The Principle And Structure of Rotary Joints （diagram）

There are three oil ports connected to the workshop pipeline, namely A port, B port, and Y port.  The three oil ports are statically connected to the outer shell of the rotary joint.  The shell and the uncoiler or coiler are kept relatively stationary.  The diameter of the spindle is smaller than the diameter of the housing, and its clearance is equivalent to the clearance between the cylinder barrel and piston of a typical hydraulic cylinder.  Due to installation reasons, the rotary joint may swing during rotation. Two bearings are used to withstand the axial force generated by the housing swing and maintain a gap between the spindle and the housing.  The material of the rotary seal is wear-resistant composite material or metal material, and the rotary seal is installed between the spindle and the housing.  Rotary seals can withstand high pressure, usually up to 200 bar, but the material is hard and the sealing performance is poor.  Oil seals are skeleton lip seals that cannot withstand excessive pressure, usually not exceeding 3 bar. However, they have good sealing performance under low pressure and high speed conditions, and their function is to seal off oil leakage from rotating seals.  The leaked oil must be drained back into the oil tank under very low pressure to protect the oil seal from damage.  Filters cannot be installed on leaking oil pipes.  The rotary joint is connected to the expansion and contraction cylinder through the rightmost flange, and the oil port of the rotary joint is docked with the oil port of the hydraulic cylinder.

There are two main types of rotary seals for rotary joints: composite material seals and mechanical seals.  The sealing performance of composite material sealing is relatively better, and it is used in situations where hydraulic cylinders have intermediate positioning.  The use of composite material sealed rotary joints can make them more compact and compact due to the smaller size of the seal itself.  The cost of composite material sealing itself is much cheaper than mechanical sealing.  The composite material rotary seal consists of two parts, consisting of an outer ring made of PTFE material and an O-ring made of NBR material.  The O-ring serves as a support for the outer ring, making it easier to install the entire rotating seal.  Sliding between the outer ring and the outer shell of the rotary joint.

Mechanical seals can theoretically achieve contactless sliding and have a long lifespan.  The production of mechanical seals is relatively complex, requires high precision, and is relatively expensive.  At present, there is no domestic production of rotary joints that use mechanical seals.  The following is a schematic diagram of the internal structure of GAT's products in Germany.  The medium is from the shell>component 1>component 3>spindle>hydraulic cylinder.  The mechanical rotary seal is mounted on the spindle, and the sealing between the spindle and the rotary joint is achieved using the composite material rotary seal mentioned above. When the mechanical rotary seal is stuck, there can be relative rotation between the mechanical rotary seal and the spindle, and the composite material rotary seal plays a safety role. The fit and sealing between the mechanical rotary seal and the housing are also achieved through the composite material rotary seal.  The mechanical rotary seal is mainly composed of four metal rings, labeled as component 1, component 2, component 3, and component 4 in the figure.  Component 1 and Component 2 are bolted together, maintaining a stationary working state with the casing. Component 1 delivers the medium to Component 2 through multiple oil channels evenly distributed along the circumference, Component 2 delivers the medium to Component 3 through multiple oil channels evenly distributed along the circumference, and Component 3 delivers the medium to the spindle through multiple oil channels evenly distributed along the circumference.  Component 3 and component 4 are bolted together, and their working state remains stationary with the spindle.  Relative rotation occurs between component 2 (1) and component 3 (4).  The structure of the mechanical rotary seal enables it to always have good static pressure oil film lubrication between the sliding surfaces, thereby achieving a long service life of the mechanical rotary seal.  

As shown in Figure a and d, there are two very small annular gaps with a gap of approximately 0.005mm, while b and c are two annular liquid chambers.  The pressure in chamber C is approximately equal to the pressure supplied from the casing. Its presence pushes components 1 and 2 to move to the right, and the medium enters chamber B. The annular area of chamber B is approximately twice that of chamber C, which is Sb=2Sc.  When the pressure in chamber B is half of the pressure in chamber C, it will push 1 and 2 to move to the left.  Therefore, the medium pressure automatically balances the pressure in chamber B with half of the external oil supply pressure.  The flow rate between flow path gap a and flow path gap d is equal, and the pressure difference between flow path gap a and flow path gap d is also equal. When Ld/La=Rd/Ra is satisfied, the size of gap a at the equilibrium point is equal to gap d, that is, Ha=Hd.  Therefore, if Ha+Hd=0.01mm, then at the equilibrium point Ha=Hd=0.005mm

The metal surface that experiences sliding friction with the composite material rotary seal should have a high surface finish, with chrome plated and polished to a mirror finish.  Its smoothness is comparable to that of the piston rod of a hydraulic cylinder.  Although the sliding surface of the mechanical rotary seal is lubricated by a static oil film, occasional contact with the sliding surface due to pressure changes can cause wear. In addition, particles with a diameter exceeding 0.005mm can also cause wear on the sliding surface.  Therefore, mechanical rotary seals should be surface hardened, and porcelain veneers on sliding surfaces can further extend the operating life of mechanical rotary seals.