Important Short Questions and Answers: Dynamics of Machines - Balancing

BALANCING

1.  Write the importance of balancing?

If the moving part of a machine are not balanced completely up then the inertia forces are set which may cause excessive noise, vibration, wear and balancing of tear of the system. So machine is necessary.

2. Why rotating masses are to be dynamically balanced? (or) Why balancing of dynamic forces is necessary?

If the rotating masses are not dynamically balanced, the unbalanced dynamic forces will cause worse effects such as wear and tear on bearings and excessive vibrations on machines. It is very common in cam shafts, steam turbine rotors, engine crank shafts, and centrifugal pumps, etc.

3.         Differentiate: static and dynamic balancing.

Static Balancing

The  net  dynamic  force  acting  on the shaft is equal to zero

It          deals  only  with  balancing  of dynamic forces.

Dynamic Balancing

The net dynamic force and the net couple due to the dynamic force is equal to zero.

It deals with balancing of dynamic force and balancing of couple due to dynamic forces.

4.         Why is only a part of the unbalanced force due to reciprocating masses balanced by revolving mass? (Or) Why complete balancing is not possible in reciprocating engine?

Balancing of reciprocating  masses is done by introducing  the balancing    mass opposite      to the  crank. The vertical component  of the dynamic force of this balancing         mass gives rise to “Hammer         b low”.  In order  to reduce  the Hammer  blow, a part  of the reciprocating  mass  is balanced. Hence  complete balancing is not possible in reciprocating  engines.        

5.         Can a single cylinder engine be fully balanced? Why?

No. A single cylinder engine can’t be fully balanced. Because the unbalanced forces due to reciprocating masses remains constant in magnitude (because of variation in Ө).

6.         What are the effects of hammer blow and swaying couple?

• The effect of hammer blow is to cause the variation in pressure between the wheel and the rail, such that vehicle vibrates vigorously.

• The effect of swaying couple is to make the leading wheels sway from side to side.

7.         List the effects of partial balancing of locomotives?

• Variation in tractive force along the line of stroke,

• Swaying couple, and

• Hammer blow

8.         Define tractive force.

The resultant unbalanced force due to the two cylinders along the line of stroke is known as tractive force.

9.         What is swaying couple?

The  unbalanced force  acting  at  a  distance          between      the  line  of  stroke  of  two cylinders,  constitute  a  couple  in  the  horizontal        direction.    This couple is  known  as swaying couple.

10.Define hammer blow with respect to locomotives?

The maximum magnitude of the unbalanced force along the perpendicular to the line of stroke is known as hammer blow.

11.State the conditions for static and dynamic balancing? (or) Write the condition for complete balancing?

• The net dynamic force acting on the shaft is zero. This is called as conditions for static balancing.

• The net couple due to dynamic force acting on the shaft is zero.

12. What are the condition to be satisfied for complete balance of in- line engine?

• The algebraic sum of the primary and secondary  forces must be zero, and

• The algebraic sum of the couples due to primary and secondary  forces must be zero.

13.Whether grinding wheels are balanced or not? If so why?

Yes, the grinding wheels are properly balanced by inserting some low density materials. If not the required surface finish won’t be attained and the vibration will cause much noise.

14.Differentiate between the unbalanced force caused due to rotating and reciprocating masses?

• Complete balancing of revolving mass can be possible. But fraction of reciprocating mass only balanced.

• The unbalanced force due to reciprocating mass varies in magnitude but constant in direction. But in the case of revolving masses, the unbalanced force is constant in magnitude but varies in direction.

15.Unbalanced effects of shafts in high speed machines are to be closely looked into –  Why?

The dynamic forces of centrifugal forces or a result of unbalanced masses are a function the angular velocity of rotation.

16.Write a note on the effect of firing order on balancing of reciprocating mass in multi-cylinder engines.

In multi-cylinder engines, there are several possibilities of the order in which firing takes place. To overcome the problem of vibrations, fuel distribution, exhaust distribution, etc., the designers are selecting different firing orders.

17.Two masses in different planes are necessary to correct the dynamic unbalance – Justify.

• If the balancing mass and disturbing mass lie in different planes, disturbing mass cannot be balanced by a single mass as there will be a couple left unbalanced.

• In such cases, at least two balancing masses are required for complete balancing and the three masses are arranged in such a way that the resultant force and couple on the shaft are zero.