Influence of Shape, Installation and Rotation Motion Errors on Tin Bronze Clearance Seal of Rotary Joint

The rotary joint is the key device of the marine clutch. Once the seal fails, the power cannot be transmitted. In view of the above problems, this research studies the influence of shape, installation and rotation motion errors on clearance seal. The results show that compared with the absence of roundness error, the roundness error of the shaft will increase the leakage by about 2.5-4.9 %. The cylindrical error of the bushing will reduce the leakage, but it will bring more serious friction, wear and life problems. The existence of radial installation error will increase the leakage, and the larger the radial installation error, the more obvious the flow field extrusion, the higher the unevenness of the pressure field in the whole flow field. The existence of angular installation error will reduce the leakage, but it will cause more serious friction and wear, resulting in a sharp increase in temperature. The leakage increases by about 0.68 % when the radial installation error is 0.1 mm, and the rotary motion error further increases the leakage by about 0.15 %. The above research can provide theoretical support for the structural design and optimization of the rotary joint tin bronze clearance seal.


Introduction
The rotary joint, as a connecting piece for transporting fluid medium from the fixed pipeline to the rotating pipeline, can realize the transmission of sealed medium under the condition of high pressure and high speed, and is widely used in military, aerospace, hydraulic and pneumatic transmission and other fields [1,2].Rotary joints generally use the clearance of matching parts to achieve sealing effect, but the size of the clearance will have an impact on the overall performance of the rotary joint.Therefore, in order to ensure the driving device accurately and effectively transfer power, it is necessary to conduct in-depth research on the bronze sleeve clearance seal of rotary joint.
A large number of research institutions and scholars have studied clearance seal.Wang et al. [3,4] found through experimental research that the clearance seal can achieve the optimal sealing effect when the ratio of the seal length to the seal diameter is a certain value.Based on the Hirs turbulent lubrication equation, Childs et al. [5] analyzed the effect of vortices on the performance of convergent conical clearance seals, which made the calculation results of Hirs more accurate.Arghir et al. [6] solved the Naiver-Stocks equation in cylindrical coordinates and obtained the solution method of incompressible fluid in seal clearance.Moore [7] used the fluid calculation program developed by himself to calculate the dynamic parameters of the seal clearance and analyze the influence of different clearance lengths on the seal performance.For Neumann and Scharer models, Joachimmiak and Krzyslak [8] proposed a correction coefficient for the test geometry.Based on the assumptions of the above model and the results obtained, the leakage rates under different sealing clearances are analyzed.
Yu et al. [9] established an oil film gap test bench and verified the simulation results.The results show that the oil film thickness on the downstream side is smaller than that on the upstream side, and the oil film thickness changes faster inside the oil seal edge than outside.Kirk et al. [10] improved the finite element algorithm and simulated the clearance seal.Tong [11] concluded that the main factors affecting the leakage of clearance seal were clearance, oil film viscosity and seal length through the analysis of clearance seal for aircraft landing gear.Zhou et al. [12] analyzed its influence on the leakage of clearance seal with the inclination of mandrel, relative movement and rotation of sealing interface, temperature and pressure as variable parameters.By analyzing the clearance seal of rotary joint, Li et al. [13] found that in the low speed range, the leakage of clearance seal will increase first and then decrease with the increase of speed, but it is not suitable for high speed.Jiang [14] adopted the method of numerical simulation to analyze the shaft sleeve clearance seal, the conical clearance seal and the rectangular slot clearance seal, and theoretically analyzed that under the same working conditions, the conical clearance seal has the best performance, while the shaft sleeve clearance seal has the worst performance.Fan et al. [15] studied the performance of engine clearance seal under the condition of rocket rotation, and concluded that the spindle speed has the greatest influence on the performance of clearance seal by means of test.Li et al. [16] simulated the flow field of the hydraulic cylinder clearance seal, and obtained the pressure distribution and leakage of the sealing clearance under different sealing gaps and inlet pressures.The research results show that the sealing clearance between the cylinder and the piston should be reduced to 0.3mm or less, which can effectively limit the leakage of the clearance seal.Chang et al. [17] conducted simulation analysis on the clearance seal in the reversing spool valve of booster pump, and determined the influence rule of clearance size, groove depth, groove width and groove number on the flow field and leakage of clearance seal.
Although the above research is helpful to reveal the working principle of clearance seal to a certain extent, it lacks the theoretical research basis of clearance seal under special working conditions of marine clutch, and cannot deeply reveal the influence law of error on tin bronze clearance seal of clutch rotary joint, and cannot provide effective technical support for solving the accident that clutch cannot be hung.Therefore, in view of the above problems, according to the actual operating conditions of marine clutch, this research studies the influence of shape error, installation error and rotation motion error on the clearance seal of tin bronze, and uses the high-pressure and high-speed rotating seal test bench for experimental verification.

