Helical compression spring: Compression springs are used to have a resistance to applied axial compression forces or to store potential energy under compression loads. In terms of energy storage per unit volume, they are amongst the most efficient spring type available. They are found in many real life applications such as automotive engines, valves, locomotives. While the most common used form of compression spring is a straight cylindrical spring made from round wire, many other shapes exist including conical, barrel and hourglass, with or without variable spacing between coils. Such forms are used to reduce solid height, buckling and surging, or to produce variable spring rate. Generally, springs are either placed over a rod or fitted inside a hole.

Axial load [F]: A load parallel and concentric to spring axis.

Wahl factor: A factor to correct shear stress to include curvature effect.

Solid height: The length of a spring with all coils closed. This is the position at which the maximum load and stress is generated.

Free length (height): The overall spring height in unloaded position.

OD (at solid height): Outer diameter of the spring when compressed to solid height.

Set removal (presetting): A process to induce useful residual stresses in the spring.

Spring Buckling: A type of failure when a slender helical spring will become laterally unstable and buckle due to the axial compressive force which exceeds a certain critical value.

Spring Rate: A parameter which shows relation between applied force and deflection. In other words, reaction force per unit deflection or spring resistance to length change.

Spring End Type: Generally 4 types of spring end is used compression spring.

Plain end
Plain end compression spring No pitch change at the end of the spring. It's cheap to manufacture. There is no inactive or dead coil. For axial stability, special end seats are necessary.
Plain and ground end
Plain and ground end compression spring Plain end spring is ground to have a flat end.
Squared (Closed) end
Closed end compression spring Last coil of the spring is formed to contact the coil just near the end coil. The two end coils are totally inactive. Grinding operation produces flat seats for more than 270° of the end coil circumference which are also nominally perpendicular to spring axis and parallel to each other.
Squared (Closed) and ground end
Closed and Ground end compression spring Similar to closed end except the end of spring is ground to have a flat end.

Spring index: The ratio of spring mean diameter to coil diameter. As a general rule, the ratio shall be between 4 and 12. Spring sizes out of this interval increases the cost and manufacturing process is harder. [Ref-1]. According to BS1726:Part 1:1987 , advised index range is between 3.5 to 16. A low index value indicates a very tightly wound spring with a relatively large wire or bar being coiled sharply around a relatively small coil diameter. This results very high axial stiffness. A high index value means an open wound spring which will be very flexible along its axis (low spring rate).

Total number of coils: Number of coils counted from one end to other. It shall include fractions of a coil.

Total number of coils in compression spring

Number of active coils: Active coils are the coils of a spring that stores and releases energy. The number of active coils cannot be directly measured . It can be calculated by subtracting the number of inactive coils from the total number of coils.

Design factor (nd):  The ratio of failure stress to allowable stress. The design factor is what the item is required to withstand .The design factor is defined for an application (generally provided in advance and often set by regulatory code or policy) and is not an actual calculation.

Factor of Safety (Safety Factor): The ratio of failure stress to actual/expected stress. The difference between the factor of safety (safety factor) and design factor is: The factor of safety gives the safety margin of designed part against failure. The design factor gives the requirement value for the design. Safety factor shall be greater than or equal to design factor.

Sinusoidal cycling loading in spring design

Static/Quasistatic Loading: Following loading cases are defined as Static/Quasistatic loading:

  • A constant loading
  • A cycling loading with torsional shear stress range up to 10 % of fatigue strength (or endurance strength)
  • A cycling loading with torsional shear stress range more than 10 % of fatigue strength (or endurance strength) up to 104 cycles

  • Dynamic Loading: A loading which varies with time with a number of load cycles over 104 and torsional stress range greater than 10 % of  fatigue strength (or endurance strength) at:

  • Constant torsional stress range
  • Variable torsional stress range

  • Reference: