Involute

File:Evolvente-parabel.svg

Two involutes (red) of a parabola

Template:Distinguish 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.<ref>Template:Cite book</ref>

The evolute of an involute is the original curve.

It is generalized by the roulette family of curves. That is, the involutes of a curve are the roulettes of the curve generated by a straight line.

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), where he showed that the involute of a cycloid is still a cycloid, thus providing a method for constructing the cycloidal pendulum, which has the useful property that its period is independent of the amplitude of oscillation.<ref>Template:Cite book</ref>

Involute of a parameterized curve

Template:See also Let <math> \vec c(t),\; t\in [t_1,t_2] </math> be a regular curve in the plane with its curvature nowhere 0 and <math>a\in (t_1,t_2)</math>, then the curve with the parametric representation

<math>\vec C_a(t)=\vec c(t) -\frac{\vec c'(t)}{|\vec c'(t)|}\; \int_a^t|\vec c'(w)|\; dw </math>

is an involute of the given curve.

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Adding an arbitrary but fixed number <math>l_0</math> to the integral <math> \Bigl(\int_a^t|\vec c'(w)|\; dw\Bigr) </math> results in an involute corresponding to a string extended by <math>l_0</math> (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 <math>a</math> and/or adding a number to the integral (see Involutes of a semicubic parabola).

If <math>\vec c(t)=(x(t),y(t))^T</math> one gets

<math>\begin{align}

X(t) &= x(t) - \frac{x'(t)}{\sqrt{x'(t)^2 + y'(t)^2}} \int_a^t \sqrt{x'(w)^2 + y'(w)^2} \,dw \\ Y(t) &= y(t) - \frac{y'(t)}{\sqrt{x'(t)^2 + y'(t)^2}} \int_a^t \sqrt{x'(w)^2 + y'(w)^2} \,dw \; . \end{align}</math>

Properties of involutes

File:Involute(in red) of parabola(dark blue).png

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 <math>s</math> to be the parameter of the given curve, which lead to the following simplifications: <math>\;|\vec c'(s)|=1\;</math> and <math>\;\vec c(s)=\kappa(s)\vec n(s)\;</math>, with <math>\kappa</math> the curvature and <math>\vec n</math> the unit normal. One gets for the involute:

<math>\vec C_a(s)=\vec c(s) -\vec c'(s)(s-a)\ </math> and

<math>\vec C_a'(s)=-\vec c(s)(s-a)=-\kappa(s)\vec n(s)(s-a)\; </math>

and the statement:

• At point <math> \vec C_a(a)</math> the involute is not regular (because <math>| \vec C_a'(a)|=0</math> ),

and from <math>\; \vec C_a'(s)\cdot\vec c'(s)=0 \;</math> follows:

• The normal of the involute at point <math>\vec C_a(s)</math> is the tangent of the given curve at point <math>\vec c(s)</math>.
• The involutes are parallel curves, because of <math>\vec C_a(s)=\vec C_0(s)+a\vec c'(s)</math> and the fact, that <math>\vec c'(s)</math> is the unit normal at <math>\vec C_0(s)</math>.

The family of involutes and the family of tangents to the original curve makes up an orthogonal coordinate system. Consequently, one may construct involutes graphically. First, draw the family of tangent lines. Then, an involute can be constructed by always staying orthogonal to the tangent line passing the point.

Cusps

This section is based on.<ref>Template:Cite book</ref>

There are generically two types of cusps in involutes. The first type is at the point where the involute touches the curve itself. This is a cusp of order 3/2. The second type is at the point where the curve has an inflection point. This is a cusp of order 5/2.

This can be visually seen by constructing a map <math>f: \R^2 \to \R^3</math> defined by <math display="block">(s, t) \mapsto (x(s) + t\cos(\theta), y(s) + t\sin(\theta), t)</math>where <math>(x(s), y(s))</math> is the arclength parametrization of the curve, and <math>\theta</math> is the slope-angle of the curve at the point <math>(x(s), y(s))</math>. This maps the 2D plane into a surface in 3D space. For example, this maps the circle into the hyperboloid of one sheet.

By this map, the involutes are obtained in a three-step process: map <math>\R</math> to <math>\R^2</math>, then to the surface in <math>\R^3</math>, then project it down to <math>\R^2</math> by removing the z-axis: <math display="block">s \mapsto (s, l- s) \mapsto f(s, l- s) \mapsto (f(s, l- s)_x, f(s, l- s)_y)</math>where <math>l</math> is any real constant.

