Springs

An elastic body whose function is to distort when loaded and to recovered its original shape when load is removed.

• It is used to provide push, pull or twisting force.
• Purpose is to store energy.
• To absorb or control energy due to shock, vibration in cars, trains, airplanes, landing gears, shock absorbers, vibration dampers.
• To measure forces in spring balance, engine indicators.

Types of Springs

1. Helical Springs 

• Made up of a wire coiled in the form of helix intended for compression and tensile loads.
• In a closely coiled helical spring, the helix angle is very small (<10-degrees) and plane containing coils is nearly ⊥ to the axis of coils.
• The major stresses produced in helix spring are shear stress due to twisting, load applied is parallel or along to the axis of spring.

There are two types of Helical Springs which are:

1. Compression Helical Spring
2. Tension Helical Spring

2. Conical & Volute Springs

Conical and Volute springs are used where spring rate increase with load is desired.

• Conical Spring  ↣  have constant pitch.
• Volute Spring  ↣  parabolidal shape with constant pitch and lead angles.
• Springs may be made partially or completely ballascoping. In each case, number of active coils gradually decreases.
• Decreasing number of active coils resulting in increases spring rate. This characteristic is utilized in vibration problem (hold a body with varying mass).

3. Torsion Springs

• Stresses produced are tensile and compressive due to bending.

There are two types of Torsion springs which are:

1. Helical Torsion Springs  ↣  Used in electrical equipment and load increases with turn.
2. Spiral Torsion Springs  ↣  Used where load increases with coils (in watches).

4. Laminated or Leaf Springs

• It consist of number of flat plates (called Leaf).
• It is mostly used in automobiles.
• Tensile and compressive stresses are produced.

5. Disc or Bellevile Springs

• It consist of conical discs held together against slipping by a central bolt and tube.
• Used where higher spring rates and compact unit are required.
• Tensile and compressive stresses are produced.

6. Special Purpose Springs

• They are air or liquid, rubber, rings springs.
• Fluid (air or liquid)  ↣  behave like compression springs.

Material Requirements for Helical Springs

Characteristics of Material for Helical Springs are given below:

• High fatigue strength.
• High Resilence.
• High Ductility.
• Creep resistant.
• Springs are usually made from oil tapered carbon steel wires containing 0.6-0.7% carbon, 0.6-1.0 manganese.
• Non-ferrous, phosphorous, bronze, copper, brass are used for increasing fatigue resistance, temperature and corrosion resistance.
• Wires of small size (< 10 mm diameter)  ↣  formed by cold working.

Creep resistance depends on the following services:

1. Severe Service  ↣  Rapid continuous load, ratio of maximum load to minimum load is one-half.
2. Average Service  ↣  Same stress range, engine governor spring or suspension systems.
3. Light Service  ↣  Static loading, safety valve springs.

Helical Compression Springs

1. Solid Length  ↣  When compression spring compressed, coils come in contact with each other, then spring is called Solid.
2. Free-Length  ↣  Length of the spring in free or unloaded condition. Maximum deflection is 15-degrees.
3. Spring Index  ↣  Ratio of mean diameter of coil to the diameter of wire.
4. Spring Rate  ↣  Load required by a unit deflection of spring.
5. Pitch  ↣  Axial distance between adjacent coils in uncompressed state. 

Springs in Series and Parallel

Total deflection of springs attached in series is given by:

Total deflection of springs attached in parallel is given by:

Design of Helical Compression Spring

Deflection Produced in Helical Compression Spring of Circular Wire

References:

• Material from Class Lectures + Book named Mechanical Engineering Design by Shigley (8th Edition) + my knowledge. 
• Pics and GIF from Google Images.  
• Videos from YouTube.