Helical groove formation of rolling/sliding pairs on the oil-lack lubrication

Wenjing Zhang^{1, 2}, Wei Chen¹, Ang Liu ²

¹School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an, CHN

²School of Mechanical Engineering, University of New South Wales, Sydney, AUS

INTRODUCTION: Pure rolling contacts in a cylindrical roller bearing are ideal and perfect, but actually contacts of a cage with rollers and other components shouldn’t be ignored. The contacts will inevitability lead to sliding friction.¹ Most frictions wears in a bearing are led by sliding movements, including circumferential sliding/rolling and axial reciprocating sliding. And a tilting angle can be used to describe the severity of misalignment.^2,3 In addition, edge stress in the line contact of a partial crowning roller will be quickly increased by tilt, which necessarily lead to surface distress but contributes to asperities flattening, even with a large slide-roll ratio². Otherwise, under some extreme work conditions, especially without satisfied oil supply, early failures of a rolling element momentarily occur.⁴ In general, a spiral groove seldom appears as a type of abrasive of bearings, but it is quite common during grinding machining process of cylindrical roller bearings.⁵ In this work, with multi-factors for extreme conditions, experimental investigation on wear mechanism and helical groove formation of rolling pairs would be conducted in both macro and micro scale. Also the effects of radial sliding on axial groove formation would be got.

METHODS: Test rig of roller bearings with sliding contacts can be seen in Figure 1. VFD of low-speed motorized spindle (4) was used to get different ring speeds n,  given at the values of 300 r/min, 500 r/min, 700 r/min and 900 r/min. Radial load F (500 N) can be got with a device (8) and measured with a sensor (7). Roller specimens with partial crowning were used in this test. Because for the radial clearances (< 10 µm) of the support bearings (5), a small tilting angle α (0.001 rad) of a roller specimen (3) can be set with a small variation of the platform height (9), while the height of inner ring (6) was invariant. At axial direction, reciprocating sliding displacements would be led by elastic deformations of the spring coupling (2), shaft clearances of support bearings and the fit allowance (≈ 0.5 mm) between the support bearings (5) and the loading device (8). At circular direction, circumferential slip ratio s can be got with different couples of ring speed and ring speed, i.e. the values of 0, 0.2, 0.4, 0.6 and 0.8. The equation of slip ratio² is, , where , , is the

diameter of a roller,is the pitch diameter of a rolling element, is the ratio of  to . In fact, speed value w( w=2πn/60 ) of inner race (6) on the test rig is the difference-value of inner ring speed  and revolution speed of roller, i.e. .

Figure 1 - Test rig of roller pairs⁶:(1) motorized spindle with high-speed (2) spring coupling (3) roller specimen (4) spindle with low-speed motorized (5) spring bearings 6000z (6) bearing inner race NU1019 (7) loading sensor (8) Loading device (9) lifting platform.

RESULTS: In this work, sliding frictions led by cage movements were concerned. Under an oil-lack condition, an interesting helical groove is led by roller tilt and sliding contacts (at axial direction 

and circumferential direction), as shown in Figure 2. Roughness profiles got by PGI 3D Profiler can be seen in Figure 3.

            (a)                            (b)                               (c)                             (d)

Figure 2 - Confocal images of helical groove on a roller (s=0.6, n= 300r/min, F=500N): (a) roller tested with sliding; (b) left edge of a roller; (c) middle part of a roller; (d) right edge of a roller.

Figure 3 - Profiles of roller grooves under different conditions

DISCUSSION: With a loss of lubricant, oil film between roller pairs becomes thinner, and elastic-plastic asperities micro-contact occur momentarily.⁴ Local overload area led by edge stress and a tilting angle was aggravated, and solid phase welding occurs at the right edge of a roller. In the effects of sliding frictions (both at axial direction and circumferential direction), the surface material at one roller edge was teared. At this stage, adhesion was the main wear mechanism. Wear debris were produced and then initial micro-grooves emerged (Figure 2 (d)), which could lead to a secondary damage. At the next stage, transfer debris was compressed and sheared by a radial load and axial sliding friction. Moreover, a tilting angle of roller pairs led to another axial force to wear particles, which attributed to the axial stretch of a circular groove. Pile-up phenomenon of transfer particles appears, and groove dimensions became larger. As a result, at a low rotating speed, with the synthetic action of roller tilt, axial sliding friction and circumferential slip ratio, a special helical groove on a roller (Figure 2) appears. Moreover, axial helix pitch of a helical groove increases with an increasing of rotating speed and a decreasing of circular slip ratio. Width values of wear grooves seem similar, while depth values of grooves become smaller, while the slip ratio increases and rotating speed decreases (Figure 3).

REFERENCES: 1. SKF. SE: Palm. Tryc. AB (1994), 2. Zhang, Ind. Lubr. Tribol. (2017), 3. Harris. CRC Pr. (2006), 4. Hutchings. Edw. Arn., Lon. (1992), 5. Shi. BRG. (2004), 6. Wu, CHN: CN1061 98019A (2016).

ACKNOWLEDGE: This study is sponsored by China Scholarship Council.