PTU PAPERS FOR B-TECH

 

MD-2

 

                                   

Bearing

 

 

Q.1.     What is bearing ?
Ans.    A Bearing is a machine part whose function.

  1. Is to support  a moving  element.
  2. Is to guide of confine its motion.
  3. Is to prevent  motion in the direction  of applied load.

Q.2.     What is function  of a bearing ?
Ans.    As above.
Q.3.     How will you classify the bearing based on nature of contact  between the working    surfaces?
Ans.    The bearing   are classified as

  1. Sliding contact  bearings.
  2. Rolling contact bearing.

Q.4.     What are plain baring.?
Ans.    Sliding contact bearings are also called plain bearing .

Q.5      How will you select bearing based on load?
Ans.    For shock loading, sliding contact baring will be preferred as rolling  contact  bearing  has lower resistance to shock loading, for other loads   rolling contact bearing may be preferred.

Q.6.     How will you select  bearing based on speed?
Ans.    For rubbing speed up to 50 m/sec i.e. for low and medium  speed rolling contact  bearing will be preferred on account of less energy loses for speed more than 50m/sec. Due to dynamic affect sliding  contact bearing will be preferred.
 
Q.7.     How will you select  bearing  based on cost .
Ans.    For mass production, cost of sliding contact bearing is small  and may be preferred.  The cost of rolling contact bearing varies little with quantity.   

Q.8.     How will you select bearing based on noise or vibration. 
Ans.    Rolling contact  bearing  have poor quality in damping of vibration and may be noisy . Hence their  use may be avoided.

 

Q.9      How will you select bearing based on starting friction.
Ans.    Based on starting friction the use of sliding  contact bearing may be avoided  as they have high coefficient  of  starting friction.

Q.10.   How will you select  bearing based on space requirement ?
Ans.    Rolling contact bearing  needs large radial dimension and sliding contact bearing need more  axial  dimensions.

Q.11.   What is bearing modulus ?
Ans.    Bearing  modulus     =                       ZN
                                                                       
                                                                        P
            Where Z = coef of viscosity (Absolute ) of lubricant used
                        N = Speed of  rotation (RPM)
                        p. = Radial Pressure  (MN/2)

Q.12.               Define Summerfield number
Ans.    Summerfield number is defined as
                        S  =   ZN                  D

                             ------------    (-------)2          X 10-6
                                    P               CD

            Where  D =  Diameter  of shaft in mm
                        CD = Clearance in mm ie difference of internal diameter of bearing and external diameter  of shaft.

Q.13.   What is bearing characteristic number ?
Ans.    Summerfield  number is also called bearing characteristic  no. and is defined as above .

Q.14.   What is difference between “housing” and “retainment”
Ans.    Retainment is a device whose function is to :

  1. support a moving element of a machine,
  2. Guide or confine the radial and axial motion of that member.

Whereas housing function also include operational effects such as
Preventing 

  1. the lubricant from being contaminated with dust and optimum use of lubrication
  2. Compensating for the  misalignment of journal.
  3. Ensuring antiweld and antiscoring properties
  4. Allowing for rapid removal of heat due to friction.

Q.15.   What is Hydrodynamic  lubrication?
Ans.    In this type of lubrication there is always a thick layer of lubrication between the working     surfaces.  The principle of this lubricant is based on the fact that t6he viscous  fluid resist being pushed  around.  The phenomena  of hydrodynamic lubrication depends  upon the selection of proper “Bearing Modulus”

Q.16.   What are the properties of  good bearing material?
Ans.    The properties are 

  1. Compatibility 2) Conformability  3) Compressive  strength  4) Fatigue strength

5) Embed ability   6) Low cost   7) Bond ability 8) Thermal conductivity
9) Low thermal expansion   10) Corrosion Resistance.
Q,17.   Antifriction bearings are  also called -------------------------------------------
Ans.    Rolling contact bearings

Q.18.   Define basic load rating?
Ans.    It is defined as the radial load that a ball bearing can withstand fore one million revolution with only 10% failure.

