Friction Circle in Journal Bearing

In a journal bearing, the friction circle is a graphical representation of the contact pressure distribution between the journal (rotating shaft) and the bearing (stationary surface). The friction circle helps to determine the maximum pressure and the resulting frictional resistance in the bearing.

Factors affecting the Friction Circle

The size of the friction circle is influenced by several factors. Let's discuss each of them and understand how they affect the radius of the friction circle:

a) Load:

The load on the journal bearing refers to the force acting on the bearing due to the weight of the rotating shaft or any external force applied. As the load increases, the pressure between the journal and the bearing also increases. This increase in pressure causes the friction circle to expand, resulting in a larger radius.

b) Radius of the Journal:

The radius of the journal directly affects the size of the friction circle. A larger radius means a larger contact area between the journal and the bearing surface. With a larger contact area, the pressure distribution is more spread out, leading to a larger radius of the friction circle.

c) Speed of the Journal:

The speed of the journal refers to the rotational velocity of the shaft. An increase in speed causes an increase in the centrifugal force acting on the journal. This centrifugal force reduces the contact pressure between the journal and the bearing surface. As a result, the radius of the friction circle decreases with an increase in the speed of the journal.

d) Viscosity of the Journal:

The viscosity of the journal refers to the resistance of the lubricating fluid present in the bearing to flow. Higher viscosity fluids have greater resistance to flow compared to lower viscosity fluids. In the case of journal bearings, the lubricating fluid forms a hydrodynamic film between the journal and the bearing, reducing the friction and wear. Higher viscosity fluids tend to form thicker hydrodynamic films, resulting in a larger radius of the friction circle.

Conclusion:

In summary, the radius of the friction circle in a journal bearing increases with an increase in the radius of the journal. This is because a larger radius leads to a larger contact area between the journal and the bearing surface, resulting in a broader pressure distribution and an expanded friction circle. The load, speed of the journal, and viscosity of the lubricating fluid also play a role in determining the size of the friction circle, but they do not directly affect the radius as significantly as the journal radius does.