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Food Tribology

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The Relationship Between Lubrication, Wear, and Friction: A Study Using the
Stribeck Curve

Introduction

Tribology is the application of science that deals with friction, wear, and

lubrication, and it has relevance to several fields and technological advancements. It is

therefore crucial to understand these elements from the perspective of food science to

work towards an attractive food product and changing consumer attitudes.

Lubrication, wear, and friction are discussed in this article, with special emphasis on

the Stribeck curve and its relevancy to food tribology.

Correlation Between Lubrication and Mouthfeel

Mouthfeel is the feel of the food product in terms of the revolved smoothness,

creaminess, and textures present in it, which are also sensory characteristics in food

science. Coating within foods like fats and oils modifies the interactions of food with

the oral surfaces for the feeling it gives to the mouth. Analyzing the influence of

lubrication on mouthfeel, it is possible to use principles of the Stribeck curve.

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Boundary Lubrication and Mouthfeel:

In the boundary lubrication regime, the feel of the food’s texture can be coarse

or abrasive because the lubricating films are incapable of entirely separating the

particles or components. This can lead to what they refer to as a less'smooth’

mouthfeel, which entails a higher perceived friction and abrasiveness.

Mixed Lubrication and Mouthfeel:

As food is being lubricated, its texture takes on better characteristics, and

when it gets to the mixed lubrication stage, the food becomes smoother. The one-step

separation of the particles by the lubricant reduces the grainy or rough feeling and

therefore enhances the mouthfeel. Thus, the texture might remain partially firm or

qualitatively characterized by friction and roughness, based on the depth and

dispersion of lubrication.

Hydrodynamic Lubrication and Mouthfeel:

In the hydrodynamic lubrication regime, where a full lubricant film is present,

the food feels smooth and creamy. The high level of lubrication ensures that particles

are well separated, minimizing friction and creating an optimal mouthfeel. This is

often desired in creamy or smooth food products where a low-friction, pleasant

mouthfeel is essential.
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Hydrodynamic zone

Lubrication Regimes Explained

There are four lubrication regimes - boundary, mixed, elastohydrodynamic and

hydrodynamic.

Boundary Lubrication

Boundary lubrication is associated with metal-to-metal contact between two sliding

surfaces of the machine. During initial start-up or shutdown of some equipment (e.g.,

journal bearings) or under heavily loaded conditions (pins and bushings of

construction equipment), the metal surfaces in a lubricated system may actually come

into severe contact with each other. If the oil film is not thick enough to overcome the

surface roughness of the metal, a lambda value of less than one results.

In general, we want to avoid boundary erasure where possible. It is agreed among

lubrication experts that friction can be at its highest during the limit lubrication mode.

This occurs during startup, shutdown, low speed or high load. Boundary lubrication

regimes occur under any condition where the asperities of two lubricated surfaces in

relative motion can come into physical contact and potential abrasion and/or adhesion

occur. Lubrication engineers and tribologists have suggested that up to 70 percent of

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wear occurs during the startup and shutdown phases of machines.The main method of

reducing marginal lubrication is to ensure the correct viscosity of the lubricant. A

lubricant with too low a viscosity cannot keep the metal surfaces separated and metal-

to-metal contact occurs. A lubricant with too high a viscosity will result in an increase

in the molecular friction of the oil. This internal shearing of the oil will cause the oil

layers to slide one after the other and consequently increase operating temperatures

and energy losses.

A backup or secondary method of reducing this phenomenon of the limit lubrication

regime is to use a fully formulated lubricant that contains anti-wear additives or

extreme pressures. These additives react with the metal asperities that have come into

contact by reacting to the high pressure and high temperature of the contact and

immediately form a changed malleable (pliable) film on the surface of the metal

(iron). This new film then acts sacrificially when the surfaces slide or roll over each

other. The chemical film created by the additive wears instead of the metal surface.

