A calculation method of thermal deformation for double helical gear

Thermal deformation caused by gear transmission is an important factor causing gear impact, vibration and partial load. Gear modification can effectively improve the effects caused by thermal deformation. The calculation of thermal deformation is the first problem to be solved before gear modification. This paper takes double helical gear as the research object and a calculation method of thermal deformation is proposed. Firstly, temperature of instantaneous meshing points on the tooth surface are measured and these discrete temperature values are fitted by the linear interpolation. Calculation formula of temperature distribution along gear radial direction is introduced. Combining both, tooth surface temperature field is obtained. Secondly, equation of tooth surface for double helical gear before and after thermal deformation is derived according to the tooth surface temperature field. Finally, an example is given. Compared with the given modification of thermal deformation, the calculated thermal deformation is almost equal to theoretical value. On this basis, the thermal deformation of double helical gear considering the installation error and machining error is calculated, which provides a theoretical basis for thermal deformation modification of double helical gear.


Introduction
Thermal deformation caused by gear transmission is an important factor causing gear impact, vibration and partial load. Gear modification is an effective way to improve its transmission performance [1][2][3][4]. The calculation of thermal deformation provides a basis for gear modification.
When calculating thermal deformation of gear, the temperature field should be determined first. It consists of two parts: the gear body temperature and the instantaneous surface temperature (it is also known as flash temperature). The instantaneous surface temperature depends on the body temperature. The traditional calculation of gear body temperature has the experience formula of ISO flash temperature criterion and integral temperature criterion, but the calculation results cannot determine the temperature distribution of gear teeth, and cannot meet the requirements of deformation calculation. Bobach [5] analyzed the regularities of distribution of temperature field inside the gear teeth. Qiu Liangheng [6] used finite element method to calculate and analyze the temperature and thermal deformation of gear. The calculated results can be used as the basis for analyzing and calculating the lubricating oil film thickness and gear tooth profile modification. Gong Xiansheng [7] applied the theories of Hertz contact, tribology, gear engagement and heat transfer, established the finite element analysis model of the temperature field of gear body, and obtained the steady-state temperature field of the planetary gear tooth. Patir [8] and Wang [9] used finite element method to estimate the temperature field of spur gear tooth, and made theoretical analysis and numerical calculation on dynamic load and oil film thickness. Li [10] proposed the concept of non-involute characteristics. When the temperature of gear is changed, the theoretical tooth profile and the practical tooth profile are not superposition. On this basis, Wang [11] proposed a method for calculating the thermal deformation of helical gear.
Present experimental measurement technology can get the meshing point temperature value of gear. Reference [1] gives a simplified formula for temperature distribution along gear radial direction, which can be applied in engineering. Based on this, the specific process of the paper is shown in Figure 1. 2 Determination of tooth surface temperature field Figure 2 showed the schematic diagram of determining tooth surface temperature distribution. The temperature values of instantaneous meshing point 1, 2, …, n are firstly measured and fitted. Together with the given temperature formula along gear radial direction, temperature field is determined.

Measurement and fitting of temperature of instantaneous meshing points on the tooth surface
Using the infrared technology, the miniature thermocouple automatic recorder and the simulated heat source infrared technology to measure the temperature of instantaneous meshing points on the tooth surface. The temperature curve in the direction of the meshing line along the tooth width is obtained by the linear interpolation.

Temperature distribution along gear radial direction
Reference [1] gives a simplified formula for temperature distribution along gear radial direction, which can be applied in engineering. The formula can be expressed as where r k is the radius of gear arbitrary circle k, its temperature is t k ; t c is the temperature of gear center; r a is the radius of addendum circle, its temperature is t a ; The unit of temperature in the formula is°C.

Determination of tooth surface temperature field
According to the fitting curve along meshing points and formula (1), the temperature t a of addendum circle along tooth surface can be obtained. And then through formula (1), the temperature of arbitrary point can be calculated, i.e., the tooth surface temperature field is determined.

Determination of thermal deformation of double helical gears tooth surface
According to the structural characteristics of double helical gear, the standard equation for tooth surface is derived. On the basis, the thermal deformation equation of tooth surface is derived according to the tooth surface temperature field.

