KDAS /DAS: Sealing Principle and Performance Pros and Cons

KDAS is a special fivepiece bidirectional piston combined seal for hydraulic cylinders. It consists of a central elastic main sealing ring, two antiextrusion backup rings and two POM wearresistant guide rings. Integrating sealing, antiextrusion and guiding support functions, it is widely applied to the doubleacting pistons of engineering machinery, injection molding machines, cranes and other equipment.

Sealing Principle

The KDAS is a pressureenergized seal. Bidirectional sealing is realized by the multilip structure of the elastic rubber main sealing ring. After assembly, the rubber ring generates initial radial pretightening force to closely fit the inner wall of the cylinder bore for basic static sealing. As hydraulic pressure rises, the medium pressure acts on the back of the sealing ring and presses the sealing lips further against the cylinder bore. Higher pressure produces stronger lip contact force, which prevents hydraulic oil from bypassing between both sides of the piston and achieves bidirectional oilsealing performance.

The two backup rings perform the antiextrusion function. Under high pressure, they prevent the rubber body from being squeezed into the clearance between piston and cylinder bore so as to avoid seal damage. The outer POM guide rings bear radial side loads of the piston and avoid direct metaltometal friction between piston and cylinder bore. Axial oil relief grooves are machined on guide rings. During direction change, pressurized oil flows through these grooves to release local trapped pressure and reduce damage caused by pressure shock. The whole assembly fits into one single piston groove and fulfills multiple functions including sealing, pressure resistance and guiding.

Main Advantages

• Reliable bidirectional sealing: One set satisfies reciprocating oil sealing for doubleacting pistons, delivering good sealing performance under both static and dynamic conditions with low leakage.
• Excellent antiextrusion performance: Backup rings protect the rubber body. It can withstand working pressure up to 2532 MPa, suitable for mediumandhighpressure hydraulic systems; the seal is resistant to overturning and twisting.
• High integration: Seals, backup rings and guide rings are assembled in one groove, shortening overall piston length, simplifying piston machining and reducing manufacturing costs.
• Easy assembly: Splitcut design for backup rings and guide rings allows installation in both open and closed grooves, facilitating disassemblyassembly and mass production.
• Good resistance to shock and pressure fluctuation: Oil relief grooves on guide rings release trapped oil. It tolerates pressure peaks and directional shocks, ideal for heavyduty reciprocating applications.

Disadvantages

• Limited permissible linear speed: The maximum allowable reciprocating speed is around 0.5 m/s. It is not suitable for highspeed cylinders, where excessive heat generation and accelerated wear may occur.
• High requirements for groove and cylinder bore surface quality. Excessive roughness of cylinder bore or grooves will shorten service life and may cause premature leakage and wear.
• Multiple components in one set. Correct assembly sequence is critical. Wrong installation order or misaligned split cuts will directly lead to seal failure.
• Moderate hightemperature resistance. Standard NBR material works from 30 ℃ to 110 ℃. FKM fluororubber must be adopted for hightemperature conditions, which increases cost.
• Limited radial compensation capacity. Severe piston eccentricity or excessive side loads accelerate wear of guide rings and indirectly shorten seal service life.

Summary

Thanks to its high integration, good pressure resistance and convenient installation, the KDAS combined seal is a preferred solution for heavyduty piston sealing under mediumandhighpressure conditions. However, it is not applicable for highspeed working conditions. To achieve optimal performance, high machining accuracy of grooves and cylinder bore as well as correct assembly sequence must be guaranteed.