# 12 General Considerations in Machine Design

Designing machine components or assembly needs to consider various factors, on these factors or Standard Data final product is designed. Final Design should involve Details information about Dimensions, Processes required, Surface finishing and tolerance details, etc. Mechanical Design engineers may use Design Databook, 3D,2D Design Softwares, research papers as a tools for Designing machine Components. In this article, we will cover some important factors on which the machine design process is dependant.

Following are the general considerations in designing a machine component:

The load, on a machine component, may act in several ways due to which the internal stresses are set up. :

• Compression- Applying forces to both ends
• Tension- Forces applied in the opposite direction
• Shear- Sliding forces that are applied in the opposite direction
• Bending- Force off-centered
• Torsional- Twisting force
• Combination – Combination of any loads

## 2. Motion of the parts or kinematics of the machine.

The successful operation of any machine depends largely upon the simplest arrangement of the parts which will give the motion required.

The motion of the parts may be:
(a) Rectilinear motion which includes unidirectional and reciprocating motions.
(b) Curvilinear motion which includes rotary, oscillatory, and simple harmonic.
(c) Constant velocity.
(d) Constant or variable acceleration.

## 3. Selection of materials

A designer must have a thorough knowledge of the properties of the materials and their behavior under working conditions. Some of the important characteristics of materials are strength, durability, flexibility, weight, resistance to heat and corrosion, ability to cast, welded or hardened, machinability, electrical conductivity, etc.

Selection of Materials for Engineering Purposes

The selection of a proper material, for engineering purposes, is one of the most difficult problems for the designer. The best material is one which serves the desired objective at the minimum cost. The following factors should be considered while selecting the material:
1. Availability of the materials,
2. Suitability of the materials for the working conditions in service, and
3. The cost of the materials.
The important properties, which determine the utility of the material, are physical, chemical, and mechanical properties. We shall now discuss the physical and mechanical properties of the material in the following articles.

Classification of Engineering Materials

The engineering materials are mainly classified as:
1. Metals and their alloys, such as iron, steel, copper, aluminum, etc.
2. Non-metals, such as glass, rubber, plastic, etc.
The metals may be further classified as:
(a) Ferrous metals and (b) Non-ferrous metals.

## 4. Form and size of the parts.

The form and size are based on judgment. The smallest practicable cross-section may be used, but it may be checked that the stresses induced in the designed cross-section are reasonably safe. To design any machine part for form and size, it is necessary to know the forces which the part must sustain. It is also important to anticipate any suddenly applied or impact load which may cause failure.

## 5. Frictional resistance and lubrication.

There is always a loss of power due to frictional resistance and it should be noted that the friction of starting is higher than that of running friction. It is, therefore, essential that careful attention must be given to the matter of lubrication of all surfaces which move in contact with others, whether in rotating, sliding, or rolling bearings.

## 6. Convenient and economical features.

In designing, the operating features of the machine should be carefully studied. The starting, controlling, and stopping levers should be located based on convenient handling. The adjustment for wear must be provided employing the various take-up devices and arranging them so that the alignment of parts is preserved. If parts are to be changed for different products or replaced on account of wear or breakage, easy access should be provided and the necessity of removing other parts to accomplish this should be avoided if possible. The economical operation of a machine which is to be used for production or the processing of material should be studied, to learn whether it has the maximum capacity consistent with the production of good work.

## 7. Use of standard parts.

The use of standard parts is closely related to cost because the cost of standard or stock parts is only a fraction of the cost of similar parts made to order. The standard or stock parts should be used whenever possible; parts for which patterns are already in existence such as gears, pulleys and bearings, and parts that may be selected from regular shop stock such as screws, nuts, and pins. Bolts and studs should be as few as possible to avoid the delay caused by changing drills, reamers, and taps and also to decrease the number of wrenches required.

Interchangeability

The term interchangeability is normally employed for the mass production of identical items within the prescribed limits of sizes. A little consideration will show that to maintain the sizes of the part within a close degree of accuracy, a lot of time is required. But even then there will be small variations. If the variations are within certain limits, all parts of equivalent size will be equally fit for operating in machines certain variations are recognized and allowed in the sizes of the mating parts to give the required fitting. This facilitates selecting at random from a large number of parts for an assembly and results in a considerable saving in the cost of production.

## 8. Safety of operation.

Some machines are dangerous to operate, especially those which are speeded up to insure production at a maximum rate. Therefore, any moving part of a machine that is within the zone of a worker is considered an accident hazard and may be the cause of an injury. It is, therefore, necessary that a designer should always provide safety devices for the safety of the operator. The safety appliances should in no way interfere with the operation of the machine.

## 9. Workshop facilities.

A design engineer should be familiar with the limitations of this employer’s workshop, to avoid the necessity of having work done in some other workshop. It is sometimes necessary to plan and supervise the workshop operations and to draft methods for casting, handling, and machining special parts.

## 10. The number of machines to be manufactured.

The number of articles or machines to be manufactured affects the design in several ways. The engineering and shop costs which are called fixed charges or overhead expenses are distributed over the number of articles to be manufactured. If only a few articles are to be made, extra expenses are not justified unless the machine is large or of some special design. An order calling for a small number of the product will not permit any undue expense in the workshop processes so that the designer should restrict his specification to standard parts as much as possible.

## 11. Cost of construction.

The cost of construction of an article is the most important consideration involved in design. In some cases, the high cost of an article may immediately bar it from further considerations. If an article has been invented and tests of handmade samples have shown that it has commercial value, it is then possible to justify the expenditure of a considerable sum of money in the design and development of automatic machines to produce the article, especially if it can be sold in large numbers. The aim of the design engineer under all conditions should be to reduce the manufacturing cost to the minimum.

## 12. Assembling.

Every machine or structure must be assembled as a unit before it can function. Large units must often be assembled in the shop, tested, and then taken to be transported to their place of service. The final location of any machine is important and the design engineer must anticipate the exact location and the local facilities for erection.

Sachin Thorat

Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

## 2 thoughts on “12 General Considerations in Machine Design”

1. Isoliments says:

Thanks for such informative content

2. Ahmed siddiqui khalifa says:

thanks

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