Machine design is a crucial aspect of mechanical engineering, focusing on the creation and development of mechanical systems. Preparing for Machine Design Interview Questions requires a solid grasp of both theoretical knowledge and practical skills. This guide provides essential questions to help you review core concepts and demonstrate your problem-solving abilities, whether you’re a newcomer or an experienced professional.

### 1. What are the different forms of threads used in a screw and explain in brief some of the specifications of it?

The different types of threads used in a screw are as follows:

• Square threads: The threads are inclined at 90 degree with respect to each other and height of such threads is p/2.
where p: pitch of thread (in m).
• Acme threads: The threads are inclined at 29 degree with respect to each other and height of such threads is p/2.
• Trapezoidal threads: The threads are inclined at 30 degree with respect to each other and height of such threads is p/2.
• Buttress threads: The threads are inclined at 45 degree with respect to each other and height of such threads is 0.75p.

### 2. How is the stress distribution non-uniform in case of bolt and how can it be eliminated?

The uneven load distribution in cases of bolts can be explained in the following points:

• The bolt has two parts, the first part is the unthreaded part which is known as shank and the other one is the threaded part. The area of the shank is uniform but the diameter of the threaded portion (core diameter) is uneven.
• The diameter of threaded portion is less and non-uniform as compared to shank. So, if the load is applied, the stress developed will be maximum in the threaded portion. In other words, the shank is a stronger part and threaded portion is a weaker part due to which the load distribution will be non-uniform.
• These issues can be resolved by making the diameter of shank equal to the core diameter. Now, when the load is acting on the bolt, it would be equally stressed in both of the portions and chances of failure will be less. Such bolt is called as bolt of uniform strength.

### 3. What are the two main classifications of ISO metric threads and how can they be differentiated?

The ISO metric threads are classified mainly as coarse and fine threads. The differentiating points are as follows:

• Static load carrying capacity is higher and have less number of threads as compared to fine threads.
• They are used where there is no vibration and where assembly and disassembly is frequent.

• Fluctuating load carrying capacity is higher and have more number of threads as compared to coarse threads.
• Fine threads are used when there are no vibrations induced in them which tend to loose the nut.

### 4. What is the meaning of the term ‘welding’ and write any three advantages of welded joints?

The process of joining two or more metal members permanently by the fusion of edges with or without applying pressure and by using a filler material, is called welding.

• Welded joints are very rigid joints and the strength and efficiency of such joints is higher.
• Welding can be done at any part of the structure and at any point and the weight of the overall structure is less.
• When welding is done for tension members, the strength of such joints is not weakened. Moreover, the process can also be automated.

### 5. What are springs and give any three applications of it?

Springs are flexible machine elements which are used primarily to deflect under load and have the ability to return to its original shape when unloaded. Applications are:

• Springs are used to store energy, measure force or absorb shocks and vibrations and also to release the energy.
• They provide a push, pull or torque and also transfer the load from the supporting surface to the body by means of any particular arrangement.
• Springs can also measure force, apply force or control motion and can also be used for maintaining force between contacting surfaces.

### 6. What do you understand by the term ‘surge’ in springs and how can they be avoided?

When a load is applied on a spring it tends to move downward and waves start to travel in a to and fro motion. This phenomenon is known as surging. They can be avoided by:

• Keeping the natural frequency of the spring 15 to 20 times higher than the surging frequency.
• One can apply friction dampers on central coils of a spring due to which propagation of wave will be reduced.
• The springs which are made up of stranded wires can reduce surge. Further, the direction of winding of the spring coils and wires must be opposite to each other.

### 7. Which are the different types of welded joints and explain each of them in short?

The different types of welded joints are given as below:

Lap or fillet joints:

• They are obtained by overlapping the plates and then welding them at edges. The cross section obtained is approximately triangular.
• Further they can be classified as parallel and transverse fillet joints depending upon the direction of load applied with respect to weld.

Butt Joints:

• They are obtained by placing the plates from edge to edge and then welding them. If thickness is less than 5 mm, beveling is not required.
• Further they can be classified as square, V, U, double V and U butt joints depending upon the thickness of plates.