Numerical Model of Clearance Seal
The numerical calculation model of clearance seal is established.The clearance seal calculation model is shown in figure 1.The 1/18 cycle model is adopted.The inlet boundary is the total temperature and total pressure, and the outlet is the average static pressure.The two circumferential boundaries are set as the interface of the rotation period for data transmission.The bottom ring surface of the fluid calculation domain is the rotor wall surface, and the top ring surface is the inner surface of the sleeve, which is a stationary fixed wall surface.The sealing diameter is 100 mm, the sealing clearance is 0.5 mm, and the clearance length is 100 mm.

Generate
ICEM CFD was used for the structured mesh division of the fluid calculation field.After the mesh sensitivity study, it was determined that there were at least 21 grid nodes with radial distribution of clearance, and the boundary layer grid was divided.High-quality computational grid was the basis and guarantee for high-precision flow field calculation and analysis, and the quality of computational grid was greater than 0.9.In the numerical calculation, the self-defined working medium can more accurately describe the properties of the working medium to ensure the accuracy and reliability of the calculation results.The working medium is 15W-40CD oil, where the density (room temperature) of the defined working medium is 890 kg/m 3 , and the specific heat capacity at constant pressure is 1880 J/kg• K.In the transport properties, the simplified dynamic viscosity changes linearly with temperature, where the dynamic viscosity is 0.0981 Pas at 40 ℃ and 0.013 Pas at 100 ℃.In the definition of working medium, kinetic model and modified Eucken model are selected as heat conduction models.

Experimental Study and Numerical Method Verification
The high pressure and high speed rotary seal test device was built, and the leakage characteristics and temperature rise characteristics of tin bronze clearance seal were investigated.The test device, mainly includes high-speed frequency conversion motor, high-pressure and large-flow oil station, test section and measurement and control system.The high-pressure and high-speed rotary seal test device and the tin bronze clearance seal test section are shown in figure 2. Tin bronze clearance seal for rotary joint, the maximum pressure under the test conditions carried out in this research is 4 MPa, the sealing diameter is 55mm, and the maximum speed is 6000 rpm. Figure 3 compares the numerical calculation and test results of clearance seal leakage.The results show that the error between the numerical calculation and test results of leakage is less than 10%.It can be considered that the numerical calculation model and numerical method are accurate and effective.

Analysis of Shape Error Influence
Table 1 shows the types of shape errors, including roundness error and cylindricity error.The shaft roundness error is 0.4 and 0.6, the cylindricity error is divided into shaft and bushing, the shaft cylindricity error is 0.2 and 0.4, and the sleeve cylindricity error is 0.2 and 0.4.Table 1.Type of shape error.

