All hydraulic cylinders must be equipped with wiper seals. During the retraction of the piston rod, the wiper seal removes contaminants adhering to its surface, protecting the sealing rings and guide bushes from damage. Double-acting wiper seals also provide an auxiliary sealing function; their inner lip wipes off the oil film on the piston rod surface, thereby enhancing the sealing effect.
Dustproof seals are critically important for protecting key components of hydraulic equipment. The ingress of dust not only wears the seals but also severely abrades the guide bushes and piston rod. Impurities entering the hydraulic fluid can impair the performance of control valves and pumps, potentially damaging these components. A wiper seal removes dust from the piston rod surface without damaging the protective oil film, which also benefits the lubrication of the sealing elements. When designing a wiper seal, it must not only fit the piston rod but also seal effectively within its groove.
For an effective wiper seal, the contact force of its lip against the piston rod must be sufficiently high. Elastic materials with relatively high hardness best achieve this effect. Polyurethane with a hardness of 94° Shore A is particularly suitable for wiper seals. Its properties include high modulus (hardness), excellent wear resistance, and low compression set. This material can be used in most applications within a temperature range of -40°C to 100°C.
Unreinforced single-acting wiper seals designed for closed installation grooves struggle to provide effective sealing in the groove. This is because they are installed with a loose fit in the groove. Only the contact between the sealing lip and the piston rod provides the sealing force against the cylindrical surface. When the temperature drops, the entire wiper seal shrinks, creating a risk of leakage between its outer diameter and the groove. This condition must be compensated for in the seal design to achieve satisfactory performance.
Common combined hydraulic seals for hydraulic cylinders in mechanical equipment are mainly composed of stacked O-rings, square rings, U-rings, Y-rings, YX-rings, and other specially shaped hydraulic sealing rings.
It is the most commonly used oil seal in hydraulic cylinders and a typical lip seal. It delivers reliable sealing performance for both piston rods and pistons.
At low pressure, the U-seal seals only through the interference deformation of its lips, resulting in relatively low friction due to the small contact area. As pressure increases, the elastic deformation of the lips increases, raising tension, compression, and bending stresses. The radial compression force of the U-seal automatically increases, and the contact length with the sealing surface extends until the entire axial length of the U-seal contacts the surface, ensuring excellent sealing under high pressure.
The U-seal offers good sealing performance but is prone to twisting when used alone, so it must be used with a tin-bronze support ring. Its friction resistance is relatively high and increases with working pressure. Therefore, U-seals are only suitable for hydraulic cylinders with low working pressure or low movement speed.
The Y-seal maintains sealing by its open lips pressing tightly against the sealing surface and can usually be used independently. It features good sealing performance, low friction resistance, high pressure resistance, stable operation, and long service life. Therefore, Y-seals are suitable for hydraulic cylinders with high-speed pressure variation, large bore, and long stroke.
The YX-seal is a modified Y-seal with a cross-section height-to-width ratio greater than 2 and unequal working and non-working lips. It is divided into piston-type YX-seals and rod-type YX-seals with identical sealing performance.
In addition to all the advantages of the Y-seal, the YX-seal never twists when used alone, further improving pressure resistance and operational stability. Thus, YX-seals are especially suitable for hydraulic cylinders with high pressure, high-speed pressure variation, and rapid movement.