Since the mapping <math>s \mapsto f(s, l-s)</math> has nonzero derivative at all <math>s\in \R</math>, cusps of the involute can only occur where the derivative of <math>s \mapsto f(s, l-s)</math> is vertical (parallel to the z-axis), which can only occur where the surface in <math>\R^3</math> has a vertical tangent plane.

Generically, the surface has vertical tangent planes at only two cases: where the surface touches the curve, and where the curve has an inflection point.

cusp of order 3/2

For the first type, one can start by the involute of a circle, with equation<math display="block">\begin{align} X(t) &= r(\cos t + (t - a)\sin t)\\ Y(t) &= r(\sin t - (t - a)\cos t) \end{align}</math>then set <math>a = 0</math>, and expand for small <math>t</math>, to obtain<math display="block">\begin{align} X(t) &= r + r t^2/2 + O(t^4)\\ Y(t) &= rt^3/3 + O(t^5) \end{align}</math>thus giving the order 3/2 curve <math>Y^2 - \frac{8}{9r} (X-r)^{3} + O(Y^{8/3}) = 0 </math>, a semicubical parabola.

cusp of order 5/2

File:Involutes of a cubic curve.svg

Tangents and involutes of the cubic curve <math>y = x^3</math>. The cusps of order 3/2 are on the cubic curve, while the cusps of order 5/2 are on the x-axis (the tangent line at the inflection point).

For the second type, consider the curve <math>y = x^3</math>. The arc from <math>x= 0</math> to <math>x = s</math> is of length <math>\int_0^s \sqrt{1 + (3t^2)^2}dt = s + \frac{9}{10} s^5 - \frac 98 s^9 + O(s^{13})</math>, and the tangent at <math>x = s</math> has angle <math>\theta = \arctan(3s^2)</math>. Thus, the involute starting from <math>x= 0</math> at distance <math>L</math> has parametric formula<math display="block">\begin{cases}

   x(s) = s + (L-s-\frac{9}{10}s^5 + \cdots)\cos\theta \\
   y(s) = s^3 + (L-s-\frac{9}{10}s^5 + \cdots)\sin\theta

\end{cases}</math>Expand it up to order <math>s^5</math>, we obtain<math display="block">\begin{cases}

   x(s) = L - \frac 92 L s^4 + (\frac 92 L - \frac{9}{10}) s^5 + O(s^6)\\
   y(s) = 3Ls^2 - 2 s^3 + O(s^6)

\end{cases}</math>which is a cusp of order 5/2. Explicitly, one may solve for the polynomial expansion satisfied by <math>x, y</math>:<math display="block">\left(x - L + \frac{y^2}{2L} \right)^2 - \left(\frac 92 L + \frac{51}{10} \right)^2 \left(\frac{y}{3L} \right)^5 + O(s^{11}) = 0</math>or <math display="block">x = L - \frac{y^2}{2L} \pm \left(\frac 92 L + \frac{51}{10} \right) \left(\frac{y}{3L} \right)^{2.5} + O(y^{2.75}),\quad \quad y \geq 0 </math>which clearly shows the cusp shape.

Setting <math>L=0</math>, we obtain the involute passing the origin. It is special as it contains no cusp. By serial expansion, it has parametric equation<math display="block">\begin{cases}

   x(s) = \frac{18}{5} s^5 - \frac{126}{5} s^9 + O(s^{13}) \\
   y(s) = -2s^3 + \frac{54}{5} s^7 - \frac{318}{5} s^{11} + O(s^{15})

\end{cases}</math>or <math>x = -\frac{18}{5 \cdot 2^{1/3}}y^{5/3} + O(y^3)</math>

Examples

Involutes of a circle

File:Evolvente-kreis.svg

Involutes of a circle

For a circle with parametric representation <math>(r\cos(t), r\sin(t))</math>, one has <math>\vec c'(t) = (-r\sin t, r\cos t)</math>. Hence <math>|\vec c'(t)| = r</math>, and the path length is <math>r(t - a)</math>.

Evaluating the above given equation of the involute, one gets

<math>\begin{align}

X(t) &= r(\cos (t+a) + t\sin (t+a))\\ Y(t) &= r(\sin (t+a) - t\cos (t+a)) \end{align}</math> for the parametric equation of the involute of the circle.