Standard tooth profile equation of double helical gear
Take one end tooth surface as an example, the coordinate of standard tooth profile is shown in Figure 3. Two tooth profile equation are expressed as Formulas (2) and (3) are the profile equation of left tooth and right tooth, respectively. Where r bi (i =1, 2) is the radius of base circle, m ki (i =1, 2) is the roll angle of point k in the involute.

Standard tooth surface equation
The tooth surfaces of double helical gear is involute helicoid. Figures 4 and 5 are its left and right tooth surface helix diagram, respectively.
The left tooth surface equation is expressed as The right tooth surface equation is expressed as Equations (4) and (5) are all the equation of involute helicoid. Where, r bi (i = 1, 2) is the radius of base circle, m ki (i = 1, 2) is the roll angle of point k in the involute, r ki (i = 1, 2) is the radius of point k in the involute, u ki (i = 1, 2) is the expansion angle of point k in the involute, g ki (i = 1, 2) is the helix angle of ascent of point k in the involute, b ki (i = 1, 2) is the helix angle of point k in the involute.

The tooth surface equation after thermal deformation
The helix diagram of left tooth surface and right surface after thermal deformation is shown in Figures 7 and 8, respectively.
The left tooth surface equation after thermal deformation is expressed as   The right tooth surface equation after thermal deformation is expressed as

Example
Taking the measured temperature gear as an example [1], the material of gear is 45 steel and the coefficient of linear expansion l = 11.6e À6 c À1 , room temperature t 0 = 25°C. Table 1 shows other parameters of the gear. The tooth surface temperature distribution measured by Zhengzhou Institute of machinery [1] is shown in Figure 9. Because the installation error and machining error of double helical gear are not considered, the gear tooth temperature distribution of two tooth surfaces is the same. According to Figure 9, the temperature equation along reference circle axial with a linear velocity of 130 m/s is deduced.   According to formulas (4), (5), (7), (8). the tooth surface before and after thermal deformation are shown in Figures 10-13 (where, the x axis describes longitudinal direction, the y axis describes tooth thickness direction and the z axis describes radial direction).
Because the installation error and machining error of double helical gear are not considered, the thermal deformation of Figures 12 and 13 is the same. Take left tooth surface as an example, the difference of the tooth surface coordinates before and after thermal deformation is shown in Figure 14, the mean value of the difference of the tooth surface coordinates before and after thermal deformation is shown in Table 2. According to reference [1], Table 3 showed the modification of thermal deformation, which corresponds to the deformation of tooth thickness. In this paper, the diameter of reference circle is 199.36 mm, which is close to 200 mm in Table 4. Therefore, the modification can be taken as 0.009 mm. In Table 3, the mean value of the difference of the tooth surface coordinates along tooth thickness direction is 0.009627 mm. The error is one order of magnitude smaller than them.
When the installation/machining error of gear exists, the meshing path of two tooth surfaces change. Reference [12] gave the meshing path of left and right tooth surfaces under installation/machining error of gear (Fig. 15). The temperature of instantaneous contact points on the left/ right tooth surface is assumed that still satisfies formula (9). The thermal deformation of two tooth surfaces is recalculated, the mean value of the difference of tooth thickness direction without and with considering installation error and machining error of gear is shown in Table 4.    It can be found that thermal deformation of left and right tooth surfaces is difference and different modifications of thermal deformation are needed.

Conclusion
The calculation of gear thermal deformation is one of the keys to gear modification. Double helical gear is taken as the research object and a calculation method of thermal deformation is proposed. The temperature field is determined according the measured temperature of instantaneous meshing point and radial temperature formula. According to the structural characteristics of double helical gear, the standard equation for tooth surface is derived. On the basis, the thermal deformation equation of tooth surface is derived according to the tooth surface temperature field. Taking a temperature measuring gears as an example, the tooth surface has changed obviously before and after thermal deformation. Compared with the relevant data given in reference [1], the result is reasonable which verifies the feasibility of the proposed method. By calculating the thermal deformation of two surfaces when the installation error/machining error of gear exists, it can be found that the deformation of two tooth surfaces is different. That is to say, the left and right tooth surfaces should be modified with different amounts.