### 8. How the phenomenon of collar friction occurs in power screw?

Collar friction is been explained in the following points below:

• When power screw is being operated, there is a thread friction which results during the transmission of power.
• Further, there are mainly two parts involved in this phenomenon which is a cup and a collar.
• Cup stays stationary but with the working of power screw, collar is attached at the annular interface and there is a relative motion between the cup and collar due to which collar friction arises.

### 9. When are multiple threads used and write any two advantages and disadvantages of the same?

Multiple threads (double or triple start threads) are similar to single start thread and are used where higher travelling speed is required.

• Efficiency of multiple start threads is higher as compared to single start thread as helix angle is increased.
• Multiple threads have larger axial motion and also speed of double start threads is twice as compared to single start thread.

• In multiple start threads, self locking property is lost and also cannot be used for applications where higher stress is dominant.
• Effort required to raise a particular load is more, which means mechanical advantage is less as compared to single-start threads.

### 10. What are the different parts or segments present in the multi-leaf spring?

Multi-leaf spring consists of several flat plates (decreasing in length) which are usually of semi-elliptical shape and are used to transfer the loads over a wide area. The parts used are:

• The top most segment of multi-leaf spring is called as Master leaf which is bent in both the directions to form the eyes of spring. Eyes are provided for attaching members at the ends.
• The amount of bend from central line, passing through the eyes, is known as Camber. It is provided so that even the maximum load should not touch the machine member.
• The central clamp is required to hold all the leaves of the spring. Through bolt holes the bolts are clamped to the leaves of spring so that it does not gets weakened. The load from master leaf to graduated leaves is shared by the means of rebound clips.

### 11. What are the differences between ball and roller bearing?

The ball and roller bearing can be differentiated in the following points:

Ball bearing:

• In this type of bearing, the rolling element is spherical ball and load carrying capacity is less.
• Nature of contact made by this bearing is point contact and power lost in friction is low.
• Ball bearing has less axial dimension and more radial dimension as compared to roller bearing.

Roller bearing:

• In this type of bearing, the rolling element is cylindrical, taper, needle or spherical roller and load carrying capacity is more.
• Nature of contact made by this bearing is line contact and power lost in friction is high.
• Roller bearing has more axial dimension and less radial dimension as compared to ball bearing.

### 12. What are the factors on which the magnitude of factor of safety depends. State any three of them?

The factor of safety (FOS) depends upon the following parameters:

• Load type: When the load acting on a machine element is static then a lower value of FOS is considered and when the load acting is dynamic or impact type, then a higher value is considered.
• Material type: When a material is of ductile type, generally yield strength is considered as a failure criterion. So, lower FOS value is selected and when a material is of brittle type, generally ultimate tensile strength is the failure criterion. So, it will require a higher value of FOS.
• Manufacturing quality: When a material is precisely manufactured, it will tend to have less failures and defects. So, lower FOS is to be chosen. Conversely, for poorly manufactured components, higher FOS value should be selected.

### 13. What is a lever and define the terms involved in it?

A lever is a mechanical device in the form a bar which rests on the fulcrum and is used to multiply or transfer the load. The terms involved in it are:

• Load, effort and fulcrum: The force which is produced by the lever at one end is called as load and in order to support or transfer that load an effort is required at the other end of the lever. Fulcrum is the pivot point on which the lever is mounted.
• Load and effort arm: The distance between the line of action of load and line of action of fulcrum is called the load arm and distance between the line of action of fulcrum and effort is called the effort arm.
• Mechanical advantage and leverage: The ratio of load to effort is known as mechanical advantage and the ratio of effort arm to load arm is known as leverage.

### 14. How will you designate ball or roller bearing?

Ball or roller bearing can be designated by considering four boxes in the following manner:

• The first box tells us about the types of bearing in which the number ‘3’ signifies double row angular bearing, ‘6’ signifies deep groove ball bearing, ‘30’, ‘31’, ‘32’ signifies taper roller bearing and ‘nil’ signifies cylindrical roller bearing.
• The second box tells us about the series of bearing in which the number ‘0’, ‘2’, ‘3’, ‘4’ signifies extra light, light, medium and heavy series of bearings, respectively.
• The third and fourth box tells us about the bore diameter of bearing (in mm) in which ‘0’ in box 3 and ‘0’ in box 4 (‘00’) signifies a bore diameter of 10 mm.
• Similarly, ‘01’, ‘02’, ‘03’ in box 3 and 4 signifies 12, 15, 17 mm of bore diameter, respectively and for ‘04’ and above, multiply the corresponding number in box 3 and 4 together by 5 to get the required bore diameter.

