Introduction to Spring

Spring is a resilient member capable of providing large elastic deformation. Spring is defined as an elastic body whose function is to distort when loaded and to recover its original shape when the load is removed.

Different functions Of Springs :

1. To absorb shocks and vibrations. – Used in Vehicle suspension springs, Railway buffer springs, Buffer springs in elevators, Vibration mounts for machinery
2. To store energy – Used in Springs used in clocks, toys. Movie-cameras, circuit breakers, and starters
3. To measure force – Used in Springs used in weighing balance and engine indicators
4. To control Motion – Used in the cam and follower mechanism, spring is used to maintain contact between two elements.
5. To apply force – Used in In engine valve mechanism, spring is used to return the rocker arm to its normal position when the disturbing force is removed. The spring used in the clutch provides the required force to engage the clutch.

Types of springs

Following are important types of springs according to their shape:
1. Helical springs
2. Conical and volute springs
3. Torsion springs
4. Laminated or leaf springs
5. Disc or Belleville springs

1. Helical Springs

• The helical spring is made from a wire, usually of circular cross-section, that is bent in the form of a helix.
• There are two basic types of helical springs: compression spring and tension spring.
• In helical compression spring, the external force tends to shorten the spring. In other words, the spring is compressed.
• In helical tension spring, the external force tends to lengthen the spring. In other words, the spring is elongated.

• It should be noted that although the spring is under compression, the wire of helical compression spring is not subjected to compressive stress.
• Also, the wire of helical tension spring is not subjected to tensile stress although the spring is under tension.
• In both cases, torsional shear stresses are induced in the spring wire.
• The words compression and tension are related to total spring and not the stresses in spring wire.

The helical springs have the following advantages:

• These are easy to manufacture.
• These are available in a wide range.
• These are reliable.
• These have a constant spring rate.
• Their performance can be predicted more accurately.
• Their characteristics can be varied by changing dimensions.

Application Of Helical Tension Spring :

1) Garage door assemblies
2) Vise-grip pilers
3) carburetors

Application Of Helical Compression Spring :

1) Ballpoint pens
2) Pogo sticks
3) Valve assemblies in engines

2. Conical and Volute Springs

• The conical and volute springs are used in special applications where a telescoping spring or a spring with a spring rate (load required per unit deflection) that increases with the load is desired.

• The conical spring is wound with a uniform pitch whereas the volute springs are wound in the form of a paraboloid with constant pitch and lead angles.
• This characteristic is sometimes utilized in vibration problems where springs are used to support a body that has a varying mass.

3.Torsion Springs

• The construction of this spring is similar to that of compression or tension spring, except that the ends are formed in such a way, that the spring is loaded by a torque, about the axis of the coils.
• Helical torsion spring is used to transmit torque to a particular component in the machine or the mechanism.
• Helical torsion spring is used in door-hinges, brush-holders, starters, and door locks.

• For example, the spring transmits a torque of (P x r).
• The helical torsion resists the bending moment (P x r) that tends to wind up the spring.
• The bending moment induces bending stresses in the spring wire.
• The term torsion spring is somewhat misleading because the wire is subjected to bending stresses, unlike torsional shear stresses induced in helical torsion or tension springs.

Application Of torsion Spring :

1. Mouse tracks
2. Rocker switches
3. Door hinges
4. Clipboards
5. Automobile starters

Spiral torsion spring:

• It is made of a band of steel wrapped around itself several times to create a geometric shape as shown in the figure.
• Its inner end is attached to an arbor and the outer end is attached to a retaining drum.
• It has a few rotations and also contains a thicker band of steel.
• It releases power when it unwinds.

Application of Spiral Spring :
• Alarm timepiece
• Watch
• Automotive seat recliners

4. Laminated Leaf Springs

• Multi-leaf spring is widely used for the suspension of trucks and railway wagons.
• It consists of a series of flat plates, usually of semi-elliptical shape. The flat plates are called leaves of the spring.
• The leaf at the top has a maximum length. The longest leaf at the top is called a master leaf.
• The leaves have graduated lengths. The length gradually decreases from the top leaf to the bottom leaf.
• It is bent at both ends to form the spring eye. Two bolts are inserted through these eyes to fix the leaf spring to the automobile body.
• The leaves are held together utilizing two U-bolts and a center clip.

• Rebound clips are provided to keep the leaves in alignment and prevent lateral shifting of the leaves during operation.
• At the center, the leaf spring is supported on the axle.
• Multi-leaf springs are provided with one or two extra full leaves in addition to master leaf.
• The extra full-length leaves are stacked between the master leaf and the graduated length leaves.
• The extra full-length leaves are provided to support the transverse shear force.

Types of leaves:

1. Full length leave
a. Full length leaves with eye,
b. Full length leave without eye.
2. Graduated leave

Applications of leaves:-

1. It is used in semielliptical leaf spring,
2. It is used in quarter elliptical leaf spring,
3. It is used in three-quarter elliptical leaf spring,
4. It is used in full elliptical leaf spring.

Nipping of Leaf Springs

• The stresses in extra full-length leaves are more than the stresses in graduated-length leaves.
• One of the methods of equalizing the stresses in different leaves is to prestress the spring.

• The pre-stressing is achieved by bending the leaves to the different radius of curvature before they are assembled with the clip.
• As shown in the above figure, the full-length leaf is given a greater radius of curvature than the adjacent leaf.
• The radius of curvature decreases with shorter leaves.
• The initial gap C between the extra full-length leaf and the graduated-length leaf before the assembly is called a nip.
• Such pre-stressing, achieved by a difference in radius of curvature, is known as nipping.

Application of Leaf Spring :

• Mainly in automobiles suspension systems.

Advantages of Leaf Spring :

1. It can carry lateral loads.
2. It provides braking torque.
3. It takes driving torque and withstand the shocks provided by the vehicles.

5. Disc Springs or Belleville springs

• A Belleville spring consists of a coned disk, as shown in the above figure.
• It is called Belleville spring because it was invented by Julian Belleville, who patented its design in France in 1867.
• Belleville spring has a typical load-deflection characteristic, as shown in the above figure.
• The variation of (h/t) ratio provides a wide variety of load-deflection curves.
• Belleville springs are used in plate clutches and brakes, relief valves, and a wide variety of bolted connections.

Belleville spring offers the following advantages:

Advantages of Belleville springs are as follows :

1. It is simple in construction and easy to manufacture.
2. It is a compact spring unit.
3. It is especially useful where a very large force is desired for small deflection of spring.
4. It provides a wide range of spring constants making it versatile.
5. It can provide any linear or non-linear load-deflection characteristic.
6. The individual coned disks of a particular size and thickness can be stacked in series, parallel or series-parallel combinations, as shown in
Figure. These combinations provide a variety of spring constants without changing the design.
7. When two Belleville springs are arranged in series, double deflection is obtained for the same force. On the other hand, when two Belleville springs are in parallel, almost double force is obtained for a given deflection.

6. Special Purpose Springs :

These springs are all together made of different materials such as air and water.

Spring Materials

The material of the spring should have high fatigue strength, high ductility, high resilience and it should be creep-resistant.

Selection of material for the spring wire depends upon the following factors:
1. The load acting on the spring
2. The range of stress through which the spring operates
3. The limitations on mass and volume of spring
4. The expected fatigue life
5. The environmental conditions in which the spring will operate such as temperature and corrosive atmosphere
6. The severity of deformation encountered while making the spring

The mainly used material for manufacturing the springs are as follows:
1. Hard drawn high carbon steel
2. Oil tempered high carbon steel
3. Stainless steel
4. Copper or nickel-based alloys
5. Phosphor bronze
6. Monel
7. Titanium
8. Chrome vanadium
9. Chrome silicon

Characteristics of some typical materials are explained below

1. Hard-drawn wire: This is cold drawn, cheapest spring steel. Normally used for low stress and static load. The material is not suitable at subzero temperatures or at temperatures above 120⁰ C.

2. Oil-tempered wire: It is a cold drawn, quenched, tempered, and general-purpose spring steel. However, it is not suitable for fatigue or sudden loads, at subzero temperatures, and at temperatures above 180⁰ C. When we go for highly stressed conditions then alloy steels are useful.

3. Chrome Vanadium: This alloy spring steel is used for high-stress conditions and at high temperature, up to 220⁰ C. It is good for fatigue resistance and long endurance for shock and impact loads.

4. Chrome Silicon: This material can be used for highly stressed springs. It offers excellent service for long life, shock loading, and for temperature up to 250⁰ C.

5. Music wire: This spring material is most widely used for small springs. It is the toughest and has the highest tensile strength and can withstand repeated loading at high stresses. However, it cannot be used at subzero temperatures or at temperatures above 120⁰ C. Normally when we talk about springs we will find that the music wire is a common choice for springs.

6. Stainless steel: Widely used alloy spring materials.

7. Phosphor Bronze / Spring Brass: It has good corrosion resistance and electrical conductivity. That is the reason it is commonly used for contacts in electrical switches. Spring brass can be used at subzero temperatures.

Spring Design

Terminology of Helical Springs

The main dimensions of a helical spring subjected compressive force are as follows:

d = wire diameter of spring
Di = inside diameter of the spring coil
Do = outside diameter of the spring coil
D = mean coil diameter
Therefore,

D= ( Di + Do ) / 2

1. Spring index (C):

The spring index is defined as the ratio of mean coil diameter to wire diameter. It is denoted by letter C.

