Question and answers

8

Explain the following types of stresses  a) Transverse shear stress b) Compressive stress c) Torsional shear stress

Explanation of stresses :

a) Transverse shear stress: When a section is subjected to two equal & opposite forces acting tangentially across the section such that it tends to shear off across the section. The stress is produced is called as transverse stress For Single shearing, Shear stress Ʈ = W/A For Double shearing, Shear stress Ʈ = W/2A

b)Compressive stress: When a body is subjected to equal & opposite axial push forces, the stress produced is called as compressive stress. It is denoted by “ σc”

2

Define machine design.

Machine design is the process of selection of the materials, shapes, sizes and arrangements of
mechanical elements so that the resultant machine will perform the prescribed task. OR
Machine Design is the creation of new and better machines and improving the existing ones

8

2) Define : a) Ductility b) Toughness c) Creep

a) Ductility: the property of material which enables it to be drawn into thin wire under the action of tensile load is called as ductility. b) Toughness: The property which resists the fracture under the action of impact loading is called as toughness. Toughness is energy for failure by fracture. c) Creep: when a component is subjected to constant stress at a high temperature over a long period of time ,it will undergo a slow& permanent deformation called creep Or it is defined as “slow and progressive deformation of material with time under constant stress at elevated temperature. E.g : Bolts & pipes in thermal power plants

2

What do you mean by creep?

When a machine part is subjected to a constant stress at high temperature for a long period of time, it will undergo a slow and permanent deformation called ‘creep’. This property is considered in designing internal combustion engines, boilers and turbines

2

Draw stress – strain diagram for brittle material.

8

(i) State and describe in brief about four ergonomic considerations in the designing of machine elements.

The different areas covered under the ergonomics are: 1. Communication between the man (user) and the machine. 2. Working environment. 3. Human anatomy and posture while using the machine. 4. Energy expenditure in hand and foot operations. Communication between man and machine  The machine has a display unit and a control unit.  A man (user) receives the information from the machine display through the sense organs.  He (or she) then takes the corrective action on the machine controls using the hands or feet.  This man-machine closed loop system in influenced by the working environmental factors such

as: lighting, noise, temperature, humidity, air circulation, etc. Working Environment  The working environment affects significantly the man-machine relationship.  It affects the efficiency and possibly the health of the operator.  The major working environmental factors are: Lighting, Noise, Temperature, Humidity and air circulation. Ergonomics Considerations in Design of Controls  The control devices should be logically positioned and easily accessible.  The control operation should involve minimum and smooth moments.  The control operation should consume minimum energy.  The controls should be painted in proper colour to attract the attention. Ergonomics Considerations in the Design of Displays  The scale should be clear and legible.  The size of the numbers or letters on the scale should be taken appropriate.  The pointer should have a knife-edge with a mirror in a dial to minimize the parallax error while taking the readings.  The scale should be divided in a linear progression such as 0 – 10 – 20 – 30… and not as 0 – 5 – 25 – 45…..  The number of subdivisions between the numbered divisions should be as less as possible.  The numbering should be in clockwise direction on a circular scale, from left to right on a horizontal scale and from bottom to top on a vertical scale.

8

Define stress concentration. What are the causes of stress concentration? State any four methods of reducing stress concentration with neat sketches.

Stress concentration: Whenever a machine component changes the shape of its cross section, the simple stress distribution no longer holds good and the neighbourhood of the discontinuity is different. This irregularity in the stress distribution caused by abrupt changes of form is called ‘stress concentration’.

Causes of stress concentration The various causes of stress concentration are as follows: (i) Abrupt change of cross section (ii) Poor surface finish (iii) Localized loading (iv) Variation in the material properties Methods of reducing stress concentration The presence of stresses concentration cannot be totally eliminated but it can be reduced, so following are the remedial measures to control the effects of stress concentration. 1. Provide additional notches and holes in tension members. a) Use of multiple notches. b) Drilling additional holes. 2. Fillet radius, undercutting and notch for member in bending. 3. Reduction of stress concentration in threaded member. 4. Provide taper cross-section to the sharp corner of member

6

What is stress concentration ? State the remedial measures to control the effect of stress concentration with neat sketches

i. Stress Concentration: Whenever a machine component changes the shape of its cross-section, the simple stress distribution no longer holds good and the neighborhood of the discontinuity is different. This irregularity in the stress distribution caused by abrupt changes of form is called stress concentration. It occurs for all kinds of stresses in the presence of fillets, notches, holes, keyways, splines, surface roughness or scratches etc.

The presence of stresses concentration cannot be totally eliminated but it can be reduced, so following are the remedial measures to control the effects of stress concentration. 1. Provide additional notches and holes in tension members as shown in fig (a) a)Use of multiple notches. b)Drilling additional holes as shown in fig(b) 2. Fillet radius, undercutting and notch for member in bending. 3. Reduction of stress concentration in threaded members as shown in fig(c) 4. Provide taper cross-section to the sharp corner of member as shown in fig(d)

8

 State any four factors that govern ‘factor of safety’.

a. Reliability of applied load. b. The extent of simplifying assumptions. c. The certainty as to exact mode of failure. d. Reliability of properties of material and change in these properties during service. e. Extent of stress concentration. f. The reliability of test results to actual machine parts. g. The extent of initial stresses set up during manufacturing. h. The extent of loss of life, if failure occurs.

