TWO MARKS QUESTIONS WITH ANSWERS ON DESIGN OF SPRINGS

SPRINGS

1.      What is a spring?

A spring is an elastic member, which deflects, or distorts under the action of load and regains its original shape after the load is removed.

2.      State any two functions of springs

a.       To measure forces in spring balance, meters and engine indicators.

b.      To store energy.

3.      What are the various types of springs?

a.       Helical springs

b.      Spiral springs

c.       Leaf springs

d.      Disc spring or Belleville springs

4.      Classify the helical springs

a.       Close – coiled or tension helical spring.

b.      Open –coiled or compression helical spring.

5.      Define : Leaf springs

A leaf spring consists of flat bars of varying lengths clamped together and supported at both ends, thus acting as a simply supported beam.

6.      Define : Belleville Springs

They are made in the form of a cone disc to carry a high compressive force. In order to improve their load carrying capacity, they may be stacked up together. The major stresses are tensile and compressive.

7.      What is spring index (C)?

The ratio of mean or pitch diameter to the diameter of wire for the spring is called the spring index.

8.      What is pitch?

The axial distance between adjacent coils in uncompressed state.

9.      What is solid length?

The length of a spring under the maximum compression is called its solid length. It is the product of total number of coils and the diameter of wire.

Ls = nt x d 

Where, nt = total number of coils.

10.  What are the requirements of spring while designing?

Spring must carry the service load without the stress exceeding the safe value. 

The spring rate must be satisfactory for the given application.

11.  What are the end conditions of spring?

a.      Plain end.

b.      Plain and ground end

c.       Squared end

d.      Squared and ground end.

12.  What is buckling of springs?

The helical compression spring behaves like a column and buckles at a comparative small load when the length of the spring is more than 4 times the mean coil diameter.

13.  What is surge in springs?

The material is subjected to higher stresses, which may cause early fatigue failure. This effect is called as spring surge.

14.  What is a laminated leaf spring?

In order to increase, the load carrying capacity, number of flat plates are placed and below the other.

15.  What semi – elliptical leaf springs?

The spring consists of number of leaves, which are held together by U- clips. The longleaf fastened to the supported is called master leaf. Remaining leaves are called as graduated leaves.

16.  What is nipping of laminated leaf spring?

Prestressing of leaf springs is obtained by a difference of radii of curvature known as nipping.

17.  What are the various applications of springs?

a.      The springs are used in various applications, they are

b.      Used to absorb energy or shocks (e.g. shock absorbers, buffers, etc.) 

c.       To apply forces as in brakes clutches, spring-loaded valves, etc.

d.      To measure forces as in spring balances and engine indicators

e.       To store energy as in watch

18.  Define free length.

Free length of the spring is the length of the spring when it is free or unloaded condition. It is equal to the solid length plus the maximum deflection or compression plus clash allowance. 

Lf = solid length + Ymax + 0.15 YMax

19.  Define spring index.

Spring index (C) is defined as the ratio of the mean diameter of the coil to the diameter of the wire. C =D/d

20.  Define spring rate (stiffness).

The spring stiffness or spring constant is defined as the load required per unit deflection of the spring. 

K= W/y 

Where 

W-load 

y-deflection

21.  Define pitch.

Pitch of the spring is defined as the axial distance between the adjacent coils in uncompressed state. 

Mathematically 

Pitch = free length/ n-1

22.  What are the points to be taken into consideration while selecting the pitch of the spring?

The points taken into consideration of selecting the pitch of the springs are

a.       The pitch of the coil should be such that if the spring is accidentally compressed the stress does not increase the yield point stress in torsion. 

b.      The spring should not be close up before the maximum service load is reached.

23.  Define active turns.

Active turns of the spring are defined as the number of turns, which impart spring action while loaded. As load increases the no of active coils decreases.

24.  Define inactive turns.

Inactive turns of the spring are defined as the number of turns which does not contribute to the spring action while loaded. As load increases number of inactive coils increases from0.5 to 1 turn.25.

25.  What are the different kinds of end connections for compression helical springs?

The different kinds of end connection for compression helical spring area.

a.       Plain ends

b.      Ground ends

c.       Squared ends

d.      Ground & square ends

26.  Write about the eccentric loading of springs?

If the load acting on the spring does not coincide with the axis of the spring, then spring is said to be have eccentric load. In eccentric loading the safe load of the spring decreases and the stiffness of the spring are also affected.

27.  Explain about surge in springs?

When one end of the spring is resting on a rigid support and the other end is loaded suddenly, all the coils of spring does not deflect equally, because some time is required for the propagation of stress along the wire. Thus a wave of compression propagates to the fixed end from where it is reflected back to the deflected end this wave passes through the spring indefinitely. If the time interval between the load application and that of the wave to propagate are equal, then resonance will occur. This will result in very high stresses and cause failure. This phenomenon is called surge.

28.  What are the methods used for eliminating surge in springs?

The methods used for eliminating surge are

a.       By using dampers on the center coil so that the wave propagation dies out

b.      By using springs having high natural frequency.

29.  What are the disadvantages of using helical spring of non-circular wires

a.       The quality of the spring is not good 

b.      The shape of the wire does not remain constant while forming the helix. It reduces the energy absorbing capacity of the spring.

c.       The stress distribution is not favorable as in circular wires. But this effect is negligible where loading is of static nature.

30.  Why concentric springs are used?

a.       To get greater spring force with in a given space 

b.      To insure the operation of a mechanism in the event of failure of one of the spring

31.  What is the advantage of leaf spring over helical spring?

The advantage of leaf spring over helical spring is that the end of the spring may be guided along a definite path as it deflects to act a structural member in addition to energy absorbing device.

