Machine Elements Design

What is a Machine?

Machine design is an important part of engineering applications, but what is a machine? Machine is the devise that comprises of the stationary parts and moving parts combined together to generate, transform or utilize the mechanical energy. All the machines are made up of elements or parts and units. Each element is a separate part of the machine and it may have to be designed separately and in assembly. Each element in turn can be a complete part or made up of several small pieces which are joined together by riveting, welding etc. Several machine parts are assembled together to form what we considering the various applications of the machines, they are classified into three main types, these are:

1) Machines generating mechanical energy: The machines generating mechanical energy are also called as prime movers. These machines convert some form of energy like heat, hydraulic, electrical, etc into mechanical energy or work. The most popular example of these machines is the internal combustion engine in which the chemical energy of the fuel is converted into heat energy which in turn is converted into mechanical work in the form of the rotation of the wheels of the vehicle. Some other examples of this group of machines are gas turbines, water turbines, steam engine etc.

2) Machines transforming mechanical energy: These machines are called converting machines because they convert mechanical energy into other form of energy like electricity, hydraulic energy etc. Some examples of these machines are electric generator in which the rotation of the shaft is converted into electrical energy, and hydraulic pump in which the rotation energy of the rotors is converted into the hydraulic energy of the fluid.

3) Machines utilizing mechanical energy: These machines receive mechanical energy and utilize it for various applications. Some examples of these machines are lathe machine that utilizes the mechanical energy to cut metals and washing machine that utilizes the rotation of the rotor for washing the clothes.

It is important to remember that all machines are limited in their efficiency by friction. No machine is 100 percent efficient in its efforts, so the mechanical advantage gained will require additional energy to accomplish the task.

What are Machine Elements?

Complex machines are made up of moving parts such as levers, gears, cams, cranks, springs, belts, and wheels. Machines deliver a certain type of movement to a desired location from an input force applied somewhere else. Some machines simply convert one type of motion to another type, such as rotary to linear.  While there is a seemingly endless variety of machines, they are all based upon simple machine elements. The elements discussed here include inclined planes, levers, wheels and axles, pulleys, and screws.

All the machines are made up of elements or parts and units. Each element is a separate part of the machine and it may have to be designed separately and in assembly. Each element in turn can be a complete part or made up of several small pieces which are joined together by riveting, welding etc. Several machine elements are assembled together to form what we call as complete machine, which performs various applications.

Classification of Machines Elements

The machines elements are classified into two main types: general purpose elements and special purpose elements. These have been described below:

1) General purpose elements: The elements which are common to various types of machines serving different applications are called general purpose elements. Examples of these elements are nuts, bolts, keys, axles, shafts, couplings, bearings etc.

2) Special purpose elements: The elements that are used only in the specific type of machine are called as special purpose elements. For instance piston and connecting rods are used in the engines and compressors, while blades are used in the turbines and blowers. Some other examples are cam shafts, push roads, crankshaft, cylinder etc.

Types of General Purpose Elements

The general purpose elements, which are common in all types of machines, are classified into two types: fasteners and elements of rotary motion drive. These are described below:

1) Fasteners: The fasteners are the machine elements that connect or join various parts of the machine. The joints can be of permanent type or temporary type. The permanent joints are the ones that cannot be separated or disassembled into individual elements without destroying or damaging them. The examples of permanent joints are welded joints, riveted joints etc. The temporary joints are the ones in which the individual elements of the assembly can be separated easily without destroying or damaging them. The joints obtained by nut and bolt, and the cotter joints are common and widely used examples of the temporary joints.

2) Elements of rotary motion drive: These are the elements that help transmit the motion or power to or from the machines. For example belt connected to the motor and pump helps running the pump. The fear box helps transmit the motion and power from the engine to the wheels of the vehicles. Other examples of elements of the rotary motion drive are rope, chain, gear, worm drives, shafts, axles, couplings, bearings etc.

Machine Design or Mechanical Design

Machine design and drawing are very important subjects of mechanical engineering. No produce can be manufactured without designing it. Here some basic concepts of machine design or mechanical design have been covered.

What is design?

The word ‘design’ comes from the Latin ‘designare’, which means to designate or make out. Design can be taken to mean all the processes of conception, invention, visualization, calculation, refinement and specification of details that determine the form of a product. Design generally begins with either a need or requirement or, alternatively, an idea. It ends with a set of drawings or computer representations and other information that enables a product to be manufactured and utilized.

The Purpose of Machine Design Course

The course in Machine Design help students understand the fundamentals of designing the most commonly used parts, elements and units of various machines. Small components of machine on assembling make a big machine, hence the machine as a whole as well as its individual components have to be designed.

The knowledge of machine design helps the designers as follows:

1) To select proper materials and best suited shapes,

2) To calculate the dimensions based on the loads on machines and strength of the material,

3) Specify the manufacturing process for the manufacture of the designed component of the machine or the whole machine.

