Involute

An involute (also known as an evolvent) is a particular type of curve that is dependent on another shape or curve. An example of the 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. 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 into mathematics and physics 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.

An involute (also known as an evolvent) is a particular type of curve that is dependent on another shape or curve. An example of the 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. 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 into mathematics and physics 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. \vec C_a(t)=\vec c(t) -\frac{\vec c'(t)} \; \int_a^t|\vec c'(w)|\; dw is an involute of the given curve. Adding an arbitrary but fixed number l_0 to the integral \Bigl(\int_a^t|\vec c'(w)|\; dw\Bigr) results in an involute corresponding to a string extended by l_0 (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 a and/or adding a number to the integral (see Involutes of a semicubic parabola). If \vec c(t)=(x(t),y(t))^T one gets \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} In order to derive properties of a regular curve it is advantageous to suppose the arc length s to be the parameter of the given curve, which lead to the following simplifications: \;|\vec c'(s)|=1\; and \;\vec c(s)=\kappa(s)\vec n(s)\; , with \kappa the curvature and \vec n the unit normal. One gets for the involute: \vec C_a(s)=\vec c(s) -\vec c'(s)(s-a)\ and \vec C_a'(s)=-\vec c(s)(s-a)=-\kappa(s)\vec n(s)(s-a)\; and the statement: At point \vec C_a(a) the involute is not regular (because | \vec C_a'(a)|=0 ), and from \; \vec C_a'(s)\cdot\vec c'(s)=0 \; follows: The normal of the involute at point \vec C_a(s) is the tangent of the given curve at point \vec c(s) . The involutes are parallel curves, because of \vec C_a(s)=\vec C_0(s)+a\vec c'(s) and the fact, that \vec c'(s) is the unit normal at \vec C_0(s) . 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. This can be visually seen by constructing a map f: \R^2 \to \R^3 defined by (s, t) \mapsto (x(s) + t\cos(\theta), y(s) + t\sin(\theta), t) where (x(s), y(s)) is the arclength parametrization of the curve, and \theta is the slope-angle of the curve at the point (x(s), y(s)) . 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 \R to \R^2 , then to the surface in \R^3 , then project it down to \R^2 by removing the z-axis: s \mapsto (s, l- s) \mapsto f(s, l- s) \mapsto (f(s, l- s)_x, f(s, l- s)_y) where l is any real constant. Since the mapping s \mapsto f(s, l-s) has nonzero derivative at all s\in \R , cusps of the involute can only occur where the derivative of s \mapsto f(s, l-s) is vertical (parallel to the z-axis), which can only occur where the surface in \R^3 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. For the first type, one can start by the involute of a circle, with equation \begin{align} X(t) &= r(\cos t + (t - a)\sin t)\\ Y(t) &= r(\sin t - (t - a)\cos t) \end{align} then set a = 0 , and expand for small t , to obtain \begin{align} X(t) &= r + r t^2/2 + O(t^4)\\ Y(t) &= rt^3/3 + O(t^5) \end{align} thus giving the order 3/2 curve Y^2 - \frac{8}{9r} (X-r)^{3} + O(Y^{8/3}) = 0 , a semicubical parabola. For the second type, consider the curve y = x^3 . The arc from x= 0 to x = s is of length \int_0^s \sqrt{1 + (3t^2)^2}dt = s + \frac{9}{10} s^5 - \frac 98 s^9 + O(s^{13}) , and the tangent at x = s has angle \theta = \arctan(3s^2) . Thus, the involute starting from x= 0 at distance L has parametric formula \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} Expand it up to order s^5 , we obtain \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} which is a cusp of order 5/2. Explicitly, one may solve for the polynomial expansion satisfied by x, y : \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 or 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 which clearly shows the cusp shape. Setting L=0 , we obtain the involute passing the origin. It is special as it contains no cusp. By serial expansion, it has parametric equation \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} or x = -\frac{18}{5 \cdot 2^{1/3}}y^{5/3} + O(y^3) === Involutes of a circle === For a circle with parametric representation (r\cos(t), r\sin(t)) , one has \vec c'(t) = (-r\sin t, r\cos t) . Hence |\vec c'(t)| = r , and the path length is r(t - a) . Evaluating the above given equation of the involute, one gets \begin{align} X(t) &= r(\cos (t+a) + t\sin (t+a))\\ Y(t) &= r(\sin (t+a) - t\cos (t+a)) \end{align} for the parametric equation of the involute of the circle. The a term is optional; it serves to set the start location of the curve on the circle. The figure shows involutes for a = -0.5 (green), a = 0 (red), a = 0.5 (purple) and a = 1 (light blue). The involutes look like Archimedean spirals, but they are actually not. The arc length for a=0 and 0 \le t \le t_2 of the involute is L = \frac{r}{2} t_2^2. The parametric equation \vec c(t) = (\tfrac{t^3}{3}, \tfrac{t^2}{2}) describes a semicubical parabola. From \vec c'(t) = (t^2, t) one gets |\vec c'(t)| = t\sqrt{t^2 + 1} and \int_0^t w\sqrt{w^2 + 1}\,dw = \frac{1}{3}\sqrt{t^2 + 1}^3 - \frac13 . Extending the string by l_0={1\over3} extensively simplifies further calculation, and one gets \begin{align} X(t)&= -\frac{t}{3}\\ Y(t) &= \frac{t^2}{6} - \frac{1}{3}.\end{align} Eliminating yields Y = \frac{3}{2}X^2 - \frac{1}{3}, 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 ). For the catenary (t, \cosh t) , the tangent vector is \vec c'(t) = (1, \sinh t) , and, as 1 + \sinh^2 t =\cosh^2 t, its length is |\vec c'(t)| = \cosh t . Thus the arc length from the point is \textstyle\int_0^t \cosh w\,dw = \sinh t. Hence the involute starting from is parametrized by (t - \tanh t, 1/\cosh t), and is thus a tractrix. The other involutes are not tractrices, as they are parallel curves of a tractrix. The parametric representation \vec c(t) = (t - \sin t, 1 - \cos t) describes a cycloid. From \vec c'(t) = (1 - \cos t, \sin t) , one gets (after having used some trigonometric formulas) |\vec c'(t)| = 2\sin\frac{t}{2}, and \int_\pi^t 2\sin\frac{w}{2}\,dw = -4\cos\frac{t}{2}. Hence the equations of the corresponding involute are X(t) = t + \sin t, Y(t) = 3 + \cos t, which describe the shifted red cycloid of the diagram. Hence The involutes of the cycloid (t - \sin t, 1 - \cos t) are parallel curves of the cycloid (t + \sin t, 3 + \cos t). (Parallel curves of a cycloid are not cycloids.) ==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. 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 ==References== ==External links== Involute at MathWorld

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