Mixed Lubrication

As sliding speed increases, the boundary lubrication decreases, causing the face to

appear as a lubricating film on the moving surface. As the possibility of rough contact

decreases and the oil film thickness increases, the coefficient of friction decreases,

reaching a mixed lubrication situation. This is a situation between the boundary

lubrication regime and the hydrodynamic/elastohydrodynamic lubrication regime, i.e.

the gray area between them. As the oil film thickness increases, the system now enters

full film lubrication, called elastohydrodynamic lubrication or hydrodynamic

lubrication.
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Hydrodynamic (HD) Lubrication

This lubrication condition occurs in sliding surfaces where a complete oil film is

supported and creates a working clearance (for example, between a rotating shaft and

a plain bearing). For complete and successful application of hydrodynamic

lubrication, there must be geometric consistency of height between the mechanical

parts (for example, the curve of the shaft and the bearing in the plain bearing are

similar) and relative pressures of moderate to low (100 to 300 psi for commercial

journals).

This lubrication condition occurs after the machine has started rotating, and the speed

and load cause a wedge of oil to be drawn from the shaft and bearing surface. This

wedge of oil moves the shaft away from the bearing surface so that there is no risk of
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hard contact. These are the best conditions for preventing friction and wear. Oil film

thickness is usually 2 to 100 microns (0.00008 to 0.004 in.). It can be larger for very

large diameters (300 microns or 0.012 in.). The lambda value (ratio of oil film

thickness to roughness) is generally greater than 2.

For hydrodynamic lubrication to be effective, the viscosity of the oil must be

sufficient to maintain the hydrodynamic conditions under all operating conditions,

such as high speed and high load, low speed and high load, low speed and low load,

etc. There may be raised or uneven metal areas. It is useful to think of hydrodynamic

lubrication as aquaplaning, when a car tire loses contact with the road. Heavy vehicles

may be powered by low viscosity fluids (water) and lose contact with the road due to

vehicle speed. Much the same thing happens with hydrodynamic lubrication.

Elastohydrodynamic Lubrication (EHL)

Elastohydrodynamic lubrication conditions occur when there is rolling motion

between moving parts and the contact surface is smooth. For example, remember that

the rolling curve in rolling season bearings is very different from the rolling contact

curve. This creates high contact pressures (hundreds of thousands of psi).

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When oil enters the contact area (rolling) between the ball and the track, the oil

pressure increases. This pressure also significantly increases the viscosity and load-

holding capacity of the oil. This concentrated load will slightly deform (flatten) the

metal of the rolling elements and roll in the area. Deformation occurs only in the

contact area and the metal elastically returns to its original state as rotation continues.

Since the viscosity of the oil is directly affected by temperature, it is obvious that

incorrect temperature or abnormal operation can affect the formation of the

elastohydrodynamic lubrication (EHL) film. High rolling contact loads occur where

rolling element bearings, gear teeth and cams come into contact (roll). Rough contact

will not occur due to the excellent properties of lubricants and metal if operating

conditions such as speed, load and temperature are not exceeded. However, EHL is

expected to operate on a saturated liquid (oil) film (high roughness of approximately

0.4 to 0.8 μm).

why is it in this specific region , the graph increases at the end in the

high velocity.
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At the high speed end of the hydrodynamic region, the graphs increase due to various

effects. As speed increases, inertial forces become more important than static forces,

reducing efficiency and providing greater yields. This change results in a transition

from laminar to turbulent flow, where strong turbulence increases mixing and

transfer. The near-surface boundary also becomes more tapered, further reducing

friction and increasing visibility. The increase in Reynolds number also indicates a

transition to a more variable, more turbulent, more powerful and faster flow.

Collectively, these effects result in the increased speed shown

Integrating the Stribeck Curve with Food Tribology

The Stribeck curve provides a useful framework for understanding the

relationship between lubrication and mouthfeel. By analyzing how different

lubrication regimes affect friction and texture, we can better predict and control the

sensory attributes of food products. For instance, achieving the desired mouthfeel in a

product may involve optimizing lubrication levels to ensure that the texture aligns

with consumer preferences.

