The Structure and Basic Working Principle of Air Cylinder

cylinder barrel, end cap, piston, piston rod, and sealing ring, etc. are fundamental parts of air cylinder

Figure 1: Each component to compose the air cylinder.

Air cylinder is composed of cylinder barrel, end cap, piston, piston rod, and sealing ring.

1.1 Cylinder Barrel

The inner diameter of the cylinder barrel determines the output force of the cylinder. The piston makes smooth reciprocating sliding in the cylinder barrel, and the surface roughness of the inner surface of the cylinder barrel shall reach ra0.8um. For the steel cylinder, the inner surface shall also be plated with hard chromium to reduce friction resistance and wear and prevent corrosion. In addition to the high carbon steel pipe, the cylinder barrel is also made of high-strength aluminum alloy and brass. Some small cylinders are made of stainless steel tubes. For cylinder with magnetic switch or cylinder used in corrosion-resistant environment its barrel shall be made of stainless steel, aluminum alloy or brass.

The piston of SMC and cm2 cylinders adopts a combined sealing ring to realize two-way sealing. The piston and piston rod are connected by pressing rivets instead of nuts.

1.2 End Cap

The end caps are designed with air inlet and outlet, and some are also provided with inside buffer mechanisms. The rod side end cap is designed 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 end cap of the rod side to enhance the guiding precision of the cylinder, bear a small amount of transverse load on the piston rod, reduce the bending amount when the piston rod is extended, and prolong the service life of the cylinder. Guide sleeve usually is made of sintered oil-impregnated alloy and forward-inclined copper castings. Malleable cast iron commonly was used to make end caps in the past. Now, in order to reduce weight and prevent rust, aluminum alloy die-casting is often used, and some micro cylinders are made of brass.

Malleable cast iron commonly was used to make end caps in the past. Now, in order to reduce weight and prevent rust, aluminum alloy die-casting is often used, and some micro cylinders are made of brass

Figure 2: The design mode of cylinder end cap.

1.3 Piston

The piston is a part bearing pressure in the cylinder. In order to prevent gas blowby between the left and right chambers of the piston, a piston sealing ring is provided. The wear-resistant ring on the piston can improve the guidance qualify of the cylinder, reduce the wear of the piston sealing ring, and reduce the friction. The wear-resistant ring is made of materials such as polyurethane, polytetrafluoroethylene, and cloth-lined synthetic resin. The width of the piston is determined by the size of the sealing ring and the necessary length of the sliding part. If the sliding part is too short, it will easily cause early wear and jamming. The piston is usually made of aluminum alloy and cast iron, and the piston of the small cylinder is made of brass.

1.4 Piston Rod

The piston rod is the most important force-bearing part in the cylinder. It is usually made of high carbon steel with hard chrome plated on the surface, or stainless steel to prevent corrosion and improve the wear resistance of the sealing ring.

1.5 Sealing Ring

Sealing parts that act the rotary or reciprocating motion is called dynamic seal, and sealing of stationary parts is called static sealing.
The main methods to connect cylinder barrel and end cap are as follows:
Integral type, riveted type, threaded connection type, flange type and pull rod type.

Cylinder piston sealing ring is essential for the normal functions of the cylinder

Figure 3: The shape and design of piston sealing ring of cylinder.

2. Working Principle of Air Cylinder

2.1 Singe Acting Cylinder

Single acting cylinder has only one chamber to input compressed air, realizing one-direction movement.

The introduction and brief description of s type single acting cylinder

Figure 4: The structure of S type single acting cylinder.

Working principle: The piston has compressed air from only one side, and the working stroke of the cylinder is confined to one direction. The piston of the cylinder can return to its original position under the action of spring, gravity or other external forces.
Its piston rod can only be pushed back with the assistance of external forces that usually include spring force, diaphragm tension, gravity, etc.

Characteristics of single action cylinder:
Only one end to inlet (exhaust) air, simple structure, and small gas consumption.
Returning with the assistance of spring force or diaphragm tension. Part of the compressed air energy is used to overcome the spring force or diaphragm tension, thus reducing the output force of the piston rod.

Springs, diaphragms, etc. are installed in the cylinder, and the stroke is generally short; Compared with the double acting cylinder with the same volume, the effective stroke is smaller.

The tension of the cylinder returning spring and the diaphragm changes with the deformation, so does the output force of the piston rod during the stroke.
Due to the above characteristics, single acting piston cylinders are mostly used for short stroke. Both thrust and movement speed are not highly required, usually applied in such occasions as air crane, positioning and clamping devices.

2.2 Double Acting Cylinder

Double acting cylinder is one t in which compressed air can be input into two chambers respectively, realizing two-way movement.

The introduction and brief description of single piston rod type double acting cylinder

Figure 5: The structure of single piston rod type double acting cylinder.

Working principle: There is air pressure on both sides of the cylinder piston to realize forward or backward action. When compressed air enters and exits alternately on both sides of the piston, the piston moves towards two directions, and the movement speed in both directions can be controlled by adjusting the air pressure.

When the cylinder block is fixed, the load it carries (such as the working surface) is integrated with the two piston rods of the cylinder, and the compressed air enters the two chambers of the cylinder in turn (one for air inlet and the other for exhaust), and the piston rod drives the working surface to move left and right. The movement range of working surface is equal to 3 times of its effective strokes. The installation takes up a lot of space and it is generally used on small equipment.

When the piston rod is fixed, for the convenience of pipeline connection, the piston rod is made hollow, the cylinder block and the load (working surface) are integrated, and the compressed air enters the two chambers of the cylinder from the left or right end of the hollow piston rod, so that the cylinder block drives the working surface to move left or right. The movement range of the working surface is 2 times its effective strokes, and it is suitable for medium and large equipment.

2.3 Air Buffer Cylinder

For those cylinders that have fast speed with approaching the end of stoke, without taking necessary measures, the piston would hit the end cap with huge force (energy), causing vibration and damage to the parts. In order to ensure piston has a smooth movement at the end of the stroke without shock, cushion devices are added at both ends of the cylinder, which is generally called a air buffer cylinder.

cushion devices are added at both ends of the air buffer cylinder to ensure piston has a smooth movement at the end of the stroke without shock

Figure 6: The internal structure of air buffer cylinder.

1, 3 -Cushion plunger 2 -Piston 4 -Cylinder barrel 5 -Guide sleeve 6 -Scraper seal 7 -Front end cover 8 -Gas port 9 -Sensor 10 -Piston rod 11-Wear ring 12 -Sealing ring 13 -Back end cover 14 -Buffer throttle valve

Working principle: When the piston moves to the right under the push of compressed air, the gas in the right chamber of the cylinder is discharged through the column hole and the air hole on the cylinder head. When the piston movement approaches to the end of the stroke, the cushion plunger on the right side of the piston blocks the column hole. When the piston continues to move to the right, the rest gas sealed in the right chamber of the cylinder is compressed and slowly discharged through the throttle valve and air hole. If the pressure energy generated by the compressed gas is balanced with all energy of the piston movement, the cushion effect will be achieved, so that the piston smooth movement at the end of the stroke and no impact are ensured. Adjusting the opening of throttle valve can control the amount of gas discharge, thus determining the pressure in the compressed volume (called cushion chamber) to adjust the cushion effect. If the piston moves in the opposite direction and compressed air is input from the air hole, the check valve can be directly opened to push the piston to the left. If the opening of throttle valve is fixed and cannot be adjusted, it is called non-adjustable air buffer cylinder.