SAL COLLEGE OF ENGINEERING

TRIBOLOGYPRESENTATION

PA-82017

Prepared by :KAUSHAL MISHRA 131130119035SOMESH PATANI 131130119052

Guided by :Asst. Professor RAKESH B. PRAJAPATI

CONTENT

CAUSES OF FRICTIONSTICK-SLIP

BEHAVIOURSLIDING & ROLLING

FRICTIONFRICTIONAL HEATINGFRICTION MEASURING

TECHNIQUES

Causes of friction

• Molecular Adhesion• Surface roughness• Deformation

Molecular adhesion• When two objects are

brought into contact, many atoms or molecules from one object are in such close proximity to those in the other object that molecular or electromagnetic forces attract the molecules of the two materials together. This force is called adhesion. Trying to slide one object across the other requires breaking these adhesive bonds. Adhesion is the essence of friction.

Surface roughness• All solid materials have

some degree of surface roughness. If you looked at what seems to be a smooth surface under a high-powered microscope, you would see bumps, hills and valleys that could interfere with sliding motion.

• In such a case, the friction is caused by surface roughness, although the adhesion effect still plays a part in the abrasion.

Deformation • When materials deform,

you must "plow" through to move, thus creating a resistive force.

• When the deformation becomes large, such that one object sinks into the other, streamlining can affect the friction, similar to what happens in fluid friction.

Stick-slip friction

• Sliding of one body over another under steady pulling force proceeds:

- sometimes at constant or near constant velocity - on other occasions at velocity which fluctuates widely, If the friction force or sliding velocity does not remain constant as a function of distance or time and produces oscillations, it could be due to a phenomenon known as stick-slip phenomenon.• Stick-slip can arise when the coefficient of static friction is markedly greater than the coefficient of kinetic friction

• During stick phase, the friction force builds up to a certain value, and once a large enough force has been applied to overcome the static friction force, slip occurs at the interface.

• Usually, a saw tooth pattern in friction force-time curve is observed during stick-slip process

• The stick-slip process can be responsible for

- squeal and chatter in bearings - jerking of brakes - chatter of windshield wipers on partly wet window glass - inaccuracies in machining and positioning.• The elasticity, damping and frictional

properties are critical in the occurrence of stick-slip process

• In any case where the coefficient of kinetic friction is less than the coefficient of static friction there will exist a tendency for the motion to be intermittent rather than smooth.

• The two contact surfaces will stick until the sliding force reaches the value of the static friction.

• The surfaces will then slip over one another with a small-valued kinetic friction until the two surfaces stick again.

• When the drive system is started, the Spring R is loaded and its pushing force against load M increases until the static friction coefficient between load M and the floor is not able to hold the load anymore.

• The load starts sliding and the friction coefficient decreases from its static value to its dynamic value. At this moment in fact the spring can give more power and accelerates M.

• During M’s movement, the force of the spring decreases, until it is insufficient to overcome the dynamic friction. At this point, the mass M stops.

Sliding Friction• Sliding friction occurs when two solid

surfaces slide over each other. • Sliding friction can be useful. • For example, you can spread sand on an icy

path to improve your footing. • Ballet dancers apply a sticky powder to the

soles of their ballet slippers so they won’t slip on the dance floor.

• And when you stop a bicycle with hand brakes, rubber pads slide against the tire surfaces, causing the wheels to slow and eventually stop.

• On the other hand, sliding friction is a problem if you fall off your bike and skin your knee!

Rolling Friction• When an object rolls across a

surface, rolling friction occurs.• Rolling friction is easier to overcome

than sliding friction for similar materials.

• This type of friction is important to engineers who design certain products.

• The ball bearings are small, smooth steel balls that reduce friction by rolling between moving parts.

Friction heating & temperature rise

• During friction, when the intermolecular bond breaks, huge energy is dissipated. This energy gets absorbed by the surrounding in the form of heat. The process is named as frictional heat.

• When there is wear or deformation on the surfaces in contact due to friction, the atoms just below the deformed layer experiences a little vibration about their own position, which results in the rise of temperature of material.

Friction Measurement Techniques

• Any device designed for measuring friction must be –

1. capable of supplying relative motion between two specimens. 2. capable of applying a known normal load between both surfaces. 3. capable of measuring the tangential load to motion.

Using Tilting Plane

Using Pin-on-disc machine

• The pin is held stationary, disc rotates.

• A pin on disc tribometer consists of a stationary "pin" under an applied load in contact with a rotating disc. The pin can have any shape to simulate a specific contact, but spherical tips are often used to simplify the contact geometry. Coefficient of friction is determined by the ratio of the frictional force to the loading force on the pin.

Pin on Flat

Pin-on-cylinder

Rectangular flats on rotating cylinder

Simple crossed cylinder arrangement

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