In this article, we will learn about the general principles of design that engineers use for the design process. This is a new subject included in the UPSC ESE Paper 1 from this year. 

Engineers apply the principles of science and mathematics to develop economical solutions to technical problems. Their work is the link between scientific discoveries and the commercial applications that meet the needs of consumer as well as society as a whole.

Many engineers develop new products. During this process, they consider several factors. For example, in developing an industrial robot, engineers precisely specify the functional requirements; design and test the robot’s components; integrate the components to produce the final design; and evaluate the design’s overall effectiveness, cost, reliability, and safety. This process applies to the development of many different products, such as chemicals, computers, power plants, helicopters, and toys.

This process of solving a design problem includes creating a new product or developing an existing product for better functioning. This process is called ‘The Engineering Process’ or ‘The Engineering Design’. This process includes a methodical series of steps that all kinds of engineers use in creating functional products and processes.


This topic focuses on the philosophy of the physics of the design of machines. With a deep knowledge of these fundamentals principles, designers can rapidly generate strategies and concepts with the greatest viability. When it comes to detailed engineering phase of the design process, fewer dead ends will be encounted. Furthermore, with a deep understanding of fundamental principles, designers can more critically evaluates machines and components. Hence, in many aspects, this topic is the foundation upon which many other which many other concepts are built.

Simplicity v/s Complexity:

Most people in today’s world would agree on the thought that the world is complex and is getting even more complex every day. People will also agree on the thought that Simple is better than Complex and that we need simplicity in the products (material or immaterial) services we use every day. When the topic of simplicity v/s complexity comes to design, most people would say that in order to achieve simplicity in design, one must simply avoid complexity.

In this topic, however, it will be learned that simplicity and complexity can actually coexist and that simplicity emerges from complexity and vice – versa.

Simplicity is not the first phase of a project but the very last one. If designers want to reach a certain level of quality, they need to go through complexity, chaos. It is a necessary step where they might feel lost for a moment. Things start to be confusing, they have too much stuff to finger out, they are not sure anymore that these inputs make any sense. But still they keep going to find order in chaos and come up with a solution. As Steve Jobs said in 1994: “when you start looking at a problem and it seems really simple, you don’t really understand the complexity of the problem. Then you get into the problems, and you see that it’s really complicated, and you come up with all these convoluted solutions. That’s sort of the middle, and that’s where most people stop. But the really great person will keep on going and find the key, the underlying principle of the problem – and come up with an elegant, really beautiful solution that works.”

A designer must design a product that is both simple as well as complex. A simple design means that most of its functionality can be judged by intuition only. The less thought and less knowledge a product requires to operate, the simpler it is. Complexity is to be intrinsic and simplicity on the outside. In some cases, designing overly simple products means a compromise on functionality whereas overly complex system on the outside bedazzle the user and increase the scope of mishandling. Therefore, engineering must design system that are simple as well as complex at the same time. Making a complex task appear simple and clear is what designers exist for.

But there is actually a place for complexity as well. Pilots need to access a variety of information and controls instantly, hence the complexity of an airplane cockpit. An aircraft could certainly be invented that was controlled through one simple screen. But if the pilot had to wade through a layered navigation interface each time he needed to check his heading and adjust, his job would become more difficult, not easier. 

Newton’s Laws:

Newton’s laws provided the foundation for the study of the mechanics of solids and fluids and catalyzed the industrial revolution. This led to the formation of engineering profession which enabled humans to engineer machines rather than develop them by trial and error. In addition Newton’s systematic method of discovery and mathematical modeling from the foundation of the scientific method, which is the basis for a deterministic design process for developing machines.

Newton’s First Law sets the stage for the motion of objects: Every body persist in its state of rest or of uniform motion in a straight line unless it is compelled to change that state by forces imposed on it. When we imply this law, we say that sum of forces on a body must be zero, and the sum of moments about a point must be zero. Force and Moment equilibrium concept arise from this law.

Newton’s Second Law is a generalized version of the first law: The acceleration of a body is directly proportional to the resultant force acting on it and parallel in the direction to this force and that acceleration, for a given force, is inversely proportional to the mass of the body. The same is true for rotary motion systems where a force F is replaced by a torque T, mass m by moment of inertia J, and linear acceleration a by angular acceleration  . 


The second law gives rise to the different expressions of motion:



Newton’s Third Law states: To every action there is always opposed an equal reaction or, the mutual reactions of two bodies upon each other are always equal and reaction or, the mutual reactions of two bodies upon each other are always equal and directed to contrary parts. Newton’s third law directly leads to the principle of conversation of linear and angular momentum: when the resultant external force acting on a system is zero, the total vector momentum of the system remains constant. This is true for both linear and angular momentum.

Whether they design moving objects (scooters, boats, compact disk players, blenders) or stationary objects (dams, bridges, stoves, sunglasses, pictures hangers), understanding Newton’s law of motion helps engineers of all disciplines quantify the “invisible” forces acting on the objects. Newton’s law are invaluable design catalysts that can help launch many ideas.

  • Conservation of linear momentum: if no force is applied, then momentum is constant.
  • Conservation of angular momentum: if no torque is applied to a body an axis, angular momentum is constant about that axis.
  • A force coincident with an axis does not apply torque about that axis.

Saint – Venant’s Principle:

In the 19th century, applied mathematicians were using a relatively new tool of calculus catalyzed by needs of the industrial revolution to develop theories on the elastic behavior of solids. One particular problem that often aroused when finding rigorous solutions in its model. For example, when determining the deflection of a cantilever beam, modeling the local deformations of the material due to force applied to the tip of beam can make the problem intractable with added complexity.

