compressed air path that expands the cylinder. A flow check valve is a valve that allows compressed air to flow unrestricted (full flow) in one direction through the valve and have the flow adjustable in the other direction through the valve. So the flow-check valve is placed with the adjustable direction of air flow into the lower end of the cylinder. The head is now closing at a controlled rate but when it contacts the garment it would be
useful to be able to control the pressure that the head exerts on the garment.
In order to do this many press manufacturers use a second cylinder called a “HIGH PRESSURE OR SQUEEZE CYLINDER” which is supplied with it’s own adjustable air pressure regulator and 4 way solenoid valve. This regulator is generally located on the table of the press and is easily set by the operator. You may well ask how the press knows when to switch to the squeeze cylinder. This is accomplished by sensing when the head closing cylinder has reached the right position. There are several methods of sensing head closing cylinder position (pneumatic or electrical limit switch, air port on the wall of the head closing cylinder). The method I’m going to use is a magnetic sensing switch attached to the side of the head closing cylinder. When the piston inside the head closing cylinder reaches a certain point a magnet attached to the piston inside the cylinder is sensed by the magnetic sensing switch outside the cylinder and an electrical signal is sent to the control board (this is a perfect example of feedback, see last month’s article). After receiving this signal the control board sends a signal to a 4 way solenoid valve controlling the squeeze cylinder by sending air from the adjustable regulator to the lower end of the squeeze cylinder thereby applying the desired pressure to the garment on the press.
Because of space constraints I’m going to leave you for a month with the head down on the trousers. The customer will no doubt claim it was half an Armani suit. Don’t despair, next month we’ll open the press head and you’ll learn some more neat pneumatic nuances…say that three times fast!
Bruce Grossman is the Chief of R&D for EZtimers Manufacturing. EZtimers is the manufacturer of the popular TATTLER steam trap tester and the SAHARA line of high purity separator water mister/evaporators. For further information on the EZtimers product line visit www.eztimers.com Please address any questions or comments for Bruce to firstname.lastname@example.org or call (702) 376-6693.