What is the difference between plain and journal bearings?

Relative motion of two surfaces in contact or separated by a thin film of fluid

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Sliding is a type of motion between two surfaces in contact. This can be contrasted to rolling motion. Both types of motion may occur in bearings.

The relative motion or tendency toward such motion between two surfaces is resisted by friction. This means that the force of friction always acts on an object in the direction opposite to its velocity (relative to the surface it's sliding on). Friction may damage or "wear" the surfaces in contact. However, wear can be reduced by lubrication. The science and technology of friction, lubrication, and wear is known as tribology.

Sliding may occur between two objects of arbitrary shape, whereas rolling friction is the frictional force associated with the rotational movement of a somewhat disclike or other circular object along a surface. Generally, the frictional force of rolling friction is less than that associated with sliding kinetic friction.[1] Typical values for the coefficient of rolling friction are less than that of sliding friction.[2] Correspondingly sliding friction typically produces greater sound and thermal bi-products. One of the most common examples of sliding friction is the movement of braking motor vehicle tires on a roadway, a process which generates considerable heat and sound, and is typically taken into account in assessing the magnitude of roadway noise pollution.[3]

Sliding friction

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Sliding friction (also called kinetic friction) is a contact force that resists the sliding motion of two objects or an object and a surface. Sliding friction is almost always less than that of static friction; this is why it is easier to move an object once it starts moving rather than to get the object to begin moving from a rest position.

F k = μ k ' N {\displaystyle F_{k}=\mu _{k}\cdot N}

Where Fk, is the force of kinetic friction. μk is the coefficient of kinetic friction, and N is the normal force.

Examples of sliding friction

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Slippery when wet signs alert drivers that they need to slow down because the kinetic friction between the tires and a wet surface is much less than that of a dry surface.

• Sledding
• Pushing an object across a surface
• Rubbing one's hands together (The friction force generates heat.)
• A car sliding on ice
• A car skidding as it turns a corner
• Opening a window
• Almost any motion where there is contact between an object and a surface
• Falling down a bowling lane

Motion of sliding friction

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The motion of sliding friction can be modelled (in simple systems of motion) by Newton's Second Law

' F = m a {\displaystyle \sum F=ma}

F E ' F k = m a {\displaystyle F_{E}-F_{k}=ma}

Where F E {\displaystyle F_{E}} is the external force.

• Acceleration occurs when the external force is greater than the force of kinetic friction.
• Slowing Down (or Stopping) occurs when the force of kinetic friction is greater than that of the external force.
  • This also follows Newton's first law of motion as there exists a net force on the object.
• Constant Velocity occurs when there is no net force on the object, that is the external force is equal to force of kinetic friction.

Motion on an inclined plane

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Free body diagram for a block subject to friction as it slides on an inclined plane

A common problem presented in introductory physics classes is a block subject to friction as it slides up or down an inclined plane. This is shown in the free body diagram to the right.

The component of the force of gravity in the direction of the incline is given by:[4]

F g = m g sin ' θ {\displaystyle F_{g}=mg\sin {\theta }}

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The normal force (perpendicular to the surface) is given by:

N = m g cos ' θ {\displaystyle N=mg\cos {\theta }}

Therefore, since the force of friction opposes the motion of the block,

F k = μ k ' m g cos ' θ {\displaystyle F_{k}=\mu _{k}\cdot mg\cos {\theta }}

To find the coefficient of kinetic friction on an inclined plane, one must find the moment where the force parallel to the plane is equal to the force perpendicular; this occurs when the block is moving at a constant velocity at some angle θ {\displaystyle \theta }

' F = m a = 0 {\displaystyle \sum F=ma=0}

F k = F g {\displaystyle F_{k}=F_{g}} or μ k m g cos ' θ = m g sin ' θ {\displaystyle \mu _{k}mg\cos {\theta }=mg\sin {\theta }}

Here it is found that:

μ k = m g sin ' θ m g cos ' θ = tan ' θ {\displaystyle \mu _{k}={\frac {mg\sin {\theta }}{mg\cos {\theta }}}=\tan {\theta }} where θ {\displaystyle \theta } is the angle at which the block begins moving at a constant velocity[5]

References

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Principle that Fundamentally connects

Sliding Motion

Both journal bearings and sliding plain bearings rely on sliding contact between two surfaces to support loads and allow relative motion. More detailed information about plain bearings you can find on Plain Bearings page.

Hydrodynamic Lubrication

In both the cases, a film of lubricant (oil or grease) is typically used to create a hydrodynamic layer that reduces direct shaft to liner contact, minimizing friction and wear.

Plain Bearings

Axial loads

Plain & Journal Bearings

Radial loads

Structure, Design and Dynamics

Basic Components

Both types consist of a shaft (journal) that rotates within a bearing surface (bushing or sleeve). The inner surface of the bearing and the outer surface of the shaft (journal) are the primary contact points.

Material Choices

They often use similar materials, such as bronze, Babbitt metal, polymers, or composite materials, PTFE, chosen for their low friction and good wear properties.
Workhorse Capabilities

Workhorse Capabilities

Load Support

Both bearings are designed to support radial loads (perpendicular to the shaft). They can handle heavy loads due to the large contact area provided by the sliding surfaces.

Self-Lubrication Options

Many sliding plain bearings, including journal bearings, can be made from self-lubricating materials like PTFE, which eliminate the need for additional lubrication.

Advanced Technologies and Innovations

Both journal bearing and sliding plain bearing utilize materials such as fiber-reinforced polymers or metal matrix composites, offering benefits like reduced weight, improved corrosion resistance, and enhanced performance under varied operating conditions.

Both journal bearing and sliding plain bearing utilize materials with inherent lubricity, such as PTFE (Teflon) or advanced polymers, to eliminate the need for external lubricants. This simplifies maintenance and reduces the risk of contamination or environmental impact.

The Bottom Line

Sliding plain bearings can be designed for radial, axial, or combined loads depending on their orientation and application however journal bearings primarily handle radial loads, but some designs can also accommodate axial loads with appropriate modifications, such as flanged designs.

Sliding plain bearings can incorporate a wide range of configurations, including linear bearings, sintered bearing, polymer bearing with shapes beyond cylindrical, while journal bearings often refer specifically to radial bearings used in rotating machinery.

Journal bearings and sliding plain bearings operate on the same fundamental principle of sliding motion between surfaces. They share similar materials, lubrication methods, and maintenance practices. Both are versatile, customizable, and suitable for a wide range of applications, with journal bearings typically being a subset of sliding plain bearings focused on supporting rotating shafts.

Contact us to discuss your requirements of Journal Thrust Plain Bearings. Our experienced sales team can help you identify the options that best suit your needs.