Tribology Characteristics of Hexagonal Shape Surface Textured Reduction Gear in Electric Agricultural Vehicle

Abstract

An experimental study was conducted on the wear and friction responses in sliding tests of a micro-textured surface on laser pattern (LP) steel as reduction gear material in electric guided vehicle. In this research, the friction characteristics of laser pattern steel under different micro texture density conditions were investigated. The friction tests were carried out at sliding speeds of 0.06 m/s to 0.34 m/s and at normal loads of 2 to 10 N. Photolithography method was used to create the dimples for surface texturing purpose. Four different specimens having different dimple densities of 10%, 12.5%, 15%, and 20% were observed respectively. In this research, friction conditions as shown in Stribeck curve were investigated. Furthermore, the microscopic surface was observed using scanning electron microscope. It was found that the dimple density had a significant role on the friction characteristics of laser pattern steel conditioned as reduction gear material in an agricultural vehicle. The duty number showed that the friction condition was hydrodynamic regime. The best performance was obtained from 12.5% dimple density with lowest friction coefficient achieved at 0.018771 under the velocity of 0.34 m/s and 10N load.

1. Introduce

Improving tribological performance can reduce losses from engine [1]. The energy loss in machine operation mainly is caused by wear and friction. Therefore, by improving tribological performance, the total energy loss can be reduced significantly.

The improvement of surface finish such as lapping [2] or superfinishing [3] is common methods to decrease the friction condition. Another promising method to reduce the friction of contact elements is surface texturing [4-7]. The micro texture can be obtained using abrasive machining or laser beam [8]. In this research, photolithography and wet etching as in [9] were used to generate the texture.

Surface textured may play a role as lubricant reservoir, increasing the thickness of the lubricant film between the mating surfaces, and decreasing friction. Furthermore, the dimples serve as pockets for wear particles, preventing debris from further damaging the substrate surface through plowing and third body abrasion. The third-body abrasion is also known to cause resistance due to the hydrodynamic disturbance of the lubricant during sliding motions, resulting in an increased coefficient of friction.

Different surfaces with different micro holes depths exist, minimizing friction at the interface. More so, the distribution of micro-dimples is an important factor against frictional characteristic [8].

The conditions underlying surface texturing were divided by worn fragments trapping, lubricant reservoir and hydrodynamic. However, it depends on the size and density of the texture, as well as the depth of the pattern even if each theory about friction characteristic on surface texturing are reasonable. As a result, it can be assumed that the surface texturing density and depth of the hole have roles to play in surface texturing for low-friction materials.

The recent studies tested the friction coefficient for 40 µm, 200 µm, and 300 µm surface dimple patterns. In several studies, grooves can be created by a mechanical process or by using a laser. A different method was applied in this study, where photolithography was used to generate the dimple. The mechanism of surface texturing was decided by a worn fragments trapping, lubricant reservoir and hydrodynamic, but each theory had propriety which are dependent on the density, depth, and dimension of the pattern. Consequently, there had been no complete study on surface texturing till date.

2. Material and Methods

The material of pin specimen used in this study was laser pattern steel. The pin had diameter and thickness of 4 mm and 1 mm respectively. Photolithography through wet etching was used to form micro-dimple patterns on the surface. The diameter of dimple circle was designed to be 100 µm. The depth of dimple was designed to be 20 µm. Hexagonal pattern was chosen to form a honeycomb-like structure. The disc was made of bearing steel material with a diameter of 60 mm, thickness of 5 mm and surface roughness Ra of 0.039 mm after polishing. The dimple density is defined as the ratio of dimple area to the total pin area. As a result, it is proportional to the number of dimples.

Fig. 1 showed the dimple pin conditions under electron microscope with 25x magnification.

Fig. 1 Scanning electron microscope dimple pin density before experiment (a) 10%, (b) 12.5%, (c) 15%, and (d) 20%.

Fig. 2 showed the dimple pattern in the hexagonal array, as observed under an electron microscope with 400x magnification. The dimples were arranged in a symmetrical hexagon, forming a honeycomb-like structure. This configuration provided symmetry in the dimple distribution.

Fig. 2 Hexagonal array observed under scanning electron microscope (SEM).

Sliding tests were performed using a pin-on-disk tribometer, as shown in Fig. 3. Several variations of normal load were applied on the pin using a metal rod. The contact area between the pin and disc was immersed in lubricant. The friction force between the specimens was monitored during the tests. The measured values obtained were stored in a data files, and the data acquisition rate was 0.1 Hz.

Fig. 3 Schematic of experimental apparatus.

Table 1 showed the test conditions with densities ranging from 10% to 20%, and the velocity ranging from 0.06 m/s to 0.34m/s.

Table 1. Summary of Test Conditions

Fig. 4 showed the typical Stribeck curve that described each friction region by lubrication parameters. In this research, the duty number was observed to identify the lubrication regime. Duty number is a dimensionless number as a function of velocity in meter per second (m/s), viscosity in Pascal-second (Pa s), load in Newton (N), and pin diameter in millimeter (mm). The duty number is expressed as follows:

duty number= (viscosity × velocity)/load

Fig. 4 Stribeck’s curve.

