Abstract:
To enhance the operational performance of the hydrostatic spindle in high-speed CNC gear shaping machines, this study explores the influence of oil groove structures, based on tribology and fluid dynamics theories. Through theoretical analysis and numerical simulations, the effects of oil grooves on oil film bearing capacity, stiffness, shear force, and maximum deformation of the spindle sleeve under various operating conditions are investigated. Additionally, transient calculations analyze the impact of oil groove structures on viscous heat generation in the oil film over ten strokes. Furthermore, the influence of different oil groove shapes on oil film bearing capacity and stiffness is explored. Experimental data from a hydrostatic guideway test platform validates the finite element simulation model. The results indicate that incorporating oil grooves increases oil film bearing capacity and stiffness by approximately 1.5 times, slightly reduces viscous resistance, and stabilizes oil film temperature. The study provides valuable insights for optimizing structural parameters and facilitating the implementation of hydrostatic spindles in high-speed gear shaping machines.
1. Introduction
With the continuous development of industrial intelligence in China, the demand for small-modulus high-precision gears has increased annually, posing higher requirements for the production efficiency and processing accuracy of high-speed gear shaping machines. Traditional gear shaping machine spindles suffer from dry friction and shaft locking under high stroke conditions, leading to reduced efficiency and limited application. The adoption of hydrostatic spindles, characterized by high stiffness, high stability, low friction, and minimal wear, effectively improves production efficiency. However, under continuous high-stroke conditions, local friction and wear issues persist in hydrostatic spindles of gear shaping machines.
2. Literature Review
Research on optimizing the structure of hydrostatic bearings to enhance performance has been extensively conducted. SATISH C.S. et al. investigated the impact of different oil chamber shapes on the dynamic and static performance of circular thrust bearings and proposed appropriate throttle compensation devices. Other researchers have focused on optimizing structures for high-speed rotating conditions of hydrostatic spindles, deriving theoretical models for bearing performance with controllable restrictors. Zhao Y.F. et al. explored the influence of surface textures on performance, proving that they can improve load-bearing capacity and reduce friction. Studies on precision gas hydrostatic spindles have also shown that incorporating pressure-equalizing grooves enhances performance. However, research on optimizing hydrostatic spindle structures for the specific motion conditions of high-speed gear shaping machines is relatively scarce.
3. Methodology
This study focuses on the hydrostatic spindle of a high-speed gear shaping machine, incorporating an oil groove structure on the circumferential oil-sealing surface. Simulations based on actual operating parameters explore the impact of oil grooves on spindle oil film bearing capacity, stiffness, and viscous heat generation, as well as the influence of different oil groove shapes.
3.1 Simulation Model and Parameters
The hydrostatic spindle is externally lubricated, with oil passing through multi-stage filters, check valves, and restrictors before forming a viscous oil film between the spindle body and sleeve. The finite element model for fluid simulation of the hydrostatic spindle parameters listed in Table 1.
| Parameter/Unit | Value |
|---|---|
| Spindle body diameter d/mm | 90 |
| Spindle clearance h0/mm | 0.02 |
| Oil chamber axial dimension L/mm | 100 |
| Axial oil-sealing edge l/mm | 10 |
| Oil density ρ/(kg/m³) | 880 |
| Oil specific heat capacity C/[J/(kg·K)] | 1884 |
| Oil chamber circumferential dimension B/mm | 51 |
| Circumferential oil-sealing edge b/mm | 19 |
| Oil chamber angle θ/° | 60 |
| Oil-sealing cavity depth H/mm | 2 |
| Oil viscosity μ/Pa·s | 0.045 |
| Thermal conductivity K/[W/(m·K)] | 0.132 |
3.2 Influence of Oil Grooves on Spindle Performance
3.2.1 Oil Film Bearing Capacity and Stiffness
Simulation models with and without oil grooves were established under the same boundary conditions. Pressure distribution in the oil film with ε=0.3. The results indicate that pressure decreases sharply from the circumferential oil-sealing edge to the outlet, remaining constant within the oil chamber. With no oil groove, pressure distributes across the pressurized oil chamber and partial oil-sealing surface; with an oil groove, pressure concentrates in the pressurized oil chamber, reducing pressure on the oil-sealing surface.
The influence of oil grooves on oil film bearing capacity. Oil film bearing capacity increases with eccentricity but the increase diminishes with higher eccentricity. At low eccentricity, the bearing capacity with an oil groove is 1.5 times that without; this ratio decreases to 1.3 at higher eccentricities. Oil film stiffness trends, decreasing with increasing eccentricity.
