What is the Application of Hydraulic Rotary point

Hydraulic power system is widely used in the industrial field by virtue of its many advantages. When the rotating equipment needs to use hydraulic medium as the transmission power, there are strict requirements on the pressure and control accuracy of the hydraulic medium, usually a pressure above 30MP, and reliable operation.

For the development and application of rotary points, it is necessary to simplify the structure of the equipment while achieving a better conversion function. The most extensive and typical application of hydraulic rotary points is in the hot-rolled wide strip steel coilers of steel rolling equipment. And on the expansion and contraction cylinders of uncoilers, coilers and other equipment on the cold-rolled strip production line.

1. Structural principle analysis of hydraulic rotary point

1.1 Structure and Principle

The hydraulic rotary point has two control oil ports and a drain port, which are respectively connected to the fixed pipelines of the hydraulic system. Each external pipeline is statically connected to the shell of the rotary point.

The shell of the hydraulic rotary point is fixed on the shell of the matching main equipment, and keeps relatively static with the main equipment base, connected with the main equipment through the mandrel. The clearance between the outer diameter of the mandrel and the inner diameter of the hydraulic rotary joint. The gap between the cylinder barrel and the piston is required to be the same as that of the hydraulic cylinder, generally 0.04mm~0.07mm. When conditions permit, lower the limit as much as possible. Each time the hydraulic system is started, a static pressure oil film will be formed in the matching gap, which protects the mandrel and the hydraulic rotary joint from wear and tear, and plays a role of sealing.

The rotating seal is installed on the mandrel of the equipment, and its main function is to isolate the oil inlet cavity and the oil return cavity to form two independent sealed spaces. Most of the materials are wear-resistant composite materials or metal materials. Rotary seals need to withstand high pressures above 30MP, and are required to be resistant to high pressure shocks, not easy to deform, and have small leakage. The rotary seal and the inner wall of the rotary joint are not in contact, and there is fluid friction between the two.

The vibration and impact generated by the high-speed operation of the main equipment are transmitted to the hydraulic rotary joint through the mandrel, resulting in a certain swing of the rotary joint. Two bearings are used to support the axial force and radial force generated by the swing of the rotary joint housing, and to realize the precise positioning of the equipment core shaft and the rotary joint.

The end face seal adopts a skeleton lip seal, which is used to seal the leaking oil from the rotating seal. The leaking oil pressure generally does not exceed 0.3MP. Once the leakage increases, it is easy to damage the end seal and cause the hydraulic oil to leak outside. The oil port of the rotary joint shell is butted with the oil port on the mandrel of the equipment.

1.2 Analysis of internal sealing structure

There are two main types of rotary seals for rotary points, composite seals and mechanical seals. The sealing performance of the composite material seal is relatively better, and it is used where the hydraulic cylinder has an intermediate positioning. The rotary point sealed with composite materials can make the rotary point more compact and exquisite due to the small size of the seal itself. The cost of the composite seal itself is much cheaper than that of the mechanical seal.

The composite rotary seal is composed of two parts, a combination of an outer ring made of PTFE and an O-ring made of NBR. The O-ring plays the role of supporting the outer ring, making the entire rotary seal easier to install, sliding between the outer ring and the housing of the rotary joint. The manufacture of mechanical seals is more complicated, the precision requirements are also relatively high, and the relative price is relatively high.

2. Installation and maintenance of hydraulic rotary points

When installing the hydraulic rotary point on the mandrel of the main equipment, ensure that the cavity of the rotary joint and the mandrel of the main equipment have good coaxiality. It is generally required that the coaxiality between the axis of the rotary joint and the axis of the equipment should be controlled within ±1mm/m. Out-of-concentricity will cause the hydraulic rotary point to produce relatively large radial swings during high-speed rotation. The radial force causes the rotary joint and the mandrel to slide periodically in the axial direction. Not only the dynamic oil film is destroyed and the wear of the mating surface is increased, but also the end face of the seal is worn. At the same time, the bearing can withstand greater external impact. Therefore, poor coaxiality will cause great damage to the internal rotating seal and bearings, and affect the service life.