### 15. What is eccentric axial loading and which stresses are involved in it?

• Generally, we assume that the line of action of force passes through the centroid (CG) of the section. But, there are components which are subjected to either tensile or compressive forces which do not pass through the CG of the section.
• The forces are subjected at a certain distance from the line of CG, we represent that distance as eccentric distance. Further, we can replace that forces by parallel forces at the line of CG and a couple will be required to compensate for eccentricity.
• Finally, there will be a couple due to which the section will be subjected with a bending stress and a force which will account for axial stress (tensile or compressive). Hence, the resultant stress will be a combination of bending and axial stress.

### 16. What are the conventions used to construct Mohr’s circle?

Mohr’s circle is the most effective method to find the principal stress and principal shear stress. The circle can be made by following these points:

• The principal and normal stresses are to be plotted on the x-axis and tensile stress which is considered as positive should be plotted on the RHS of origin and compressive force (negative) on the LHS of origin.
• The principal and shear stresses are to be plotted on the y-axis and a pair of shear stress is considered to be positive if they rotate the element clockwise and negative if they rotate it anti-clockwise.
• It can also be concluded that the maximum shear stress will always be the radius of the given Mohr’s circle.

### 17. Which theory of failures are used for ductile materials?

The theories of failure can be selected by considering the following points:

• In ductile materials, the tensile and compressive strength of the components are almost same. Further, the failure criterion of such materials is yielding.
• In maximum shear stress and distortion energy theory, we assume that the yield strength in tension and compression are same and also yielding is the failure criterion.
• Maximum shear stress theory is applied when the dimensional tolerances are not a major priority and distortion energy theory is used when accuracy needs to be maintained. So, these theory of failures are to be selected for ductile materials.

### 18. What are the positions of fulcrum, load and effort in three types of levers. Also, give an application of each type?

The positions of fulcrum, load and effort vary according to the type of lever as:

• For the first type of lever, fulcrum is located between the effort and load and effort arm is less than or equal to the load arm. Application: Scissors.
• In the second type of lever, load is between the fulcrum and effort and effort arm is greater than the load arm. Application: Wheelbarrow.
• In the third type, effort is located between the fulcrum and load and the load arm is greater than the effort arm. Application: Biceps curls.

### 19. What happens to a component when it is subjected with thermal stresses?

The phenomenon of thermal stresses is explained below:

• The thermal stresses occur in a component due to change in the temperature. When an element is subjected with temperature change, it will either expand or contract.
• If the machine element is allowed to expand or contract on its own or freely, it will not get subjected to any stress.
• However, if the component is not allowed to contract or expand freely or if the expansion or contraction is restricted, stresses will get induced in the component. Such restrictions are generated due to change in the temperature which further cause stress, known as thermal stresses.

### 20. Which theory of failure is used for brittle materials?

The theory of failure for brittle materials is called the maximum principal stress theory:

• For brittle materials, the compressive strength is much greater as compared to the tensile strength. Also, the ultimate strength is the failure criterion for such materials.
• Hence, the theory of failure selected should not have the compressive and tensile strengths to be the same, it must show a difference between the magnitudes of compressive and tensile strength.
• So, due to these considerations and reasons, the maximum principal stress theory is the ideal choice for brittle components.

### 21. What is stress concentration and explain any three causes of it?

Stress concentration is the localization of stresses which gets developed due to abrupt changes in cross section and irregularities present in the component. The causes of it are:

• Changes in material properties: In a component there may be variations like internal cracks like blow holes, cavities in welds, foreign inclusions which acts as discontinuities and causes stress concentration.
• Load application: Machine components are subjected to forces. The forces act either at a small or wide area. Generally, the forces are point load due to which as the area decreases, the stress concentration increases.
• Scratches: In a component there may be machining scratches, inspection scratches, stamp marks, which acts like surface irregularities and results in stress concentration.

