In mathematics, an involute (also known as an evolvent) is a particular type of curve that is dependent on another shape or curve. An involute of a curve is the locus of a point on a piece of taut string as the string is either unwrapped from or wrapped around the curve.[1]

Two involutes (red) of a parabola

It is a class of curves coming under the roulette family of curves.

The evolute of an involute is the original curve.

The notions of the involute and evolute of a curve were introduced by Christiaan Huygens in his work titled Horologium oscillatorium sive de motu pendulorum ad horologia aptato demonstrationes geometricae (1673).[2]

Involute of a parameterized curveEdit

Let   be a regular curve in the plane with its curvature nowhere 0 and  , then the curve with the parametric representation


is an involute of the given curve.

The string acts as a tangent to the curve  . Its length is changed by an amount equal to the arc length traversed as it winds or unwinds. Arc length of the curve traversed in the interval   is given by


where   is the starting point from where the arc length is measured. Since the tangent vector depicts the taut string here, we get the string vector as


The vector corresponding to the end point of the string ( ) can be easily calculated using vector addition, and one gets


Adding an arbitrary but fixed number   to the integral   results in an involute corresponding to a string extended by   (like a ball of wool yarn having some length of thread already hanging before it is unwound). Hence, the involute can be varied by constant   and/or adding a number to the integral (see Involutes of a semicubic parabola).

If   one gets


Properties of involutesEdit

Involute: properties. The angles depicted are 90 degrees.

In order to derive properties of a regular curve it is advantageous to suppose the arc length   to be the parameter of the given curve, which lead to the following simplifications:   and  , with   the curvature and   the unit normal. One gets for the involute:


and the statement:

  • At point   the involute is not regular (because   ),

and from   follows:

  • The normal of the involute at point   is the tangent of the given curve at point  .
  • The involutes are parallel curves, because of   and the fact, that   is the unit normal at  .


Involutes of a circleEdit

Involutes of a circle

For a circle with parametric representation  , one has  . Hence  , and the path length is  .

Evaluating the above given equation of the involute, one gets


for the parametric equation of the involute of the circle.

The   term is optional; it serves to set the start location of the curve on the circle. The figure shows involutes for   (green),   (red),   (purple) and   (light blue). The involutes look like Archimedean spirals, but they are actually not.

The arc length for   and   of the involute is

Involutes of a semicubic parabola (blue). Only the red curve is a parabola.

Involutes of a semicubic parabolaEdit

The parametric equation   describes a semicubical parabola. From   one gets   and  . Extending the string by   extensively simplifies further calculation, and one gets


Eliminating t yields   showing that this involute is a parabola.

The other involutes are thus parallel curves of a parabola, and are not parabolas, as they are curves of degree six (See Parallel curve § Further examples).

The red involute of a catenary (blue) is a tractrix.

Involutes of a catenaryEdit

For the catenary  , the tangent vector is  , and, as   its length is  . Thus the arc length from the point (0, 1) is  

Hence the involute starting from (0, 1) is parametrized by


and is thus a tractrix.

The other involutes are not tractrices, as they are parallel curves of a tractrix.

Involutes of a cycloidEdit

Involutes of a cycloid (blue): Only the red curve is another cycloid

The parametric representation   describes a cycloid. From  , one gets (after having used some trigonometric formulas)




Hence the equations of the corresponding involute are


which describe the shifted red cycloid of the diagram. Hence

  • The involutes of the cycloid   are parallel curves of the cycloid

(Parallel curves of a cycloid are not cycloids.)

Involute and evoluteEdit

The evolute of a given curve   consists of the curvature centers of  . Between involutes and evolutes the following statement holds: [3][4]

A curve is the evolute of any of its involutes.


The involute has some properties that makes it extremely important to the gear industry: If two intermeshed gears have teeth with the profile-shape of involutes (rather than, for example, a traditional triangular shape), they form an involute gear system. Their relative rates of rotation are constant while the teeth are engaged. The gears also always make contact along a single steady line of force. With teeth of other shapes, the relative speeds and forces rise and fall as successive teeth engage, resulting in vibration, noise, and excessive wear. For this reason, nearly all modern gear teeth bear the involute shape.[5]

Mechanism of a scroll compressor

The involute of a circle is also an important shape in gas compressing, as a scroll compressor can be built based on this shape. Scroll compressors make less sound than conventional compressors and have proven to be quite efficient.

The High Flux Isotope Reactor uses involute-shaped fuel elements, since these allow a constant-width channel between them for coolant.

See alsoEdit


  1. ^ Rutter, J.W. (2000). Geometry of Curves. CRC Press. pp. 204. ISBN 9781584881667.
  2. ^ McCleary, John (2013). Geometry from a Differentiable Viewpoint. Cambridge University Press. pp. 89. ISBN 9780521116077.
  3. ^ K. Burg, H. Haf, F. Wille, A. Meister: Vektoranalysis: Höhere Mathematik für Ingenieure, Naturwissenschaftler und ..., Springer-Verlag, 2012,ISBN 3834883468, S. 30.
  4. ^ R. Courant:Vorlesungen über Differential- und Integralrechnung, 1. Band, Springer-Verlag, 1955, S. 267.
  5. ^ V. G. A. Goss (2013) "Application of analytical geometry to the shape of gear teeth", Resonance 18(9): 817 to 31 Springerlink (subscription required).

External linksEdit