An Introduction To Spring Types, Materials ,Selection And Applications

An Introduction To Spring Types, Materials ,Selection And Applications

Introduction

Springs are among the most important and most often used mechanical components. Many
mechanisms and assemblies would be virtually impossible to design and manufacture without
the use of springs in one form or another. 

Springs are constructional elements designed to retain and accumulate mechanical energy, working on the principle of flexible deformation of material. Springs belong to the most loaded machine components and are usually used as:
        ∎ energy absorbers for drives and reciprocating devices
        ∎ interceptors of static and dynamic forces
        ∎ elements to create force joints
        ∎ shock absorbers in anti-vibration protection

spring types

Based on the deformation pattern, springs can be divided into the following three types:
    1- springs with linear characteristics
    2- springs with degressive characteristics
    3- springs with progressive characteristics
Metal springs can be divided into groups according to many aspects. Division according to load type and structural design of a spring can be considered as basic. The most common spring types are described in detail as follows:
    ∎ Springs for axial forces load (compression/tension)
         - Helical (coil) springs
         - Belleville springs and washer springs
         - Ring (annular) springs
         - Constant force springs
    ∎ Springs for transversal forces load (flexion)
         - Leaf springs
         - Curved springs
    ∎ Springs for torque load
         - Torsion bar springs
         - Spiral springs
         - Helical (coil) springs

Helical cylindrical compression springs

FIG 1

A helical compression spring is an open-pitch spring which is used to resist applied compression forces or to store energy. It can be made in a variety of configurations and from different shapes of wire, depending on the application. Round, high carbon- steel wire is the most common spring material, but other shapes and compositions may be required by space and environmental conditions.

Springs of cylindrical shape made of helically coiled wires, with constant clearance between the active coils, able to absorb external counter-acting forces applied against each other in their axis. Springs with wire diameter up to approx. 16 mm are usually cold wound. Hot forming shall be used for the production of heavily loaded springs of greater sizes with a diameter of the over 10 mm. Compression springs are usually made of wires and rods of round section. Springs of rectangular wire are most often used in applications where low constructional height of the spring (springs with b>h) is required together with relatively high load.
Specific properties
   ●  suitable for low and medium load forces
   ●  linear working characteristics
   ●   relatively low spring constant
   ●   easy mounting and dismantling
   ●   low production costs

Helical conical compression springs

FIG 2

Springs of conical shape made of helically coiled wires, with constant clearance between the active coils, able to absorb external counter-acting forces applied against each other in their axis. Springs with wire diameter up to approx. 16 mm are usually cold wound. Hot forming shall be used for the production of heavily loaded springs of greater sizes with a diameter of the over 10 mm. Conical springs are usually used if the spring constant is to rise together with its progressing compression.

Specific properties
 ●  suitable for low and medium load forces
 ● nonlinear (progressive) working characteristics
 ● relatively low spring constant
 ●  easy mounting and dismantling
 ● low production costs

Belleville springs

FIG 3

Belleville springs are used in two broad types of applications. First, they are used to provide very high loads with small deflections, as in stripper springs for punch press dies, recoil mechanisms, and pressure-relief valves. Second, they are used for their special nonlinear load-deflection curves, particularly those with a constant load portion. In loading a packing seal or a live center for a lathe, or in injection molding machines, Belleville washers can maintain a constant force throughout dimensional changes in the mechanical system resulting from wear, relaxation, or thermal change.

Annular rings of hollow truncated cone, able to absorb external axial forces counter-acting against each other. The spring section is usually rectangular. Springs of larger sizes (t > 6 mm) are sometimes made with machined contact flats.
Belleville springs are designed for higher loads with low deformations. They are used individually or in sets. When using springs in a set it is necessary to take account of friction effects. Friction in the set accounts for 3 – 5% of loading per each layer. Working load must then be increased by this force.
Stress occurring in the Belleville spring is rather complex. Maximum stress (compressive) develops in the inner top edge. Tensile stress occurs on the bottom outer edge. Maximum compressive stress serves for strength check of springs subjected to static load. In the springs subjected to cyclic (fatigue) load the pattern of tensile stresses is checked.
Specific properties
    ●suitable for large loading forces
    ●nonlinear (degressive) working characteristics
    ●high spring constant (stiffness)
    ●low space requirements
    ●easy mounting and dismantling
    ●low production costs

Helical cylindrical tension springs

FIG 4

Helical extension springs store energy and exert a pulling force. They are usually made from round wire and are close-wound with initial tension. They have various types of end hooks or loops by which they are attached to the loads.