### 22. What are the methods to reduce stress concentration?

Stress concentration can be reduced in the following ways:

• Notched and holes in tensile members: If a component has a single notch, then it will get subjected to a higher stress concentration. To reduce it, double notches can be implemented, or additional holes can be drilled, undesired material can be removed so that flow line follows a smooth curve.
• Fillets, undercuts, notch for bending members: If a component like a circular bar with shoulder is subjected with bending moment, then stress concentration will be higher due to sharp points at the base of shoulder, to reduce it, fillets with large radius, notch and undercuts can be implemented to follow a smooth curve.
• Undercuts in threaded members: In a threaded member, the flow line bends as it passes from the shank to the threaded portion which results in stress concentration. To reduce it, an undercut with a fillet radius can be introduced between shank and threaded portion so that the flow line follows a smooth curve.

### 23. Which curves are used when a component is subjected with fluctuating stresses?

Whenever a component is subjected with fluctuating stress, the different curves used are as follows:

• Gerber line: A parabola joining endurance limit (Se) on the y axis (stress amplitude) and ultimate strength (Sut) on the x axis (mean stress), is called Gerber line.
• Soderberg line: A straight line joining endurance limit (Se) on the y axis (stress amplitude) and yield strength (Syt) on the x axis (mean stress), is called Soderberg line.
• Goodman line: A straight line joining endurance limit (Se) on the y axis (stress amplitude) and ultimate strength (Sut) on the x axis (mean stress), is called Goodman line.

### 24. What are the advantages of using a power screw?

A power screw is a device which converts rotary motion into linear motion and is used in the transmission of power. It offers the following advantages:

• The overall dimension of a power screw is small, simple to design, compact in construction and has a large load carrying capacity.
• The manufacturing process of a power screw is easy and does not require any kind of special machinery. Further, it also provides a large mechanical advantage.
• There are very few parts present in a power screw which reduces its cost and increases its reliability. Moreover, it can also be designed with self-locking property.
• A power screw gives precise and highly accurate linear motion required in various applications and it provides a smooth and noiseless service without any maintenance.

### 25. What are the limitations of square threads over trapezoidal threads?

• Square threads are difficult to manufacture. They are generally manufactured on a lathe machine by using a single point cutting tool which is expensive as compared by machining with a multi-point cutting tool.
• The thickness of thread at core is directly proportional to its strength. As square threads have less thickness than trapezoidal threads, the load carrying capacity of screw reduces.
• The wear of thread surface is an essential parameter in the service life of power screw. It is not possible to compensate for its wear in square threads. So, whenever worn out either nut or screw requires replacement.

### 26. Why should one use buttress threads?

• The manufacturing of buttress threads can be initiated on a thread milling machine and can be done economically. Further, it has higher efficiency as compared to trapezoidal threads.
• These threads are used where heavy axial force is required in one direction only and axial wear at the surface of thread can be compensated by means of a split-type nut.
• Buttress threads are stronger than square and trapezoidal threads. Because, the thickness of buttress threads at the base is larger as compared to these two threads.

### 27. What are the terminologies involved in a power screw?

The main terminologies involved in a power screw are discussed in the following points:

• Pitch and Lead: The distance measured parallel to the axis from one point of thread to the corresponding point on adjacent thread, is called pitch. The distance measured parallel to the axis in which the nut will advance in one revolution of the screw, is known as lead.
• Nominal and Core diameter: The largest diameter of a screw is called nominal diameter or major diameter (d). The shortest diameter of a screw is called core diameter or minor diameter (dc).
• Helix angle: The angle made by the helix of a screw thread and a plane perpendicular to the axis of a screw, is known as helix angle. It is denoted by (α).

### 28. What are re-circulating ball screws?

The re-circulating ball screws are different from other conventional screws and they are explained in the following points:

• A re-circulating ball screw consists of a nut and a screw. The surface of screw and a nut are separated by a series of balls. The screw and nut have semi-circular thread profiles as compared to conventional thread profiles like square and trapezoidal.
• As the screw is rotated, the balls advance in the grooves of the screw and nut. After operation, the balls are collected at the end of the nut and returned back.
• These screws are pre-loaded and are given accurate motion due to the elimination of backlash. Further, there is no heat generation due to negligible friction and are used for high speed applications.

### 29. What is fusion welding and how it can be classified?

The welding in which heat alone is used, with or without using a filler material, is called fusion welding. It can be classified into thermit, gas and electric arc welding:

• Thermit welding: In thermit welding, a mould is being placed around the joint and thermit is placed in the reservoir of mould. Thermit consists of a mixture of iron oxide and aluminum. When thermit is ignited, iron oxide gets converted to molten steel, which flows into the mould and liquefies the parts and forms the joint on solidifying.
• Gas welding: In gas welding, a mixture of oxygen-hydrogen or oxygen-acetylene gas is burned in the presence of a torch to form a pointed flame. The intense heat of the flame heats the parts of the joints to be joined and simultaneously melts the filler rod to supply the molten metal to the joint.
• Electric arc welding: In electric arc welding, heat required for joining the metals is generated by an electric arc with the help of a power supply and also an electrode is used which may be of consumable or non-consumable type.

### 30. What are the different types of a butt joint?

A butt joint is a type of joint in which two components are joined by placing their ends together. The different types of butt joints are discussed below:

• Square butt joint: When the thickness of plates is less than five (5) mm, there is no special preparation required (beveling) before welding the edges. Further, the weld formed is in the shape of a square with respect to the plates.
• Single V joint: When the thickness of plates is between 5 to 25 mm, beveling of edges is required before welding. Further, the weld form is in the form of V shape with respect to the plates and welding is done from only one side.
• Single U joint: When the thickness of plates is more than 20 mm, beveling of edges is required before welding. Further, the weld form is in the form of U shape with respect to the plates and welding is done from only one side.
• Double V joint: When the thickness of plates is more than 30 mm, beveling of edges is required before welding. Further, the weld form is in the form of V shape with respect to the plates and welding is done from both the sides.

### 31. What are the methods to find deflection in a beam and explain Castigliano’s and Mohr’s methods?

The methods to find deflection in a beam are double integration method, Macaulay’s method, Mohr’s 1st and 2nd theorem, strain energy and Castigliano’s methods:

• Castigliano’s method: The partial derivative of the total strain energy in a structure with respect to any force at a point is equal to the deflection at that point in the direction of force.
• Mohr’s 1st theorem: The change in slope between any two points ‘M’ and ‘N’ on the elastic curve is equal to area of the bending moment diagram between these two points divided by the flexural rigidity.
• Mohr’s 2nd theorem: The vertical deviation of any point ‘M’ on the elastic curve from the tangent of a point ‘N’ is equal to the first moment of area of bending moment diagram between ‘M’ and ‘N’ about point ‘M’ divided by the flexural rigidity.

### 32. Why are theories of failure used and elaborate any three?

Theories of failure are used to relate stresses with the strength of material under bi-axial or tri-axial stress condition:

• Maximum normal stress theory (Rankine’s theory): A machine element subjected to bi-axial or tri-axial stress fails, when the maximum normal stress at a point in the machine element reaches the value of maximum normal stress in a standard specimen of the simple tension test during failure.
• Maximum shear stress theory (Tresca’s and Guest’s theory): A machine element subjected to bi-axial or tri-axial stress fails, when the maximum shear stress at a point in the machine element reaches the value of maximum shear stress in a standard specimen of the simple tension test during failure.
• Maximum strain energy theory (Haigh’s theory): A machine element subjected to bi-axial or tri-axial stress fails, when the strain energy per unit volume at in the machine element reaches the value of strain energy per unit volume in the standard specimen of the simple tension test during failure.

### 33. What are the differences between the functions of a flywheel and governor?

The basic differences between the functions of a flywheel and governor are as follows:

• The governor controls the mean speed of the engine by varying the fuel supply to the engine. The flywheel has nothing to do with mean speed, it limits the cyclic speed fluctuations.
• The flywheel stores kinetic energy, the whole of the kinetic energy can be totally converted into work without having any friction. On the contrary, the governor mechanism involves losses by friction.
• The governor does not operate if the load on the engine is constant as mean speed for every cycle remains constant. But, the flywheel will remain in operation continuously due to cyclic fluctuations.