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Stribeck Curve

Stribeck curve is a fundamental and one of the most widely known concepts in

tribology and lubrication fields. The concept represents the friction behavior in

lubricated contacts as a function of viscosity of the lubricant, entrainment speed and

roughness (sometimes, just roughness). Richard Stribeck along with Mayo Hersey are

accredited as the pioneer researchers of the 20th century, who introduced Stribeck

curve by extending their work on the topic of friction within the railway industry;

however the results of the research were in coherence with the previous researches.

1. Boundary lubrication (point A):

 Speed: 0.1 m/s

 Load: 100 N
 Coefficient of friction: 0.6
Explanation: At low speeds and loads, a thin lubricant film provides metal-to-metal
contact, providing a friction coefficient of 0.6. Surface roughness and insufficient
lubrication cause friction and wear.

2. Mixed lubrication (point B):

 Speed: 0.5 m/s
 Load: 200 N
 Coefficient of friction: 0.4
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Explanation: With As the speed and load increase, the thickness of the lubricant film,
which is part of the separation area, also increases. Due to the combination of
boundary lubrication and hydrodynamic lubrication mechanisms, the friction
coefficient is reduced to 0.4.
3. Full film lubrication (point C):
 Speed: 1.0 m/s
 Load capacity: 300 N
 Coefficient of friction: 0.2

Description: At higher speeds and loads, the thick oil separates from the surface,
causing the coefficient of friction to drop to 0.2. Hydrodynamic lubrication, which
provides good surface separation, prevails.
4. Minimum friction (point D):
 Speed: 1.5 m/s
 Load: 400 N
 Coefficient of friction: 0.1

Description: minimum friction (0.1) environment performs best, the lubricant film
thickness is ideal to reduce friction and wear. Surface roughness is reduced and an oil
film is applied for smooth sliding of the surface. Improved performance and improved
durability.

Practical Applications in Food Science

The practical applications of the Stribeck curve in food science are extensive.

For example, the balance of fat and water content in dairy products is important to

ensure desired oral intake. By measuring the coefficient of friction of different

formulations, dairy companies can adjust the fat content to hydrodynamically

condition the product and provide the creamy, smooth texture that the customer

prefers. Understanding the Stribeck curve can help reproduce the mouthfeel of animal

products. Plant-based formulas often struggle to achieve the same smoothness and

creaminess due to differences in fat content. By examining the lubrication properties

and adjusting the food accordingly, manufacturers can create products that taste

similar to animal products. By making assumptions and comparing the necessary data

with tribological data, researchers can validate the Stribeck Curve findings and ensure

that the prediction is applicable to customer needs. This process involves a detailed
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evaluation in which trained experts evaluate the texture and mouthfeel of the food

under control.

Tribological tests quantify a broad range of materials. Sensory testing along

with tribological analysis enhances the insight into the impact of lubrication on the

mouth, thus resulting in improved predictions and, therefore, better products and, yes,

the ability to analyze Stribeck curves for food uses. If the tribometer to be used has

special sensors, it is possible to read out data that can be used to plot the Stribeck

curve since it can determine the coefficient of friction in the present situation. This

device can imitate mouth-related conditions by sensing the mouth’s circumstances,

such as temperature, pressure, and movement. Changes in surface microstructure.

They enable the investigators to visualize the actions of the lubricants on food, thus

being beneficial to decipher specifics of the Stribek curve as well as its effect on the

dental health of people. There are many research prospects investigating the Baker

curve for more effective learning. It is also possible for future studies to examine the

impact of various classes of lubricants (oil, fat, and emulsion) on the Stribek curve

and the mouth. Other research could be conducted to discover correlations between

temperature and other factors and their impacts on the lubrication as well as the

coefficient of friction. For instance, research could be carried out to devise optimized

biopolymers, hydrocolloids, and other natural materials to manufacture improved

lubricants that improve the taste and do not affect the nutritive value and smell.

