Bearing

The third generation HBU is a definitive evolution of this component. Now the hub bearing is part of the wheel hub in a unique component. This reduces the cost for maintenance, increases durability, but results in heavy components for race cars. Although it is compact, it is not feasible to apply racing wheel hubs. However as this component is largely used in mass production vehicles, it is possible to find some racing series in which the cars use this kind of hub. For instance, touring cars and grand touring cars.

wheelhub中文

The second generation hub bearing unit is characterised by the simplification of the assembling upright, wheel bearing and wheel hub. Usually the outer of the bearings is machined or assembled in the upright or in the hub. As these components, in the racing field, are designed with focus in a lightweight construction, this results in a lightweight complete assembling of upright, bearing and wheel hub. However, it requires a more precise production process, since the section of the bearing must be precisely machined. In addition, the maintenance of this kind of HBU requires more attention to the tolerance and the bearing nut torque. In addition, the cost of this is higher than the first generation HBU.

HUB

Other important features are related to the section of the wheel hub where important components are brake disc bell, pegs and the rim. This section usually experiences a great transition from smaller diameter to a greater one, thus a stress concentration will occur at that point. In addition to this, there are also some technological features such as the notch which avoid an excessive stress concentration. For this reason the amount of material, so the hub thickness, at that section is increased.

The wheel pegs work during braking or accelerating. At the first condition, these transfer the brake torque to the wheel. However, at this condition the wheel nut helps the pegs creating an axial force. The design of the peg usually assumes a conservative approach, which the total brake torque flows through the pegs. The most common peg assembling is through threads in the wheel hub. The problem with this concept is that these generates an unnecessary stress in the hub, since pegs are not fixing components. In fact, pegs are exposed to a small pressure, but the critical load is the shear one. However, the approach using slots and the form fitting results in optimized and lighter hub and pegs. In addition the stress concentration from the threads gives a place to a controlled tolerance stack-up.

Wheel hubassembly

Wheel hubs designed for racing applications are usually by high strength steel. They are basically cyllinders which provides the connection between the transmission and the wheel. In racing, the wheel hub has thin walls to reduce its weight. Although it has a simple shape, a wheel hub concentrate many functions that request some important features and technologies in its manufacturing. This article proposes a brief review of all features of racing cars wheel hubs and also present some design strategies for this component.

wheelbearing是什么

There are some procedures to define the required torque on the bearing nut for a proper fixture. An excessive torque will generate overheating of the spheres of the bearing. This changes their properties and makes them weak. There is no such sensor to monitor the temperature of the bearings, but it is possible to ensure their proper operation, without excessive friction, with the fixing on the wheel nut and the bearing nut. Usually the bearing nut is defined when the wheel is fastened on the hub. A torque-metre is used to register the torque to rotate the wheel. The wheel is rotating while the bearing is being fastened, as soon the wheel torque increases until the threshold value, the bearing nut is defined. Once the bearing nut and the wheel nut is properly fastened, the hub is exposed to the loads generated by these, they are called Clamping Loads. They are traction forces that stretch the wheel hub. In addition they also generate a bending moment on these, which is the critical part of these loads. However the clamping loads are analysed in the static condition, once the car is running, these are added by the wheel loads.

There is also a gap between the peg and the surface of the hub pocket. This one is filled when the wheel nut is tightened. The wheel is pressed against the hub flange and the gap from the bearings and the pegs is zeroed. The gap between the lateral walls is left for maintenance reasons. In fact, this hub configuration is usually adopted in NASCAR or Endurance race cars, which may require the changing of the brake disc during the race. This is critical maintenance during the race because the pegs and the new disc brake are at different temperatures. Hence the pegs expands due to thermal dilatation, thus the tolerance of these must also account that at this situation, the disc must enter easily. The pegs are designed to facilitate the change of the disc brake in this situation, in which the hub is usually hotter than the flange of the disc brake. In fact, one is made from high strength steel and the disc brake flange is made from aluminium. Another important detail about the pegs is its length. It must be long enough to avoid the mechanics beginning to fasten the wheel nut with the wheel over the peg.

Wheelbearing

In some literature the wheel hub is referred to as the component which transfers the torque from the engine or brake system to the wheel. This is not totally true. Actually in the upright assembling (Figure 3), the wheel hub is fixed by the wheel nut and the bearing nut. However, there are other components which act as an auxiliary to the hub assembling. They are a kind of bolt from the brake disc flange and the wheel peg or pin. This last one is a high strength steel pin with a press-fitting assembly. The objective of this pin is to provide a robust means to transfer the torque to the wheel. Although this function is from the wheel nut, pegs are an auxiliary component in case of loss of the clamping force from the wheel nut. However, normally the pegs do not carry any load when the nuts are properly assembled.

In the automotive field there are mainly three kinds of wheel hub bearings, they are basically the evolution of these components. These are called 1st, 2nd and 3rd generation wheel bearings. The main difference between them is related to the tracks. The first generation wheel hub, also called Hub Bearing Unit, basically is a ball bearing assembled between the upright and the hub. Hence the characteristics of these one is usually found in catalogues. Its advantage is the maintenance costs, which are low. In addition the cost of a first generation HBU is lower relative to the other generations. It will not require adjustments or spacers, it is just assembly. To work properly, the balls must be lubricated. This is made by grease. Hence each bearing has it inside the track and between the spheres. If the temperature gets too high (due to friction between balls and tracks), this grease becomes liquid and leaks from the bearings. When this is sealed, the result is more torque to rotate the wheel, because the sealing avoids the leakage of the grease and the entering dirt from the environment. However, this same sealing creates a significant resistance to rotate, while bearings without sealing, just a cap, this resistance is lower. On the other hand, the durability is lower, because the protection provided by the cap against dirty and humidity is also lower. Hence it is a trade-off.

