MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) (ISO/IEC - 27001 - 2005 Certified) SUMMER – 13 EXAMINATION Subject Code: 12089 Model Answer __________________________________________________________________________________________________ Page 1 of 29 Important Instructions to examiners: 1) The answers should be examined by key words and not as word-to-word as given in the model answer scheme. 2) The model answer and the answer written by candidate may vary but the examiner may try to assess the understanding level of the candidate. 3) The language errors such as grammatical, spelling errors should not be given more Importance (Not applicable for subject English and Communication Skills). 4) While assessing figures, examiner may give credit for principal components indicated in the figure. The figures drawn by candidate and model answer may vary. The examiner may give credit for any equivalent figure drawn. 5) Credits may be given step wise for numerical problems. In some cases, the assumed constant values may vary and there may be some difference in the candidate’s answers and model answer. 6) In case of some questions credit may be given by judgement on part of examiner of relevant answer based on candidate’s understanding. 7) For programming language papers, credit may be given to any other program based on equivalent concept.
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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous)
iv) Fluctuation of Energy : The variation of energy above or below mean torque line is known as fluctuation
of energy
Fluctuation of speed: The variation of speed above or below mean speed of m/c during a cycle is called
the fluctuation of speed. ( 01 mark for each definition)
v) Classification of brakes ( 02 marks for appropriate answer)
BRAKES
Shoe Brake Band Brake
External Shoe brake Internal Shoe brake Band Brake Band and Block brake Mechanical Hydraulic Pneumatic Simple Differential Simple Differential
vi) Types of cams ( 02 marks for appropriate answer)
Cam
According to follower motion According to the shape According to the manner of constraint of follower Dwell – Rise Dwell – Rise Rise - Return Dwell cam Return – Dwell cam Rise cam Pre- loaded spring cam Positive drive cam Gravity cam
Wedge cam or Radial or Cylindrical or Flat cam Disc cam Barrel cam
vii) Angle of Lap: The angle of lap is defined as the angle subtended by the portion of the belt which is
in contact at the pulley surface of the pulley.
Slip :If the difference in tensions between tight and slack sides of the belt is too large to be resisted by
friction between belt and the pulley, whole or the portion of the belt which is in contact with the pulley
begin to slide. This results in relative motion between the belt the pulley and is termed as slip.
( 01 mark for each definition)
MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous)
Mechanism Structure 1) A mechanism is a kinematic chain which 1) There is no relative motion between its links
transmits and transforms motion. which transmits force only. 2) Mechanism is Skeleton diagram of a machine 2) It serves to modify and transmit the forces only
to transmit the definite motions between and no energy is transmitted for doing any various links. mechanical work.
3) The motion & Force transmission is desirable 3) Only the Force transmission is desirable
d) Pantograph : ( 02 marks for sketch, 02 marks for explanation) A Pantograph is an instrument used to reproduce to an enlarged or a reduced scale and as exactly
as possible the path described by a given point. (See Fig.) The link ABQ is parallel to link OC and OA is parallel to link BC.
A line is drawn joining O to Q. It cuts BC at point P. It will be seen that points Q and P describe similar paths but to different scales. For the paths to be similar the essential requirement is that triangle OPC should be similar or triangle BPQ for all positions. For another position when point P has moved to P’ in the process of tracing the path of a circle, it is seen that Q also traced the path of a circle from Q to Q’. The ratio
arc PP’ = OP For any position the line will OQ will intersect line BC at point P such that. arc QQ’ OQ BQ = PB = PQ OC PC OP and since OC and BQ are fixed lengths, the ratio PC = OP
PB PQ
is always same for all positions.
MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous)
5 a Difference between Flywheel & Governer (Full marks any 4 differences)
Flywheel Governor
i) Flywheel is a device which stores or i) Governor is a device controls the supply energy when produced in excess & fuel to engine & controls mean speed
releases when required by m/c
ii) It regulates fluctuation of speed when ii ) It regulates speed of engine when there there is a variation in cyclic torque of m/c is a external load variation.
iii) It acts by virtue of its inertia iii) It acts as a mechanism to control
fuel supply
iv) If torque variation is high, flywheel iv) If external load variation is higher,
required is larger size more control necessary on fuel supply
v) Used in Engines, forging m/c, Sheet v) Used in Engines
metal press, shearing m/c
MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous)
6a) Causes of Vibrations (Full marks to any 4 points )
There are various factors those act as disturbing means to the machines from their equilibrium conditions. These are the disturbing forces that cause vibrations in machines. From safe running & life point of view, the vibrations are dangerous. Vibrations can result from a number of conditions, acting alone or in combination. Vibration problems might be caused by auxiliary equipment, not just the primary equipment.
Following are some causes of vibrations
i) Imbalance—An unbalanced mass in a rotating component will cause vibration when the unbalanced weight rotates around the machine’s axis, creating a centrifugal force. Imbalance could be caused by manufacturing defects (machining errors, casting flaws) or maintenance issues (deformed or dirty fan blades, missing balance weights). As machine speed increases the effects of imbalance become greater. Imbalance can severely reduce bearing life as well as cause undue machine vibration.
ii) Misalignment/shaft run-out—Vibration can result when machine shafts are out of line. Angular misalignment occurs when the axes of (for example) a motor and pump are not parallel. When the axes are parallel but not exactly aligned, the condition is known as parallel misalignment. Misalignment can be caused during assembly or develop over time, due to thermal expansion, components shifting or improper reassembly after maintenance. The resulting vibration can be radial or axial (in line with the axis of the machine) or both.
iii) Wear—As components such as ball or roller bearings, drive belts or gears become worn, they might cause vibration. When a roller bearing race becomes pitted, for instance, the bearing rollers will cause a vibration each time they travel over the damaged area. A gear tooth that is heavily chipped or worn, or a drive belt that is breaking down, can also produce vibration.
iv) Looseness / Clearance —Vibration that might otherwise go unnoticed can become obvious and destructive if the component that is vibrating has loose bearings or is loosely attached to its mounts. Such looseness might or might not be caused by the underlying vibration. Whatever its cause, looseness can allow any vibration present to cause damage, such as further bearing wear, wear and fatigue in equipment mounts and other components.
v) Excessive load- This is also one reason for cause of vibrations. If load on m/c is beyond its capacity, vibrations are caused.
vi) Extreme speeds- If there is an extreme variation in operational speeds of the m/c , then also vibrations are created.
vii) Foundation of m/c- This is basic cause if it is not proper to absorb vibrations.
MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous)