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Journal lagers
Journal lagers zijn mechanische onderdelen die roterende assen en assen ondersteunen en laterale stabiliteit bieden. Ze zijn cruciaal om wrijving, sli…
Journal bearings are used to provide lateral stability to rotating shafts and axles.
Consider a rope winch with dry or partially lubricated journal bearings.
The clockwise rotation of the spool causes the shaft to roll up the bearing's inner surface until it slips and undergoes stable rotation.
A free-body diagram of the shaft is drawn. The forces acting on the system include the shaft's weight, the clockwise couple, and the bearing's reaction force.
The non-collinear reaction force is equal and opposite to the weight and acts at an angle relative to the surface normal. This angle is called the angle of kinetic friction.
The line of action of the reaction force is always tangent to the circle of friction.
The moment equilibrium condition about point O yields the shaft moment.
For a small kinetic friction angle, the sine term can be approximated to the tangent of the kinetic friction angle.
As the tangent of the kinetic friction angle equals the coefficient of kinetic friction, the moment required to overcome the bearing's frictional resistance is obtained.
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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.