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 10o Helix angle is
large (>10o)
