8.16
Les paliers de journal
Les paliers de journal sont des composants mécaniques qui soutiennent et assurent la stabilité latérale des arbres et essieux rotatifs. Ils sont essen…
Les paliers lisses sont utilisés pour assurer la stabilité latérale des arbres et des essieux en rotation.
Envisagez un treuil à câble avec des paliers lisses secs ou partiellement lubrifiés.
La rotation dans le sens des aiguilles d'une montre de la bobine fait rouler l'arbre sur la surface intérieure du roulement jusqu'à ce qu'il glisse et subisse une rotation stable.
Un schéma de corps libre de l’arbre est dessiné. Les forces agissant sur le système comprennent le poids de l'arbre, le couple dans le sens des aiguilles d'une montre et la force de réaction du roulement.
La force de réaction non colinéaire est égale et opposée au poids et agit à un angle par rapport à la normale de surface. Cet angle est appelé angle de frottement cinétique.
La ligne d’action de la force de réaction est toujours tangente au cercle de frottement.
La condition d’équilibre des moments autour du point O donne le moment de l’arbre.
Pour un petit angle de frottement cinétique, le terme sinusoïdal peut être approximé à la tangente de l’angle de frottement cinétique.
Comme la tangente de l'angle de frottement cinétique est égale au coefficient de frottement cinétique, le moment nécessaire pour surmonter la résistance au frottement du roulement est obtenu.
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Q1: What is the primary function of journal bearings in rotating machinery?
Journal bearings support and provide lateral stability to rotating shafts and axles while reducing friction, wear, and vibration. They work by forming a thin lubricant film between the bearing surface and the rotating shaft, which minimizes direct contact and reduces frictional forces in machinery such as engines, turbines, and pumps.
Q2: How does a shaft behave when it rotates inside a journal bearing?
As the shaft rotates clockwise, it initially rolls up the inner surface of the bearing until it slips and undergoes stable rotation. During this process, the shaft experiences forces including its weight acting downward, the applied clockwise couple representing torque, and the bearing's reaction force, which acts at an angle relative to the surface normal.
Q3: What is the angle of kinetic friction in journal bearing analysis?
The angle of kinetic friction is the angle at which the bearing's non-collinear reaction force acts relative to the surface normal. The line of action of this reaction force is always tangent to the circle of friction, which represents the locus of all possible contact points between the shaft and bearing surface.
Q4: How is the moment required to overcome bearing friction calculated?
By applying moment equilibrium about the shaft center, the required moment can be determined. For small kinetic friction angles, the sine term approximates to the tangent of the kinetic friction angle. Since the tangent of the kinetic friction angle equals the coefficient of kinetic friction, this relationship yields the moment needed to overcome the bearing's frictional resistance.
Q5: What forces are included in a free-body diagram of a journal bearing system?
A free-body diagram of a shaft in a journal bearing includes three main forces: the shaft's weight acting vertically downward, the clockwise couple representing applied torque, and the bearing's reaction force. The reaction force is equal and opposite to the weight and acts at the angle of kinetic friction relative to the surface normal.
Q6: Why is the circle of friction important in journal bearing design?
The circle of friction represents the locus of all possible contact points between the shaft and bearing surface. The reaction force's line of action is always tangent to this circle, making it essential for determining how forces distribute during rotation and for calculating the moment required to maintain stable shaft rotation.
Q7: How does lubrication affect the performance of journal bearings?
Lubrication creates a thin film between the bearing surface and rotating shaft, minimizing direct contact and significantly reducing frictional forces. This film reduces wear and vibration in the system, improving overall machinery efficiency and performance while lowering the moment required to overcome bearing friction.