Springs of cylindrical shape made of helically coiled wires, with constant clearance between the active coils, able to absorb external axial forces counter-acting from each other. Springs with wire diameter up to approx. 16 mm are usually cold wound. Hot forming shall be used for the production of heavily loaded springs of greater sizes with a diameter of the over 10 mm. Tension springs are usually made of wires and rods of round section. Springs made of rectangular wire are used very rarely.
With regards to the considerable effects of the shape and design of fixing eyes on reduction of the spring's service life and impossibility of perfect shot peening of the spring, it is not advisable to use tension springs exposed to fatigue loading. If it is necessary to use a tension spring with fatigue loading, it is advisable to avoid use of fixing eyes and choose another type of fixing of the spring.

Specific properties
  ●  suitable for low and medium load forces
  ● less suitable for cyclic (fatigue) load
  ● linear working characteristics
  ● relatively low spring constant
  ● easy mounting and dismantling
  ● low production costs

Tension springs are used in two basic designs:
1- Spring with prestressing.
Cold formed tension springs are preferably produced with prestressing, thus with close-coiled active coils. The spring prestressing has considerable effects on increase in the loading capacity of the spring. For deformation of the spring to the desired length, it is necessary to use a higher loading than with springs without prestressing. Prestressing appears in coils of the spring in the course of coiling of the spring wire, and its size depends on the used material, spring index and the manner of coiling.
2- Spring without inner prestressing.
If necessary due to technical reasons, it is possible to use loose-coiled tension springs without prestressing, with gaps between the active coils. The coil pitch of a free spring is usually in the range 0.2*D < p < 0.4*D.

Leaf  or flat springs

FIG 5

The classification flat springs applies to a wide range of springs made from sheet, strip, or plate material. Exceptions to this classification are power springs and washers. Flat springs may contain bends and forms. Thus the classification refers to the raw material and not to the spring itself.

Springs based on the principle of long slander beams of rectangular section subjected to bending. They are used as cantilever springs (fixed at one end), or as simple beams (fixed at both ends). The leaf springs can be used either independently or in sets (laminated leaf springs).

Specific properties

Single springs
    ● suitable for low and medium load forces
    ● linear working characteristics
    ● relatively low spring constant
    ● considerable length requirements, otherwise minimum space needed
    ● low production costs

Laminated leaf springs

FIG 6

     ● suitable for higher loading forces
     ● theoretically linear working characteristics (friction between the leaves causes hysteretic pattern          of the working curve)
     ● relatively higher spring constant (stiffness)
     ● high space requirements
     ● demanding maintenance (lubrication and cleanness)

Torsion bar springs

FIG 7

Torsion bars used as springs are usually straight bars of spring material to which a twisting couple is applied. The stressing mode is torsional. This type of spring is very efficient in its use of material to store energy. The major disadvantage with the torsion bar is that unfavorable stress concentrations occur at the point where the ends are fastened.

Springs based on the principle of long slender bars of circular or rectangular section subjected to torsion. The ends of bars with circular section are mostly fixed by means of grooving. Sometimes one end is square-shaped in order to facilitate attachment. Torsion bar springs must be secured against bending stress.
Specific properties
     ● suitable for higher loading torques
     ● linear working characteristics
     ● high spring constant
     ● considerable length requirements, otherwise minimum space needed
     ● low production costs

Spiral springs

FIG 8

The spring made of a strip with rectangular section wound into the shape of Archimedes spiral, with constant spacing between its active coils, loaded with torque in the direction of the winding.
Specific properties
      ● suitable for low loading torques
      ● linear working characteristics
      ● low spring constant
      ● low production costs

Helical cylindrical torsion springs

FIG 9

Springs of cylindrical shape made of helically coiled wires, with constant spacing between the active coils, able to absorb external forces applied in the planes perpendicular to the winding axis through a torque in the direction of winding or unwinding. Springs with wire diameter up to approx. 16 mm are usually cold wound. Hot forming shall be used for the production of heavily loaded springs of greater sizes with a diameter of the over 10 mm.
Specific properties
    ● suitable for low and medium loading torques
    ● linear working characteristics
    ● relatively low spring constant
    ● low production costs
Basic Formulae Used in Designing of Springs