Machine Design is the application of: mathematics, kinematics, statics, dynamics, mechanics of materials, engineering materials, mechanical technology of metals and engineering drawing. It also involves application of other subjects like thermodynamics, electrical theory, hydraulics, engines, turbines, pumps etc. Machine drawing is the integral part of the machine design, since all the components or the machines that have been designed should be drawn to manufacture them as per the specifications. Without machine drawing the subject of machine design is incomplete.

Factors to be considered while Designing Machine

When the designer designs the elements of the machine or the complete machine, they have to consider several important parameters. Here are some of the important factors to be considered while doing machine design:

1) Cost: Cost has always been the major factor of consideration while designing the machine elements or machine and in this age of competition it has become more important. The best machine design is the one which helps get the finished product with all the major functionalities and highest possible quality at the lowest possible cost. Gone are the days when expensive and bulky materials were used for making the machine elements.

2) High output and efficiency: Earlier machines used to be very heavy and consume lots of power. Now the trend is of full functional machines consuming low power and giving high output in terms of the number of the of products manufactured. Some computer controlled machines can manufacture the components very fast and are highly efficient.

3) Strength: The machine elements or the machine should be strong enough to sustain all the forces it is designed for so that it is not damaged or permanently deformed during its life time. Right at the time of the designing the machine the designer should consider the force machine can be applied to and consider all the relevant factors that could affects its life.

4) Stiffness or rigidity: The machine should be rigid enough so that under the effect of applied forces for which it is designed there is no deformation of the machine or machine elements beyond the specified limits. If there is excessive deformation, there are chances of the failure of the machine elements and the whole machine.

5) Wear resistance: Wear is the removal of the material from the metallic surface when two surfaces rub with each other. If there is more removal of the material, the component will become weaker and eventually break. The wear of the contacting surfaces can be reduced by the lubrication of the surfaces, increasing the strength or the hardness of the working surfaces. The effect of wear can also be reduce by increasing the surface, so that during the lifetime of the mating machine elements they will not fail even if there is some wearing between them.

6) Lubrication: Lubrication between the two mating surfaces of the elements of the machine help reducing friction between them and wearing of the two surfaces, which results in the increase in life of the components of the machine.

7) Operational safety: For the safety of the operator of the machine, the hazard producing things from the machine should be eliminated and the design should confirm to the safety codes.

8) Ease of assembly: The elements of the machine should be such that the machine can be assembled very easily. For the mass production of the complex machines like automobiles, type writers etc, the concept of unit assemblies are common. The unit assemblies are assembled together to form the complete machine.

9) Ease and simplicity of disassembly: Like assembly, the disassembly of the machine also should be easy so as to easily carry out replacement of the parts, and repair and maintenance of the machine and machine elements.

10) Ease and simplicity of servicing and control: The machine and its element should be simple enough so that very little maintenance and servicing is required. The repair and maintenance of the machine should be easy and cheap and simple replacements should be available.

11) Light weight and minimum dimensions: The machine elements and machine should be strong, rigid and wear resistant with minimum weight and least dimensions. This can be achieved by using light weight rolled sections and hardening the metals. Using high strength grades of cast iron and light alloys can further help getting light materials and minimum dimensions of the machine elements. Improving the design in this direction is very important.

12) Reliability: The reliability of the machine is a very important if the machine has to find the huge market in the business.

13) Durability: The longer the life of the machine more it develops the reputation of being the dependable machine and more will be its sale. Hence the right at the time of designing reliability and durability should be given priority. For this the machine should be designed for least maintenance requirements and long-life.

14) Economy of performance: For the proper economic performance of the machine correct mechanical, hydraulic, thermodynamic and other principles should be applied while designing the elements of the machine and the whole machine.

15) Accessibility: The machine elements and machine the whole should be easy to handle and access.

16) Processability: The shape and the materials for the elements of the machine should be such that they can the processing costs and labor costs are lowest possible.

17) Compliance with state standards: Following the standards makes designing easier and availability of various parts faster and easier.

18) Economy of repairs and maintenance: While designing the machine elements and machine the designing should be such that least amount of repairs and maintenance will be required for the machine.

19) Use of standard parts: There should be maximum possible standard parts in the design of the machine. This will help reduce the cost of the machine and ensure easy availability of the parts. With standard parts the design can be modified easily.

20) Use of easily available materials: Materials selected for the machine elements during the design should be available easily and lowest possible costs.

21) Appearance of the machine: While designing the machine the aesthetics and ergonomics of the machine should be given due consideration without affecting its functionality.

22) Number of machines to be built: Designing of the machine will depend a lot on the number of machines to be manufactured. If few numbers of machines are to be manufactured then expensive materials and high production costs can be considered, but for the mass production economy of the machine should be top priority.

