Short Answer:

Bearing stress is the contact stress developed between two surfaces when one surface presses against another. It mainly occurs in connections like bolts, rivets, and pins where one component bears the load from another over a limited contact area.

In simple terms, bearing stress is the pressure experienced by a material when it is in contact with another body and subjected to a compressive load. It helps engineers determine whether the contact area is sufficient to safely carry the load without causing damage, deformation, or crushing at the point of contact.

Detailed Explanation :

Bearing Stress

Bearing stress is a type of compressive stress that occurs when two components are in contact and transmit a load through their surfaces. It is a localized stress that acts perpendicular to the contact area between the two surfaces. This stress ensures that the load is distributed evenly across the contact region, preventing excessive deformation or failure.

When one part presses against another — such as a bolt inside a hole, a shaft inside a bearing, or a rivet in a plate — the load is transmitted through a small contact area. The intensity of this contact pressure is known as the bearing stress.

Mathematically, bearing stress is defined as:

Where,

σb = bearing stress
P = load applied or transmitted between surfaces
A = projected bearing area (depends on contact dimensions)

Since bearing stress depends on load and contact area, it is expressed in N/mm² (MPa).

Explanation with Example

Consider a bolt passing through a hole in a metal plate. When the bolt is tightened or when the plate is loaded, the bolt exerts a compressive force on the surface of the hole. This pressure acting on the contact area between the bolt and the plate hole is called bearing stress.

The bearing area in this case is the product of the bolt diameter (d) and the thickness (t) of the plate:

Thus,

This means that the larger the contact area between the bolt and the plate, the smaller the bearing stress will be for the same load. If the bearing stress becomes too high, it can lead to permanent deformation or elongation of the hole, loosening of the bolt, or even failure of the joint.

Importance of Bearing Stress

Bearing stress plays an important role in mechanical design and analysis, especially in joints and connections where components transmit loads through surface contact. The design must ensure that the bearing stress remains below the allowable limit for the materials in contact.

If the bearing stress exceeds the safe value, the material may experience local crushing, distortion, or wear, reducing the strength and lifespan of the joint. Therefore, engineers carefully calculate the bearing stress and provide an adequate contact area to prevent such failures.

Applications of Bearing Stress

Bearing stress occurs in many mechanical systems, including:

Riveted Joints:
The rivet bears against the plates, and the contact area between the rivet and the plate hole experiences bearing stress.
Bolted Joints:
The shank of a bolt presses against the sides of the bolt hole, developing bearing stress.
Pin Joints:
Pins in connecting rods or chain links experience bearing stress where they contact the link holes.
Shafts and Bearings:
The surface of a shaft in contact with a bearing is subjected to bearing stress due to the transmitted load.
Couplings and Keys:
Bearing stress is also present at the surfaces of key and keyway connections, where torque is transmitted.

In all these cases, the magnitude of bearing stress must be limited to prevent damage or failure at the contact points.

Factors Affecting Bearing Stress

Magnitude of Load:
Bearing stress increases with the applied load. A higher load produces greater compressive stress on the contact area.
Contact Area:
A larger bearing area reduces the stress. Hence, increasing the contact surface helps in distributing the load more uniformly.
Material Strength:
The ability of materials to resist bearing stress depends on their compressive strength. Harder materials like steel can handle higher bearing stresses than softer ones like aluminum.
Fit and Tolerances:
The clearance or tightness between mating parts affects stress distribution. A tighter fit ensures even load distribution and reduces stress concentration.
Load Direction:
Uniformly distributed loads produce uniform bearing stress, whereas eccentric or misaligned loads create uneven stress concentration.

Comparison with Other Stresses

Bearing stress is different from other types of stresses such as tensile, compressive, or shear stress because it acts perpendicular to the contact surface and is not uniformly distributed. The contact pressure is usually highest near the load application point and gradually decreases away from it.

For example:

Tensile stress acts to pull materials apart.
Shear stress acts tangentially to surfaces, trying to slide layers over each other.
Bearing stress acts perpendicularly at the contact area between surfaces in contact.

Understanding these differences helps in proper design and failure analysis.

Design Considerations

To ensure safety and efficiency, engineers follow certain design considerations:

The bearing stress should not exceed the permissible bearing stress of the materials.
The contact area should be sufficiently large to distribute the load evenly.
The component dimensions, such as hole diameter and plate thickness, should be chosen to provide enough resistance.
High-quality surface finishes and accurate alignment should be maintained to reduce stress concentrations.

Conclusion

In conclusion, bearing stress is the compressive contact stress that occurs between two surfaces when one presses against the other. It is a crucial factor in the design of joints like bolts, rivets, pins, and bearings. Proper control of bearing stress ensures the joint can carry loads safely without causing material failure, deformation, or wear. Understanding and calculating bearing stress accurately is essential in mechanical engineering for achieving strong and durable connections.