Reciprocating seals are the most common sealing requirements in hydraulic rotating, pneumatic components and systems. Reciprocating seals are used on the power cylinder piston and cylinder block, piston intervention cylinder head and various slide valves. The gap is formed by a cylindrical rod and a cylindrical hole, and the rod moves axially in the cylindrical hole. The sealing element acts to limit the axial leakage of the fluid. When used as a reciprocating seal, the pre sealing effect and self sealing effect of the O-ring are the same as those of the static seal, and due to the elasticity of the Jiulong seal O-ring itself, it has the ability to automatically compensate for wear. However, when sealing with liquid medium, the situation is more complicated than that of static sealing due to the rod movement speed, liquid pressure and viscosity. 

When the liquid is under pressure, the liquid molecules interact with the metal surface. The 'polar molecules' contained in the oil are closely and orderly arranged on the metal surface, forming a strong boundary layer oil film between the sliding surface and the seal, and producing great adhesion to the sliding surface. The liquid film always exists between the sealing element and the reciprocating surface. It also plays a sealing role and is very important for the lubrication of the moving sealing surface. But it is harmful for leakage. However, when the reciprocating shaft is pulled out, the liquid film on the shaft will be pulled out together with the shaft. Due to the 'wiping' effect of the seal, when the reciprocating shaft retracts, the liquid film will be blocked outside by the seal element. As the number of reciprocating strokes increases, more liquid is retained outside, and finally oil droplets are formed, which is the leakage of reciprocating seal device. As the viscosity of hydraulic oil decreases with the increase of temperature, the oil film thickness decreases accordingly, so the hydraulic equipment. 

As a reciprocating seal, O-ring has compact structure and small size, which can reduce the price of components. Mainly used for: 
 1) Low pressure hydraulic components are generally limited to short stroke and medium pressure of about 10MPa.
 2) In small diameter, short stroke and medium pressure hydraulic spool valves.
 3) Pneumatic slide valve and pneumatic cylinder.
 4) As the elastomer in the combined reciprocating sealing device.

As a reciprocating seal, O-ring is most suitable for small diameter, short stroke, medium and low pressure applications, pneumatic cylinders, pneumatic slide valves and other reciprocating components. In hydraulic components, O-ring is used as the main dynamic seal, which is generally limited to short stroke and about 10MPa medium and low pressure. O-ring is not suitable for reciprocating seals with very low speed and high-pressure reciprocating seals alone. This is mainly due to the large friction under this condition, which will lead to premature failure of the seal. In any type of application, it shall be used according to the rated data or capacity of the sealing element, and shall be properly assembled to obtain satisfactory performance.