Structural principles and basic functions of cylinders

1. Type of cylinder
In pneumatic transmission, it is a pneumatic actuator that converts the pressure energy of compressed gas into mechanical energy. There are two types of cylinders: those that perform reciprocating linear motion and those that perform reciprocating swing. Cylinders for reciprocating linear motion can be divided into four types: single-acting, double-acting, diaphragm and impact cylinders.​
① Single-acting cylinder: There is a piston rod at only one end. Air is supplied from one side of the piston to accumulate energy to generate air pressure. The air pressure pushes the piston to generate thrust and extends, and returns by spring or self-weight.​
②Double-acting cylinder: Air is supplied alternately from both sides of the piston to output force in one or two directions.​
③Diaphragm cylinder: A diaphragm is used instead of a piston, which only outputs force in one direction and is returned by a spring. It has good sealing performance but short stroke.​
④Impact cylinder: This is a new type of component. It converts the pressure energy of the compressed gas into the kinetic energy of the piston's high-speed (10-20 meters/second) movement to perform work. The impact cylinder has a middle cover with a spout and a drain port. The middle cover and piston divide the cylinder into three chambers: air storage chamber, head chamber and tail chamber. It is widely used in various operations such as blanking, punching, crushing and forming. A cylinder that swings back and forth is called a swing cylinder. The inner cavity is divided into two by a blade, and air is supplied to the two cavities alternately. The output shaft swings with a swing angle of less than 280°. In addition, there are rotary cylinders, gas-liquid damping cylinders and stepping cylinders.​

2. The function of the cylinder: Convert the pressure energy of the compressed air into mechanical energy, and the driving mechanism performs linear reciprocating motion, swing and rotation motion.​

3. Classification of cylinders: cylinders for linear motion and reciprocating motion, swing cylinders for swing motion, air claws, etc.​

4. Cylinder structure: The cylinder is composed of cylinder barrel, end cover, piston, piston rod and seals. Its internal structure is shown in the figure below.

5. Cylinder structure principle
1. Cylinder barrel. The inner diameter of the cylinder barrel represents the output force of the cylinder. The piston should reciprocate smoothly in the cylinder, and the surface roughness of the inner surface of the cylinder should reach Ra0, 8um. For steel tube cylinders, the inner surface should also be plated with hard chromium to reduce frictional resistance and wear and prevent corrosion. In addition to using high-carbon steel pipes, the cylinder tube material is also made of high-strength aluminum alloy and brass. Small cylinders use stainless steel tubes. For cylinders with magnetic switches or cylinders used in corrosion-resistant environments, the cylinder barrel should be made of stainless steel, aluminum alloy or brass. The SMC CM2 cylinder piston uses a combined sealing ring to achieve bidirectional sealing, and the piston and piston rod are connected by riveting without nuts.​
2. End cover. The end cover is equipped with intake and exhaust ports, and sometimes there is a buffer mechanism inside the end cover. The rod side end cover is equipped with a sealing ring and a dust ring to prevent air leakage from the piston rod and external dust from mixing into the cylinder. There is a guide sleeve on the rod side end cover to improve the guiding accuracy of the cylinder, bear a small amount of lateral load on the piston rod, reduce the amount of bending of the piston rod when it extends, and extend the service life of the cylinder. Guide sleeves are usually made of sintered oil-containing alloy, forward-inclined copper castings. In the past, malleable iron was commonly used for end caps. Nowadays, in order to reduce weight and prevent rust, aluminum alloy die-casting is often used, and brass materials are sometimes used for micro cylinders.​
3. Piston. The piston is the pressure-bearing part in the cylinder. In order to prevent the left and right chambers of the piston from channeling air, a piston sealing ring is provided. The wear-resistant ring on the piston can improve the guidance of the cylinder, reduce the wear of the piston seal ring, and reduce friction resistance. The wear-resistant ring length uses materials such as polyurethane, polytetrafluoroethylene, cloth-filled synthetic resin, etc. The width of the piston is determined by the size of the sealing ring and the necessary length of the sliding part. The sliding part is too short, which can easily cause early wear and jamming. The materials of piston are commonly used aluminum alloy and cast iron, and the piston of small cylinder is made of brass.
4. Piston rod. The piston rod is the most important force-bearing part in the cylinder. High carbon steel is usually used, and the surface is plated with hard chromium, or stainless steel is used to prevent corrosion and improve the wear resistance of the sealing ring.
5. Seal ring, the seal of the rotating or reciprocating parts is called dynamic seal, and the seal of the stationary part is called static seal. The main methods of connecting the cylinder barrel and the end cover are as follows: integral type, riveted type, threaded connection type, flange type, and tie rod type.​
6. When the cylinder is working, it relies on the oil mist in the compressed air to lubricate the piston. There are also a small number of lubrication-free cylinders.

6. Working principle of cylinder
The push force and pull force on the piston rod are determined according to the force required for the work. Therefore, the cylinder should be selected so that the output force of the cylinder has a slight margin. If the bore diameter is selected to be small, the output force will not be enough and the cylinder will not work properly.