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Causes of friction

Friction is caused by the roughness of the materials rubbing against each other, deformations in the materials, and a molecular attraction between materials.

Most friction results because the surfaces of materials in contact are not completely smooth. If both surfaces become ultra-smooth and flat, the friction from surface roughness becomes negligible, but then friction from molecular attraction comes into play, often becoming greater than the normal friction.

Friction Theories

Friction is the resistance to relative motion, which is experienced whenever one solid body slides over another. The resistive force, which is parallel to the direction of motion, is called the friction force.

If the solid bodies are loaded together and a tangential force is applied, then the value of the tangential force, which is required to initiate sliding, is the static friction force. The tangential force required to maintain sliding is the kinetic friction force. Kinetic friction is generally lower than static friction.

The Laws of Friction:

First law states that the friction is independent of the apparent area of contact between the contacting bodies.

Second law states that the friction force is proportional to the normal load between the contacting bodies.

Third law states that the kinetic friction is nearly independent of the speed of sliding.

First two laws are often referred to as Amontons laws and Coulomb introduced the third law.

Coefficient of Friction:

The second law states that the friction force F is proportional to the normal load W.

Therefore F = u W

Where ‘u’ is a constant known as the coefficient of friction and is a constant only for a given pair of sliding materials under a given set of ambient conditions and varies for different materials and conditions

We know that nearly all surfaces are rough on a microscopic scale and real contact is obtained over a small fraction of the apparent contact area. Thus the real area of contact is independent of the apparent area of contact so the first law of friction is explained that friction is related to the real area of contact and independent of the apparent area of contact.

Causes of Friction:

When two surfaces are loaded together they can adhere over some part of the contact and this adhesion is therefore one form of surface interaction causing friction. If no adhesion takes place then the only alternative interaction which results in a resistance to motion is one in which material must be deformed and displaced to accommodate the relative motion.

We have two types of interaction

1. Adhesion
2. Material displacement - either due to asperity interlocking or macro displacement

Adhesion Theory of Friction

When metal surfaces are loaded against each other, they make contact only at the tips of the asperities. Because the real contact area is small the pressure over the contacting asperities is assumed high enough to cause them to deform plastically. This plastic flow of the contacts causes an increase in the area of contact until the real area of contact is just sufficient to support the load.

Ploughing Effect

Ploughing caused by asperities of a hard metal penetrating into a softer metal and ploughing out a groove by plastic flow in the softer material. This is a major component of friction during abrasion processes and also it is probably important in cases where the adhesion term is small.

For asperities on most metal surfaces, the ploughing term is negligible in comparison with the adhesion term. For rough surfaces where is larger, the ploughing term can be comparable to the adhesion term.

XPC AND FRICTION

XPC CAPITALISES UPON THE FRICTIONAL FORCE THAT CAN BE GENERATED BETWEEN SMOOTH SURFACES IN CONTACT WHEN THERE IS HIGH NORMAL FORCE.

THE NORMAL FORCE IS GENERATED BY APPLYING PRELOAD VIA A THREADED OUTBOARD SECTION OR OTHER MEANS. XPC IS DESIGNED SO THAT THE HOLDING STRENGTH IN A GIVEN MATERIAL IS PROPORTIONAL TO THE APPLIED PRELOAD

IT MAKES FOR A BETTER JOINT IF THE TUBE BORE CAN BE MAINTAINED SMOOTH IN AS-FORMED CONDITION, THUS SURFACE ROUGHNESS AND RIBS AND OTHER FEATURES THAT CAUSE STRESS CONCENTRATIONS ARE ENTIRELY AVOIDED

THE HOLDING FORCE, EVEN BETWEEN SMOOTH SURFACES, IS LIMITED ONLY BY THE APPLIED PRELOAD AND THE TUBE BURST STRENGTH

ONLY XPC FASTENER HAS EXPLORED THE POSSIBILITIES OF HOLDING BY INTIMATE SURFACE CONTACT FRICTION ALONE

IT HAS BEEN PROVEN AT XPC THAT ANY KIND OF RIBBING ON THE COLLAR WEAKENS THE JOINT VERY SIGNIFICANTLY - ESPECIALLY IN BENDING, BY CREATING STRESS CONCENTRATIONS

SMOOTH IS STRONGER - UP TO THE STRENGTH OF THE TUBE.

A JOINT WHICH IS STRONGER THAN THE TUBE IS MISMATCHED

XPC OFFERS UNIQUELY HIGH HOLDING STRENGTH IN TENSILE, BENDING, SHEAR AND TORSION

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THE WEAKEST SECTION OF THE TUBE IS THE OUTER EDGE, SO THE FURTHER FROM THE EDGE THAT XPC IS PLACED THE STRONGER THE JOINT.

AND THE HOOP STRENGTH OF THIN OR WEAK TUBE CAN BE ENHANCED BY MEANS OF RESTRAINING COLLAR AS SHOWN BELOW