By: Justin Uhm
In this post, we introduce the different types of pneumatic valves in the market today and how are they used.
Direct Control Valve
The direct control valve controls the direction of airflow. The direction control valve generally refers to a conversion valve, but more specifically, it includes the conversion valve (direct conversion valve), check valve, stop valve, and shuttle valve. When classified by function, it can be classified as shown in Figure 1.
A control valve is used to control the supply of pressure or to stop pneumatic pressure to pneumatic circuits, and to supply or release pneumatic pressure. Also, it is used to convert the direction of pneumatic pressure for the repetitive operation of the driving apparatus, such as a cylinder.
A check valve directs the airflow to one side so that pneumatic pressure does not backflow into the pneumatic circuits.
A shuttle valve is a type of valve that allows liquids to flow through it. Generally, they are used in pneumatic systems. Sometimes, shuttle valves will be found in hydraulic systems.
Classification by the Number of Ports that Connect to the Main Pipe on a Direct Conversion Valve
In conversion valves, the number of ports represents the number of passages controlled. Valve ports are classified into air passage joints and pilot ports for manipulating signals. However, the number of valve ports only counts the number of main air pipelines and the number of air holes on the valve.
The number of ports on ordinary conversion valves is shown in Table 1 below. As shown in the table, 4-port and 5-port valves have two outlets, and only one of these valves can control the direction of double-acting cylinders. The difference between 4-port and 5-port valves is that 4-port valves have one common exhaust port, while 4-port valves have multiple exhaust ports. Each exhaust port is assigned to one outlet, which allows the throttle valve to control the speed of the actuator.
Table 1. Number of Ports on Conversion Valves
Structure of the Main Valve (Conversion Valve)
Based on their structure, conversion valves can be classified into poppet, spool, and slide type. Poppet-type valves have the most basic structure and are widely used with FRLs, flow control valves, and direction control valves. Slide-type valves are not widely used, but they are occasionally used on direction control valves.
With spool type valves, the pneumatic motion inside the valve has almost no influence on the manipulation of the valve, which means it can operate the valve with a relatively smaller force than the poppet type. Poppet type influences the valve because of its pneumatic motion, which is against the operation of the valve, and requires a greater force with higher pneumatic pressure.
The spool type has a structure that must allow a certain amount of air leakage, whereas the poppet type has no air leakage because the valve body is closely attached to the valve seat.
Types of Pneumatic Valves
The poppet-type valve shown previously in Figure 1 is a type in which a flat sheet or cone-shaped valve body moves perpendicular to a valve seat.
A spool-type valve moves a spool that precisely fits in a sleeve according to the axis, changes the flow inside the valve. Spool valves are mostly used on direction control valves with three or more ports because they can serve various purposes according to the type of spool and the number of ports. They do not require a large manipulative force.
A slide type valve’s body slides through the top of the valve seat to convert the flow of air.
Classification by the type of seal
A spool valve’s slide and seal are air-tightened by metal-against-metal friction. Elastomer seals (permeable seal), use synthetic rubber.
Generally, a metal seal uses a spool-sleeve structure that combines a spool with a cylindrical sleeve. The seal allows a few microns of gap between the spool and sleeve, causing a little bit of air leakage. Although the amount of leakage is less than 1%, it can cause a problem in severe environment conditions where no leakage can be tolerated. A metal seal has a sleeve that is attached to the value body using an O-ring that buffers any deflection of the valve body by a large outer force, which allows the spool to operate properly.
An elastomer seal type can attach onto the spool. Also, the shape of the elastomer seal can be classified into squeeze seals and lip seals. Elastomer seals have an extremely small gap compared to metal seal, and they ensure excellent airtightness without leakage. However, this type of seal is not as resistant to abrasion because it is made of synthetic. This means elastomer seals are less durable than metal seals.
We hope this has given you a good idea of the different types of valves on the market and its applications. Here at TPC Automation, we keep an inventory of all different types of valves to handle all your pneumatics needs.
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