3.2.2 Oil Film Viscous Resistance
The main resistance in stroke motion is the viscous shear force generated by the viscous oil film between the spindle body and sleeve. Shear force distribution with V=2m/s. Oil film shear force decreases with increasing oil film thickness. The oil film shear force at the oil chamber is significantly smaller than at other locations.
Flow monitoring results show uneven flow distribution on the same oil-sealing surface, with flow at the minimum oil film thickness significantly less than other locations. Incorporating an oil groove concentrates viscous shear force on the oil-sealing surface opposite the direction of motion, reducing the oil film area and increasing oil flow, thereby improving the overall lubrication environment. Viscous resistance trends with different eccentricities.
3.2.3 Oil Film Temperature Variation
During cutting, the main motion of the hydrostatic spindle is high-speed reciprocating stroke motion. As stroke number increases, oil film temperature rises due to viscous shear heat generation. Some heat exits with the oil film outlet flow; however, accumulated heat at varying stroke speeds can elevate oil film temperature. Temperature variation trends. Initially, oil film temperature rises; as strokes increase, the average temperature of oil films without oil grooves gradually increases, while those with oil grooves fluctuate around 28°C.
3.2.4 Sleeve Deformation
Excessive hydrostatic oil film pressure can deform the cast aluminum bronze alloy sleeve, affecting spindle performance. Deformation distributions for models without and with oil grooves, respectively. The maximum deformation position differs with eccentricity. Without oil grooves, maximum deformation occurs at the circumferential oil-sealing surface at low eccentricity (ε<0.1), transitioning towards the oil-sealing cavity center with increasing load. At high eccentricities, maximum deformation occurs at the pressurized oil-sealing cavity center and partial circumferential oil-sealing surface.
3.3 Influence of Oil Groove Shape on Bearing Performance
Models with rectangular, circular, triangular, and trapezoidal cross-sectional oil grooves were simulated under the same conditions. Oil film bearing capacity trends for different shapes. The influence of oil groove shape on bearing capacity is minimal. However, stiffness varies with eccentricity, with circular and trapezoidal grooves showing initial slight increases followed by decreases, while rectangular and triangular grooves decrease gradually, with triangular grooves showing the fastest attenuation.
4. Experimental Validation
To validate the simulation model, a cylindrical hydrostatic guideway stiffness testing platform was built.
5. Conclusions
Addressing the unclear impact of oil return groove structure on the performance of hydrostatic spindles in gear shaping machines, various oil film models with different oil return groove configurations were established, and finite element method (FEM) simulations were conducted. Based on actual operating parameters, the influence of oil return groove structure on the performance of hydrostatic spindles in high-speed gear shaping machines was investigated.
The study drew the following conclusions:
- Compared to oil films without oil return grooves, those with oil return grooves exhibit a more concentrated pressure distribution at the oil sealing chamber. The addition of oil return grooves can increase the oil film’s carrying capacity and stiffness by 1.3 to 1.5 times. Under the same operating conditions, the shear force of the oil film on the oil sealing surface positioned on the same side of the movement direction is greater than that on the opposite side. The oil return groove structure can effectively enhance oil flow, reduce the oil film area on the oil sealing surface, and decrease the viscous resistance during spindle movement.
- Incorporating oil return grooves on the oil sealing surface can effectively mitigate the viscous shear heat generation during the spindle’s stroking motion. The oil return groove structure reduces the area of concentrated oil film temperature on the oil sealing surface and increases the oil flow within the sealing surface, facilitating the removal of viscous temperature rise with the oil flow. Compared to structures without oil return grooves, those with oil return grooves can maintain the average oil film temperature at a relatively stable level of approximately 28°C over multiple strokes.
- When the hydrostatic spindle is not eccentric, the maximum deformation of the hydrostatic sleeve without oil return grooves occurs at the circumferential oil sealing surface, while with oil return grooves, it occurs at the center of the oil sealing chamber. When the hydrostatic spindle is loaded and eccentric, the maximum deformation of the sleeve with oil return grooves remains at the center of the oil sealing chamber, whereas for the sleeve without oil return grooves, it occurs at both the oil sealing chamber and part of the oil sealing surface. At lower eccentricity rates, the sleeve deformation is relatively larger for structures without oil return grooves; at higher eccentricity rates, the deformation is relatively larger for structures with oil return grooves.
- The shape of the oil return groove has a relatively minor impact on the oil film’s carrying capacity but can significantly affect the oil film’s stiffness at certain eccentricities.
In subsequent research, the authors aim to explore the influence patterns of actual operating conditions on the lubrication characteristics of hydrostatic spindles in high-speed gear shaping machines, with the goal of enhancing the machine’s work efficiency and prolonging the service life of its spindles.