Basic Elements of Machine Design

In this tutorial, you will learn the basics of machine design, including its definition, why is it used, standardization, and standard norms involved. In short, you will understand the basic criteria with which designing a component starts and what is expected from a good design.

Contents:

1. What is Machine Design?
2. Basic Process of Designing a Machine Element
3. Requirements of a Designed Machine Element
4. What is Standardization?
5. What are Preferred Numbers?
6. Using Basic and Derived Series
7. Aesthetic Considerations of a Design
8. Ergonomic Considerations of a Design

What is Machine Design?

Machine design in essence is the application of scientific principles, technical information, and a little imagination in the creation of a machine or a system with the aim to perform a specific task efficiently and economically.

• A designer in mind uses the principle of basic engineering sciences like physics, mathematics, statistics, etc. to come up with the concept of the design.
• Designer keeps in mind the task for which the machine is to be designed and gather technical information for the other basic elements of the machine, including fastening devices, chains, dates and gears, springs, nuts, and bolts, etc.
• He uses his skills and imagination to create a configuration which is a combination of these basic elements and serves the required purpose.
• The outcome of such a design process consists of a description of the machine in the form of drawing, details of dimensions for each element, material information and information about their assemblies.

Basic Process of Designing a Machine Element

The following diagram shows the general process involved in designing a machine element.

The basic procedure of designing a machine element consists of a step-by-step process, from giving specification about the functional requirements of the product to the complete description in the form of drawings of the final product.

• The first step in the process consists of creating a complete list of the requirements of the product. These include the output capacity of the machine, service life, cost, reliability, performance characteristics, operations and control, and much more.
• After careful study of this list, the designer creates various sketches of possible iterations, which are compared against each other and with existing models in the market. They are compared depending upon cost competitiveness, availability of raw materials and manufacturing facility, the best possible activation is selected for the product.
• A Block diagram is prepared to show the general layout of the selected iteration. In the step designer also specifies the details, such as the joining methods. Rough sketches are created for the individual parts.
• The next step involves design of individual component of the machine. It is characterized by heavy brainstorming and detailed stress and deflection analysis.
• The last stage is to prepare drawings of the assembly and individual components. Often a prototype model is also prepared for the product and thoroughly tested before finalizing the assembly drawings.

Requirements of a Designed Machine Element

A well-designed machine element is expected to preserve its operating capacity during the stipulated service life with minimum manufacturing and operating costs. In short, the element must satisfy several basic requirements.

• It should not fail under the effect of the forces that act on it and must have sufficient strength to avoid failure due to fracture or yielding.
• It should be rigid and must not deflect too much on the action of the force. In certain cases, maximum permissible deflection and permissible angle of twist are criteria for design.
• It should be sufficiently strong, rigid and wear resistant, and at the same time have minimum possible dimensions and weight to reduce costs.
• Machine part should conform to the national or international standards covering its profile, dimensions, grade and material.
• The element should be highly reliable during its lifespan and should be easy to service or repair in case of damage.
• Lastly, the designed machine element should be easy for fabrication and assembly. The shape should be such that it can be easily produced at a minimum labor cost.

What is Standardization?

Standardization refers to the obligatory norms to which various characteristics of a product should conform. These include material, dimensions, method of testing, middle of marking, packing and storing of the product, etc.

• Standards are often classified based on their scope of usage.
  • Company standards are limited to within a particular company or a conglomerate.
  • National standards are unique each country, like the I.S. (Bureau of Indian standards) or the AISI.
  • International standards are prepared by the international organization for standardization.
• Standardization is important in the industry as it allows for mass manufacturing by reducing the types and dimensional variances for a particular component. Manufacture of a standard company on a mass scale reduces the cost.
• Standardization allows for interchangeability of machine elements. This makes replacement of worn-out parts easier. This reduces the maintenance cost in turn.
• Application of standard machine elements and especially standard units reduces the time and effort required to design a new machine from scratch. There is no requirement to design the standard elements which can be directly selected from the manufacturers catalogue.

What are Preferred Numbers

In Engineering design, many times the designer has to specify the size of the product. The size is a general term for different characteristics of the machine-like power transmission, load capacity, speed, etc. Preferred numbers are thus used to specify the size of the product in these cases.

The following table lists the basic series and their series factors.

• The system was first introduced by French balloonist and engineer Charles Renard. It uses geometric progression to the develop a set of numbers.
• There are five basic series, namely R5, R10, R20, R40 and R80, which increases in steps of 58%, 26%, 12%, 6% and 3% respectively.
• Each series has its own series factor. The series is established by taking the first number and multiplying it by the series factor to get the second number. This process continues until the complete series is built up.
• Preferred numbers minimize unnecessary variations in sizes. They assist the designers in avoiding selection of sizes in an arbitrary manner.

Using Basic and Derived Series

There are two dogs which are frequently used in relation to provide numbers, the basic series, and the derived series. R5, R10, R20, R40 and R80 form the basic series. Any series formed based on a basic series is called a “derived series”.

• There are two methods of forming a derived series. In the first method, the derived series is obtained by taking every nth element of the basic series. The nth value is specified next to the basic series after a ‘/’. For example, R10/3 takes every third element of the series. Often the limit of the series is specified in parentheses.
• In the second method, the derived series is obtained by increasing the numbers of a particular basic series. For example, R5(1, … 10) is {1, 1.6, 2.5, 4, 6.3, 10}. The next numbers may be derived by multiplying the series by 10 and so on, resulting in the derived series {1, 1.6, 2.5, 4, 6.3, 10, 16, 25, 40, 63, 100}. This series can be increased depending on requirement.
• The advantage of derived series is that one can obtain the geometric series for any range of numbers, with any value of the first and the last numbers. Also, one can have any intermediate numbers between the two limits.

Aesthetic Considerations of a Design

When there are several products in the market, having the same functional characteristics, the customer is attracted to the most athletically pleasing product. External appearance is thus an important feature which dominates the sales in a market for consumer durables like automobiles, household appliances and audiovisual equipment.

• There is a relationship between functional requirement and appearance of the product. The streamlined shape of the Boeing airplane is a result of aerodynamic strides, while chromium plating of wheel is necessitated due to the risk of corrosion in its absence.
• Collection of proper color is an important consideration in product ethics. The choice of color should be compatible with a conventional idea of the operator, for example red usually catches the eye and is used for emergency switches.
• The external characteristic of the machine is also affected by factors like surface finish, motion of individual component, manufacturing methods, materials, and noise reduction.

Ergonomic Considerations of a Design

Ergonomics is the relation between man and machine. It is a process of finding the point where these two elements work most efficiently to produce functional work.

• Ergonomists carry our experiments to determine this point of best working behavior. Various iterative models are used in conjunction with humans to test for the best result.
• Controls used to operate the machines are often areas of heavy ergonomic studies. Positioning of levers, shape of control components, color scheme for buttons and switches, etc. are considered.
• The aim of ergonomics is to reduce operational difficulties present in the man-machine joint system and thereby reduce the physical and mental stresses.