C = D/d

• The spring index indicates the relative sharpness of the curvature of the coil.
• A low index means high sharpness of curvature.
• When the spring index is low (C < 3), the actual stresses in the wire are excessive due to the curvature effect.
• Such a spring is difficult to manufacture and special care in coiling is required to avoid cracking in some wires.
• When the spring index is high (C > 15), it results in a large variation in coil diameter.
• Such a spring is prone to buckling and also tangles easily during handling.
• Spring index from 4 to 12 is considered better from manufacturing considerations. Therefore, in practical applications, the spring index usually varies from 4 to 12.
• However, the spring index in the range of 6 to 9 is still preferred particularly for close tolerance springs and those subjected to cyclic loading.

2. Solid length:

• When the compression spring is compressed until the coils come in contact with each other, then the spring is said to be solid.
• The solid length of a spring is the product of the total number of coils and the diameter of the wire.

Mathematically, Solid length of the spring formula,

Ls = n’ * d

n’ = Total number of coils, and
d = Diameter of the wire.

3. Free length:

The free length of a compression spring is the length of the spring in the free or unloaded condition.

It is equal to the solid length plus the maximum deflection or compression of the spring and the clearance between the adjacent coils (when fully compressed).

Mathematically, Free length of the spring formula,

Lf= Solid length + Maximum compression + Clearance between adjacent coils

Lf = n’d + δmax + 0.15 δmax

4. Spring Rate OR Spring Stiffness :

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

Mathematically, Spring rate formula,

k = W / δ

where W = Load, and
δ = Deflection of the spring

5. Pitch:

It is defined as the axial distance between adjacent coils in an uncompressed state.

Mathematically, spring pitch formula

p= Lf / ( n’- 1 )

Buckling of Compression Springs

It has been found experimentally that when the free length of the spring (LF) is more than four times the mean or pitch diameter (D), then the spring behaves like a column and may fail by buckling at a comparatively low load as shown in the below figure.

The critical axial load (Wcr) that causes buckling may be calculated by using the following relation, i.e.

Wcr = k × KB × LF

k = spring rate or stiffness of the spring = W/δ,
LF = Free length of the spring, and
KB = Buckling factor depending upon the ratio LF / D.

How to prevent buckling?

• Free length (LF) should be less than 4 times the coil diameter (D) to avoid buckling.
• Material should be selected having higher stiffness.
• It order to avoid the buckling of spring, it is either mounted on a central rod or located on a tube.
• When the spring is located on a tube, the clearance between the tube walls and the spring should be kept as small as possible, but it must be sufficient to allow for an increase in spring diameter during compression.

Surge in Springs

• 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 take up the 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 to the deflected end.
• This phenomenon can also be observed in a closed water body where a disturbance moves toward the wall and then again returns back to the starting of the disturbance.
• This wave of compression travels along the spring indefinitely
• If the applied load is of fluctuating type 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.

How to prevent surging?

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

1. By using friction dampers on the center coils so that the wave propagation dies out.
2. By using springs of high natural frequency (the operational frequency of the spring should be at least 15-20 times less than its fundamental frequency).
3. By using springs having a pitch of the coils near the ends different than at the center to have different natural frequencies.

Advantages Of Springs :

Advantages of Springs are explained below :

• Normally Springs are maintenance-free
• Springs are available in large variety and shapes hence application area is large.
• Spring can minimize machine vibration. Springs can eliminate the undesirable effects of vibrations.
• Springs are easy to manufacture.
• Springs are highly reliable.
• Mostly we use spring to avoid vibration because the sudden vibrations in a car may affect the human and it causes vomiting
• It helps in storing energy as in the case of watches and toys.

Disadvantages Of Springs :

• If the deflection of the spring exceeds some critical value than the spring will buckle.
• It is difficult to replace the spring.
• Ones the spring is damaged it is difficult to repair.

Application Of Springs :

The following are some applications of springs.

1. To absorb or control energy in automobiles suspension springs, vibration. dampers, railway buffers.
2. To apply forces in brakes, clutches, valves of IC engines.
3. To store the energy in watches and toys.
4. To measure forces in spring balances, gauges.
5. To provide clamping force in toolings like jigs and fixtures, etc.
6. To control motion by maintaining contact between two elements, as in the case of cam and follower, etc.
7. T o exert a force, as in spring-loaded safety valve
8. To support moving masses or to isolate vibration.
9. Tension springs are used in Industrial Robots, door locks.
10. Torsion springs are used in clothes pins, garage doors