32.  Write notes on the master leaf & graduated leaf?

The longest leaf of the spring is known as main leaf or master leaf has its ends in the form of an eye through which bolts are passed to secure the spring. The leaf of the spring other than master leaf is called the graduated leaves.

33.  What is meant by nip in leaf springs?

By giving greater radius of curvature to the full length leaves than the graduated leaves, before the leaves are assembled to form a spring thus a gap or clearance will be left between the leaves. This initial gap is called nip.

34.  What is the application of leaf spring?

The leaf springs are used in automobiles as shock absorbers for giving suspension to the automobile and it gives support to the structure.

35.  Define flat spiral spring.

A flat spiral spring is a long thin strip of elastic material wound like a spiral. These springs are frequently used in watch springs, gramophones, etc.

36.  What are the differences between helical torsion spring and tension helical springs?

Helical torsion springs are wound similar to that of tension springs but the ends are shaped to transmit torque. The primary stress in helical torsion spring is bending stress whereas in tension springs the stresses are tensional shear stresses.

37.  Define helical springs

The helical springs are made up of a wire coiled in the form of a helix and are primarily intended for compressive or tensile load.

38.  What are the different types of helical springs?

The different types of helical springs are.

a.       Open coil helical spring 

b.      Closed coil helical spring

39.  The extension springs are in considerably less use than the compression springs

OR

Why extension springs are in less use than compression springs

a.       Extension springs are subjected to additional stress- concentration

b.      Extension springs are wound with initial tension and this cannot be cold set without losing their tension

c.       They may be overstresses by being stretched too far.

40.  What condition must be satisfied for concentric spring?

a.       Maximum shear stress developed in the concentric springs is same for same material and free length

b.      Deflection is same for both the springs

c.       Solid length of both the springs is same

41.  Why pre stressing is done in laminated leaf spring? Explain.

By giving a greater radius of curvature to the full length leaves than graduated leaves, before the leaves are assembled to form a spring. By doing so, a gap or clearance will be left between the leaves. This initial gap is called nip. When the central bolt, holding the various leaves together, is tightened, the full length leaf will bend back and have an initial stress in a direction opposite to that of the normal load. The graduated leaves will have an initial stress in the same direction as that of the normal load. When the load is gradually applied to the spring, the full length leaf is first relieved of this initial stress and then stressed in opposite direction. Consequently, the full length leaf will be stressed less than the graduated leaf. The initial gap between the leaves may be adjusted so that under maximum load condition the stress in all the leaves is equal, or if desired, the full length leaves may have the lower stress. This is desirable in automobile springs in which full length leaves are designed for lower stress because the full length leaves carry additional loads caused by the swaying of the car, twisting and in some cases due to driving the car through the rear springs

42.  Define strain energy. How is it used to find number of coils in helical springs?

When an elastic material is deformed, work is done. This work is stored as elastic strain energy in the material. The close coiled helical spring when subjected to axial load is subjected to torsion. This gives rise to strain energy stroed in the spring. Strain energy stored in a close soiled helical spring under axial load

      U = (4W² D³ n) / Gd⁴

Deflection, δ = (8 W D³ n) / Gd⁴

Then number of coils, n = (δ Gd⁴ ) / (8 W D³)

43.  What are the desirable properties of spring materials?

a.       High fatigue strength

b.      High ductility

c.       High resilience

d.      High creep

44.  What is curvature effect in helical springs?

When the wire is bent in the form of a helical coil, the length of the inside fibre is less than the length of outside fibre. This results in stress-concentration at the inside fibre of the coil. Due to the greater curvature on the inside surface the resultant stress is more on the inside fibres. This take care of by the Wahl’s stress concentration factor which accounts for curvature and direct shear stress effects.

45.  Explain the utility of centre bolt and rebound clip in leaf spring.

The centre bolt holds the leaves together. The U-clips reduces the bending stresses in the central part of the spring. The rebound clips serve to distribute some of the load to the shorter leaves which otherwise would be taken by the master leaf alone.

46.  Explain surging in spring

When one end of a helical spring is resting on a rigid support and the other end is loaded suddenly, then all the coils of the spring will not suddenly deflect equally, because some time is required for the propagation of stress along the spring wire. A little consideration will show that in the beginning, the end coils of the spring in contact with the applied load takes up whole of the deflection and then it transmits a large part of its deflection to the adjacent coils. In this way, a wave of compression propagates through the coils to the supported end from where it is reflected back to the deflected end. This wave of compression travels along the spring indefinitely. If the applied load is of fluctuating type as in the case of valve spring in internal combustion engines and if the time interval between the load applications is equal to the time required for the wave to travel from one end to the other end, then resonance will occur. This results in very large deflections of the coils and correspondingly very high stresses. Under these conditions, it is just possible that the spring may fail. This phenomenon is called surge.

It has been found that the natural frequency of spring should be atleast twenty times the frequency of application of a periodic load in order to avoid resonance with all harmonic frequencies upto twentieth order. The natural frequency for springs clamped between two plates is given by

where d = Diameter of the wire,

D = Mean diameter of the spring,

n = Number of active turns,

G = Modulus of rigidity,

g = Acceleration due to gravity, and

ρ = Density of the material of the spring.

The surge in springs may be eliminated by using the following methods:

By using friction dampers on the centre coils so that the wave propagation dies out.

By using springs of high natural frequency.

By using springs having pitch of the coils near the ends different than at the centre to have different natural frequencies

47.  What are the differences between closed coil & open coil helical springs

Closed coil helical spring                                          Open coil helical spring

The spring wires are coiled very                                 The wires are coiled such that there

Closely, each turn is nearly at right                             is a gap between the two consecutive

Angles to the axis of helix                                          turns.

Helix angle is less than 10^o                              Helix angle is large (>10^o)