Pneumatics 102

Last month I had to leave your pant legger with the head down on a pair of “alleged” Armani trousers. Well let’s continue finishing them observing the behavior of the pneumatic system components as the pressing cycle proceeds.

Most automatic pant leggers have the following sequence of operations carried out right after the head closes on the buck of the press and the HIGH PRESSURE OR SQUEEZE CYLINDER is activated (see last month’s article for more information on the squeeze cylinder). Each manufacturer recommends different lengths of time for each of the following finishing cycle operations but the order of the operations are similar.

1. A shot of head steam.

2. A short pause to allow steam to condition the trousers.

3. Open the vacuum valve to begin to dry the trousers, the first vacuum.

4. Shut the vacuum valve before the press head opens.

5. Open the head of the press.

6. Open the vacuum valve for the final drying of the trousers, the second vacuum.

7. Shut the vacuum valve.

Let’s refresh our memories by paraphrasing a portion of last month’s article. The magnetic sensing switch attached to the side of the head closing cylinder has sensed the piston inside the head closing cylinder has reached the head closed position and an electrical signal is sent to the control board. After receiving this signal the control board sends a signal to a solenoid valve controlling the squeeze cylinder. This solenoid opens and allows compressed air to enter an adjustable regulator then to the squeeze cylinder. This applies the desired head pressure to the garment on the press. After a momentary delay an electronic SEQUENCING TIMER (a sequencing timer is a device that goes through a series of operations or events one after another like links following each other on a chain) is turned on.

1. A shot of head steam- This sequencing timer first activates a THREE WAY AIR SOLENOID VALVE (a three way electrically activated valve switches compressed air into a cylinder via the ACTIVATION PATH, or allows compressed air to leave the cylinder via the EXHAUST PATH) which allows compressed air to flow into a small 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) called the head steam actuating cylinder. This causes the cylinder piston to extend and push on a lever (this is a lever activated type of mechanical action) attached to the handle of the head steam valve, forcing it to open allowing steam to escape and condition the trouser leg. After reaching the end of the time set for head steam the three way air solenoid valve shuts off stopping the compressed air supply to the head steam actuating cylinder and provides an exhaust path from the cylinder. As the compressed air leaves the cylinder a spring inside the cylinder is allowed to expand forcing the piston back into the cylinder. When the end of the piston retracts from the lever on the head steam valve handle a spring in the head steam valve assembly pushes the head steam valve shut which stops the escape of steam.

2. A short pause- The next operation of the sequencing timer is simply a pause in which nothing new happens. This allows the steam to penetrate and condition the trouser leg.

3. Open the vacuum valve- After the pause the sequencing timer activates a three way air solenoid valve which allows compressed air to flow into a single acting cylinder called the vacuum valve actuating cylinder. Inside this cylinder a spring pushes against a piston keeping the piston extended. Attached to the end of the piston is a disk which blocks the connection between the pipes going to the air vacuum and the buck of the press, similar to a cork in a bottle (this is a close coupled type of mechanical action). When the compressed air enters the cylinder it pushes the piston back, compressing the spring and opening the connection between the pipe going to the air vacuum and the buck of the press, thereby opening the vacuum valve and allowing room air to be pulled through the trouser leg and into the buck of the legger. This dries the fabric and sets the crease.

4. Shut the vacuum valve before the press head opens- In order to prevent the opening press head from lifting the trouser leg off the buck of the legger, the vacuum valve must shut before the press head moves open. This is done by de-energizing the three way air solenoid valve shutting off the supply of compressed air (activation path) to the vacuum valve actuating cylinder and setting up a path for the compressed air to exit from the vacuum valve actuating cylinder (exhaust path). As the compressed air leaves the cylinder the spring is allowed to expand forcing the piston in the direction which closes the vacuum valve, forcing the cork back in the bottle. Keep in mind when dealing with cylinders that it is just as important to provide for compressed air flow out through an exhaust path as it is to provide an activation path allowing air into the cylinder.

5. Open the head of the press- In order to open the press head first the high pressure or squeeze cylinder must be exhausted and then the piston in the head closing cylinder must be contracted to lift the head off of the buck. The solenoid valve supplying compressed air to head pressure regulator is de-energized allowing an exhaust path from the squeeze cylinder. Since the squeeze cylinder is usually very large, a separate valve called a QUICK EXHAUST VALVE (a valve that contains a diaphragm or disk that closes a large diameter port to the atmosphere when compressed air is supplied to an attached cylinder, and allows that port to open when the supply of compressed air is interrupted. This allows the rapid de-pressurization of the cylinder. A muffler is usually installed in the port to the atmosphere in order to silence the explosive discharge of compressed air). Next the 4 way solenoid valve controlling the head closing cylinder switches the activation and exhaust paths to the head closing cylinder contracting the piston into the cylinder and lifting the head off of the buck.

Steps 6 and 7 are just duplicates of steps 3 and 4.

Well folks, there you have it. A complete legger pant pressing cycle. Hope you enjoyed the journey. That’s it for this month; next month tell me what you would like explained in the realm of machine repair, operation or control. Are you having a problem that we can troubleshoot and solve in an article for next month? Address your requests to or call me at (702) 376-6693, it will turn out to be fun, as well as a useful educational exercise.


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 Please address any questions or comments for Bruce to or call (702) 376-6693.