FIG 10

Compression Springs, and Tension Springs without Initial Tension

Where :
d   - Diameter of Material (mm )
D1 - Inner Diameter of a Coil( mm) 
D2 - Outer Diameter of a Coil (mm )
D  - Coil Mean Diameter= (D1+D2)/2 (mm) 
Nt - Total Number of Winding 
Na - Number of Active Winding  
L - Free Length(Length) (mm) 
HS - Solid Length (mm) 
p - Pitch (mm )
Pi - Initial Tension (N{kgf})
c - Spring Index c= D/d  
G - Shear Modulus of Elasticity N/mm² {kgf/mm² } 
P - Load on Spring N{kgf} 
𝞭- Spring Deflection (mm )
k- Spring Constant N/mm{kgf/mm}
𝝉0- Torsional Stress N/mm² {kgf/mm² } 
𝞃 - Corrected Torsional Stress N/mm² {kgf/mm² } 
𝝉i - Initial Stress N/mm2 {kgf/mm² } 
x- Stress Correction Factor 
f - Frequency Hz 
U - Spring-Retained Energy (N·mm){kgf·mm} 
𝜔- Per Unit Volume Material Weight (kg/mm³)
W - Mass of Moving Parts( kg )
g- Gravitational Acceleration (1) (mm/s²)
Tension Springs with Initial Tension (Where:P>Pi)

Shear Modulus of Elasticity

Number of Active Winding
Compression Springs 

 Na=Nt−(X1+X2)

Where 
X1 and X2: are the number of turns at each end of the coil.
(a) When only the end of the coil is in contact with the next free coil [Corresponding to (a) ~ (c) in Fig.11] 

FIG 11

X1=X2=1 Therefore, Na=Nt-2 

(b) When the end of the coil is not in contact with the next coil, and the spring end has of a turn. [Corresponding to (a) ~ (e) in Fig.11] 

X1=X2=0.75 Therefore, Na=Nt-1.5 

Tension Springs 
The number of active winding can be determined as follows. But hooks are ignored. 

Na=Nt

 Solid Length 
The solid length of a spring can normally be obtained by using the following simplified formula. Generally, the purchaser of a compression spring does not specify the solid length of the spring.

 As for those compression springs, both ends of which are shaped as shown in (b), (c), (e) or (f) of Figure 11 and for which the solid length needs to be specified, the following formula can be used to obtain the maximum solid length. However, the actual maximum solid length can be greater than the value thus calculated depending on the shape of the spring in question. 

Initial Tension of Tension Springs 
Cold-formed solid-coiled tension springs are subjected to initial tension (Pi) The initial tension can be obtained using the following formula. 

SELECTION OF SPRING MATERIALS

Springs are resilient structures designed to undergo large deflections within their elastic range. It follows that the materials used in springs must have an extensive elastic range.

Some materials are well known as spring materials. Although they are not specifically designed alloys, they do have the elastic range required. In steels, the medium and high-carbon grades are suitable for springs. Beryllium copper and phosphor bronze are used when a copper-base alloy is required. The high-nickel alloys are used when high strength must be maintained in an elevated-temperature environment.

The selection of material is always a cost-benefit decision. Some factors to be considered are costs, availability, form ability, fatigue strength, corrosion resistance, stress relaxation, and electric conductivity. The right selection is usually a compromise among these factors. 

Surface quality has a major influence on fatigue strength. This surface quality is a function of the control of the material manufacturing process. Materials with high surface integrity cost more than commercial grades but must be used for fatigue applications, particularly in the high cycle region.

The spring material should be designed with regards to the method of loading the spring and the operational conditions.
As regards the strength check and the service life, there are the following two types of metal spring loads:
1- Static loading. 
Springs loaded statically or with lower variability, i.e. with cyclical changes of loading, with the requirement of a service life lower than 105 working cycles.
2- Fatigue loading. 
Springs exposed to oscillating (dynamic) loading, i.e. with cyclical changes of loading, with the requirement of a service life from 10⁵ working cycles up.

The ultimate tensile strength of the cold drawn spring wires of some materials is considerably dependent on the wire diameter. Material strength increases with decreasing diameter of the wire.material values for the calculation uses minimum ultimate stress values of the selected material for wires of the largest diameters (approx. 15 mm, 5/8 in).

Approximate values of the ultimate strength depending on the wire diameter can be found in the graphs:

Ultimate tensile strength - ASTM

Ultimate tensile strength - EN