What Is an Axial Load?

The axial direction refers to the direction along the axis of the shaft, and the radial direction perpendicular to it is along the radial direction of the shaft. Therefore, the axial load of the bearing refers to the load generated in the axial direction of the bearing. In simple terms, the force that pushes the inner ring of the bearing out of the outer ring.

Axial load exists widely in various structural vibrations. It not only has important research value in theory, but also has extensive engineering background. For example, during the flight of a rocket missile, there is a large axial compressive load. This axial compressive load has different degrees of influence on the lateral vibration characteristics of the missile.

At present, the influence of axial load on structural vibration characteristics has received great attention at home and abroad, mainly reflected in the dynamic response of bending and torsional coupling of thin-walled beams and stability analysis of axial loads; and the British JRBanerjee et al. Used the dynamic stiffness matrix method to study the axial Influence of load on coupled torsional vibration characteristics of composite beams.

Studies have found that when elastic waves propagate in a periodic composite or structure, the elastic waves undergo periodic modulation and cannot propagate in a specific frequency range. This frequency range is called a band gap, and this periodic composite or structure with an elastic wave band gap is called a phononic crystal. Because the band gap characteristics of phononic crystals can effectively control the propagation of elastic waves, and the band gap frequency range can be designed artificially, phononic crystals have broad application prospects in acoustic devices and vibration and noise reduction.

Analysis of the axial load characteristics of car wheel bearings: Aiming at the actual driving conditions of cars, based on the static analysis method, a simplified model of a single track of a car is established to analyze the axial load characteristics of car wheel bearings.

The external load of car wheel bearings is complex and changeable. In actual driving, road conditions, driving speed, turning radius, and tire characteristics all significantly affect the life and performance of wheel bearings. We have formulas for calculating the radial and axial tire load of a car, but do not give the corresponding calculation methods or results of the car in a turning state. Start by establishing a linear car rigid body that ignores curve driving resistance, tire tangential force, and rolling resistance.

The external load of the wheel bearing is applied to the wheel bearing through the tire, that is, in the actual driving process of the car, the radial and axial loads of the road surface on the wheel indirectly act on the wheel bearing. According to the force transmission characteristics, it can be considered that the external load of the hub bearing is equivalent to the external load of the tire. The lateral acceleration of a car is directly determined by the turning radius and driving speed of the car. Among them, the turning radius of the car is closely related to the steering angle of the front wheels.

Analysis shows that most of the natural frequency of the body structure is lower than 15Hz, so when the vibration frequency of the car is lower than 15Hz, the body motion can be assumed to be rigid body motion. In addition, the mass coupling relationship between the front and rear parts of the car is not obvious. Ignore non-linear characteristics such as inertia, damping, elasticity of the car steering system, tires and suspension, and car yaw. Assume that the car's center of mass is on the road, and the system is linear. The size, that is, the car has only two front and rear tires, and the center of mass is on a rigid frame connecting the front and rear wheels. The car movement is simplified into a basic linearized rigid body movement, and a rigid automobile linear monorail model is established.

The coordinate system of a car is fixed at the center of the tire, and its origin coincides with the geometric center of the tire. Set the x-axis to point directly in front of the tire, the y-axis to the left, and the z-axis to point directly upward through the tire's center of mass. Considering the general situation, as a representative of the previous wheel-drive car, the car turns at a steady steering angle and a constant speed v on a slope with a cross-slope angle , and a rigid monorail model is established as shown in the figure below. ^[3]

When a radial thrust bearing is subjected to a radial load R, since the contact line between the contact line of the rolling elements and the raceway and the bearing axis, the reaction force Ni of each rolling element does not point in the radial direction, but follows the contact. The normal direction of a point can be decomposed into a radial component and an axial component. Using Pi to represent the radial component of a rolling element reaction force, the corresponding axial component Fdi should be equal to Pi tan. The combined force R 'of all radial component forces Pi is balanced with the radial load R; the sum of all axial component forces Fdi constitutes the derived axial force Fd of the bearing, which forces the journal (together with the bearing inner ring and rolling elements) toward Move to the right and finally balance with the axial force Fa.

When only one rolling element is loaded, the load angle and the contact angle are equal. When the number of rolling elements to be loaded increases, although the same radial load Fr acts, the derived axial force Fd will increase. Because the directions of the radial reaction forces Pi acting on the rolling elements are different at this time, although their vector sum R 'is balanced with Fr, its algebraic sum must be greater than Fr, and the derived axial force Fd is determined by each The axial force Fdi derived from Pi is synthesized, and its value should be calculated according to the algebraic sum of Fdi. Therefore, under the same radial load Fr, the axial force Fd synthesized by the axial forces derived from Pi acting on each rolling element will be greater than the axial force derived when only one rolling element is loaded. ^[1]

The above analysis shows:

(1) Radial thrust bearings must work under the combined action of radial load R and axial force A.

(2) For the same bearing, under the same radial load R, when the number of rolling elements being loaded is different, different axial forces S are derived, and different axial forces A are needed to balance it. Or conversely, under the condition that the radial load R is constant, when the axial force is gradually increased from the minimum value (Fa = Fr * tan) by a rolling body (that is, the angle increases), This means that the number of rolling elements in contact with the bearing is gradually increasing.

For the actual working radial thrust bearing, in order to ensure that it can work reliably, it should be at least fully loaded in the lower half of the rolling body. Therefore, when installing this type of bearing, there must be no large axial movement.