SALIENT FEATURES OF SPRINGS
CLASS NOTES FOR MECHANICAL
Spring is an important part in almost every machinery. These springs
perform different functions in different applications. These absorb shocks
in vehicles running on uneven roads. These store energy in toys and circuit
breakers. These give cushion effect in vehicles. Springs are of different
types namely close and open coiled helical springs, laminated and spiral
TYPES OF SPRINGS
(a) Based on wire and plate springs
(i) Helical springs
(ii) Flat springs
(iii) Spiral springs
(b) Based on use
(i) Extension springs
Spring is made from wire which is wound into a cylindrical or conical shape. It is in the form of a coil whose turns normally touch each other. When a tensile force is applied, the coils opens.
(ii) Compression spring
It is initially open coiled spring with space between successive turns. When a compression force is applied, the spring shortens and coil turn touch each other.
(iii) Torsion spring is open coiled spring. A force is applied to twist the coil. As a result of this, diameter of the coil is considerably reduced. Torsion springs are used in garage doors, clipboards and butterfly hair clips.
(iv) Flat Spiral spring
This spring is coiled into a flat spiral rather than a cylindrical or conical shape. One end of the spring is attached at the center of the spiral. The other end is connected at its outer edge. These are used in watches and toys.
(v) Leaf spring
These springs do not have coils. It is like a shallow arch. This is made from leaves. Springs of this type are commonly used in automobile suspension systems.
(vi) Disc spring
It contains a washer which is shaped like a truncated cone. Very small size springs are used to support keys on cellular phone touch . Very large size springs are used to support an entire building to protect it from the earthquake vibration.
ADVANTAGES OF HELICAL SPRINGS
(i) Their manufacturing is easy and less costly.
(ii) Springs are easily available over a wide range of sizes.
(iii) These give better and reliable performance.
(iv) They are easy to install in various applications.
Materials for springs
(i) Carbon steel: low carbon steel, medium carbon steel and high carbon steel
(ii) Oil-tempered low-carbon steel
(iii) Chrome silicon steel
(iv) Chrome vanadium steel
(v) Stainless steel.
Manufacture of springs
It consists of the following processes:
(i) Coiling or making of coils
It is done by cold winding of a wire only up to 18 mm diameter. It consists of winding the wire around a shaft called a mandrel or an arbor. Coils can also be made without a mandrel on CNC machine. Extension and torsion springs are made by cold working. When coiling is complete, the ends are bent into the desired hook or straight end for connections.
(ii) Hot winding
Thicker wires up to 75 mm diameter are coiled into springs by heating the wire to make it flexible. The steel wire is coiled around a mandrel while red hot. Then it is oil quenched cool to harden it. Immediately after it is tempered.
(iii) Heat treatment
During cold or hot working, stresses are locked in the coils. To relieve these pre-stresses, it is tempered. Tempering consists of heating in an oven. It is hold at that temperature for a predetermined time. Then spring is cooled slowly.
(iv) Grinding or finishing
The ends are ground with a grinding wheel.
(v) Shot Peening
In this, the entire spring is compressed with tiny steel balls to increase its fatigue resistance.
That Spring is fully compressed to make coils touch each other. Desired length and pitch of the spring is obtained.
The Spring is coated with zinc or chromium to make it corrosion resistant.
TESTING OF SPRINGS
t Different springs are tested for required hardness and stiffness.
Small springs are used in computers, cell phones and medical devices like catheter or an endoscope.
Leaf springs are used in vehicles.
Helical Coiled springs are used in suspension system of all types of vehicles. These are also used in engine indicators, railway buffers, pressure gauges, and balances.
These are used to absorb shock and to act as vibration dampers.
Springs are used brakes, clutches and spring loaded valves.
To maintain contact between two moving elements such as cam and a follower.
A single spring can perform more than one function. A spring is used to keep a component to bear against a surface. It helps to maintain a contact with a surface. It can engage or disengage a device as in a thermostat for making the operation automatic. Springs are used to counter balance a force acting on a component. A Spring may be used to make a process cyclic.
All springs store energy as in watches and toys.
These are used in weighing balances and pressure gauges. Spring absorbs shock in vehicles. It gives a cushioning effect in sofas, seats of cycles and bikes.
Suction and discharge valve openings and closings in internal combustion engines are controlled with springs.
In refrigerators, springs support compressor to reduce vibrations and noise.
Different springs are made from a wire, strip or plate.
Helical wire springs are made by winding a wire around a mandrel. Helical springs are made in the form of a helix. Turns of the coil having a slope with respect to the horizontal axis.
Spiral springs are made from a strip wound in the form of a spiral around a fixed point called spindle.
Wire Springs are of two types
(a) Helix angle is zero for Close coiled helical spring. (b) open coiled helical springs have some value of helix angle.
Under axial load, close coiled helical spring develop torsion shear stress and are designed on the basis of pure torque. Wahl’s stress concentration is applied to account for minor effect of bending.
When axial torque is applied, close coiled helical spring develops pure bending stress. Hence these are designed on the basis of pure bending.
Close coiled helical spring develop bending as well as shear stress when subjected to axial torque and axial load simultaneously. These are designed on the basis of principal stresses and maximum shear stress.
Performance parameters for any type of spring are stress, stiffness and resilience.
A single spring can perform more than one function. A spring helps a component to maintains a contact with a surface, Springs counter balance force acting on a component. A spring may make a process cyclic.
Mostly springs are light weight.
Different springs are used in small devices to very heavy machinery.
Small springs are made from pre-hardened stock. The larger size springs are made from annealed steel and hardened later.
Springs requiring strong corrosion resistance are made from phosphor Bronze and titanium. Springs carrying electrical current are made from Beryllium copper because of its low electrical resistance.
When the springs are connected in series, then their stiffness will decrease as 1/k = 1/k1 +1/k2
If the springs are connected in parallel, then their stiffness will increase as k = k1 +k2
Total number of turns is the sum of active and inactive turns, nt = na + nia. However number of inactive turns depends on the end condition of the spring i.e.
Firstly for plain ends, nia = 0.5
Secondly for plain and grounded ends, nia =1
Thirdly for squared ends, nia = 1
Fourthly for squared and grounded ends,nia = 2