Name: friction moment in journal bearings
Uploaded: 2023-09-22T11:49:30.000Z
Duration: 1 min 23 s
Channel: JoVE

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&#39;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&#39;s weight, the clockwise couple, and the bearing&#39;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&#39;s frictional resistance is obtained.

journal bearings

Journal bearings are mechanical components that support and provide lateral stability to rotating shafts and axles. They are crucial in reducing friction, wear, and vibration in machinery such as engines, turbines, and pumps. The principle behind journal bearings is forming a thin lubricant film between the bearing surface and the rotating shaft, which minimizes direct contact and reduces frictional forces.
To better understand the concept of journal bearings, consider a rope winch with dry or partially lubricated journal bearings supporting its rotating shaft. As the spool rotates clockwise, the shaft rolls up the inner surface of the bearing until it slips and undergoes stable rotation.
<img alt="Static equilibrium, force diagram, ΣF=0, torque analysis, rotational motion, vector forces, mechanics" src="/files/ftp_upload/14326/14326_Figure_1v2.svg" style="text-align: center; height: 300px;" data-altupdatedat="1761830439000" data-alt="Figure 1">
A free-body diagram of the shaft can be drawn to analyze the forces acting on the system. These forces include the weight of the shaft, acting vertically downward, the clockwise couple, representing the torque applied to the shaft, and the reaction force exerted by the bearing on the shaft. The non-collinear reaction force is equal and opposite to the weight of the shaft and acts at an angle relative to the surface normal. This angle is referred to as the angle of kinetic friction. The line of action of the reaction force is always tangent to the circle of friction, which represents the locus of all possible points of contact between the shaft and the bearing. By applying the moment equilibrium condition about point O (the center of the shaft), we can determine the shaft moment needed to maintain stable rotation.
<img alt="Static equilibrium, equation: M₀ - (R sin ϕₖ) r = 0, mechanical stability analysis." src="/files/ftp_upload/14326/14326_Equation_1v2.svg" style="text-align: center; height: 13px;" data-altupdatedat="1761830439000" data-alt="Equation 1">
The sine term in the equation can be approximated to the tangent of the kinetic friction angle for a small kinetic friction angle.
<img alt="Static friction formula, equation: sin(ϕₖ) ≈ tan(ϕₖ) ≈ μₖ, used in physics calculations." src="/files/ftp_upload/14326/14326_Equation_2v2.svg" style="text-align: center; height: 13px;" data-altupdatedat="1761830439000" data-alt="Equation 2">
As the tangent of the kinetic friction angle equals the coefficient of kinetic friction, we can calculate the moment required to overcome the bearing's frictional resistance. This moment is an essential factor in determining the efficiency and performance of the system.
<img alt="Mo=Rrμk formula in physics, representing rotational equilibrium concepts, equations." src="/files/ftp_upload/14326/14326_Equation_3v2.svg" style="text-align: center; height: 13px;" data-altupdatedat="1761830439000" data-alt="Equation 3">

Journal Bearings

Journal bearings are mechanical components that support and provide lateral stability to rotating shafts and axles. They are crucial in reducing friction, ...

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Mechanical Engineering

Unit 1

Friction

KEY TERMS AND CONCEPTS

friction moment in journal bearings

Journal bearings support rotating shafts and axles while helping keep them stable. They are used in machines such as engines, turbines, and pumps. Their job is to reduce friction, wear, and vibration.

A journal bearing works by keeping a thin film of lubricant between the bearing surface and the rotating shaft. This film limits direct contact and lowers frictional force. The video uses a rope winch with dry or only partly lubricated journal bearings to show the idea.

As the spool turns clockwise, the shaft first rolls up the inner surface of the bearing. It then slips and settles into stable rotation. A free-body diagram of the shaft helps show the forces in the system.

The diagram includes the weight of the shaft, a clockwise couple that represents the applied torque, and the reaction force from the bearing. The reaction force is not collinear with the weight of the shaft. It acts at an angle to the surface normal, and that angle is called the angle of kinetic friction.

The line of action of the reaction force is tangent to the circle of friction. The circle of friction is the set of possible contact points between the shaft and the bearing. By using moment equilibrium about point O, the center of the shaft, the needed shaft moment can be found.

For a small kinetic friction angle, the sine term in the equation can be approximated by the tangent of that angle. Since the tangent of the kinetic friction angle equals the coefficient of kinetic friction, the required moment can be calculated from it. This moment helps describe the bearing’s frictional resistance and its effect on efficiency and performance.

Friction Moment in Journal Bearings

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key terms and concepts

Learn how journal bearings support rotating shafts, reduce friction, and create the reaction force and moment used to analyze stable rotation.

Friction Moment in Journal Bearings | JoVE K12

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mechanical engineering

Friction Moment in Journal Bearings

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