INTRODUCTION TO THE DESIGN OF MACHINE COMPONENTS

WHAT IS A MACHINE ?

1. Mechanical device: a device with moving parts, often powered by electricity, used to perform a task, especially one that would otherwise be done by hand a washing machine

2. Simple unpowered device: a simple device used to overcome resistance at one point by applying force at another point, e.g. a lever, pulley, or an inclined plane

3. Powered form of transportation: an engine-driven means of transportation, e.g. an aircraft, car, or motorcycle

4. Group of people in control: an organized group of people that controls or directs something, especially a political group. the party machine

5. Complex system: a complex system structured so as to accomplish a particular goal. the war machine.

6. Somebody who behaves mechanically: somebody who is regarded as behaving like a mechanical device, e.g. somebody who is efficient but uncreative. men trained as deadly machines.

 

A machine is composed of three elements

Main parts of machine

•       Power Block: Supplies the required energy plus power for mechanical motion.

•       Transmission/Drive: Connects the power the executive organ.

•       Executive organ: Performs both primary and secondary motions required for the transformation process, determines the diversity of technological functions performed by the machine.

Machine Classification

Machine can be simple or complex;

   

Examples of simple machines are; the wedge, Lever, Inclined plane, the screw jack, pulley.

   

Examples of complex machines are; Industrial Robots, Manipulators, Space rockets, aero plane, Computer Numerical Control (CNC), Machine centers, combined Harvesters etc.

 

Machine Classification

   

    Machine can be as small as a coin and as big as a 10 storey building e.g. hydraulic press or as large as 500 metres sailing ship.

    Machine can also be manual (mechanical), semi automatic (electronic) or fully automatic, and by functions, electrical, hydraulic, pneumatic vacuum, magneto hydrodynamic, nuclear powered.

 

Basic Definition

 

    Machines, mechanisms, equipment, apparatus, fixtures, instruments and similar products of primary and secondary industries are made from smaller components (details) and sub-assemblies.

 

    A detail is a machine component made of uniform material of one make without the need for any assembly operation (e.g. bolt, nuts shaft, gear etc.)

 

Basic Definition cont’d.

   

    Two or more details, assembled together in the factory to form part of a machine, equipment or instrument is referred to as an assembly unit (e.g. bearing, reductor, speed box, clutch).

   

    A machine may contain hundreds, thousands and even millions of components depending on its complexity.

Basic components of machines

•       Examples of these are: shafts and axle, springs, bolts, Nuts, studs, screws, rivets, gears, bearings, clutches and shaft couplings, keys, and splines, brakes, gaskets, washers, body or carcass, pulleys, sprockets, ratchet, cam, tappet push rods, bushings racks, worms, instrument dials etc.

•       Machine components are as diverse as the types of machine.

•       Some components, however, are found more frequently in machine than others. These are referred to as the basic components.

 

Machine components can be grouped into

1. A Fasteners: they are divided into

(i) Temporary such as Bolt, Nut, Screw, Studs, etc. and

(ii) Permanent Joint: Welding, Riveting, Glue, Brazing, Soldering, etc.

2. Joints such as Fits: Press fit, Forced fit, Interference fit, Pin & Cotter, Loose fit etc.

3. Transmission elements: Belt, Gear, Chain.                                                                            Gear drive is very important and versatile in transmission. Examples are Hypoid (for heavy load), Spur, Bevel, Worm & Wheel, Rack & Pinion.

4. Transmission Support: Shaft, Pulley, Bearings, Clutches and Couplings. Seals, springs and lubricators, Keys & Spline.

5. Motor Power Block (which are normally based on the power need)

 

Machine functionality

Ability to perform its function effectively within the machine is one of the most important demands of a component.

•       Criteria for functionality are:

•       Strength: Ability to resist destroying force, a most important criterion

•       Rigidity : Ability to retain its form

•       Wear Resistance

•       Vibration resistance

 

Approach to Machine Design

Machine design is a creative multi-phased process with its own peculiarity. It is characterized by

•       Multi-variant decision making.

•       Conformity with existing standard and industry norms.

•       Conformity with both general and specific demands to the construction.

•       Orientation towards innovativeness and to accommodate as many new ideas as possible.

 

Stages in Machine Design.

The following are the 5 stages of a new machine design project:

Stage 1: Prepare a Technical Brief

Stage 2: Preparation of Technical & techno-economic feasibility based on the clients brief.

Stage 3: Preparation of initial design sketches

Stage 4: Preparation of Final Design which includes design drawings

Stage 5: Preparation of final working document

•       Stage 1: Prepare a Technical Brief: – This document contains the name, main function, specifications, quality indices, economic indices and any special demand of the client. (In preparing this document, use is made of the client or customer brief (need), latest information on the latest and most modern and similar products, patent search, scientific research and prognosis pertaining to such machine).

•       Stage2: Preparation of Technical & techno-economic feasibility based on the clients brief. Preliminary sketches of possible design variants are brought forward for selection and confirmation of the best choice by the client and his consultants.

Stage 3: Preparation of initial design sketches showing more details as to the general view, working principle, dimension and basic parameters of the selected variant.

Stage 4: Preparation of Final Design which includes design drawings and working documents showing full view (2D&3D), assembly drawings of sub-assemblies which must reflect best practices and bench marked against the best of its type anywhere in the world. At this stage also consideration is given to reliability, safety, storage and transportation issues.

Stage 5: Preparation of final working document. Full detail drawings of components to be produced. Full design, reliability and economic analysis to justify production.

     Please note that use of computer is highly recommended at every stage considering the very difficult calculations and drafts involved. Computer aided Design project Management Software should be used.