Machine Elements Procedures:

There is no fixed machine design procedure for when the new machine element of the machine is being designed a number of options have to be considered. When designing machine one cannot apply rigid rules to get the best design for the machine at the lowest possible cost. The designer who develops the habit of following a fixed line of steps for designing the machine or machine elements cannot come out with the best product. When the new product is to be developed the problems keep on arising at design stage, and these can be solved only by having flexible approach and mportanct Steps of Designing Machine

Though the machine design procedure is not standard, there are some common steps to be followed; these can be followed as per the requirements wherever and whenever necessary. Here are some guidelines as to how the machine design engineer can proceed with the design:

1) Making the written statement: Make the written statement of what exactly is the problem for which the machine design has to be done. This statement should be very clear and as detailed as possible. If you want to develop the new produce write down the details about the project. This statement is sort of the list of the aims that are to be achieved from machine design.

2) Consider the possible mechanisms: When you designing the machine consider all the possible mechanisms which help desired motion or the group of motions in your proposed machine. From the various options the best can be selected whenever required.

3) Transmitted forces: Machine is made up of various machine elements on which various forces are applied. Calculate the forces acting on each of the element and energy transmitted by them.

4) Material selection: Select the appropriate materials for each element of the machine so that they can sustain all the forces and at the same time they have least possible cost.

5) Find allowable stress: All the machine elements are subjected to stress whether small or large. Considering the various forces acting on the machine elements, their material and other factors that affect the strength of the machine calculate the allowable or design stress for the machine elements.

6) Dimensions of the machine elements: Find out the appropriate dimensions for the machine elements considering the forces acting on it, its material, and design stress. The size of the machine elements should be such that they should not distort or break when loads are applied.

7) Consider the past experience: If you have the past experience of designing the machine element or the previous records of the company, consider them and make the necessary changes in the design. Further, designer can also consider the personal judgment so as to facilitate the production of the machine and machine elements.

8) Make drawings: After designing the machine and machine elements make the assembly drawings of the whole machines and detailed drawings of all the elements of the machine. In the drawings clearly specify the dimensions of the assembly and the machine elements, their total number required, their material and method of their production. The designer should also specify the accuracy, surface finish and other related parameters for the machine elements.

Safety factor

A component subjected to a solitary load will be considered in the first instance. This load is interpreted in the context of the component's nature and duty - thus   load usually implies a transverse force in the case of a beam component, or a longitudinal compressive force in a column, or a torque in the case of a shaft, or a pressure in a fluid containment vessel, and so on.

There are two completely different manifestations of the load, which have important consequences for the component :

• the extrinsic  actual load is the load exerted on the component by its surrounds, and
• the intrinsic   maximum load is the largest load that the component can withstand without failure; the maximum load is a property of the component, a function of its dimensions and material.

Each of these two loads is expressed usually by its   nominal value.   ( "Nominal" means "in name only"; a nominal value of something is merely a convenient label. For example, 500mm pipes are made with an outside diameter of 508mm and various wall thicknesses - and hence come in various internal diameters - but they are all referred to by the convenient rounded "500"  nominal diameter as a descriptor of size. )

Clearly a component is safe only if the actual load applied to the component does not exceed the component's inherent maximum sustainable load. The degree of safety is usually expressed by the   safety factor,   n :-

( 1 )      n    =    maximum load / actual load    =    F_max / F

. . . . and it follows that :

if n = 1 then the component is on the point of failure

if n < 1 then the component is in a failed state

if n > 1 then the component is safe.

The safety factor is usually expressed as a ratio of nominal loads. A higher value of the safety factor seems to indicate a safer component - however this is not necessarily the case as the inevitable variations must be kept in mind.

IF - and only IF - the stress in a component is proportional to the actual load on the component, then the safety factor may be interpreted also as a stress ratio :-

( 1a )      n    =    S / σ

where S is the strength of the component's material, and σ is the stress in the component due to the actual load, F.

SUGGESTED SAFETY (DESIGN) FACTORS FOR ELEMENTARY WORK
based on yield strength - according to Juvinall & Marshek op cit.
1	1.25 - 1.5	for exceptionally reliable materials used under controllable conditions and subjected to loads and stresses that can be determined with certainty - used almost invariably where low weight is a particularly important consideration
2	1.5 - 2	for well-known materials under reasonably constant environmental conditions, subjected to loads and stresses that can be determined readily.
3	2 - 2.5	for average materials operated in ordinary environments and subjected to loads and stresses that can be determined.
4	2.5 - 3	for less tried materials or for brittle materials under average conditions of environment, load and stress.
5	3 - 4	for untried materials used under average conditions of environment, load and stress.
6	3 - 4	should also be used with better-known materials that are to be used in uncertain environments or subject to uncertain stresses.
7		Repeated loads : the factors established in items 1 to 6 are acceptable but must be applied to the endurance limit (ie. a fatigue strength ) rather than to the yield strength of the material.
8		Impact forces : the factors given in items 3 to 6 are acceptable, but an impact factor (the above   dynamic magnification factor ) should be included.
9		Brittle materials : where the ultimate strength is used as the theoretical maximum, the factors presented in items 1 to 6 should be approximately doubled.
10		Where higher factors might appear desirable, a more thorough analysis of the problem should be undertaken before deciding on their use.