About the “Spring selection”

A simple word “spring” denotes different objects with various shapes and a wide range of uses and
operations. What shape is most suitable for such use and such operation? What strength is required?
It is necessary to consider many factors one-by-one when designing a spring.

If you need a spring, do not hurry and wait a little before ordering a coil spring!

If a spring is not designed appropriately, without confirming and verifying specific usage environment and,
of course, required strength, it will rust or break in a moment.
In this text we will introduce the knowledge necessary for proper spring selection./p>

Since this text contain simple expressions to make understanding easier for all readers, please be sure to
contact us when actually designing or selecting a spring.

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Contents

1. Spring definition and functions

・1-1 Spring definition

・1-2 Spring functions

　・1-2-1 Fixed relationship between load and deformation

　・1-2-2 Accumulation of elastic energy

　・1-2-3 Natural vibration frequency

　・1-2-4 Vibration absorption

　・1-2-5 Impact absorption

2. Types and characteristics of springs

・2-1 Classification of springs by material

・2-2 Classification of springs by shape

3．Designing a spring

・3-1 Spring lifespan

・3-2 Spring design methods

 

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1．Spring definition and functions – Is the word “spring”
derived from “spring up”?

Although the word “spring” is often used casually, this word has a long history. There are different opinions
about its etymology, but the prominent theory is that this word is derived from the word “spring up”.
In English the word “spring” also contains the meaning of “spring up” or “leaping”, and indeed,
such movement is often associated with spring operation.

1-1. Spring definition

It is difficult to imagine a real object when hearing the definition of a spring.
The Japanese Industrial Standard defines spring as a “machine element whose main purpose is to utilize the energy accumulated by elasticity or deformation of a material body”(spring terms in JIS B0103).

Putting simply, spring is an element that has the ability to deform under applied force and then use this force to return to its original shape. Conversely, any object that can return to its original shape after being
deformed by applied force, can be considered a spring.

Thus, why do substances have restoring force to return to original shape after deformation?

Two different forces act on atoms that make up a substance: attracting force, trying to pull them to each
other, and a repulsing force that tries to separate them. Normally these two forces are balanced, keeping
a certain distance between atoms. When an external pushing force is applied in this balanced condition,
atoms and molecules are forcibly pushed closer together, increasing the repulsive force. It is the same image, as with a rubber, which has stronger resistance the more it is squeezed.

When the external pushing force disappears, the substance returns to its original state similar to separating magnets.
Conversely, when a pulling force is applied, the substance stretches, increasing the attracting force between molecules. Likewise, when the pulling force disappears, the material shrinks, returning to its original state,
like rubber returns to original shape.

Springs likewise have the restoring force. However, this deformation was measured mathematically only
recently, in the second half of 17th century. British physicist Robert Hooke discovered a linear relationship
between force and material deformation that was named Hooke’s Law. And the constant of the proportional relationship in the Hooke’s Law is the spring constant.

1-2. Spring functions

The restoring force of a spring was mentioned in the spring definition, but in reality there are five “abilities”
or “functions” of spring related to this restoring force.

1-2-1. Fixed relationship between load and deformation

As stated by the Hooke’s Law mentioned earlier, there is a fixed relationship between force (load) and
deformation. Depending on spring type, it may not always be a uniformly beautiful straight line, but there is
always a certain relationship between force and deformation in any spring.

1-2-2. Accumulation of elastic energy

When a force is applied to a spring, the spring is in a state of accumulating energy. This energy is called
elastic energy, which is converted into kinetic energy.
When the force applied to the spring is gradually removed, the accumulated elastic energy is also removed. When the force is removed completely, the elastic energy also becomes zero.

1-2-3. Natural vibration frequency

When force applied to a spring is suddenly removed, the spring vibrates. This vibration depends on
“mass of the spring” and “spring constant” and has characteristic (natural) frequency
See calculation of natural frequency here.

The vibration may have significant effect in actual use of springs, such as in buildings and structures.
Therefore, when designing a spring, not only the spring constant
but also the natural frequency of the spring must be reliable calculated.

1-2-4. Vibration absorption

Contrary to what was said above, there is a function that uses the vibration frequency.
Vibration exists not only in springs, but also in machines and other objects.
Vibration of large machines may adversely affect buildings and other devices.
To prevent transmission of such machine vibration, springs are installed on the machines,
absorbing the vibration and reducing the effect on buildings and other machinery.

1-2-5. Impact absorption

Similarly to the vibration absorption, springs can be used to absorb impacts. As undoubtedly often seen in
everyday life, springs ere very effective in absorbing impacts, and are used for this purpose everywhere.

2. Types and characteristics of
springs
– there are many classifications!

When speaking about springs, we usually imagine a coiled round spring. However, there are many kinds of
spring used in industrial word, and they can be classified in different ways.

2-1. Classification of springs by material

While springs are generally made from metal, springs from other materials, such as ceramics and carbon
fibers, are also gaining popularity recently.
The main advantage of non-metallic springs is their lightness.
In addition, air springs containing gas like a balloon are widely known as springs used in suspension
of trains due to low spring constants and simplicity of the mounting height adjustment.

Table. Classification of springs by material

2-2. Classification of springs by shape

Next, we will explain the classification of springs by shape. Like the classification by material, springs can be
classified by shape in many ways!

Coil springs

The majority of springs in the world are represented by coil springs. The shape of the springs is simple and
suitable for mass-production.
The coil springs can be further divided into 3 types: compression coil springs, tension coil springs
and torsion coil springs.