### 34. What are thin and thick pressure vessels and write the major differences involved in the analysis of it and give one example of each where analysis can be applied?

When the ratio of thickness to inner diameter of vessel is less than or equal to twenty, it is called thin pressure vessel and when the ratio is greater than 20, it is known as thick vessel:

• In thin cylinders, the tangential stress in the vessel is assumed to be distributed uniformly over the thickness of cylinder, whereas in thick cylinders, the tangential stress decreases from the inner surface to the outer surface of cylinders.
• In thin type of cylinders, the radial stress is neglected as longitudinal and circumferential or hoop stresses are dominant as compared to radial stress. In thick type of cylinders, the radial stress is significant, hence, cannot be neglected.
• The analysis for thick pressure cylinder can be applied in high pressure pipes, hydraulic cylinders and in gun barrels. Whereas, for thin pressure cylinder can be applied in boiler shells, low pressure pipes and tanks.

### 35. What is autofrettage and explain the methods of the same?

Autofrettage is a process involved in cylinders to pre-stress it before operating it in service. The methods for pre-stressing the cylinder are:

• The cylinder can be pre-stressed by using a compound cylinder. A compound cylinder consists of two concentric cylinders where the outer cylinder is attached to the inner cylinder. As a result, the outer cylinder will induce compressive stresses in the inner cylinder.
• The other method is to overload the cylinder before its service. The pressure of overloading is adjusted such that the outer portion of cylinder lies in the elastic range and the inner portion in plastic range. When the pressure is released, the outer portion starts contracting and exerts pressure on the inner portion.
• In third method, a wire is held under tension and it is closely wound around the cylinder vessel. Due to the tension in wire, the cylindrical vessel starts to contract, which results in compressive stresses.

### 36. What are the guidelines to select a wire rope?

The various guidelines for the selection of wire ropes are given in the below points:

• Select the wire rope’s strength according to the tensile strength of the individual wires. Also, the usage of fiber cores instead of steel cores will increase the strength of wire ropes to a certain extent.
• Flexibility of wires becomes an important parameter where the rope makes bends or where sheaves are small. Flexibility can be increased by using a number of small diameter wires.
• The wire rope will get dragged by a gritty material or by a stationary object. So, here abrasion resistance becomes an important factor to consider. Usage of large diameter wires will give better wear resistance.

### 37. What is a column and what are the rules for buckling of a column?

A column is a machine component which has high slenderness ratio, i.e, length of column is greater than the lateral dimensions (depth, diameter, or width). The rules are:

• A column which is made of a ductile material and whose linear dimension (length) is eight (8) times greater than the least lateral dimensions (depth, diameter, or width) is likely to buckle and will be treated as a column.
• A column which is made of a brittle material and whose linear dimension (length) is six (6) times greater than the least lateral dimensions (depth, diameter, or width) is likely to buckle and will be treated as a column.
• If the yield strength of a material is greater than the critical strength (critical strength corresponds to the critical load or buckling load) of a material, then, it is likely to buckle and will be treated as a column.

### 38. What are design and natural tolerances and how be the percentage of rejected components be decided?

Natural tolerances are the limits within an allowable fraction of items fall. Design tolerances are set up by the designer for proper functioning of the assembly. The criteria is:

• When the manufacturing process is such that the design tolerance is less than 3σ (where, σ: standard deviation), then the percentage of rejected components will be there. As a result, one cannot avoid it.
• When the manufacturing process is such that the design tolerance is slightly greater than 3σ (where, σ: standard deviation), then the percentage of rejected components will not be there even if the process is off-centered.
• When the manufacturing process is such that the design tolerance is equal to 3σ (where, σ: standard deviation), then the rejection will not be there, provided the process is centered. For off-centered process, some components might be rejected.