A French mechanician and mathematician Adhemar Jean Claude Barre de Saint – Venant (popularly known as Saint – Venant) devised a theory what is now called as Saint – Venant’s Principle which made analytical methods more tractable. This principle states that several characteristic dimensions away from an effect, the effect is essentially dissipated. Saint – Venant demonstrated this by using a pair of pliers to squeez a rubber bar, and the deforming and stress effects become very small 3 to 5 bar thickness away.

The applications of Saint – Venant’s principle are very apparent in many aspects of machine design from bearing to structures to bolted joints. For example, when mouting bearings to support a shaft, the bearings should be spaced 3 to 5 shaft diameters apart if the bearings are to effectively resist moments applied to the shaft. In a machine tool structure, for example, if one is to minimize bending, the length of the structure should not be more than 3 to 5 times the depth of beam. When bolting components together, the bolt’s strain (stress) cones should overlap.

The strain cone emanates from 45 to 60 degrees under the bolt head. The strain cones typically overlap if the bolts are spaced less than 3 to 5 bolt diameters apart.

Golden Rectangle:

The Golden Rectangle was discovered by Pythagoras in ancient Greece. The golden rectangle has proportion of its sides such that when a square is cut from the rectangle, the remaining rectangle has the same proportions.



ab + b2 = a2

a2 - b2 – ab = 0

a = (keeping b = 1)

a = 1.618

The process of dividing subsequent rectangles into square and rectangles can continue till infinity in both the directions. If a quadrant of a circle is drawn in all the obtained squares, an infinite spiral is obtained.

The golden rectangle can help engineers initially in sketching concepts that have a greater chance of being realizable. When designing a machine, engineers can either start from outside, with a sketch of the overall envelop, and work inwards, or they can start from within with the critical module, and design outwards. The former tends to result in machines that are too spacious. A compromise from the beginning can be achieved with either method by initially sketching concepts while keeping the proportions of the golden rectangle in mind.

Saint – Venant’s principle should be considered when initially laying out proportions and spacing of machine elements, whereas Golden Rectangle helps with overall proportions of systems.

Golden ratio has been used in design of structure from ancient times and is used till today in designing buildings with modern architecture.


Abbe’s Principle:

In the 19th century, Carl Zeiss, a preeminent designer and manufacturing of precision microscope sought the analytical help of Dr.Ernst Abbe who was co – owner of Carl Zeiss AG Dr. Abbe developed theory of Abbe relates to tools for the design and manufacture of precision instruments. The most enduring theory of Abbe relates to angular errors causing increasing translational errors as one moves away from the source. In the other words, the translational error δ in a system at a distance between the axis of measurement, and the axis of the intended a sine error. Hence, the Abbe Error is the product of the Abbe Offset and the sine of the angular error in the system. The source of the angular error is typically geometric error motions in moving mechanical components.

Another theory of Abee coroally to sine error is consine error. This theory implies a less direct effect, but one that is still important in very high precision systems, such as photolithography. This principle states that in measuring the length of any part with a scale, if the measuring scale is inclined to the true line of the dimension being measured, then there will be what is called consine error.

The amplification of angular motion to create large translational motions is one of the foremost principles in the design of precision and robust machines. The Abbe Principles resulted from observations about measurement errors in the manufacturing of microscopes. If errors in parallax are to be avoided, then the measuring system must be placed coaxially with the axis along which the displacement is to be measured on the workpiece. When an angular error is amplified by a distance, e.g., to create an error in a machine’s position, the strict definition of the error is a sine or cosine error. The implications of this observations on the design of instruments and machine are profound. Always try to place the measurement system as close as close to the line of action (the process) as possible. Always try to place bearings and actuators as close to the line of action the process) as possible.

Self Principles:

Self principle in beginning design utilize the phenomenon that machine is itself trying to solve its problem. The technique is just to rephrase the problementic phenomenon; just say “ I have a problem with” and replace it with “Self-“ and start exploring the solutions. For example systems can be envisioned which have a need of the capability for Self – Reinforcing, Self – Balancing, Self – Limiting, Self – Protecting, Self – Banking, Self – Starting, Self – Releasing, Self – Servicing, etc. These principles of Self – Help introducing in machines are called Self Principles.

As an example of Self – Reinforcing, consider early in the industrial revolution when boilers started to gain popularity to generate steam for power. It became apparent that the boilers needed to be cleaned to prevent buildup of crud which reduced efficiency and also led to corrosion with disastrous side effects; however, to make a boiler, it had to be riveted tightly together so there would be no leaks. To put an access door in the structure would he to invite leaks from the interval pressure which was sure to deform any door so the steam could escape.

As an example of Self Balancing, consider your washing machine, where it is near impossible to ensure that the clothes will be properly distributed to obtain dynamic during the spin cycle. Somewhere, someone figured that if elements were placed in the drum that were fee to move, they could be designed to the proper place in the presence of dynamic imbalancing: hence was born the three balancer which uses three balls free to move in a groove along the outside of the drum.

As an example of Self Limiting is the flyball governor, which is what James Watt really invented in regardes to making steam engines. As the speed of the engine increase, which would cause centrifugal stresses to become too great in many parts of the engine, centrifugal forces cause balls attached to leavers to move outwards, and the levers gradually close a valve controlling steam flow; hence velocity is controlled.

An as example of Self Damaging are price labels that are applied with microcuts in them, so they look fine, but when they are removed, they come apart and thus are very hard to pull off. People who would change the price tags in stores before check out hate them!

As an example of Self Braking consider the need to rapidly stop an object. If a tapered brake element is gently pushed into the system such that frictional forces pull the tapered element in even harder, then the system will stop itself very quickly.

So, this is all that you should understand about the general principles of design. In the next article, we will learn about design for quality.