The seal on the piston of a hydraulic cylinder significantly influences its performance. Poor sealing can affect piston functionality and life.
In severe cases, it can also affect the overall use and longevity of the hydraulic cylinder. Therefore, the sealing of hydraulic cylinder pistons must receive due attention.
The seal, normally installed on the piston, mainly serves to seal the gap between the piston and the cylinder, preventing hydraulic oil leakage. The basic requirements for the sealing device include excellent sealing performance that improves with increasing pressure.
In addition, it should have low friction resistance, oil resistance, corrosion resistance, wear resistance, long service life, and be easy to manufacture and disassemble.
Hydraulic cylinders mainly use O-rings for sealing. Commonly used sealing rings include O-shaped, V-shaped, Y-shaped and composite types. The materials used for these seals are oil-resistant rubber, nylon, and polyurethane.
1. Sealing ring
O-ring is mainly used for static sealing. It is conveniently installed and economical, capable of operating in a temperature range of -40 to 120°C. However, compared to lip seals, it has greater resistance to movement and is prone to twisting during dynamic sealing. Therefore, it is generally not used independently for dynamic sealing of hydraulic cylinders, but combined with other sealing components.
During the installation of any seal, an adequate amount of pre-compression must be ensured. If it is too small, it will not be able to seal; if it is too large, the friction increases and it is more likely to be damaged. Consequently, the groove size and surface accuracy for seal installation must be strictly safeguarded in accordance with the data provided in the relevant manuals.
In dynamic sealing, when the pressure exceeds 10 MPa, the O-ring may be squeezed into the gap and damaged. To prevent this, a back-up ring made of polytetrafluoroethylene (PTFE) or nylon, with a thickness of 1.25-2.5 mm, must be placed on the low pressure side of the O-ring.
If it is subjected to high pressure in both directions, support rings must be added on both sides. The structure is represented in the following diagram.
2. V-shaped seals
The cross-section of a V-ring, also known as a V-shaped seal, is V-shaped, as shown in the diagram below.
The V-shaped seal assembly consists of a compression ring, a V-ring and a support ring. When the operating pressure exceeds 10MPa, the number of V-rings can be increased to increase the sealing efficiency. During installation, the opening of the V-ring should face the side with the greatest pressure.
V-shaped seals feature excellent sealing performance, high pressure resistance and long service life. By adjusting the clamping force, optimal sealing effects can be achieved.
However, V-shaped seals have relatively high frictional resistance and large structural dimensions, and are mainly used for reciprocating motion sealing of piston rods. They are suitable for operation in conditions with working pressure of p>50MPa and temperatures ranging from -40 to 80°C.
3. Y-shaped sealing rings
Y-rings (YX), also known as Y-rings, have a Y-shaped cross-section and are categorized as lip-type seals. These seals are known for their excellent sealing properties, stability, pressure resistance, low friction and long service life, making them widely used. Y-rings are mainly used for reciprocating seals.
Depending on the ratio of length to width in cross-section, Y-rings can be classified into two shapes: wide cross-section and narrow cross-section. The figure below illustrates a Y-shaped sealing ring with a wide cross section.
The sealing function of the Y-ring depends on the intimate contact of its edge with the mating surface and generates significant contact pressure under the influence of hydraulic oil to achieve the sealing purpose. As the hydraulic pressure increases, the lip edge adheres more firmly to the coupling surface, the contact pressure is higher, and the sealing performance is better.
When installing the Y-ring, the mouth end surface of the lip should face the side with high hydraulic pressure. When the pressure changes significantly or the sliding speed is high, a support ring must be used to fix the sealing ring, as shown in the figure below.
Wide cross-section Y-rings are typically suitable for applications where the working pressure is less than 20 MPa, while narrow cross-section Y-rings are generally suitable for operations under a working pressure of less than 32 MPa.
4. Selection of hydraulic cylinder piston seals
Typically, Y-type seals are selected for hydraulic cylinder pistons. If the cylinder operates at high speed and under medium to high pressure, a Glyd ring should be chosen – this is a combination of a rubber ring and a PTFE ring.
For high-pressure cylinders operating at slower speeds, where internal leakage needs to be strictly controlled and pressure retention is required, V-type combination seals can be selected.
If high speed and high pressure are required, along with pressure retention, a custom seal design will be required – the specific plan must be determined based on the specific circumstances. For low pressure cylinders that require pressure retention, rubber material should be selected.