Q.19.   In a full journal bearing the angle of contact of the bearing with the journal is -----------------------------------------------------------------------------------
Ans.    360

Q.20.   In thrust bearing the load acts --------------------
Ans.    In the direction of axis of rotation.

Q.21.   What is cycloidal curve /
Ans.    It is the curve  traced by a point on the circumference of a circle which rolls without slipping  on a fixed straights line

Q.22.   What is Involute curve ?
Ans.    It is curve generated by a point on a tangent  to a circle which rolls on the circle without slipping

Q.23.   What is pressure angle?
Ans.    It is the angle between the common normal to two gear teeth at the point of contact and the common tangent at the pitch point.

Q.24    The standard pressure angles are   --------------------------------------------
Ans.                            1*
                        14 ---------------           and 20*
                                    2         

Q.25.   Define Module
Ans.    It is the ratio of pitch circle diameter in mm to the number of teeth.
                        Or  m = D/T

Q.26.   From Lewis equation what is determined?
Ans.    Beam strength of gear teeth
Q.27.   Name the main advantage of gear drive.
Ans.    The main advantage are

  1. positive drive  i.e. no slipping  2) Compact drive  3) high efficiency

4)Longest  service life  5) can give precise timings.

Q.28.   Name three main limitations of a gear drive.
Ans.    The main limitations are

  1. Manufacture of gear is complex  2) Noisy  3) Not suitable for large distances

Q.29.   Name the main classification of gear .
Ans.    The main classification  is  1) Spure gear   2) Helical gear  3) Bevel gear

  1. Worm and worm wheel.

Q.30.   What is interference in “Gear”
Ans.    The tooth portion inside the base circle is just useless .  Now if for a given pressure angle . the  number of teeth on pinion is decreased below a certain limit.  The tooth portion below the base circle of the pinion will interfere with the addendum of the large gear.
Q.31.   How interference in gear can be avoided
Ans.    1) By selecting  no. of teeth more than the minimum permissible

  1. By undercutting of the teeth.

Q.32.   Compare the advantage and  disadvantages  of spur and helical gears.
Ans.    The advantage of spur gear are simplicity in  design, economy of manufacture and absence of end  thrust on bearings.  These are noisy and known as slow speed gears.  Helical gear are high speed gears  and can take higher loads.  Their motion is smoother and quieter.  But they require thrust bearing. 
Q.33.   Herringbone gears are --------------------------------------------
Ans.    Double helical gears.
 
Q.34    What are bevel gears?
Ans.    These gear are meant to transmit power between two shaft whose axes intersect at a certain angle  but in the same plane.

Q.35    When do we use worm gear drive.
Ans.    When motion is to be transmitted between shaft whose axes arenon- kparallel, non-intersecting.

Q.36.   What consideration are taken in

  1. Lewis equation
  2. Barth factor.
  3. Buckingham equation.

Ans.    Lewis  equation  is based on the static strength of the teeth in bending.Stress concentration at the tooth root was not considered when Lewis first proposed his equation.  It was meant for low pitch  line velocity.  As the pitch line velocity increased, the resulting dynamic  factor in the Lewis equation at the later stage called Barth factor.
Buckingham  proposed a new method of gear design based on dynamic  load, endurance limit of a material and wear load.

Q.37.   The size of gear is specified by --------------------------------------
Ans.    Pitch circle diameter.

Q.38.   The static load should be  ------------------ the dynamic load  .
Ans.    Greater then

Q.39.   The helix angle for single helical gear range from -----------------------
Ans.    20*  -35*

Q.40.   What are the advantage  of 20* pressure angle over       1*            pressure angle
                                                                                 14 -------
                                                                                                     2.
Ans.    1) More Strength
            2). Help in reducing  minimum no of teeth.

Q.41.   What is the primary function of the spring ?
Ans.    1) To absorb or control energy due either to impact  or shock loads .

