Module 1 : Introduction

Introduction to Tribology

What is Tribology ?

• Tribology is derived from the Greek word “Tribos”. Meaning of Tribos is Rubbing. • Tribology is a science that deals with friction, lubrication and wear in all contacting pairs. • Tribological knowledge helps to improve service life, safety and reliability of interacting machine components; and yields substantial economic benefits.

Few Examples requiring tribological knowledge :

Let us consider few failed machine components, failure of which could had been avoided using tribological knowledge.

Example 1 : Seal

As shown in Fig. 1.1 carbon graphite seal is employed to avoid leakage of steam from rotary joints of paper industry. Failure of this component occurs due to adhesive wear. Adhesive wear causes uneven surface that leads to reduction in mechanical contact area. For same imposed load, reduction in mechanical contacts, increases the level of stress and hence chances of failure.

Fig. 1.1: Carbon graphite seal.

Example 2: Cam

Example 2 is related pitting wear on the cam surface(as shown in Fig. 1.2). Cams are used to transmit rotary motion in reciprocating motion. These components are subjected to jerks in sliding distance, which leads to form some pits on the cam surface. Creation of pits on cam surface increases noise pollution and reduces mechanical performance. Understanding the mechanism of pit formation helps to estimate the life of component and find methods to reduce such pitting failures.

Fig. 1.2: Pitting of cam surface.

Example 3: Journal Bearings

The following figures(Fig. 1.3(a) and Fig. 1.3(b)) are examples of two journal bearing. Left hand side is photograph of centrally grooved engine journal bearing. It appears that bearing is worn out due to foreign particles. Right hand side is a photograph of an aluminum bearing subjected to heavy load, which causes shaft surface to run over bearing inner surface. In these examples of journal bearing, wear increases the clearance between shaft and bearing and leads to reduction in load support capacity of the bearing. Often such failures occur in absence of sufficient lubricant hydrodynamic film thickness due to relatively low speed. Learning tribology cultivates an understanding that at low speeds, the main purpose of oil is the lubrication and high viscosity oil will be preferred to low viscosity oil, while at high speeds the major purpose of oil is to act as a coolant and low viscosity lubricants are preferred to carry away frictional heat of operation. Here lubrication is a secondary consideration.

Fig. 1.3(a): Abrasive wear and Fig. 1.3(b): Rubbing wear

Example 4: Magnetic Bearing

Magnetic bearings are known as non-contact levitation. In the figure given below(Fig. 1.4) a repulsive type permanent magnetic bearing is shown. Due to improper design and external noise factors, bearing failed within three hours of operation at relative speed of 115 rpm.

Fig. 1.4: Wear scar due to edge loading

Example 5: Multi-row Roller Bearing

Cracking of outer ring is shown in Fig. 1.5. Here cracking means deep cracks which breaks outer ring in number of pieces. Such failure occurs due to faulty manufacturing and wrong assembly of roller bearing. Tribological relations help estimating increase in contact stresses due to misalignment of shaft and improper mounting of bearing surfaces. Hence an approximation on reduction in service life can be estimated.

Fig. 1.5: Failure of large size roller bearing

Example 6: Gear

A pit on the surface of gear tooth is shown in Fig. 1.6. The pit generally occurs due to excessive contact stress. Understanding the effect of contact stress helps in developing an equation for estimation of perspective gear life.

Fig. 1.6(a): Gear teeth removed from gear.

Fig. 1.6(b): Pits on gear teeth.

Studies of fluid film bearings, rolling element bearings, seals, gears, cams, and brakes are some of the applications in which tribology is required.

Basic knowledge gained by Tribology course is very useful for industries related to power, steel, cement, oil etc. Practicing such knowledge in problems ranging from house hold appliances to large size ships earns great economic benefits. Therefore tribology course is often named as : “Industrial Tribology”, “Applied Tribology”.