Hunger can be viewed as one of the primary needs that is met through relationships of

an object-oriented nature.
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Credible Sources

1. Tribology: Friction, Lubrication, and Wear" by George E. Totten, Hiren C.

Pachwa, and Tracy L. Kennedy (2016)

This book introduces the concepts and applications of tribology, including its

various types: These are friction, lubrication and wear, and the mechanisms associated

with them. This also incorporates many facets of tribology that may be applied in

automotive, aerospace, as well as the environmental sector. Static friction, kinetic

friction, and its subtypes, namely rolling friction and sliding friction, impact friction.

They also talk about categories of lubrication such as boundary lubrication,

elastohydrodynamic lubrication, and hydrodynamic lubrications and different lubriant

products used across industries. Types of wear such as adhesive wear, abrasive wear

and fatigue wear, and techniques of minimizing wear for instance by finishing and

wear- resistant materials. The authors also describe numerous prospects of tribology,

such as the engine design, the design, and the friction layers Furthermore, the authors

provide a list of additional literature. Tribology and Its Applications is aimed at

students, engineers, and researchers interested in tribology and its many uses.

2.Tribology: Materials, Surfaces, and Design," by W. M. Rainforth and M. H.

Stone (2019)

This book presents the subjects and creations of tribology, from the finished

material selections for explicit uses to the most elevated machining edges and the

design decisions that bring down rub and wear. It also treats interesting issues like

nanotribology and biomimetic tribology. Examples of proxies include roughness, the

materials used, and lubrication. They also differentiate various kinds of lubricant and

how it influences friction as well as wear. Machining, grinding, and polishing

operations that are performed on the surface of materials and their consequences on
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friction and wear. The authors also explain several design techniques to minimize and

very the friction and wear, including surface finishes, enhanced interfaces, and

friction layers. Tribology ideas and approaches that are also a list of key terms and

sources. It is intended for students, engineers, and researchers who would like to

develop, improve, or implement tribology and its uses.

3.Fundamentals of Tribology and the Mechanics of Friction Systems" Author:

D. Dowson (1998)

Regarding this book, it offers readers some general understanding about

thermology, such as several types of friction, mechanisms of lubrication, and

procedures of wear. It also encompasses different uses of tribology and addresses new

findings and innovations in the said branch. The two types of friction are sliding

friction and impact friction. They also demystify the classifications of lubrication

where they look at the boundary lubrication, elastohydrodynamic lubrication,

hydrodynamic lubrication, and the various lubricants used for different purposes.

Adhesive wear, abrasive wear, and fatigue wear and measures that could be taken and

used to minimize the wear, like finishing and wear-resistant materials. Tribology is

also presented by the authors with specifications on application such as engine design,

design, and friction layers, as well as the list of further readings. Directed at students,

engineers, and researchers that focus on the subject of tribology and also its various

uses.

4. Tribology: Friction and Lubrication of Mechanical Components" Author: J.

F. Archard (1980)

In this book, the basic concepts of tribology, different kinds of friction,

lubrication, and such other wear phenomena are presented along with their
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application. It also describes the fields of tribology applications and includes

information about new studies and advancements in tribology science prior to the

publication date. It explains the various types of friction, such as rolling friction,

sliding friction, impact friction, etc. The different types of layers of oil film are also

described, which include boundary lubrication, elastohydrodynamic lubrication, and

hydrodynamic lubrication, and various types of oil for various applications are also

described. Techniques of minimizing wear: adhesive wear, abrasive wear, fatigue

wear.

References

Bhushan, B. (2013). Fundamentals of Tribology. Springer.

Rao, M. A. (2014). Food Texture and Viscosity: Concept and Measurement.

Academic Press.

Stone, H., & Sidel, J. L. (2004). Sensory Analysis of Food: Improving Quality.
CRC Press.

Dowson, D., & Wright, R. N. (2014). Tribology: Friction, Lubrication, and

Wear. Springer.
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Dealy, J. M., & Larson, R. G. (2007). Structure and Rheology of Complex

Fluids. Oxford University Press.

Munro, P. A., & Hughes, C. L. (2000). Food Texture and Viscosity: Concept
and Measurement. Academic Press.