← MES Jr. Grade
Past Questions
Which of the following screw thread is adopted for power transmission in either direction?
A)
Acme threads
B)
Square threads
C)
Buttress threads
D)
Multiple threads
To determine the correct screw thread for power transmission in either direction, we must analyze the geometry and efficiency of the options provided:
1. **Square Threads (Option b):** These threads have a profile where the flanks are perpendicular to the axis. Because the thread is symmetrical, it offers the highest efficiency (lowest friction) and is designed to transmit power equally well in both directions. It is the ideal choice for bidirectional power transmission.
2. **Acme Threads (Option a):** These are a modification of square threads with a trapezoidal shape. While they can transmit power in both directions, they are primarily used for ease of manufacture and adjustment (like in lead screws of lathes) rather than maximum efficiency in power transmission. They are less efficient than square threads due to the flank angle.
3. **Buttress Threads (Option c):** These are specifically designed to transmit a very high force in one direction only. One flank is nearly perpendicular to the axis while the other is slanted. If force is applied in the opposite direction, the thread is highly inefficient and prone to failure.
4. **Multiple Threads (Option d):** This refers to the number of starts (lead) of a thread, not the profile. Multiple threads increase the lead (distance moved per revolution) but do not determine the directionality of power transmission efficiency; that is determined by the thread profile (Square, Acme, etc.).
Therefore, for bidirectional power transmission with maximum efficiency, Square threads are the correct choice.
In a thin shell, the ratio of longitudinal stress to the circumferential stress is
A)
1/2
B)
3/4
C)
1
D)
2
Based on established principles in the relevant subject area, the correct and most appropriate choice is '1' (Option C).
Resilience can also be termed as
A)
Stress energy
B)
Strain energy
C)
Modulus
D)
Tenacity
Resilience is defined as the capacity of a material to absorb energy when it is deformed elastically and then, upon unloading, to have this energy recovered. In technical terms, the energy stored in a body due to deformation is called 'Strain Energy'. Therefore, resilience is essentially the strain energy stored in a material within the elastic limit.
Analysis of other options:
- a. Stress energy: This is not a standard technical term used in strength of materials.
- c. Modulus: This refers to the stiffness of a material (e.g., Young's Modulus), which is the ratio of stress to strain, not the energy absorbed.
- d. Tenacity: This refers to the ability of a material to resist fracture; it is the total energy absorbed by the material up to the point of rupture (the area under the entire stress-strain curve), whereas resilience only considers the elastic region.
Principal planes are those planes on which
A)
Normal stress is maximum
B)
Normal stress is minimum
C)
Normal stress is either maximum or minimum
D)
Shear stress is maximum
In the study of stress analysis (Mechanics of Materials), a principal plane is defined as a plane upon which the shear stress is zero. According to Mohr's Circle and the equations for plane stress, the normal stresses acting on these planes are the extreme values of normal stress possible for that point in the material. Specifically, there are typically two principal planes: one where the normal stress is at its maximum (Major Principal Stress, σ₁) and another where the normal stress is at its minimum (Minor Principal Stress, σ₂). Therefore, a principal plane is any plane where the normal stress is either the maximum or the minimum. Option (a) and (b) are only partially correct as they describe only one of the two principal planes. Option (d) is incorrect because the plane of maximum shear stress is oriented at 45 degrees to the principal planes.
The rotating shafts tend to vibrate violently at whirling speeds because
A)
the system is unbalanced
B)
bearing centre line coincides with the axis
C)
the shafts are rotating at very high speeds
D)
resonance is caused due to the heavy mass of the rotor
Whirling of shafts occurs when the axis of rotation of a shaft coincides with its natural frequency of transverse vibration. In a perfectly balanced and perfectly straight shaft, there would be no centrifugal force to pull the shaft away from its axis of rotation. However, in real-world scenarios, no shaft is perfectly balanced; there is always some eccentricity (the center of mass does not coincide with the geometric center). This 'unbalance' creates a centrifugal force as the shaft rotates. When the rotational speed reaches the 'critical speed' (whirling speed), this centrifugal force excites the shaft at its natural frequency, leading to resonance and violent vibrations.
Analysis of other options:
- Option (b) is incorrect because if the bearing center line coincided perfectly with the axis and there was no unbalance, there would be no excitation force to cause whirling.
- Option (c) is incorrect because high speed alone does not cause violent vibration; vibration occurs specifically at the critical speed (resonance), regardless of whether that speed is considered 'high' or 'low' for that specific system.
- Option (d) is incorrect because while mass affects the natural frequency, the 'heavy mass' itself is not the cause of the vibration; the cause is the eccentricity (unbalance) of that mass.
