The Effect of Outer Ring Distortion on Wheel Bearing Friction Torque.
Wheel Bearing Drag
Vehicle fuel economy is comprised of many factors, such as driving behavior, environmental influences, and physical component-level features. One such component-level factor that contributes to fuel economy is wheel bearing friction torque ("drag"). Total wheel bearing drag consists of multiple aspects, including ball or roller bearing friction, preload, seal friction, and internal lubrication characteristics. During the design phase, wheel bearing manufacturers strive to optimize these key features. However, external interactions are also present which affect the wheel bearing drag and thus the contribution to the overall vehicle fuel economy.
The importance of the relationship between wheel bearing drag and fuel economy can be quantified by considering the value of brake drag reduction to vehicle manufacturers. The value of a 1 Nm drag reduction for an average vehicle can range from $20.74 to $30.98 . Since wheel bearing drag is contained within this value, and four bearings are used per vehicle, even small improvements can have a large effect on overall fuel economy.
It should be noted that though wheel bearings are produced in a number of different forms, for the purpose of this study, Generation 3 ball bearings were analyzed. Additionally, since the torque of the CVJ nut could potentially have an effect on drag, this factor needs to be considered during any drag comparison study. Therefore, driven bearings were used during the study in order to include CVJ shafts and nuts.
Interaction of Drag and Durability Performance
The desire to improve fuel economy through lower wheel bearing drag must be balanced with the requirement for wheel bearings to maintain long-term durability performance. These two constraints are often in conflict with each other. For example, one key factor in wheel bearing design is seal lip pressure. Higher lip pressure will improve seal performance for durability, but may have a negative effect on drag. Therefore, it is important to recognize those design factors which can have a positive effect on both drag and durability.
Outer Ring Raceway Roundness Effect on Durability
The wheel bearing outer ring raceway roundness has a direct influence on durability, from both a sealing and a raceway integrity standpoint . Raceway distortion occurs when wheel bearings are assembled to knuckles and secured with fasteners. Raceways that have a highly distorted shape such as the one shown in figure 2 are more susceptible to spalling failure modes and reduced sealing performance.
In order to counteract these effects, the mounting flange design must be optimized in order to minimize raceway distortion. This is often achieved through concavity of the flange, as shown in figure 3.
One case study in particular demonstrates the effectiveness of the mounting flange design optimization. Figure 4 shows the warranty performance of front wheel bearings on a particular vehicle, represented by cumulative IPTV (incidents per thousand vehicles) over 36 months in service. An improvement of approximately 33% was achieved by reducing the raceway distortion through the optimization of the flange design.
OUTER RING DISTORTION EFFECT ON DRAG
In addition to outer ring raceway roundness being a key factor for durability performance, it also has a significant effect on wheel bearing drag. When the raceway is distorted, the localized stress is increased and the balls must expend more energy to travel around the raceway, thus leading to a higher drag value.
One way to influence the roundness of the raceway is through the torque value of the fasteners used for mounting the bearing. Figure 5 shows the difference in measured drag values when using the proper specified mounting bolt torque versus using a very low mounting bolt torque.
However, there are several other factors that could contribute to the raceway roundness and therefore the overall drag performance. In order to assess the effects and potential interactions of these various factors, an L9 design of experiments (DOE) was performed, using a bench-level loaded wheel bearing drag test. The parameter diagram describing the DOE study is shown in figure 6.
The wheel bearing component-level drag is measured using a bench test that was developed to measure drag under various conditions. As shown in figure 7, the bearing is mounted to a rigid fixture and loaded to the vehicle corner weight at a fixed wheel offset, then drag is measured at various speeds in both the clockwise and counterclockwise directions.
DESIGN OF EXPERIMENTS STUDY
The L9 DOE is capable of measuring up to 4 variables at 3 levels . In this experiment, only 3 factors were used and the fourth was set aside as an interaction factor. Table 1 shows the factors and levels used during the DOE.
Test machine size constraints did not permit the actual knuckles to be used during drag measurement. In order to induce various levels of raceway distortion using a steel fixture on the bench test, a method was developed to alter the knuckle mounting flange flatness. Different shim thicknesses and placement locations were experimented with, in order to attain raceway distortion that corresponded with levels seen in actual vehicle conditions.
In the most successful arrangement, three different thicknesses of shims were inserted between the knuckle mounting flange and the test machine fixture as shown in figure 8. This orientation allowed the axial alignment between the bearing and the test fixture to be maintained, and allowed the raceways to be distorted in a constrained manner. This method replicated the distortion seen in vehicle environments and allowed the amount of distortion to be controlled so as to study the effect.
In order to investigate the effect of the knuckle bolt torque as shown in figure 5, the torque was varied from the nominal production specification value to two values that were 10% and 25% lower than nominal. Torque values that are too low may lead to inadequate clamping loads at this fastener joint. Higher torque values were not considered due to the risk of fastener yield.