The <math>a</math> term is optional; it serves to set the start location of the curve on the circle. The figure shows involutes for <math>a = -0.5</math> (green), <math>a = 0</math> (red), <math>a = 0.5</math> (purple) and <math>a = 1</math> (light blue). The involutes look like Archimedean spirals, but they are actually not.

The arc length for <math>a=0</math> and <math>0 \le t \le t_2</math> of the involute is

<math>L = \frac{r}{2} t_2^2.</math>

File:Evolvente-np.svg

Involutes of a semicubic parabola (blue). Only the red curve is a parabola. Notice how the involutes and tangents make up an orthogonal coordinate system. This is a general fact.

Involutes of a semicubic parabola

The parametric equation <math>\vec c(t) = (\tfrac{t^3}{3}, \tfrac{t^2}{2})</math> describes a semicubical parabola. From <math>\vec c'(t) = (t^2, t)</math> one gets <math>|\vec c'(t)| = t\sqrt{t^2 + 1}</math> and <math>\int_0^t w\sqrt{w^2 + 1}\,dw = \frac{1}{3}\sqrt{t^2 + 1}^3 - \frac13</math>. Extending the string by <math>l_0={1\over3}</math> extensively simplifies further calculation, and one gets

<math>\begin{align} X(t)&= -\frac{t}{3}\\

Y(t) &= \frac{t^2}{6} - \frac{1}{3}.\end{align}</math>

Eliminating Template:Mvar yields <math>Y = \frac{3}{2}X^2 - \frac{1}{3},</math> 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 Template:Slink).

File:Involute.gif

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

Involutes of a catenary

For the catenary <math>(t, \cosh t)</math>, the tangent vector is <math>\vec c'(t) = (1, \sinh t)</math>, and, as <math> 1 + \sinh^2 t =\cosh^2 t,</math> its length is <math>|\vec c'(t)| = \cosh t</math>. Thus the arc length from the point Template:Math is <math>\textstyle\int_0^t \cosh w\,dw = \sinh t.</math>

Hence the involute starting from Template:Math is parametrized by

<math>(t - \tanh t, 1/\cosh t),</math>

and is thus a tractrix.

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

Involutes of a cycloid

File:Evolvente-zy.svg

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

The parametric representation <math>\vec c(t) = (t - \sin t, 1 - \cos t)</math> describes a cycloid. From <math>\vec c'(t) = (1 - \cos t, \sin t)</math>, one gets (after having used some trigonometric formulas)

<math>|\vec c'(t)| = 2\sin\frac{t}{2},</math>

and

<math>\int_\pi^t 2\sin\frac{w}{2}\,dw = -4\cos\frac{t}{2}.</math>

Hence the equations of the corresponding involute are

<math>X(t) = t + \sin t,</math>

<math>Y(t) = 3 + \cos t,</math>

which describe the shifted red cycloid of the diagram. Hence

• The involutes of the cycloid <math>(t - \sin t, 1 - \cos t)</math> are parallel curves of the cycloid

<math>(t + \sin t, 3 + \cos t).</math>

(Parallel curves of a cycloid are not cycloids.)

Involute and evolute

The evolute of a given curve <math>c_0</math> consists of the curvature centers of <math>c_0</math>. Between involutes and evolutes the following statement holds: <ref>K. Burg, H. Haf, F. Wille, A. Meister: Vektoranalysis: Höhere Mathematik für Ingenieure, Naturwissenschaftler und ..., Springer-Verlag, 2012,Template:ISBN, S. 30.</ref><ref>R. Courant:Vorlesungen über Differential- und Integralrechnung, 1. Band, Springer-Verlag, 1955, S. 267.</ref>

A curve is the evolute of any of its involutes.

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Application

The most common profiles of modern gear teeth are involutes of a circle. In an involute gear system, the teeth of two meshing gears contact at a single instantaneous point that follows along a single straight line of action. The forces the contacting teeth exert on each other also follow this line and are normal to the teeth. The involute gear system maintaining these conditions follows the fundamental law of gearing: the ratio of angular velocities between the two gears must remain constant throughout.

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 planar gear systems are either involute or the related cycloidal gear system.<ref>V. G. A. Goss (2013) "Application of analytical geometry to the shape of gear teeth", Resonance 18(9): 817 to 31 Springerlink (subscription required).</ref>

File:Two moving spirals scroll pump.gif

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 also

• Envelope (mathematics)
• Evolute
• Goat grazing problem
• Involute gear
• Roulette (curve)
• Scroll compressor

External links

• Involute at MathWorld