①Compression coil springs

These springs are used in objects which are subjected to pressing force (compression load) applied from
above. While generally there are many round coil springs with a nice cylindrical shape,
there are also conical coil springs, barrel-shaped coil springs, springs wound with uneven coil pitch
and so on. In normal cylindrical springs there is a proportional relationship between load and
deflection with the spring constant as the proportionality constant. However, when the shape changes,
the relationship loses its linearity and becomes non-linear. There are many applications utilizing
this non-linear characteristic.

②Tension coil springs

Unlike to compression coil springs, tension coil springs are used in applications where a pulling force
(tensile load) is applied. Hooks are often mounted on both ends of the spring to facilitate pulling.
The hooks themselves do not act as springs, but since a pulling force is applied, the strength of the
hooks must also be considered during design.
As with compression springs, there is a proportional relationship between load and deflection
with spring constant used as the proportionality constant.

③Torsion coil springs

The main difference of torsion coil springs from compression and tension coil springs is their usage;
the torsion coil springs are used where rotational motion occurs. The operation of torsion coil springs
can be understood by looking at clothes-pin and safety pin.
Unlike
compression coil springs and tension coil springs, torsion coil springs often have arms whose shape is
determined by the product to which the torsion coil spring is attached.

Disc springs

The disc spring is a cone-shaped spring with a hole drilled in the center of the spring like 5 yen coin.
The name (disc spring) of the spring is derived from its shape.
(Large size disc springs can be cut from metal parts.) Comparing to coil springs, the disc springs
can bear a very large load in a small space and, depending on design methods, can be designed with
non-linear load characteristic.

Leaf springs

Leaf (plate) springs are the springs with the largest number of shapes in the world. This is because almost
any used plate material acts like a leaf spring.
All plates that act like a spring, even bent ones and plates with holes, can be considered springs.
Disc springs made as plates can also be viewed as a kind of leaf spring.

Torsion bars
As the name suggests, the torsion bar is a rod. A spring rod with spring properties tries to return to original
shape when wrung. This force is used in torsion bars.
The elastic energy stored per unit volume is large compared to other springs,
so even light bars exert significant force.
There are stabilizers with arms attached to torsion bars which are used to stabilize cars.

Spiral springs

Spiral springs, also known as power springs, are springs using a force that tries to unwind a wound long thin
strip. A widely known toy ChoroQ uses this force to move forward.
The force of power springs is also used to pull back cord of vacuum cleaners with one switch.
These springs can accumulate large energy in limited space.

Spring washers, retaining rings

A spring washer placed between nut and bolt, in addition to the role of a washer, also serves as a stopper
between the nut and the object to be tightened. Bolts and nuts are less likely to become loose
when a spring washer is placed between them.
A wave washer looks like a normal washer with bulges and dents. It is stronger than a normal washer
and do not scratch objects it contacts. There are also wave washers made like compression coil springs,
called coiled wave springs.
As the name suggests, the retaining rings are springs used to fasten objects.
These springs are frequently used in the industry to fasten the parts that must be fixed.
All springs described above that are used simply to fasten some parts are called “fastener springs”.

Mesh springs

A mesh spring also known as a knit mesh spring, is made from thin spring material knitted like a cloth. This is a generic term for items used for cushioning. It has a belt shape in knitted state, but is made into cylinder
or donut shape when used.
Comparing to general cushioning material, the mesh spring has a higher ability to absorb vibration,
so it is often used in foundations for machines, engines and other devices prone to vibration.

Volute springs

Volute spring got its name from the similar looking shell-fish. The spring is made from wound long
rectangular stripe, similar to the spiral (power) spring, but unlike the spiral (power) spring,
it rises in a conical shape. When applied load increases (deflection becomes larger),
the spring constant gets stronger, which makes the spring ideal for absorbing large shocks.

Ring springs

The ring spring is a stacked up alternating combination of inner and outer rings with conical face.
When compressive load is applied to the ring spring, these two kinds of rings expand and contract,
exhibiting spring characteristics.
Since the frictional force is generated between two rings in addition to the expansion and contraction,
the design becomes considerably complicated.

There are also springs with non-fixed shape, like flat springs, wire springs, etc. A flat spring is a general thin
flat plate detail with spring action. A wire spring is a generic name for wires of various shapes with
round or irregular cross-section acting like a spring, excluding coil springs.

Thus, there are many types of springs divided by shape and material only.
It means that there is an appropriate spring for each application.

3. Designing a spring

Returning to the spring definition presented in section 1-1, we can say that in addition to restoring force,
springs also possess such functions as the fixed relationship between load and deformation,
the ability to store the elastic energy, the natural vibration frequency and the ability to dampen
vibrations and shocks!
It is natural that springs are used for purposes that lie among the spring functions listed above, but if a spring breaks after single use in actual usage conditions, it must be replaced each time.
Thus, the long continuous usage is the most desirable. The important thing is the lifespan of the spring.

3-1. Spring lifespan

The spring lifespan is the time period until spring breaks or loses its design characteristic as a spring.

The lifespan of a spring can be estimated to some extent by what kind of movement the spring performs,
its shape (degree of applied stress) and the environment the spring is operating in. For example,
a spring that maintains the fixed height almost without movement and a spring that stretches and
strings many thousand times will have completely different lifespans even with the same load applied.
Depending on shape, some springs may lose their characteristics with applied force on the order of
several tons, and some can support hundreds of tons. When used in special environment with high
temperature and humidity, springs of the same shape but made from different materials will have
different lifespans.
That is why it is necessary to design a spring.

3-2. Spring design methods

It is easy to determine the relationship between applied force and deflection and the lifespan of a spring to
some extent.
There is a wide range of free software, so please try it.

 
However, in practice the lifespan and load often do not act according to calculations, stressing the
importance of actual experience. If you have concerns about the design, please do not hesitate to contact us.