Improved Sealing Structure of Commercial Vehicle Wheel Hub Bearing Units and Its Experimental Verification

Abstract;

Commercial vehicle wheel hub bearing units are maintenance-free, placing higher demands on their sealing structures. The grease leakage problem in the original sealing structure was analyzed as the primary cause, which was lubricating oil entering the bearing interior through the outer and middle seals. Improvements were proposed, including controlling the spiral pattern on the outer diameter surface of the inner ring at the outer seal position, adding an oil return line, and modifying the middle seal from a square to an O-ring + support seat structure. The improved sealing structure was subjected to an oil inlet bench test and a mud and water test on the wheel hub bearing unit. No oil inlet occurred, demonstrating the effectiveness of the improved structure.

The primary function of the seal in commercial vehicle wheel hub bearing units is to prevent contaminants from entering the bearing and grease loss during operation. To ensure the long life and maintenance-free performance of the wheel hub bearing unit, higher requirements are placed on its sealing performance.

1 Analysis of Wheel Hub Bearing Unit Seal Failure 1.1 Original Seal Structure Wheel hub bearing units are grease-lubricated. The sealing structure prevents external mud, water, and lubricating oil from entering the bearing interior, as well as grease overflow, which could cause grease dilution failure; and prevents external debris and dust from entering the bearing interior, which could accelerate bearing fatigue failure. The original sealing structure is shown in Figure 1, which mainly includes outer, middle and inner seals. The sealing structures in different positions will also be different. The outer seal and inner seal adopt a composite sealing structure with an annular spring and a stamped retaining ring to prevent impurities such as mud and water from entering the bearing and grease from overflowing, and minimize the friction caused by the seal.

The three sealing structures and their functions of the rear axle wheel hub bearing unit are shown in Table 1. The inner sealing ring is assembled by automatic line or manually pressed. The main lip of the sealing ring cooperates with the L-shaped retaining ring, and the auxiliary lip, L-shaped retaining ring and bearing flange are interference fit, thus achieving a sealing effect. Table 1 Sealing structure and function of each position of the wheel hub bearing unit

The inner ring of the wheel hub bearing unit and the shaft generally have a clearance fit. The lubricating oil in the drive axle differential can easily enter the bearing through the gap between the half-shaft and the bridge housing, causing grease dilution and failure. Solving the grease leakage problem at the end face fit of the two inner rings is a technical requirement put forward by the axle manufacturer for bearings. 1.2 Analysis of oil inflow failure After disassembling the wheel hub bearing unit that was leaking grease, it was found that there was oil stain on the outer surface of the inner seal, while there was no oil stain on the outer surface of the outer seal, indicating that the grease mainly leaked from the inner seal. At the same time, there was diluted grease inside the bearing. This was due to the intrusion of lubricating oil or the leakage of the base oil of the internal grease, causing the grease to become thinner and overflow from the inner seal. The main function of the inner seal is to prevent foreign matter such as mud and water from entering, and to prevent grease leakage. However, after the lubricating oil enters the bearing, the oil will leak from the inner seal. If the outer seal and the middle seal are not filled with oil, the inner seal will not leak grease. Therefore, this article improves the structure of the outer seal and the middle seal to solve the oil inflow problem.

2 Improvement measures 2.1 Improvement of the outer seal structure

 1) Control the spiral pattern on the outer diameter surface of the inner ring. During centerless grinding, the guide wheel and the grinding wheel make relative axial movement, which will cause spiral patterns on the surface of the inner ring. The lubricating oil on the left side of the outer seal will enter the bearing through the spiral patterns on the end face of the inner ring large rib (Figure 2). Therefore, it is proposed to improve centerless grinding to plunge grinding and strictly control the spiral patterns on the end face of the inner ring large rib. The spiral patterns are measured using the plumb method. For details, please refer to JB/T 14580-2023 "Rolling Bearings, Commercial Vehicle Wheel Hub Bearings and Units". The twist measuring instrument can achieve accurate measurement of the spiral patterns.

