Helical or
coil springs designed for tension
A spring is a flexible elastic object used to store mechanical
energy. Springs are usually made out of hardened steel.
Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication. Some non-ferrous metals are also used including phosphor
bronze for parts requiring corrosion resistance and beryllium copper for springs
carrying electrical current (because of its low electrical resistance).
The rate of a spring is the change in the force it exerts,
divided by the change in deflection of the spring. That is, it is the gradient of the force versus deflection curve. For an extension or compression spring it has the units of
lbf/in, N/mm, or similar. For a torsional spring it has the units of N·m/rad or ft·lbf/degree, for
example. The inverse of spring rate is compliance, that is if a spring has a rate of 10 N/mm, it has a compliance of 0.1 mm/N.
The stiffness (or rate) of springs in parallel adds, and the compliance of springs in series, adds.
History
Simple non-coiled springs were used throughout human history. In the bronze age more sophisticated spring devices were used,
as shown by the spread of tweezers in many cultures. The Greek engineer Ctesibius of
Alexandria developed a method for making bronze with spring-like characteristics by
producing an alloy of bronze with an increased proportion of tin, and then hardening it by hammering after it is cast.
Coiled springs were introduced in the 15th century.[1]
Types
A volute spring. Under compression the coils slide over each other, so affording longer travel.
The most common types of spring are:
- Coil spring or helical spring - a spring (made by winding
a wire around a cylinder) and the conical spring - these are types of torsion spring, because the wire itself is twisted when the spring is compressed or stretched. These are
in turn of two types:
- Tension springs are designed to become longer under load. Their turns are normally touching in the unloaded position,
and they have a hook, eye or some other means of attachment at each end.
- Compression springs are designed to become shorter when loaded. Their turns are not touching in the unloaded position,
and they need no attachment points. A volute spring is a compression spring in the form of
a cone so that under compaction the coils are not forced against each other, thus permitting longer travel.
- Spiral spring or 'clock spring' - a spring of the type as used in clocks, galvanometers, and places where electricity must be carried to
partially-rotating devices such as steering wheels.
Other types include:
- Belleville washer or Belleville spring - a disc shaped spring commonly used to
apply tension to a bolt (and also in the initiation mechanism of pressure-activated landmines).
- Spring washer - used to apply a constant tensile force along the axis of a
fastener.
- Torsion spring - any spring designed to be twisted rather than compressed or
extended.
- Rubber band - a tension spring where energy is stored by stretching the material.
Physics
Two springs attached to a wall and a mass. In a situation like this, the two springs can be replaced by one with a spring
constant of
keq=
k1+k
2.
Hooke's law
-
Springs that are not stretched or compressed beyond their elastic limit obey
Hooke's law, which states that the force with which the spring pushes back is linearly proportional to the distance from its
equilibrium length:
where
- x is the distance the spring is elongated by,
- F is the restoring force exerted by the spring, and
- k is the spring constant or force constant of the spring.
Simple harmonic motion
-
Since force is equal to mass, m, times acceleration, a, the force equation looks like:
-
The displacement,
x, as a function of time. The amount of time that passes between peaks is called the
period.
But acceleration is just the second time derivative of x, so
-
Re-arranging results in a differential equation
-
the solution of which is the sum of a sine and cosine:
-
The graph of this function is displayed in the image on the right.
Theory
In classical physics, a spring can be seen as a device that stores
potential energy by straining the bonds between the atoms
of an elastic material.
Hooke's law of elasticity states that the
extension of an elastic rod (its distended length minus its relaxed length) is linearly proportional to its tension, the force used to stretch it. Similarly, the contraction
(negative extension) is proportional to the compression (negative tension).
This law actually holds only approximately, and only when the deformation (extension or contraction) is small compared to the
rod's overall length. For deformations beyond the elastic limit, atomic bonds get
broken or rearranged, and a spring may snap, buckle, or permanently deform. Many materials have no clearly defined elastic limit,
and Hooke's law can not be meaningfully applied to these materials.
Hooke's law is actually a mathematical consequence of the fact that the potential energy of the rod is a minimum when it has
its relaxed length. Any smooth function of one variable approximates a quadratic
function when examined near enough to its minimum point; and therefore the force — which is the derivative of energy with respect to displacement — will approximate a linear function.
Contrary to popular belief, springs do not appreciably "creep" or get "tired"
with age.[citation needed] Spring steel has a very high resistance to creep under normal loads. Say,
in a car engine valve spring typically undergoes about a quarter billion cycles of compression-decompression over engine's life
time without noticeable change in length or loss of strength. The sag observed in some older automobiles suspension is usually due to the springs being occasionally compressed beyond their yield point,
causing plastic deformation. This can happen when the vehicle hits a large bump or pothole, especially when heavily loaded. Most
vehicles will accumulate a number of such impacts over their working life, leading to a lower ride height and eventual
bottoming-out of the suspension.
Uses
References
- ^ Springs How Products Are Made, 14 July 2007.
External links
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