In designing a key, it is assumed that the distribution of forces along the length of key
A)
Varies linearly
B)
Is uniform throughout
C)
Varies exponentially, being more at the torque-input end
D)
Varies exponentially, being less at the torque-input end
In the standard design and analysis of keys (such as sunk keys) used to transmit torque between a shaft and a hub, the fundamental simplifying assumption is that the shear stress and the resulting force are distributed uniformly along the entire length of the key. While in reality, there may be slight stress concentrations at the edges or variations due to the fit, for all practical engineering calculations used in competitive exams and textbook design formulas (where Torque T = Force F × Radius r × Length L), the force is assumed to be constant (uniform) across the length of the key.
- Option (a) is incorrect because linear variation is not the standard design assumption for keys.
- Options (c) and (d) are incorrect because exponential distribution is not applicable to the mechanical interaction of a key in a keyway.
A solid circular shaft has a diameter d. Its polar modulus will be
A)
d2 / 16
B)
d3 / 64
C)
d3 / 16
D)
d3 / 32
Based on established principles in Mechanical Engineering, the correct and most appropriate choice is ' d3 / 16' (Option C).
The surface of the gear tooth below the pitch surface is called
A)
addendum portion
B)
dendendum portion
C)
flank
D)
face
In gear terminology, the tooth profile is divided into two main regions relative to the pitch circle:
1. The portion of the tooth surface above the pitch circle is called the 'Face'.
2. The portion of the tooth surface below the pitch circle is called the 'Flank'.
Evaluating the options:
- Option (a) 'addendum portion' refers to the radial distance from the pitch circle to the top of the tooth, not the surface itself.
- Option (b) 'dedendum portion' (misspelled as dendendum) refers to the radial distance from the pitch circle to the bottom of the tooth space.
- Option (c) 'flank' is the correct term for the surface below the pitch surface.
- Option (d) 'face' is the surface above the pitch surface.
In a gear train, where the axes of gears have motions, is called
A)
simple gear train
B)
compound gear train
C)
epicyclic gear train
D)
reverted gear train
In mechanical engineering, gear trains are classified based on the relative motion of their axes:
1. **Simple Gear Train**: All gears are mounted on fixed parallel axes. Each shaft carries only one gear.
2. **Compound Gear Train**: Like a simple gear train, the axes are fixed. However, at least one shaft carries two or more gears.
3. **Reverted Gear Train**: A type of compound gear train where the input and output shafts are coaxial (they lie on the same line), but the axes remain fixed in space.
4. **Epicyclic Gear Train**: Also known as a planetary gear train. In this arrangement, one or more of the gears (called planet gears) move around another gear (called the sun gear). Therefore, the axes of some of the gears are not fixed but move (rotate) relative to the frame.
Since the question specifically asks for a gear train where the axes of gears have motion, the correct answer is the epicyclic gear train.
If a composite bar is cooled, then the nature of stress in the part with high coefficient of thermal expansion will be
A)
tensile
B)
zero
C)
compressive
D)
none of these
In a composite bar consisting of two different materials bonded together, the total deformation must be the same for both materials to maintain the integrity of the bond.
1. **Thermal Expansion/Contraction**: The change in length due to temperature change is given by $\Delta L = L \alpha \Delta T$, where $\alpha$ is the coefficient of thermal expansion.
2. **Cooling Effect**: When the composite bar is cooled ($\Delta T$ is negative), both materials want to contract. The material with the higher coefficient of thermal expansion ($\alpha_{high}$) will attempt to contract more than the material with the lower coefficient ($\alpha_{low}$).
3. **Internal Constraint**: Because they are bonded, the material with the lower $\alpha$ prevents the material with the higher $\alpha$ from contracting fully.
4. **Resulting Stress**: Since the material with the higher $\alpha$ is 'held back' from its natural tendency to shrink, it is effectively being pulled or stretched relative to its desired cooled state. This results in **tensile stress** in the part with the higher coefficient of thermal expansion and **compressive stress** in the part with the lower coefficient of thermal expansion to maintain equilibrium.
The value of Poisson’s ratio for steel varies from
A)
0.20 to 0.25
B)
0.25 to 0.35
C)
0.35 to 0.40
D)
0.40 to 0.55
Poisson's ratio (ν) is the ratio of lateral strain to longitudinal strain when a material is stretched or compressed. For most structural metals, this value typically falls between 0.25 and 0.35. Specifically for steel, the value is generally accepted to be around 0.27 to 0.30.
- Option (a) 0.20 to 0.25: This is generally too low for standard structural steel.
- Option (b) 0.25 to 0.35: This range accurately encompasses the standard value for steel (approx. 0.3).
- Option (c) 0.35 to 0.40: This range is more typical for materials like aluminum or certain plastics.
- Option (d) 0.40 to 0.55: Values approaching 0.5 are characteristic of perfectly incompressible materials like rubber. Steel is far more rigid and has a lower ratio.