### 39. What are the elements which need to be considered in the definition of reliability?

The various elements required to be considered in the definition of reliability are explained in the given points:

• The performance of the product is one of the basic elements which needs to be considered for the definition of reliability. For example, a simple ball bearing used for a required amount of load capacity must support the shaft and should ensure relative motion to transfer the load from the shaft to the housing.
• The time period of operation of product is the second important element which needs to be considered for the definition of reliability. For example, a ball bearing must support the rotating shaft and must transfer the desired amount of load at a particular angular speed (rpm) throughout its operational time (in hours).
• The third significant element to be considered is the operating conditions of a product. For example, a ball bearing should be lubricated during operation and the lubricant should be replaced periodically, oil seals must be coated to protect the bearings from moisture or dust particles, the bearing must not exceed the limiting speed, etc.

### 40. How will you differentiate standard normal distribution and normal distribution curves?

The differentiating points between standard normal distribution and normal distribution curves are elaborated in the following points:

• In normal distribution curve, frequency is plotted against with random variable ‘x’ and the curve formed is symmetric and also known as bell shaped or Gaussian curve. Whereas, in standard normal distribution curve, frequency is plotted against with random variable ‘z’ and the curve is symmetric along frequency axis.
• In standard normal distribution curve, the area under the curve which extends from minus infinity to plus infinity along the random variable (z) axis is always one. Whereas, in bell shaped curve, the area under the curve along the random variable (x) axis is equal to the total population.
• In Gaussian curve, the mean (µ) and standard deviation (σ) has a certain value. On the contrary, in standard normal distribution curve, the value of mean (µ) is equal to zero and standard deviation (σ) always equal to one.

### 41. What is a cylinder liner and write the advantages of it?

Cylinder liner acts as an inner wall of piston cylinder and it retains the lubricant within it. The advantages of cylinder liner are:

• Whenever the cylinder is worn out, it can be easily replaced without replacing the whole assembly which consists of jacket, frame and cylinder. So, the usage of cylinder liners is considered to be economical.
• For the whole assembly which consists of jacket, frame and cylinder, different materials can be used for jacket and frame (ordinary cast iron) and better grade material only for cylinder liner (wear resistant cast iron). No need of using better grade materials for jacket and frame.
• With the usage of cylinder liner, heat transfer will take place. Due to the transfer of heat, thermal stresses will set up which in turn will expand into all direction. As a result, the cylinder liner will allow longitudinal expansion.

### 42. What are the differentiating points between elasticity and plasticity of a material?

Elasticity and plasticity of a material can be differentiated in the following ways:

• The ability of a metal to regain its original shape under an external load after a temporary deformation is made, is known as elasticity. The ability of a metal to retain or continue its deformation even after the load is removed, is known as plasticity.
• Whenever an elastic deformation is taking place, the atoms for a short period of time
leave their positions but again come back to their original positions when the load is removed. But, in plastic deformation, even after the load is removed, the atoms cannot come back to their original positions, they take up new positions.
• For most of the materials, the stress strain relationship is approximately linear in the elastic range and mostly non-linear in the plastic range. Further, elasticity becomes an important consideration for components made by machine tool and plasticity for components which are made by press working operations.

### 43. What should be the properties of a material that should be selected for cylinders and cylinder liners?

While selecting a material for cylinders and cylinder liners one should look into the following properties:

• The material which is to selected should be strong enough to withstand high gas pressure which is set up in the piston cylinder during the combustion of fuel and the material should be corrosion resistant.
• The material which is to selected should be sturdy enough to withstand the thermal stresses which are set up in all direction of the cylinder due to the transfer of heat.
• The material when used for cylinder liners must have good surface finish in order to reduce friction and wear during the motion of piston. Further, the material must be hard enough to counteract wear caused by the piston movement.

### 44. What are the various functions performed by a piston in an IC engine?

Piston in an IC engine is a reciprocating part which performs different functions such as:

• Piston during the complete cycle, it moves in upward and downward motion to transmit the gas pressure which is created inside the cylinder to the crankshaft with the help of a connecting rod.
• It protects and seals the inner portion of cylinder with the help of piston rings from the crankcase and also the piston compresses the gas during the compression stroke.
• Due to the obliquity of the connecting rod, the piston takes the side thrust from it. Further, it also dissipates or ejects a large amount of heat from the combustion chamber to the walls of cylinder.