3. Result and Discussion

3.1 Friction Characteristics

The experimental results revealed several graphical tendencies in each variation. In the initial stage of the experiment, the sliding speed was 0.06m/s. Fig. 5 depicts the friction coefficient as a function of load at 0.06m/s. A dimple density of 12.5% provided the lowest friction force at a load ranging from 2N to 10N. The highest friction force was observed at 10% pin density. At higher speed, the friction force was stable and there was no significant fluctuation experienced in the friction coefficient under these conditions.

Fig. 5 Friction coefficient as a function of density at 0.06m/s.

Fig. 6 Friction coefficient as a function of density at 0.22m/s.

Fig. 7 Friction coefficient as a function of density at 0.34m/s.

At 15% and 20% dimple density, variations in the load affected the friction force. For 15% and 20% dimple density, the friction force increased according to the dimple density. This phenomenon indicated that pin density had important role to control the friction force in the pin surface.

More so, velocity and load increment played significant roles in this test, as both factors had an effect on the friction force. The graph indicated that the friction force differed with the area density of the dimples. Among the four pin specimens considered, the pin with 12.5% density showed the best performance. The lowest friction force was observed in all sliding speed range.

The friction initially increased, reaching a maximum, and subsequently tended to a constant with an increment in the velocity and load. The dimpled pin with 10% had the lowest slope, which is the ratio between friction force and load. Accordingly, it can be concluded that 10% density has carrier load higher than the other pins. Higher carrier load gave the pin its ability to maintain the friction force stable with lower slope even though the load increased significantly.

There are three kinds of friction as classified on the basis of the Stribeck curve: boundary friction, mixed friction, and hydrodynamic friction. Boundary friction occurs when two surfaces in contact rub against each other. Under this condition, third-body contact may occur.

The dimpled pin surface played the main role in the friction, under the boundary friction conditions. The third-body contact was potentially reduced in this case, as shown in Fig. 8, of the graph for 15% dimple density. Under a load of 2 N and sliding speed of 0.06-0.34 m/s, two regions of friction were seen. The friction coefficient at the lowest sliding speed was initially high, and then decreased to a minimum with an increased sliding speed. The minimum friction coefficient was observed when the sliding speed varied between 0.06 to 0.34 m/s. Other pin densities showed only one region of friction, which developed a fully hydrodynamic lubrication.

Fig. 8 Friction coefficient as a function of duty number at 2N.

Fig. 9 showed another condition with a different load. The graph tendencies were similar in the entire sliding velocity variations. Changes in the graph tendencies showed in Fig. 10, can be seen on pin densities of 12.5%, 15% and 20% .

Fig. 9 Friction coefficient as a function of duty number at 6N.

Fig. 10 Friction coefficient as a function of duty number at 10N.

3.2 Scanning electron microscope image

Fig. 11 showed the scanning electron microscope (SEM) images of the dimpled pin before and after the test, under 400x magnification. Fig. 11 (a) showed the image of the pin surface covered with dimples.

Fig. 11 SEM image of dimpled pin (a) dimple condition before test and (b) dimple condition after test.

Under solid friction conditions, the dimples are in contact with the disc surface. The free area among the dimples acted as a reservoir. The lubricant (paraffin oil) was trapped in this area which resulted in some advantages. The pressure provided by load increment at lower speeds compressed the lubricant, thereby causing the formation of a thin film between the contact surfaces.

The areas that were not covered by dimples acted as trap for worn fragments, such that they would not be welded into the contact surface. Small fragments may be disadvantageous by possibly allowing third-body contact. This condition was shown in Fig. 11 (b). The fragments trapped can be seen clearly in the Fig. 11 (b), and also the sliding direction was indicated by pin surface scratch. Dimples prevented the worn fragment from being welded together with the contact surface; hence, the third body contact was minimized considerably.

4. Conclusion

In this study, the following results were obtained by performing frictional tests on a hexagonal array:

1. The friction coefficient increased with an increase in the sliding speed and decreased with an increment of load.

2. The minimum friction coefficient was generally observed at the sliding with velocity of 0.34 m/s and 10N load.

3. The pin with different dimple densities showed two friction regimes: mixed and hydrodynamic regime lubrication.

4. The friction coefficient of the specimen with 12.5% dimple density was the most effective to reduce the friction for the velocity and load condition studied.

5. Based on these results, it can be indicated that there is a considerable opportunity for the applications of a hexagonal shaped surface textured material to be replicated as reduction gear material in an agricultural vehicle for a sustainable operation condition under minimum quantity lubrication where textured materials are anticipated to lessen material interface friction as well as minimize the equipment wear whilst the re-deposited materials at the rim of the dimple pose as micro-chip breakers.

Acknowlegements

This work was supported by Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, Forestry and Fisheries (IPET) through Agriculture, Food and Rural Affairs Research Center Support Program, funded by Ministry of Agriculture, Food and Rural Affairs (MAFRA) (716001-7)