Hydraulic transmission is a way of energy conversion, transmission and control with pressurized fluid as the working medium. Hydraulic gear pump is the energy conversion element in hydraulic transmission system, widely used in engineering, mining, forklift and other machinery.
Hydraulic gear pump dynamic, static seal are often used: dynamic seal with rotary mechanical seal, rubber lip seal; The static seal has gaskets, O-ring seals and sealants. Hydraulic gear pump is the most commonly used external seal (internal pressure) for O-ring seal, seal boundary is mostly a planar ring end face.
1. Sealing and leakage
Sealing is strictly a matter of degree, it is never absolute, sealing technology can solve the problem is only to prevent or reduce leakage. There is no absolute seal in the physical sense, and any gap, small as it is, allows the liquid molecule to have a channel in both directions, colloquially known as leakage. The most frequent mode of leakage in hydraulic systems is caused by pressure flow and occurs as dripping or flowing fluid.
The form and size of the seal gap vary greatly in a variety of sealing systems, and the film thickness of the sealing interface ranges from about 0.1μm to 1mm. In the contact seal, the seal gap is very small, the sealing interface relative sliding, sealing interface dynamic fluid film formation, this dynamic fluid film thickness is usually 0.1 ~ 1μm, that is, with the roughness and roughness of the sealing interface equivalent. Since the size of oil molecules is no larger than about 1nm (0.001μm), the fluid molecules are still small relative to the thinnest dynamic film, and leakage is inevitable.
2. O-ring sealing mechanism and selection
2.1 Sealing mechanism
O-ring seals in hydraulic gear pump is widely used, the materials for rubber (from fluid sealing Angle to analyze the sealing mechanism can also be called elastomer), it has the following advantages: (1) low elastic modulus E and large fracture elongation (100% or higher), namely rubber adaptive performance is strong, after the initial installation of the contact stress in an acceptable range; (2) The high Poisson's ratio ν with the theoretical limit close to 0.5 reflects the incompressibility of rubber; ③ Low shear modulus G, does not change the volume of the case is easy to change the shape, to adapt to different cavities.
The O-ring sealing mechanism depends on the elasticity and incompressibility of the material and the presence of initial interference or precompression. In the free state, only the sealing of the compression and symmetric extrusion surface is considered. After the O-ring is assembled into the sealing groove, the contact surface produces a certain load δ0 before pressure. When the pressure is applied as δP, the fluid pressure acts on the exposed surface of the seal and promotes the displacement of the O-ring to the low pressure side. At the same time, the elastic deformation is further increased, and the contact stress on the surface of the O-ring has a parabolic distribution.
2.2 Matters needing attention for selection
(1) O-rings may lose elasticity due to thermal aging or chemical erosion, and may also suffer embrittlement, surface cracks, expansion or contraction, so the selection of O-rings should pay attention to the compatibility with the sealing environment (consult suppliers).
(2) The main material of O-ring rubber has high thermal expansion, and its filler has low thermal expansion, design high or low temperature, need to pay attention to the volume change of the groove of the O-ring relative and linear interference change.
(3) When the O-ring is subjected to tensile stress, if the temperature of the use environment is too high, the Goff-Joule effect will occur, and the O-ring is more inclined to shrink and easy to leak.
(4) On the sliding contact surface or the contact surface with poor surface roughness or the surrounding area of non-uniform friction distribution, the O-ring may slip in a certain position, a certain position is stuck, resulting in uncontrollable torsion, leading to the spiral cracking of the O-ring, even if the O-ring is not damaged, it will also produce leakage. Good solutions are: seals with anti-rolling profiles, such as rectangular rings.
(5) The leakage of gear oil pump just started in winter in low temperature areas indicates that the glass transition temperature Tg of seals is high, especially for seals with high fluorine compounds. At this critical temperature, the O-ring is more like leather than rubber, and the static pressure of fluids cannot be transmitted, leading to leakage.
(6) The rectangular area of the O-ring installation groove should be greater than 25% of the cross-sectional area of the O-ring (except for vacuum sealing, it should be 100%). After the O-ring is installed in the sealing groove, the end face of the O-ring is generally subjected to 8% ~ 30% compression deformation. The initial compression during static sealing is 15% ~ 30%, that is, the tank full rate value should be 70 ~ 85%. In the dynamic seal (hydraulic) initial compression 10% ~ 18%. It should be noted that the static seal takes the larger compression rate value, because the synthetic rubber compresses at low temperature, so the precompression of the static seal O-ring should be considered to compensate for its low temperature compression.
(7) Under high pressure, the O-ring in the groove is pressed by the liquid to the non-pressure side, which becomes an approximate rectangle and squeezes into the gap at the low pressure end. When there is pressure pulsation, the O-ring may continue to be slightly peeled off. In order to minimize the risk of extrusion and ensure the small gap of the main sealing surface, it is better to assemble the anti-extrusion ring with higher strength between the O-ring and the extrusion gap, commonly known as "retaining ring" or "retaining bar". Such as: the internal seal on the side plate or sleeve of the gear pump, the axial clearance is certain, when the fluid pressure is high, the effect of preventing the O-ring extrusion damage is significant.
3. Conclusion
Although the O-ring is not the leading technology, but it is one of the key technologies in hydraulic gear pump, it determines the safety, reliability and durability of hydraulic gear pump. In the reduction of leakage, reduce wear, improve reliability and stability, prolong the service life of the advantages of O-ring seal in hydraulic gear pump irreplaceable position.
