8.16
Rolamentos de manivela
Rolamentos de manivela são componentes mecânicos que suportam e fornecem estabilidade lateral a eixos e eixos rotativos. Eles são cruciais para reduzi…
Os rolamentos de mancal são usados para fornecer estabilidade lateral a eixos e eixos rotativos.
Considere um guincho de cabo com mancais secos ou parcialmente lubrificados.
A rotação no sentido horário do carretel faz com que o eixo enrole a superfície interna do rolamento até que ele escorregue e sofra rotação estável.
Um diagrama de corpo livre do eixo é desenhado. As forças que atuam no sistema incluem o peso do eixo, o par no sentido horário e a força de reação do rolamento.
A força de reação não colinear é igual e oposta ao peso e atua em um ângulo em relação à superfície normal. Esse ângulo é chamado de ângulo de atrito cinético.
A linha de ação da força de reação é sempre tangente ao círculo de atrito.
A condição de equilíbrio de momento em torno do ponto O produz o momento do eixo.
Para um pequeno ângulo de atrito cinético, o termo senoidal pode ser aproximado da tangente do ângulo de atrito cinético.
Como a tangente do ângulo de atrito cinético é igual ao coeficiente de atrito cinético, obtém-se o momento necessário para superar a resistência ao atrito do rolamento.
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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.