4

Define Endurance limit and draw typical S-N curve for steel.

Endurance Limit: It is defined as maximum value of the completely reversed bending stress which a polished standard specimen can withstand without failure, for infinite number of cycles (usually 107 cycles).It is known as endurance or fatigue limit (ϭe).

4

What is stress concentration? State its significance in design of machine elements

Whenever a machine component changes the shape of its cross section, the simple stress distribution no longer holds good. This irregularity in the stress distribution caused by abrupt changes of form is called as stress concentration

In most machine elements have some forms of discontinuity, namely sudden change in cross section, grooves ,holes, keyways and other changes in sections. these continuity in machine element alter the stress distribution in the neighborhood so that the elementary stress equations no longer described the actual state of stress in the part, such discontinuity is called stress raisers and in the region in which these occur is called the area of stress concentration. Internal cracks and flaws, cavities in welds, blowholes, and pressure in certain points are the common examples of stress raisers.

8

 State applications of maximum shear stress theory and principal normal stress theory

(i) Applications of maximum shear stress theory : for ductile material , crank shaft, propeller shafts , c frames (ii) Applications of maximum principle normal stress theory : for brittle material , machine spindle, machine beds , c frames, overhang crank

4

State four examples of ergonomic considerations in the design of a lathe machine.

Ergonomics consideration in the design of Lathe machine Any 4 1) The controls on lathe should be easily accessible and properly positioned. 2) the control operation should involve minimum motions. 3) Height of lathe should be match with worker for operation 4)Lathe machine should make less noise during operation. 5) force& power capacity required in turning the wheel as per operation or human being can apply normally. 6) should get required accuracy in operation.

4

What are the steps involved in general design procedure? Explain.

1. Recognition of need. First of all, make a complete statement of the problem, indicating the need, aim or purpose for which the machine is to be designed. 2. Synthesis (Mechanisms). Select the possible mechanism or group of mechanisms which will give the desired motion. 3. Analysis of forces. Find the forces acting on each member of the machine and the energy transmitted by each member. 4. Material selection. Select the material best suited for each member of the machine. 5. Design of elements (Size and Stresses). Find the size of each member of the machine by considering the force acting on the member and the permissible stresses for the material used. It should be kept in mind that each member should not deflect or deform than the permissible limit. 6. Modification. Modify the size of the member to agree with the past experience and judgment to facilitate manufacture. The modification may also be necessary by consideration of manufacturing to reduce overall cost. 7. Detailed drawing. Draw the detailed drawing of each component and the assembly of the machine with complete specification for the manufacturing processes suggested. Prepare assembly drawing giving part numbers, overall dimensions and part list. The component drawing is supplied to the shop flow for manufacturing purpose, while assembly drawing is supplied to the assembly shop 8. Production. The component, as per the drawing, is manufactured in the workshop

8

Explain with neat sketch the stress-strain diagram for ductile material.

Point A: Proportional limit Point B: Elastic limit Point c: Upper yield point Point D: Lower yield point Point E: Ultimate tensile stress point Point F: Breaking Stress point. 1. Proportional limit. We see from the diagram that from point O to A is a straight line, which represents that the stress is proportional to strain. Beyond point A, the curve slightly deviates from the straight line. It is thus obvious, that Hooke's law holds good up to point A and it is known as proportional limit. It is defined as that stress at which the stress-strain curve begins to deviate from the straight line. 2. Elastic limit. It may be noted that even if the load is increased beyond point A upto the point B, the material will regain its shape and size when the load is removed. This means that the material has elastic properties up to the point B. This point is known as elastic limit. It is defined as the stress developed in the material without any permanent set. 3. Yield point. If the material is stressed beyond point B, the plastic stage will reach i.e. on the the load, the material will not be able to recover its original size and shape. A little consideration will show that beyond point B, the strain increases at a faster rate with any increase in the stress until the point C is reached. At this point, the material yields before the load and there is an appreciable strain without any increase in stress. In case of mild steel, it will be seen that a small load drops to D, immediately after yielding commences. Hence there are two yield points C and D. The points C and D are called the upper and lower yield points respectively. The stress corresponding to yield point is known as yield point stress. 4. Ultimate stress. At D, the specimen regains some strength and higher values of stresses are required for higher strains, than those between A and D. The stress (or load) goes on increasing till the point E is reached. The gradual increase in the strain (or length) of the specimen is followed with the uniform reduction of its cross-sectional area. The work done, during stretching the specimen, is transformed largely into heat and the specimen becomes hot. At E, the stress, which attains its maximum value is known as ultimate stress. Itis defined as the largest stress obtained by dividing the largest value of the load reached in a test to the original cross-sectional area of the test piece. 5. Breaking stress. After the specimen has reached the ultimate stress, a neck is formed, which decreases the cross-sectional area ofthe specimen, as shown in Fig. The stress is, therefore, reduced until the the specimen breaks away at point F. The stress corresponding to point F is known as breaking stress.

4

 State the meaning of following colour codes in aesthetic consideration while designing the product: (1) Red (2) Orange (3) Green (4) Blue

1) Red: Danger,Hot 2) Orange: Possible Orange 3) Green : Safe 4) Blue: Cold