2) Add an oil return line to the seal ring. Because the outer seal lacks an oil return line, allowing oil to easily enter, a proposal was made to add an "8"-shaped oil return line to the outer seal, as shown in Figure 3. This allows the lubricating oil pumped into the lip to return to the oil seal side. Fault reproduction tests showed that when the oil return line is 0.1 mm from the main lip, the oil return effect is better.

2.2 Improvement of the intermediate seal structure The original square intermediate seal was mainly designed with reference to the Schaeffler bearing. During installation, the seal is squeezed axially but not radially. Lubricating oil can easily enter the bearing through the gap between the half-shaft and the bridge housing and between the two inner rings, resulting in grease dilution and failure. In addition, due to the lack of radial extrusion, slippage is likely to occur during installation and under bending loads, leading to flipping. The intermediate seal was improved to an O-ring + support seat structure, and the mating part with the small end of the inner ring was processed into a crescent groove shape, as shown in Figure 4. Through design, the O-ring is subjected to pressure generated by interference fit in both the axial and radial directions during installation, thereby better fitting with the crescent groove. The average radial compression rate of the intermediate O-ring is 23.6%, which meets the requirements of GB/T 3452.3-2005 "Groove dimensions of O-ring rubber seals for hydraulic and pneumatic applications" and has a good sealing effect.

3 Experimental Verification

3.1 Outboard Seal Oil Inlet Bench Test

The effectiveness of adding an oil return line can be verified through a fault reproduction test, as described in Section 2.1. This test primarily compared the sealing performance of hub bearing units with and without spiral grooves. The center seal employed an improved structure, and an oil return line was added to the outboard seal. The test conditions are shown in Table 2. The two steps were repeated cyclically, with grease maintained at the centerline of the seal ring. The test was terminated after 1012 hours. The hub bearing unit with spiral grooves before the modification exhibited oil ingress, while the hub bearing unit without spiral grooves after the modification did not exhibit oil ingress, demonstrating the effectiveness of the improved outboard seal.

3.2 Intermediate Seal Oil Intake Bench Test

The outer seal adopts an improved structure. An intermediate seal oil inlet bench test device, shown in Figure 5, was developed to improve the existing test equipment. This device adds an oil reservoir between the bearing and shaft. After the bearing is installed, oil is added through the oil filling hole in the fixture. The oil flows through the hole in the center of the fixture and fills the space between the bearing inner diameter surface and the shaft. Since oil only accumulates here, and both end faces are sealed, the bearing can only receive oil from the intermediate seal. After the test, the grease condition is observed to determine whether the intermediate seal has received oil.

The grease used was Hande special grease HD-R02. The test was terminated after 696 hours of operation under operating conditions with an axial load of 45 kN and a radial load of 135 kN. The bearings before the modification experienced oil ingress, while the bearings after the modification did not, demonstrating the effectiveness of the improved intermediate seal.

3.3 Mud and Water Test of Wheel Hub Bearing Unit Seals The test method was based on JB/T 13353-2017, "Rolling Bearings - Test and Evaluation Methods for Automobile Wheel Hub Bearing Units," and JB/T 10238-2017, "Rolling Bearings - Automobile Wheel Hub Bearing Units." The axial tightening torque was 1000 N·m. During the pre-operation phase, the axial load was 0 kN, the radial load was 56.35 kN, the speed was 200 r/min, and the loading time was 4 hours. The test was conducted in two phases. The operating conditions are shown in Tables 3 and 4. A mud testing machine for heavy-duty truck wheel hub bearing units was used. The test principle is shown in Figure 6. The angle between the water spray direction and the horizontal direction is 65°, the distance between the water spray port and the end face of the sealing ring is (60±10) mm, the water spray port flow rate is 4 L/min, the water spray port diameter is 5 mm, the number of water spray ports is 2, and the axial and radial loading offsets (the distance from the loading center to the inner bearing end face) are 526 and 70 mm, respectively.