One important feature of some applications of the wheel bearings is the chamfer usually machined in the inner track of it. This is done due to the position at which the bearings are mounted. They are usually positioned near to the hub flange, which is a section of a high stress concentration. A notch is machined in the hub to reduce this critical point, however the bearing edges could injure the notch and result in a failure since that section is highly requested. For this reason, a chamfer is usually machined in bearing and other components assembled at that hub section.

There are two critical points due to the gaps between the hub pocket and the peg. The first one is relative to the gap between the circular wall of the pocket and the peg. The objective is to design in press-fitting which makes it about 0.2 – 0.3 mm from the wall. As the peg is being radially stressed, it is more efficient to keep the contact point, instead of distributing stress over the wall which will not provide any torque.

The design for stiffness is a procedure which takes into account the section of the component that is exposed to the highest amount of load. Since there is the Haigh equation, its effect in a component can be a different amount of material over the cross-section as can be seen in the hub drawing. A hub of a race car is full of features due to the many functions that it performs. As already known the critical point is the section where the wheel nut, the rim and the disc bell are mounted. The reason is the flow of the loads at that section. Although there are important features, such as the chamfers on the disc bell and rim, these components can be considered a big block attached to the wheel hub.

Wheelrim

Wheel hubbearing

This exerts a big force at the clamping points of the hub, thus a considerable bending stress at that section, as already seen. Since that section is so requested, the decision of the design to use Kt or Kf is very important. At that point, the amount of material is increased on purpose to afford the loads and the stress concentrations. The main objective of this approach is to guarantee a hub that won’t deform when at service. If this happens, the contact between the bearings and the hub will be compromised. This must be uniform over the entire length of the inner track. When the hub distorts at that section for any reason, the bearing will be in contact with one portion of the inner track. The result is a point of stress concentration.

An estimation of the stresses over the time when the vehicle is under operation can also be drawn. Considering a straight forward movement, the stresses can be easily estimated since the hub is a cylindrical component which rotates thus the stress variation should be near to a synodal. As can be seen, the components of the dynamic wheel loads (Fz) are constantly oscillating while the component due to the pre-load is constant. If these are summed, it can be seen how the total stress oscillates as the wheel hub rotates. An important conclusion about these assumptions is that there is no need to request a finite element analysis to retrieve some information about the stresses in the critical section of the wheel hub, just the beam theory and some tables. The concern is until which point these diagrams and calculations are close enough to the problem in consideration. In fact, the most important point of this analysis is the stress concentration factor, given by Kt and Ks.

These can be given by tables developed after years of research.  Hence, once it was assumed that the analysis is based in the beam theory, the table of the stress concentration can be used. This one accounts for two important features that are in the hub, different diameters and notches. Usually, the variation of the diameter in an axle, shaft or beam, results in an abrupt cross-section variation. This is directly related to the moment of inertia of the component. In addition, the point between the smaller and the bigger diameter is separated by a sharp edge. However, the use of notches shows that the stress concentration is drastically reduced. Hence, the stress concentration of the hub depends on the notch radius and the ratio between the bigger and the smaller diameter (D/d). Once the Kt and the Ks are found it is possible to calculate the maximum bending stress due to the forcing on the hub. Actually, Kt is the ratio between the maximum bending stress and the nominal bending stress. The last one is straight forward calculated by the formula and depends on the moment generated due to forcing and the moment of inertia. However, the maximum bending moment depends on the stress concentration factor. Therefore, the most critical point in the hub is always at the point where the cross-section changes and where there are bores and notches.

The wheel hubs have the fixing points of the wheel. Hence it is possible to understand that there is some stress at this region, due to the clamping load generated by the wheel nut or wheel bolts. What makes these so important is the traction generated by the fixing components. One of these is the wheel nut and the other is the bearing nut. Both fix the hub at the upright, but first one fixes the wheel rim and then provides the fixture for the bearing. A spacer is introduced between the bearings, these are assembled inside the upright. As soon as the bearings are at their housing and the hub is assembled, the spacer presses the inner track of the inner bearing which results in little gap between this and the ball. Finally, the wheel nut is fastened, the force generated over the inner track of the inner bearing overcomes the spacer one, which behaves like a spring, since this one is made of steel and has elastic properties. The result is the filling of the gap between the inner track and the ball. Hence, there is an excessive friction torque to rotate the wheel.

Understanding that the wheel hub is statically exposed to a pre-load caused by the clamping loads is not enough to define its total stress. In fact, dynamically the situation is different. These are accounted for together with the lateral and vertical components of the wheel loads. At these first considerations, the misuse loads are not taken into account. For a rapid hand calculation, the beam theory is enough to retrieve qualitatively good numbers. The analyst should have in mind the main section of the hub, which is the one in which there is its flange, and verify the effort at that portion. This section is critical due to some factors such as, the diameter variation, notches radii, bores and clamping loads. Hence, a section drawing from this hub can be drawn together with loads under consideration to illustrate this. Considering that the pre-loads (from bearings and clamping loads) are the static loads and vertical and lateral (Fz and Fy, respectively) are the dynamic ones, the drawing can be seen in Figure 11. If the loads are displaced to the level of the centre of gravity of the hub, the analysis is easier to be performed. However, for that it must be considered additional moments to balance the system. Hence the free body diagram assumes two moments due to the vertical and the lateral loads. The components of the preload are considered only the shear stress, while the bending load generated by it is neglected due to its magnitude, too low if compared with the vertical and lateral components. It is also possible to visualise all the features of a wheel hub, its notches, bores and thickness. These are all in the critical section which result in a high stress concentration factor.