Pneumatic Systems (Part 2) Muhajir Ab. Rahim School of Mechatronic Engineering

Pneumatic Circuits Before pneumatic circuits are covered it may be beneficial to refresh of some common component applications. Flow controls Directional control valves

Flow Controls Variable restrictors — These sometimes are used to control the volume of airflow discharged through a needle cooler tube, thread blower tube or some type of material guidance blower. It is important to remember that a variable restrictor will restrict the airflow equally in both directions.   Variable restrictor with a check valve, also called a "speed control" — These are sometimes used to control the speed of an air cylinder. A speed control will restrict the airflow in one direction but allow the air to flow freely in the other direction.

Directional Control Valves 2-position, 2-way—Used primarily to activate needle coolers, thread blowers, etc.   2-position, 3-way — Used to activate single-acting air cylinders or for any component with one air connection that must be allowed to exhaust the air so it can return to its original position.   2-position, 4-way — Activates double-acting air cylinders or any component with two air connections. A 4-way valve will connect pressurized air to one end, and allow exhaust air to escape from the other end of a component in one position, but in the other position it will reverse the pressurized and exhaust connections.

Figure 1 shows a circuit that could be used to raise a presser foot for lockstitch machine. This circuit consists of: A filter separator to remove contaminants and water from the air. A regulator to reduce the air pressure to the correct pressure.   A lubricator could be used at this point, however, since the control valve is the knee operated type, the exhaust air would contain some oil which could stain the operator's clothing. For this reason lubricators are sometimes left out of this type of circuit.   A 2-position, 3-way manually operated directional control valve.  This must by a 3-way valve so that the exhaust air from the air cylinder can escape, allowing the presser foot to drop.   A single-acting air cylinder.  Since this cylinder is lifting a presser foot it is not necessary to have a spring-return cylinder as the presser foot spring will return the air cylinder as it pushes the foot back down.

Next slides, to show a circuit that could be used to control the speed of a single acting air cylinder in one direction. This circuit consists of: A 2-position, 3-way solenoid activated directional control valve, a speed control, and a single-acting, spring-return air cylinder.

In Figure 3A the solenoid valve has been activated In Figure 3A the solenoid valve has been activated. This allows the air to flow through the valve to speed control #1.  Because of the orientation of the speed control the air entering will push the check ball off of its seat, allowing the air to flow through freely. The air then flows to the air cylinder causing the air cylinder to extend with full force.

In Figure 3B the solenoid valve has been deactivated In Figure 3B the solenoid valve has been deactivated. This removes the high pressure air from the circuit and allows the spring in the air cylinder to retract the cylinder rod. As the spring retracts the cylinder it pushes the air out of the cylinder and to speed control #1. Since the air is now entering the right side it will push the check ball into its  seat, blocking this passage for airflow.

This means that all of the air being exhausted out of the cylinder must flow through the restrictor. By adjusting the restrictor we can  control the speed of the air cylinder as it retracts. From speed control #1 the air goes to the directional control valve and is exhausted out the exhaust port. Whenever possible the speed of an air cylinder should be controlled by controlling the exhaust air from the air cylinder. This, however, is not always possible. In Figure 3A and 3B controlling the exhaust air will only control the speed of the air cylinder while it is retracting. In order to control the speed of this air cylinder as it extends we must also control the air going into the cylinder.

Figure 4A and 4B illustrate a circuit for controlling the speed of a single-acting air cylinder in both directions. In Figure 4A the directional control valve is activated which allows the air to flow to speed control #1. Since the air entering pushes the check ball off of its seat the air is allowed to flow freely through this speed control without restriction. As the air enters speed control #2 it pushes the check ball into its seat preventing the air from flowing through the check valve. This means that all of the air must flow through the variable restrictor. From speed control #2 the air flows to the air cylinder causing the cylinder rod to extend. Adjusting the variable restrictor of speed control #2 will control the speed of the air cylinder as the rod extends.

In Figure 4B the directional control valve has been deactivated. This removes the high pressure air from the circuit and allows the spring in the air cylinder to retract the cylinder rod. As the spring retracts the cylinder rod it  pushes the air out of the air cylinder and to speed control #2. As the exhaust air enters it pushes the check ball off of its seat allowing the exhaust air to flow through freely. From speed control #2 the air flows to speed control #1, where the variable restrictor is adjusted to control the speed of the air cylinder, as previously described for Figure 3A and 3B.

Quiz Describe the symbol of the control directional valve.