The housing of the rotary joint is fixed to prevent it from rotating synchronously with the spindle, so as long as it can be prevented from rotating in a circle.

Do not use more constraints. Radial or axial force acting on the shell will be transmitted to the bearing of the hydraulic rotary joint and the internal rotating seal through the shell, forcing the bearing or seal to wear or damage.

When connecting external oil pipes, strictly abide by the hydraulic equipment installation specifications. In particular, it is necessary to strictly check the cleanliness of each oil port to prevent external contaminants and machined burrs from being brought into the rotary joint. Due to the complex internal structure of the hydraulic rotary joint and the small matching clearance, once external contaminants enter the hydraulic rotary joint, it is easy to cause damage to the oil film, mechanical seals and bearing jams, and serious leakage.

The skeleton oil seal of the rotary joint is used to seal the rotating mandrel, so that the leaking oil of the rotary seal is drained from the leaking oil pipe to the oil tank. The pressure resistance of the oil seal is usually no more than 3bar, so the leaking oil pipe must be returned to the oil smoothly.

During installation, the leaking oil port of the rotary joint must not be blocked. If there is a valve on the leaking oil pipe, it must be opened before the medium is introduced, otherwise the oil seal will inevitably be squeezed out. It is also not possible to incorporate the leaking oil pipe into the oil return pipe, because the pressure of the oil return pipe usually exceeds 3 bar.  Do not install a filter on the leaking oil pipe. Rotary joints usually have leakage, so they can’t be used in occasions where pressure is required. The leakage of mechanical seals is greater.

Servo control can be considered when it is required to control the stroke of the hydraulic cylinder to compensate for the leakage. When the hydraulic cylinder is not positioned in the middle, the problem will be simpler, as long as the hydraulic cylinder continues to supply the medium after the hydraulic cylinder moves to the limit position.

Rotary joints are generally lubricated and cooled by medium, so it is not possible to test or drive without passing the medium. Make sure to open the leaking oil pipe after overhauling the rotary joint or related equipment. The wear or damage of hydraulic rotary joints can be estimated by measuring the leakage. The leakage of rotary joints should be monitored and tracked regularly to monitor the operating conditions of rotary joints.

3. Common faults of hydraulic rotary joints

In actual use, hydraulic rotary joints mainly have two types of faults. One fault is manifested as the internal bearing damage of the rotary joint, and the other is the external leakage of the rotary joint.

Analyze the causes of internal bearing damage, there are three main points:

1) The hydraulic oil has poor cleanliness and granularity during installation or during use, resulting in serious wear and failure of the rolling elements of the bearing;

2) When the rotary joint is installed, the installation accuracy requirements are not met, resulting in uneven force under the static load state of the bearing and excessive vibration value under the dynamic load state, causing damage to the bearing;

3) The selected hydraulic rotary joint design and manufacturing quality do not meet the requirements of the working conditions of the equipment. The reasons for leakage outside the rotary joint are:

The machining accuracy of the mating surface of the installation seal is low, which cannot meet the accuracy requirements of the seal;

The selection and installation of the seal are not suitable for the requirements of the working conditions;

The installation accuracy of the rotary joint is poor, causing excessive vibration and damage to the seal.

4. Conclusion

In practical applications, due to the complex working conditions of the main equipment and the changeable environment. When selecting a specific application, a rotary joint suitable for a wide range of loads should be selected according to the specific working conditions. In order to meet the requirements of the main equipment from low pressure and low speed to high pressure and high speed in a wide range of working conditions. The high-precision mechanical seal hydraulic rotary joint can better realize the purpose of small leakage, reliable and durable operation.

By improving the processing accuracy and technical requirements of the parts, and improving the installation accuracy, the ability of the rotary joint to adapt to high speed and high pressure can be greatly improved, and the service life can be prolonged.

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