 

Mechanisms and their functions

    Mechanisms form parts of machines when the working process is purely mechanical motion.

    They have chain structure and may have two or more connected members called links.

Main types of mechanisms used in machines:

•       Lever mechanisms (which may have one or more translator or rotary kinematics pair included in the chain).

•       Cam mechanisms which consist of the forced contact of a cam and a tappet follower. Can give any displacement, velocity or acceleration. It is a very versatile mechanism. Examples are Drum cam, Disc cam, etc.

•       Friction mechanisms e.g. braking system.

•       Gear mechanisms (spur, gear drive, bevel gear)

•       Maltese-cross (Geneva stop)

•       Hydraulic/ Pneumatic mechanisms

 

Functional Characteristics of Machine

•       Quality of performance

•       Reliability

•       Ergonomics (Aesthetics quality) composition of the machine.

•       Technological – ease of maintenance & repair; works easily.

•       Economical – cheap, justifies cost.

 

Three basic models for machine component analysis.

According to strength of materials / solid mechanics, there are 3 models in application.

(a) Material model i.e. assumption of uniformity (isotropy not anisotropy) of material.

(b) Computational model for the material i.e.

•       Elasticity

•       Plasticity

•       Yield strength

(c) Model of Geometric form i.e.

•       Cylindrical, Plate, disc, Ring, Spherical, Massive, Bar, Round.

 

Theoretical principles

•       Hook’s law

•       Principle of Superposition & Independence of forces

•       The elements of a machine in the process of operation experience different externally applied forces which can damages or deform them.

•       Hence, methods used are;

•       Stress – computation of the stressed and strained condition.

•       Strain – computation of the extent of deformation

•       Structural stability test (Slenderness ratio)

•       Rigidity.

 

Types of Deformation of machine component

•       Tensile & Compressive

•       Shear & Twist

•       Bending & Compound Resistance

•       Lateral / Structure Stress under dynamic/impact load

•       Fatigue wear & Failure of shafts & rotors

•       Buckling of Columns

 

Design Criteria

    The most important criterion for selection of machine component is that of allowable or limiting stress.                                                                                                                                             

   Where  stress at some point in the biggest load
 is limiting stress due to the material and type of details.

   E.g. To determine the diameter of bolt in a bolted joint

   If the bolt experiences a force F, then

s    dp= F/A = 4F/ ²

   Let [s] the limiting stress due to the material which is given,

   Then d² sp= 4F/

  

Design Criteria cont’d.

Another criterion is that of safety factor.                                      In this case n = s_min/s_max . Where n = safety factor 

s     = limiting stress or

s    _max = maximum stress at the most dangerous point in the detail or structure.

 

FINALLY.

   The design process is clearly a systematic process that calls for creativity.

   However, creativity also involves risk.

Basic Characteristics of Steel

•       Pure iron is a relatively soft, ductile, low strength metal with few practical engineering applications.

•       The addition of Carbon to pure iron increases strength and hardenability to useful levels. However it decreases ductility.

•       Since mechanical behaviour is directly related to the phases present it is important to study these phases and how they are influenced by temperature. A study of the Iron- Carbon phase diagram is used for this purpose.

Iron-carbon phase equilibrium

•       notes on iron and what happens when iron /carbon mixtures are cooled from liquid to solid.   The notes are based on the Iron Phase Diagram (equilibrium diagram ).  

•       A "Phase" is a form of material having characteristic structure and properties. It is a form of the material which has identifiable composition, structure and boundaries separating it from other phases in the material volume.

•       This phase diagram tells us the various phases a particular alloy of Iron and Carbon will go through when allowed to cool down to room temp.

•       In general iron carbon alloys up to 2% are known as steels while from 2% upwards the alloys are identified as cast iron.

•       The diagram below shows the phases present when when Fe-C alloys (C up to 7%) are cooled from liquid to solid.
The left side of the diagram represents pure iron and the right hand of the diagram represents an alloy with 6,67% C. which result on cooling in the formation of Cementite.  This is a intermetallic compound (iron carbide-Fe₃ C) which although not 100% stable, but is to all practical purposes a stable phase.  The phase diagram shown is therefore a meta-stable phase.

 

Different reference sources indicate the Eutectoid point at 0,8% C and 0,77% C.

Steels

•       If the carbon content of the cooled solid is less than Eutectoid (about 0,8% C) the solid is identified as a hypoeutectoid steel: most steels are this form.   If the carbon content is more then 0,8% then the solid is a hypereutectoid steel.   Hypereutectoid steels with carbon content over 1,2% C are very brittle.  Few steels are made with carbon contents over 1,2%.

    Generally in order to increase the strength of steel other alloying elements are added which increase the strength while retaining toughness and ductility.

CONCLUSION

 

*    There are basically two approaches to engineering design

– Forward and

– Reverse engineering

(developing & developed countries)

 

*    The general knowledge of engineering materials helps in material selection for professional engineering design.

 REFERENCES

       Thomson, Ross (2009), Structures of Change in the Mechanical Age: Technological Invention in the United States 1790-1865, Baltimore, MD: The Johns Hopkins University Press, ISBN 978-0-8018-9141-0

      Zelinski, Peter (2013-11-08), "Hybrid machine combines milling and additive manufacturing", Modern Machine Shop.

      Robert L. Norton, Machine Design, (4th Edition), Prentice-Hall, 2010

      Matthews, Clifford; American Society of Mechanical Engineers (2005), ASME engineer's data book (2nd ed.), ASME Press, p. 249, ISBN 978-0-7918-0229-8.

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