The CVJ nut torque was also included as a factor, with values that varied from the nominal production specification to values that were 10% lower and 10% higher than nominal. This aspect was considered due to the fact that during wheel bearing validation testing, drag values are often lower when the CVJ is not present, as demonstrated by the example component data in figure 9.
Two noise factors were considered for the experiment. Rotational direction of the bearing can sometimes have an effect on the drag value, so data were recorded in both clockwise and counter-clockwise directions and those were treated as two different runs for the same test condition. In addition, data were recorded at four speeds (950 rpm, 800 rpm, 600 rpm, and 50 rpm); though in order to simplify the data analysis, only data points from the highest speed were used for comparison purposes.
Response plots from the experiment are shown below. The mean graph, shown in figure 10, shows the effect of the control factors on the drag. As expected, the shim thickness, which simulates raceway distortion, has the largest effect. Knuckle bolt torque in essence had no effect, which is perhaps due to the fact that the bearing was mounted to a rigid steel fixture instead of an aluminum knuckle. The CVJ nut torque had a slight effect, particularly at the higher value. The signal-to-noise graph, shown in figure 11, shows the sensitivity of each control factor. A higher value is desirable since this indicates lower variation between tests for that factor.
Outer Ring Distortion Effect on Drag
After the drag was recorded, the parts were disassembled and the raceway roundness of each part was measured while under the same test conditions used on the drag test bench. In order to simulate the mounting strategy used during the drag test, the bearing outer ring was mounted to a steel plate with the same shim thickness and knuckle bolt torque used during the test, as shown in figure 12, and the measurements were performed with a roundness trace machine. This permitted the drag to be plotted with respect to the roundness value of a particular measurement point. The relationship between the drag (averaged between clockwise and counterclockwise values) and the roundness of a point on the raceway is shown in figure 13. A baseline data point is also included for comparison purposes, from a sample that was tested with no shims, nominal knuckle bolt torque, and nominal CVJ nut torque. For this set of data, the drag increased approximately 5% for each additional micron of distortion.
There is a clear relationship between outer ring raceway distortion and drag. Higher distortion after mounting the bearing to the vehicle leads to higher drag values.
Although knuckle bolt torque and CVJ torque must be specified in order to provide adequate fastening performance, optimizing these values can help minimize outer ring distortion when the bearing is mounted into the knuckle. Consequently, the effect on wheel bearing drag should be considered when determining or modifying these fastener bolt torques.
While sealing performance and drag are often in conflict during wheel bearing design, low raceway distortion is one factor that has a positive effect on both. The testing performed shows the importance of maintaining the outer ring raceway roundness in order to reduce friction within wheel bearings. The DOE also shows that increased distortion in the bearing increases the variation in drag. Therefore, any potential distortion effects must be minimized in order to optimize vehicle fuel economy.
[1.] Antanaitis, D., "Vehicle Level Brake Drag Target Setting for EPA Fuel Economy Certification," SAE Int. J. Passeng. Cars--Mech. Syst. 9(3):1157-1171, 2016, doi: 10.4271/2016-01-1925.
[2.] Lee, S., Lee, N., Lim, J., and Park, J., "The Effect of Outer Ring Flange Concavity on Automotive Wheel Bearings Performance," SAE Int. J. Passeng. Cars--Mech. Syst. 9(3):1264-1269, 2016, doi:10.4271/2016-01-1958.
[3.] Taguchi, G., and Konishi, S., S., "Taguchi Methods: Orthogonal Arrays and Linear Graphs--Tools for Quality Engineering,". American Supplier Institute, Center for Taguchi Methods, Allen Park, MI:, ISBN 0-941243- 01-X, 1987.
The author gratefully acknowledges the assistance of Mr. Jon Washington and Mr. Robert Sutherlin of the General Motors Brake Group, and Mr. Christopher Evitts, Mr. Rick McFarland, and Mr. Mike Nellett of the General Motors Vehicle Development Technical Support Group.
Generation 3--Type of wheel bearing utilizing two raceways where the outboard raceway is integrated into the wheel hub
CVJ--Constant velocity joint, also called a half-shaft
DOE--Design of experiments
drag--Wheel bearing friction torque
driven--Type of wheel bearing that contains CVJ splines and is used with a half-shaft
IPTV--Incidents per thousand vehicles
L9--For this paper, an orthogonal array with 9 test runs that permits up to 4 variables at 3 different levels
rpm--Revolutions per minute
General Motors LLC
Table 1. Summary of DOE control factors. Control Factor Level 1 Level 2 Level 3 A Shim Thickness 100 [micro]m 300 [micro]m 500 [micro]m B Knuckle Bolt Torque -25% -10% Nominal C CVJ Nut Torque -10% Nominal +10%
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|Publication:||SAE International Journal of Passenger Cars - Mechanical Systems|
|Date:||Oct 1, 2017|
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