The type of stresses developed in the key is/are
A)
both shear and bearing stresses
B)
bearing stress alone
C)
shear stress alone
D)
shearing, bearing and bending stresses
A key is a machine element used to connect a rotating machine element (like a gear or pulley) to a shaft to prevent relative rotation between them. When torque is transmitted, the key is subjected to several types of stresses:
1. **Shearing Stress**: The torque creates a force that tends to shear (cut) the key across the plane where the shaft and the hub meet. This is the primary failure mode for most keys.
2. **Bearing (Crushing) Stress**: The sides of the key are pressed against the keyway of the shaft and the hub. This compressive stress is known as bearing or crushing stress.
3. **Bending Stress**: In many practical scenarios, the key does not fit perfectly or the pressure is not uniformly distributed across the entire depth. This causes the key to act like a cantilever beam, inducing bending stresses. This is particularly significant in 'sunk keys'.
While basic textbook problems often simplify calculations to only shear and bearing, a comprehensive engineering analysis of a key's failure must account for all three: shearing, bearing, and bending. Therefore, option (d) is the most complete and accurate answer.
What is the correct formula for Euler’s equation of motion? if, q = density of the fluid, p = pressure force, g = acceleration due to gravity, v = velocity of the fluid
A)
(p / q) + (g / q) + (v / q) = 0
B)
(p / q) + (g / q) + (v dv) = 0
C)
(p / q) + (g dz) + (v dv) = 0
D)
(p dp) + (g dz) + (v dv) = 0
Based on established principles in Mechanical Engineering, the correct and most appropriate choice is '(p / q) + (g / q) + (v / q) = 0' (Option A).
The type of rotary compressor used in gas turbine is of
A)
Centrifugal type
B)
Axial flow type
C)
Radial flow type
D)
None of this
Based on established principles in Mechanical Engineering, the correct and most appropriate choice is 'None of this' (Option D).
Gases have poor
A)
tranmissitivity
B)
absorptivity
C)
reflectivity
D)
emissivity
Based on established principles in Mechanical Engineering, the correct and most appropriate choice is 'emissivity' (Option D).
Supercharging is the process of.
A)
Providing the forced cooing air
B)
Raising exhaust pressure
C)
Suppling the intake of an engine with air at a density greater than the density of the surrounding atmosphere
D)
Suppling compressed air to remove combustion product fully
Based on established principles in Mechanical Engineering, the correct and most appropriate choice is 'Providing the forced cooing air' (Option A).
In fluid mechanics, the continuity equation is a mathematical statement embodying the principle of
A)
conservation of momentum
B)
conservation of mass
C)
conservation of energy
D)
none of the above
Based on established principles in Mechanical Engineering, the correct and most appropriate choice is 'conservation of energy' (Option C).
Thermal conductivity of powderly and porous materials
A)
Decreases with increasing temperature
B)
Increases with increasing temperature
C)
Is independent of temperature change
D)
None of the above
Based on established principles in Mechanical Engineering, the correct and most appropriate choice is 'Increases with increasing temperature' (Option B).
The temperature profile for heat conduction through a wall of constant thermal conductivity in the presence of a heat source is
A)
a straight line
B)
parabolic
C)
logarithmic
D)
hyperbolic
Based on established principles in Mechanical Engineering, the correct and most appropriate choice is 'hyperbolic' (Option D).
Transfer of heat energy takes place in accordance with
A)
Zeroth law of thermodynamics
B)
First law of thermodynamics
C)
Second law of thermodynamics
D)
Third law of thermodynamics
Based on established principles in Mechanical Engineering, the correct and most appropriate choice is 'Second law of thermodynamics' (Option C).
A nozzle is designed for
A)
maximum pressure at the outlet
B)
maximum discharge
C)
maximum pressure and maximum discharge
D)
maximum kinetic energy at the outlet
Based on established principles in Mechanical Engineering, the correct and most appropriate choice is 'maximum pressure at the outlet' (Option A).
In thermosyphon system there is (are)
A)
no pump
B)
one pump
C)
two pumps
D)
three pumps
Based on established principles in Mechanical Engineering, the correct and most appropriate choice is 'one pump' (Option B).
Biot number is defined as
A)
k/hL
B)
kL/h
C)
hL/k
D)
h/kL
Based on established principles in Mechanical Engineering, the correct and most appropriate choice is 'kL/h' (Option B).
If the position of metacentre M remains lower than c.g. of the body, G, the body will remain in a state of
A)
stable equilibrium
B)
unstable equilibrium
C)
neutral equilibrium
D)
any of the above
Based on established principles in Mechanical Engineering, the correct and most appropriate choice is 'neutral equilibrium' (Option C).
A fluid does not flow
A)
in the presence of pressure
B)
in the absence of pressure
C)
in the presence of shear stress
D)
in the absence of shear stress
Based on established principles in Mechanical Engineering, the correct and most appropriate choice is 'in the absence of pressure' (Option B).