### 45. What should be the requirements if one wants to design a piston?

The several requirements in order to design a piston are discussed in the following points:

• Whenever a piston is to be designed, it should have enough strength to withstand the gas pressure resulted from the combustion of fuel and should also handle the inertia forces which arises due to the reciprocating parts.
• The piston should have sufficient capacity for the dissipation of heat from the piston head or crown to the walls of cylinder through the skirt and piston rings. In addition, the piston must have adequate rigidity to withstand mechanical and thermal distortions.
• The piston must create a seal to prevent the leakage of flue gases to the crankcase from the combustion chamber. Also, the leakage of lubricating oil into the combustion chamber must be avoided with the help of a piston.

### 46. What are the advantages of using cast iron as a material for piston?

If cast iron is used as a material for piston it offers the following advantages:

• As compared to aluminium alloy piston, cast iron has higher amount of strength. Due to which it is possible to provide thin sections for the parts of piston made by cast iron. Further, as the temperature will increase, the strength of aluminium alloy piston will decrease rapidly as compared to cast iron piston.
• As the piston moves in upward and downward direction, there are chances that the piston will wear out. So, if wear strength is concerned, the material used for the designing of piston should be cast iron as it has higher wear resistance than aluminium alloy piston.
• The coefficient of thermal expansion of cast iron is approximately half of the aluminium alloy piston. Therefore, the piston made from cast iron does not need much clearance between the piston rings and cylinder walls.

### 47. Why the piston head is provided with a number of ribs?

The following reasons are there for the piston head to be provided with a number of ribs:

• When the piston heads are equipped with the ribs, it strengthens the head of piston against the pressure of gas in the combustion chamber. Ribs also increase the rigidity of the piston head and also avoid the distortion of it.
• A huge amount of combustion heat is transferred to the piston rings from the piston head. Due to this reason, a decrease in temperature difference is created between the edge and center of the piston head.
• Due to the obliquity of the connecting rod, a side thrust is introduced and is transferred to the piston at the piston pin. Further, with the help of skirt the thrust is then transmitted to the walls of cylinder.

### 48. What is standardization and how it can be beneficial?

There are certain obligatory norms to which several features or characteristics of a product should conform, which is known as standardization. It offers the following advantages:

• In a centralized process, it is possible to manufacture a component on mass basis, due to the reduction in dimensions and types of identical components. As the component is manufactured on a mass scale basis, it will eventually help in reducing the cost.
• As the standard component is manufactured by the specialized factory, so, it does not involve the laborious work made by the workers in the machine building plant to manufacture the same component. The standard components have decreased the use of manufacturing facilities to an extent.
• If standardization is being followed up, the quality of specifications and procedures of testing of machine elements will get improved and reliability of the same will also get enhanced. Hence, design of machine will get more optimized.

### 49. What are the ergonomic considerations in the design of controls?

There are certain ergonomic considerations which needs to be followed in the design of controls:

• The controls should be such that it should be easily accessible and positioned logically. The operations related to controls must involve minimum motions in order to avoid absurd movements.
• The shape of the control component which is in contact with the hands should be such that it should have conformity with the shape or anatomy of the human hands. It should not misfit or the operator must not feel uncomfortable while using it.
• The controls should be such that it should have proper colors like it should be painted with the red color in the grey background of machine tools to call or watch out for attention as proper color codes produces favourable psychological effects.

### 50. How will you differentiate resilience and toughness of a material?

The differentiating points between resilience and toughness are elaborated in the following points:

• The ability of a material to absorb strain energy within the elastic range, is known as resilience. Whereas, toughness is the ability of a material to absorb the strain energy within the both elastic and plastic range.
• Modulus of resilience is the area under the curve of stress strain graph in a set up of tension test till the yield point. But, modulus of toughness is the area under the curve of the whole stress strain graph (till the fracture point).
• Resilience becomes an important property mostly where elasticity is concerned like in applications of a spring. Whereas, toughness becomes significant where the components are subjected to impact loads, twisting and bending. Example: Toughness property is used in steel structures and resilience property in used in spring steels.

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