  1. To damp vibration.
  2. To store energy as in clocks.
  3. To measure force as in spring balances.
  4. To control motion by maintaining contact as in cam and its follower or clutch and brakes .

Q.42.   What is wahl ‘s factor?
Ans.    This factor takes into account the effect of both direct shear and wire curvature in the design of  helical spring.  This factor “K”

                                                            4S  - 1                         0.615
                                                K =   ---------------         +         ------------
                                                             4S-4                                 S

                        Where  S  = Spring index.
]
Q.43.   What is meant by surge in springs.
Ans.    When once end of the helical spring is resting on a rigid support  and the oter end is loaded  suddenly, then all the coils of the spring will not suddenly deflect equally due to requirement of time  for the propagation of stress along the spring wire.  This result in a wave traveling along the spring  modefinitely.If the applied load is of fluctuating type and the time interval between the load application is equal to the time required for the wave to travel from one end to the other end, then resonance will  occur.  This is called surge “ This  result  in large deflection and stresses in spring.

Q.44.   How surge in spring may be eliminated ?
Ans.    1)  By using friction damper

  1. By using spring of high natural frequency
  2. By using spring having pitch of the coils near the ends different than at the center.

Q.45.what is the solid length of the spring ?
Ans.    When is the solid lengthy of the spring is compressed.  The coils come in contact with each other, then the spring is said to be solid and  solid length   = nd
                        Where  n = no of coils
                                    D = dia of wire.
                                   
Q.46.   what is free length of a spring ?
Ans.    The free length of compression spring is the length of the springing unloaded condition.  It is    =  Solid length  + max. compression + clearance between coils.
            n.d + 6 max =(n-1)X 1 mm
           
Q.47.   What is spring index ?
Ans.    Spring indeed is the ratio of  D/d
                        Where  D =  Mean coil dia.
                                    D =  dia. of wire.

Q.48.   What is the pitch of coil ?
Ans.    It is the axial distance between adjacent coil in uncompressed state.
                                                Free length
                                    P =   --------------------
                                                  n-1
                                                Where  n  = no. of coil.

Q.49.   How deflection  of the helical spring is found.
Ans.                                        8WD3n
                                    δ          ----------
                                                  cd4

Q.50.   What is usual value of Wahl’s factor.

Ans.    For spring index   =  8  (usual value)

                                    4X8-1             .615
                        K =  -----------  +   --------------

                                    4X8-4             8

            =         31                    .615
                    -----------       +   --------------         =  1.12 (apporx.)
                        28                      8

 

Q.51.   What is a brake?

Ans.    The brake is a friction  device whose  primary function  is to control the motion of the machine or a machine member.

Q.52.   What is difference between a brake and a clutch.

Ans.    A  brake connects a moving member to a stationary member whereas in a clutch both the member to be engaged are in motion. 

Q.53.   How total energy to be absorbed by the brake can be found?

Ans.    Total energy to be absorbed by brakes is sum of

            1) Kinetic energy of translation  =  w2
                                                                   -------
                                                                        2g

            2) Kinetic energy of rotation   =  Iw2
                                                               -------
                                                     2
            3) Potential energy.               =  Wh

Q.54.   What are self energizing brakes?
Ans.    When the friction acts so as to assist in applying the brake, the brake is said to be self energizing .

Q.55.   What are self actuating brakes.
Ans.    Self energizing brakes are also called self actuating brakes.
Q.56.   What are self locking brakes
Ans.    When the self energizing effect is to the extent that no external  that no external force is required to apply the brake, the brake is called self locking.

Q.57.   Enumerate the  various type of clutches.
Ans.    The clutches may be classified as

  1. Mechanical clutch  2) Fluid clutch 3) Electro magnetic fluid and power clutch

Mech. Clutches may be classified as
1) friction clutches   2) positive contac6t clutches.