Shape error
Remarks (unit: mm) The roundness error is 0.4 Large shaft radius is 100.2, small shaft radius is 99.8, sleeve radius is 100.5 The roundness error is 0.6 Large shaft radius is 100.3,small shaft radius is 99.7, sleeve radius is 100.5The cylindricity error is 0.2 (shaft) / The cylindricity error is 0.4 (shaft) / The cylindricity error is 0.2 (sleeve) / The cylindricity error is 0.4 (sleeve) / Figure 4 shows the influence of shaft roundness error on leakage.The existence of roundness error will increase leakage.Compared with the absence of roundness error, the existence of roundness error will increase leakage by about 2.5-4.9 %.At this time, the leakage channel is not a standard ring, but a non-uniform ring with an ellipse inside and a circle outside.As the roundness error increases from 0.4 to 0.6, the leakage amount increases, compared with the roundness error of 0.4, the leakage of the seal increases by about 2.2-2.3 % when the roundness error is 0.6.And the leakage amount caused by the shaft roundness error increases with the increase of the pressure difference between the inlet and outlet.The influence of cylindricity error of shaft and sleeve on leakage is given in figure 5.It can be seen from the figure that the influence of different cylindricity errors on leakage is consistent under different pressure differences.The shaft cylindricity error of 0.2 will significantly increase the leakage, and with the increase of the shaft cylindricity error, it will reduce the leakage, which is the sleeve clearance channel becomes a conical clearance channel at this time.The cylindricity error of the sleeve will reduce the leakage, but the conical clearance formed at this time will bring many other problems, such as more serious friction, wear and life problems.

Analysis of Installation Error Influence
The flow field calculation model of clearance seal considering radial installation error is given in figure 6.The radial eccentricity is 0.1mm and 0.2mm respectively, and the corresponding radial eccentricity is 1 ‰ and 2 ‰ respectively, that is, the radial installation error is 1 ‰ and 2 ‰.The blue part shown in figure 6 is the outer surface of the eccentric shaft (rotor).The black solid line represents the inner surface of the bushing (stator), and the black dotted line represents the outer surface of the shaft (rotor) without radial installation error.Figure 7 is a schematic diagram of angular installation error, where the angular installation error is ± 1/1000 and ± 2/1000.
Figure 8 shows the influence of radial installation error on the leakage of clearance seal.The radial installation error will increase the leakage.Under the same operating conditions, the larger the eccentricity, the greater the leakage.When the radial error is 1 ‰, the leakage increases by 0.65-0.72 %, and when the radial error is 2 ‰, the leakage increases by 2.5-3.1 %.When the inlet pressure is low, the leakage increase is more obvious.Figure 9 shows the influence of angular error on leakage.When there is an angular installation error, a conical gap will be formed.For every 1 ‰ increase in angular error, the leakage will be reduced by about 8 %.The main reason for the decrease in leakage is the formation of a conical clearance, which changes the internal flow.However, the angular installation error will cause more serious friction and wear, resulting in a sharp increase in temperature, and once wear occurs, the clearance will cause greater leakage, resulting in seal failure and reducing seal life.The influence of radial error on the pressure distribution of the clearance cross section is given in figure 10.The radial installation eccentricity of the rotor will squeeze the seal flow field, resulting in a circumferential non-uniform pressure field inside the seal flow field.There is a minimum clearance near the eccentric side of the rotor, and a maximum clearance is formed far away from the eccentric side of the rotor.The local high pressure zone and local low pressure zone of the sealing flow field are formed in the whole range.However, the local high pressure area does not completely coincide with the minimum clearance area, and the local low pressure area does not completely coincide with the maximum clearance area.This is because the rotation of the rotor entrains the sealing fluid with a certain tangential velocity, so there is an angle between the pressure distribution area and the clearance area.Under the same operating conditions, the greater the radial installation error, the more obvious the extrusion of the flow field, and the higher the non-uniformity of the pressure field throughout the flow field.At the same time, the feedback force (fluid force) of the gap seal flow field on the rotor is also greater.
Figure 11 shows the force diagram of the oil film on the rotor under radial error.Under the condition of radial installation error, the rotor is eccentric, which is equivalent to a squeezing effect of the rotor on the flow field, forming a non-uniform clearance flow field.At this time, a non-uniform pressure distribution is formed in the clearance flow field.Under the action of circumferential pressure difference, the rotor is subjected to the feedback force of the sealing fluid, and the direction of the force is from the high pressure zone to the low pressure zone.The force is obtained by integrating the pressure along the circumferential direction, which is the resultant force of the whole sealing fluid on the whole circumferential surface of the rotor.The surface force is equivalent to a concentrated force of F, which acts on the center of the rotor.There is a component force Fx in the X-axis direction and a component force Fy in the Y-axis direction.Table 2 shows the force of the oil film on the rotor under different radial errors, where the negative sign represents the direction of the force is opposite to the coordinate axis.Table 2.The force of oil film on rotor under radial error.
Force of clearance oil film on rotor Fx (N) Fy (N) The eccentricity is 0.1 mm -410 -848 The eccentricity is 0.2 mm -776 -1850

Analysis of Rotational Motion Error Influence
Figure 12 shows the influence of radial installation error and rotation motion error on leakage.Both radial installation error and rotation motion error will cause leakage to increase.The radial installation error is 0.1mm, which will cause leakage to increase by about 0.68 %.The rotor whirl (rotation motion error) causes the leakage to further increase by about 0.15 %.As the whirling speed increases, the leakage does not change significantly.The rotation motion error of the rotor is mainly concerned with two characteristic quantities, namely, whirling speed and amplitude.Figure 13 shows the influence of whirling speed on leakage under different rotating speed conditions.It can be seen from the figure that the higher the rotating speed, the smaller the leakage.Under the same rotating speed, the leakage is not sensitive to the whirling speed.Under the same amplitude condition, with the increase of whirling speed, the leakage is basically unchanged.Figure 14 is concerned with the influence of different vibration amplitudes on the leakage.The abscissa is the rotational speed, and the ordinate is the leakage.As the rotational speed increases, the leakage decreases.When the rotor has a rotation motion error, the leakage increases, and the leakage increases with the increase of the amplitude of the rotation motion error.When the amplitude is 0.1 mm, the leakage increases by about 1 %.When the amplitude increases to 0.2 mm, the leakage increases by about 5.2-10.8%. Figure 15 shows the influence of clearance oil film on rotor force under the condition of rotation motion error.Table 3 shows the magnitude of the gas force on the rotor under the rotation motion error.The higher the whirling speed, the greater the fluid force of the oil film on the rotor.The specific force analysis, the relationship between the direction of the fluid force and the

Conclusion
In this research, the tin bronze clearance seal of marine clutch rotary joint is studied.The specific conclusions are as follows: (1) Compared with the absence of roundness error, the roundness error of the shaft will increase the leakage by about 2.5-4.9 %.The cylindrical error of the sleeve will reduce the leakage, but it will bring more serious friction, wear and life problems.
(2) The radial installation error will increase the leakage.When the radial error is 1 ‰, the leakage increases by 0.65-0.72 %, and the greater the radial installation error, the more obvious the extrusion of the flow field, and the higher the non-uniformity of the pressure field throughout the flow field.For every 1 ‰ increase in angular error, the leakage is reduced by about 8 %.However, the angular installation error will cause more serious friction and wear, resulting in a sharp increase in temperature.
(3) When the radial installation error is 0.1 mm, the leakage will increase by about 0.68 %, and the rotor whirl (rotation motion error) will further increase the leakage by about 0.15 %.When the rotor has a rotation motion error, the leakage increases with the increase of the amplitude of the rotation motion error.When the amplitude is 0.1mm, the leakage increases by about 1 %.

Figure 1 .
Figure 1.Numerical model of clearance seal fluid field.

Figure 2 .
Figure 2. The high-pressure and high-speed rotary seal test device and the tin bronze clearance seal test section.

Figure 3 .
Figure 3. Leakage result of tin bronze clearance seal.

Figure 4 .
Figure 4.The influence of shaft roundness error on leakage.

Figure 5 .
Figure 5.The influence of cylindricity error on leakage.

Figure 8 .
Figure 8.The influence of radial installation error on leakage.

Figure 9 .
Figure 9.The influence of angular error on leakage.

Figure 10 .
Figure 10.The influence of radial error on the pressure distribution of clearance cross section.

Figure 11 .
Figure 11.The force of oil film on rotor under radial error.

F
rotor motion, and the influence of the fluid force on the rotor stiffness, damping and rotor stability need further in-depth research.

Figure 12 .
Figure 12.Comparison of the influence of dynamic and static eccentricity on leakage.

Figure 13 .
Figure 13.The influence of whirling speed on leakage.

Figure 14 .Figure 15 .
Figure 14.The influence of rotation motion amplitude on leakage.

Table 3 .
The magnitude of the fluid force acting on the rotor under the rotation motion error.