laws of rolling friction1

8/4/2019 Laws of Rolling Friction1

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The laws for sliding friction cannot be applied to rolling bodies inequally quantitative terms, but the following generalities can be

given:

(a) The rolling friction force F is proportional to the load L and

inversely proportional to the radius of curvature r, or F = r L/r,where r is the coefficient of rolling resistance, in meters (inches).

As the radius increases, the frictional force decreases.

(b) The rolling friction force F can be expressed as a fractional power of

the load L times a constant k, or F = kLn where the constant k and

the power n must be determined experimentally.

(c) The friction force F decreases as the smoothness of the rolling

element improves.


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When a body rolls over another body, the places of contact get

deformed and a slight bump is formed.

The bump caused in front has to be constantly overcome. It is like

constantly climbing an incline. Secondly, the adhesive forces

between the two surfaces also have to be overcome constantly. It is

due to these two factors that rolling friction arises. It is denoted by fr

and is given by


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Where

r= coefficient of rolling friction

L = normal reactionr = radius of rolling body

This equation is applicable only for pure rolling and no sliding. It also

must be noted that the coefficient of rolling friction, r, has the

dimensions of length


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Friction aids walking. Our feet pushes the floor back and the floor

pushes our feet forward, by using friction to do so (every action has

equal and opposite reaction).

Friction between tyres of vehicles and the road make controlled

motion possible.

Many machine parts stay in place and are also driven by friction.

All kinds of brakes in vehicles use friction for braking.

Nails stay in place in the walls because of friction.


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Friction causes wear and tear in machine parts.

A large amount of fuel and energy in vehicles and machines is used

to overcome friction.

Friction between the bone joints, especially legs in old people,causes discomfort, pain and various ailments.


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The friction are measured by the following methods

Weight ratio

Spring balance Tilted plane


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= Ff / N = mdead weight /

mblock


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Spring balance = Fspring/Fnormal

= Fspring/(mblock g), g=9.81 m/s


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Tilted plane = tan = Ff/F


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In the measuring methods discussed above the friction

coefficient is measured in fresh contacts, not after running in.

The coefficient of friction may change significantly during

first half hour of sliding. The time necessary to obtain a

stable value of the coefficient of friction can be observed in amotorized Tribometers by monitoring the friction over time.

This method is common for measuring the specific wear rate

and the contact temperature during operation.


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laws of rolling friction1

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