 

Q.58.   State the advantage of friction clutches.
Ans.    1) Minimum shock

  1. Can be used at the high speed
  2. Under shock loading, they provide cushioning.

Q.59.   State  the advantage  of positive  contact clutches  over friction clutches.
Ans.    1) They are lighter and less costly for comparable torque capacity.

  1. They are simple and do not require adjustment for wear .
  2. They do not develop  heat appreciably during use.
  3. They will not slip to the point of destruction.

Q.60.   What type of clutches are used in automobile.
Ans.    Disk clutch.

Q.61.   What is a wet clutch ?
Ans.    Wet clutch is clutch used in oily conditions.  The wet friction is roughly  half of dry  friction which result in lower torsion capacitor. 

Q.62.   What  are advantage of flat belt drive compared to other drive.
Ans.    1)shock absorption at high speed.
            2)It allows long distances between shafts .
            3)Simplicity.
            4)Low cost
            5)Noiseless as compared to gears.
            6)Smoothness of operation.
            7)Long life.
            8)Excellent performance in serpentine drive.
            9)Comparative low efficiency.

Q.63.   What are the disadvantage  of flat belt drive compared to other drives.
Ans.    1) Velocity ratio is not constant  2) Large size  3) Noisy then other belt drive
            4)Slip problem   5) Comparative low efficiency.

Q.64.   Name various type of flat belt drives.
Ans.    1)Open belt 2)Crossed belt  3)Quarter twist 4) Right angle 5)Stepped pulley

  1. Fast and loose pulley 7) Jockey pulley drive 8) Compound drive 9) Reverse drive.

Q.65.   What is another name of belt drive with idler pulley .
Ans.    Jockey pulley drive.

Q.66.   Name common material for belt drive.
Ans.    1) Leather 2) Fabric and canvas 3) Rubber 4)Balata 5) Camel’s hair.

Q.67.   What are the speed limit for longer belt life.
Ans.    15  -  20 m/s.
Q.68.   What is the belt slip.
Ans.    There should be firm grip between the belt and pulleys. When the frictional grip  becomes   insufficient, there may be forward motion of driver pulley without carrying the belt with it. This is called Belt strip.

Q.69.What is belt creep
Ans.    Belt creep is caused by Easticity of the material.   The belt is stretched more on the tight side then  on the slack side due to elasticity.  As a result there is a difference in the length being received and being delivered by a pulley.  This  is called creep.

Q.70.   What is the value of combined effect of slip and creep
Ans.    3% max.

Q.71.   What are the various stresses in the belt ?
Ans.    The various stress are

            1) Due to maximum working tension          F
                                                                           -------------
                                                                                    bt
            2)Due to bending of belt over the pulley jb    =     Et
                                                                                          -----
                                                                                            D
            3) Due to centrifugal forces     =     jc    =  f v2
            Where  b = with of  belt . t  = thickness     E =  modulus of elasticity of belt material   D = dia. of pulley.                                                

Q.72.   Why the face of pulley is crowned.
Ans.    The face of pulley is crowned to keep the belt on the pulley . Inaccurate alignment 
Of the pulley causes the belt to run side ways.  The crown will force the belt to return to the center of pulley.

Q.73.   What is the conditon for the transmission of  maximum power by the belt
Ans.    T = 3 Tc
            Where  T  = Max tension to which belt can be subjected
            Tc  =  Centrifugal  tension

Q.74    What is the velocity of belt for maximum power.
Ans.    v = (T/3m) 1/2

Q.75.   What are the limits of safe central distance for the flat belt drive
Ans.    6-7.5 m
 
Q.76.   What  is safe velocity ration  for “V” Belt drive.
Ans.    7 – 8

Q.77.   What are  advantage  of v-belt drive over flat belt.
Ans.    1) High velocity ration  2) Long life 3) Easily installed  & removed

  1. No. joint trouble  5) Possibility of using with small central distance
  2. Less slip 7) Compactness 8) Quier 9)Higher shock absorption

10) can transmit higher torque.
 
