Spring (device)

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Helical or coil springs designed for tension
The English longbow - a simple but very powerful spring made of yew, measuring 2 m (6 ft 6 in) long, with a 470 N (105 lbf) draw force
Military boobytrap firing device from USSR (normally connected to a tripwire) showing spring-loaded firing pin

A spring is an 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 and titanium 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. An extension or compression spring has units of force divided by distance, for example lbf/in or N/m. Torsion springs have units of force multiplied by distance divided by angle, such as N·m/rad or ft·lbf/degree. 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 is additive, as is the compliance of springs in series.

Depending on the design and required operating environment, any material can be used to construct a spring, so long the material has the required combination of rigidity and elasticity: technically, a wooden bow is a form of spring.

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[edit] History

Simple non-coiled springs were used throughout human history e.g. the bow (and arrow). In the Bronze Age more sophisticated spring devices were used, as shown by the spread of tweezers in many cultures. 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 appeared early in the 15th century,[1] in clocks.[2] The first spring powered-clocks appeared in that century[3][2][4] and evolved into the first large watches by the 16th century.

In 1676 British physicist Robert Hooke discovered the principle behind springs' action, that the force it exerts is proportional to its extension, now called Hooke's law.

[edit] Types

A spiral hair spring
A volute spring. Under compression the coils slide over each other, so affording longer travel.
Tension springs in a folded line reverberation device.

Springs are classified according their properties.

Depending on load they may be classified as:

  • Tension/Extension spring
  • Compression spring
  • Torsional spring

In tension/extension and compression there is axial load. On the other hand in the torsional spring there is torsional force.

Depending on spring material it can be classified as:

  • Wire/Coil spring
  • Flat spring

The most common types of spring are:

  • Cantilever spring - a spring which is fixed only at one end.
  • 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:
    • 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, designed so that under compression the coils are not forced against each other, thus permitting longer travel.
    • Tension or extension 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.
Vertical volute springs of Stuart tank
Leaf spring on a truck

Other types include:

A torsion bar twisted under load
  • Torsion spring - any spring designed to be twisted rather than compressed or extended. Used in torsion bar vehicle suspension systems.
  • Negator spring - a thin metal band slightly concave in cross-section. When coiled it adopts a flat cross-section but when unrolled it returns to its former curve, thus producing a constant force throughout the displacement and negating any tendency to re-wind. The commonest application is the retracting steel tape rule.[5]
  • Wave spring - a thin spring-washer into which waves have been pressed.[6]

[edit] 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+k2.

[edit] Hooke's law

Main article: Hooke's law

Most springs (not stretched or compressed beyond the 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:

F=-kx, \

where

x is the displacement vector - the distance and direction in which the spring is deformed
F is the resulting force vector - the magnitude and direction of the restoring force the spring exerts
k is the spring constant or force constant of the spring.

Coil springs and other common springs typically obey Hooke's law. There are useful springs that don't: springs based on beam bending can for example produce forces that vary nonlinearly with displacement.

[edit] Simple harmonic motion

Main article: Harmonic oscillator

Since force is equal to mass, m, times acceleration, a, the force equation for a spring obeying Hooke's law looks like:

F = m a \quad \Rightarrow \quad -k x = m a. \,
The displacement, x, as a function of time. The amount of time that passes between peaks is called the period.

The mass of the spring is assumed small in comparison to the mass of the attached mass and is ignored. Since acceleration is simply the second derivative of x with respect to time,

- k x = m \frac{d^2 x}{dt^2}. \,

This is a second order linear differential equation for the displacement x as a function of time. Rearranging:

\frac{d^2 x}{dt^2} + \frac{k}{m} x = 0, \,

the solution of which is the sum of a sine and cosine:

x(t) = A \sin \left( t \sqrt{\frac{k}{m}} \right) + B \cos \left(t \sqrt{\frac{k}{m}} \right). \,

A and B are arbitrary constants that may be found by considering the initial displacement and velocity of the mass. The graph of this function with B = 0 (zero initial position with some positive initial velocity) is displayed in the image on the right.

[edit] 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 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.

Force of fully compressed spring

Fmax = (Ed4(L − nd)) / (16(1 + nu)(D − d)3n)

where

E - Young's modulus
d - spring wire diameter
L - free length of spring
n - number of active windings
nu - Poisson ratio
D - spring outer diameter

[edit] Zero-length springs

"Zero-length spring" is the standard term for a spring that exerts zero force when it has zero length. In practice this is done by combining a spring with "negative" length (in which the coils press together when the spring is relaxed) with an extra length of inelastic material. This type of spring was developed in 1932 by Lucien LaCoste for use in a vertical seismograph. A spring with zero length can be attached to a mass on a hinged boom in such a way that the force on the mass is almost exactly balanced by the vertical component of the force from the spring, whatever the position of the boom. This creates a pendulum with very long period. Long-period pendulums enable seismometers to sense the slowest waves from earthquakes. The LaCoste suspension with zero-length springs is also used in gravimeters because it is very sensitive to changes in gravity. Springs for closing doors are often made to have roughly zero length so that they will exert force even when the door is almost closed, so it will close firmly.

[edit] Uses

[edit] References

  1. ^ Springs How Products Are Made, 14 July 2007.
  2. ^ a b White, Lynn Jr. (1966). Medieval Technology and Social Change. New York: Oxford Univ. Press. ISBN 0195002660. , p.126-127
  3. ^ Usher, Abbot Payson (1988). A History of Mechanical Inventions. Courier Dover. ISBN 048625593X. http://books.google.com/books?id=xuDDqqa8FlwC&pg=PA305&sig=_SRpwfz0YBAjt2aGxXhmRkZ16GQ. , p.305
  4. ^ Dohrn-van Rossum, Gerhard (1997). History of the Hour: Clocks and Modern Temporal Orders. Univ. of Chicago Press. ISBN 0-226-15510-2. http://books.google.com/books?id=53K32RiEigMC&pg=PA121&sig=5huN81ukYRbSlxq4MsToTDIXYDY. , p.121
  5. ^ Samuel, Andrew; Weir, John (1999). Introduction to engineering design: modelling, synthesis and problem solving strategies (2 ed.). Oxford, England: Butterworth. p. 134. ISBN 0750642823. 
  6. ^ Davis, Thomas Beiber; Nelson, Carl A. Senior. Audel Mechanical Trades Pocket Manual (4 ed.). Hoboken, NJ: Wiley. p. 275. ISBN 978-0-7645-4170-4. 

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