Q.78.   What are disadvantage  of v-belt over flat belts.
Ans.    1) Can not be used for large central distances.
2)Can not be used with timing devices due to creep .

  1. Less durable.
  2. Complicated design.
  3. Reduced service life.
  4. Belt life  above 32 + c and below 15 + c  significantly shortened .
  5. Can not be used for speed more then 90 m/s due to severe centrifugal forces.

Q.79.V-belts are particularly suitable  for ---------drives.
Ans.    Short.
Q.80.   The included angle for the  V- belt is usually ----------
Ans.    30* - 40

Q.81.   Name same practical uses of ropes
Ans.    Ropes are used in
            1) Elevators 2) Cranes 3)Conveyors 4)handling device and suspension bridges
            5)Mine hoists

Q.82.   When ropes are used.
Ans.    When a large amount of power is to be transmitted  over long distance ( up to 150 m apart)

Q.83.   What are advantage  of ropes.
Ans.    1)High efficiency
            2)Lighter in weight.
            3)Silent operation.
            4)More reliable.
            5)Low cost.
            6)More durable.
            7)Can withstand shock loads.

Q.84.   Name common materials of fiber ropes.
Ans.    1) Hemp or Manilla 2) cotton.

Q.85.   Name common materials for wire ropes.
Ans.    1) Wrought iron 2) cast steel  3) Alloy steel.

Q.86.   How are wire ropes designated.
Ans.    Wire ropes are designated by the number of strands and  the number of wires  in each strand  e.g. 6x7 rope means 6 strands and 7 wires in each strand
.          
Q.87.   Wire rope makes contact at ----------groove of the pulley
Ans.    Bottom.

Q.88.   What are advantages of chains. ?
Ans.    1) Can be used both for short and long distances.          
            2)Compact size as compared to belt drive.
            3)Efficiency 98 – 99 %
            4)Low load of shaft as compared to belt drive.

Q.89.   What are disadvantage  of chain. ?
Ans.    1)Due to wear of joint, the chain gets stretched resulting in velocity fluctuation.
            2)Noisy operation.
            3)Require more servicing.
            4)higher cost as compared to belts.
            5)Complicated design as compared to belts.

90.       Name the common chains used.
Ans.    1)Roller chain.
            2)Bush chain.
            3)Silent chain.

Q.91.   Are silent chains exactly silent?
Ans.    No, silent chains are not exactly  silent but are much smoother and quieter then roller chains.

Q.92.   What are the function of flywheel in a machine.
Ans.    The primary function of a fly wheel is to
            1) To absorb energy when demand of energy  is less than the  supply
            2)To give out energy when demand of energy is more than the supply.

Q.93.   Why stresses are set up in the flywheel  rims.
Ans.    1)Centrifugal stresses.
            2)Shrinkage while casting.
3)Change of speed  & Load.
 
Q.94.   Why flywheel is used in I.C. engines.
Ans.    In I.C or steam engines the energy is developed during power stroke, no energy is developed during suction, compression and  exhaust strokes in 4-stroke engine.  It helps the crank shaft to run at  uniform speed by performing its primary  function.

Q.95.   What is the difference in function of a governor and a flywheel.
Ans.    Governor regulates the mean speed of an engine when there are variation in load by changing the  supply of working fluid. Flywheel does not maintain a constant speed.  It reduces the fluctuations.
 
Q.96.   Coeff. Of fluctuation of speed  is ----------- of maximum fluctuation of speed and the man speed.
Ans.    Ration.

Q.97.   Due to centrifugal forces acting on the rim, the flywheel arms will be subjected to----------- stresses.
Ans.    Tensile.

Q.98.   Why flywheel arms are usually  elliptical.
Ans.    This  helps in more section modulus  for the same weight.  This results in more strength  for a circular  section.

Q.99.   Under what consideration, the shaft for a flywheel is designed.
Ans.    It is designed under shear stresses produced due to  combined action of torsion and bending moment.
           
 

 

 

 

 

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