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Involute
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<h2 id="involute-of-a-parameterized-curve">Involute of a parameterized curve</h2> <p class="note">See also: <a href="/facts/Arc_length/QgTrmLxY">Arc length</a></p> <p>Let c → ( t ) , t ∈ [ t 1 , t 2 ] {\displaystyle {\vec {c}}(t),\;t\in [t_{1},t_{2}]} be a <a href="/facts/Regular_curve/xBQ0pTBW">regular curve</a> in the plane with its <a href="/facts/Curvature_(mathematics)/RFCAEsj8">curvature</a> nowhere 0 and a ∈ ( t 1 , t 2 ) {\displaystyle a\in (t_{1},t_{2})} , then the curve with the parametric representation </p><p> C → a ( t ) = c → ( t ) − c → ′ ( t ) | c → ′ ( t ) | ∫ a t | c → ′ ( w ) | d w {\displaystyle {\vec {C}}_{a}(t)={\vec {c}}(t)-{\frac {{\vec {c}}'(t)}{|{\vec {c}}'(t)|}}\;\int _{a}^{t}|{\vec {c}}'(w)|\;dw} </p><p>is an <i>involute</i> of the given curve. </p> <table><tbody><tr><td></td></tr></tbody></table> <p>Adding an arbitrary but fixed number l 0 {\displaystyle l_{0}} to the integral ( ∫ a t | c → ′ ( w ) | d w ) {\displaystyle {\Bigl (}\int _{a}^{t}|{\vec {c}}'(w)|\;dw{\Bigr )}} results in an involute corresponding to a string extended by l 0 {\displaystyle l_{0}} (like a ball of wool <a href="/facts/Yarn/fhMTsWg2">yarn</a> having some length of thread already hanging before it is unwound). Hence, the involute can be varied by constant a {\displaystyle a} and/or adding a number to the integral (see Involutes of a semicubic parabola). </p><p>If c → ( t ) = ( x ( t ) , y ( t ) ) T {\displaystyle {\vec {c}}(t)=(x(t),y(t))^{T}} one gets </p> X ( t ) = x ( t ) − x ′ ( t ) x ′ ( t ) 2 + y ′ ( t ) 2 ∫ a t x ′ ( w ) 2 + y ′ ( w ) 2 d w Y ( t ) = y ( t ) − y ′ ( t ) x ′ ( t ) 2 + y ′ ( t ) 2 ∫ a t x ′ ( w ) 2 + y ′ ( w ) 2 d w . {\displaystyle {\begin{aligned}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{aligned}}} <h2 id="properties-of-involutes">Properties of involutes</h2> <p>In order to derive properties of a regular curve it is advantageous to suppose the <a href="/facts/Arc_length/QgTrmLxY">arc length</a> s {\displaystyle s} to be the parameter of the given curve, which lead to the following simplifications: | c → ′ ( s ) | = 1 {\displaystyle \;|{\vec {c}}'(s)|=1\;} and c → ″ ( s ) = κ ( s ) n → ( s ) {\displaystyle \;{\vec {c}}''(s)=\kappa (s){\vec {n}}(s)\;} , with κ {\displaystyle \kappa } the <a href="/facts/Curvature/RFCAEsj8">curvature</a> and n → {\displaystyle {\vec {n}}} the unit normal. One gets for the involute: </p> C → a ( s ) = c → ( s ) − c → ′ ( s ) ( s − a ) {\displaystyle {\vec {C}}_{a}(s)={\vec {c}}(s)-{\vec {c}}'(s)(s-a)\ } and C → a ′ ( s ) = − c → ″ ( s ) ( s − a ) = − κ ( s ) n → ( s ) ( s − a ) {\displaystyle {\vec {C}}_{a}'(s)=-{\vec {c}}''(s)(s-a)=-\kappa (s){\vec {n}}(s)(s-a)\;} <p>and the statement: </p> <ul><li>At point C → a ( a ) {\displaystyle {\vec {C}}_{a}(a)} the involute is <i>not regular</i> (because | C → a ′ ( a ) | = 0 {\displaystyle |{\vec {C}}_{a}'(a)|=0} ),</li></ul> <p>and from C → a ′ ( s ) ⋅ c → ′ ( s ) = 0 {\displaystyle \;{\vec {C}}_{a}'(s)\cdot {\vec {c}}'(s)=0\;} follows: </p> <ul><li>The normal of the involute at point C → a ( s ) {\displaystyle {\vec {C}}_{a}(s)} is the tangent of the given curve at point c → ( s ) {\displaystyle {\vec {c}}(s)} .</li> <li>The involutes are <a href="/facts/Parallel_curve/37v4lMc6">parallel curves</a>, because of C → a ( s ) = C → 0 ( s ) + a c → ′ ( s ) {\displaystyle {\vec {C}}_{a}(s)={\vec {C}}_{0}(s)+a{\vec {c}}'(s)} and the fact, that c → ′ ( s ) {\displaystyle {\vec {c}}'(s)} is the unit normal at C → 0 ( s ) {\displaystyle {\vec {C}}_{0}(s)} .</li></ul> <p>The family of involutes and the family of tangents to the original curve makes up an <a href="/facts/Orthogonal_coordinates/LLoa2lAJ">orthogonal coordinate system</a>. 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. </p> <h3>Cusps</h3> <p>This section is based on.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> </p><p>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. </p><p>This can be visually seen by constructing a map f : R 2 → R 3 {\displaystyle f:\mathbb {R} ^{2}\to \mathbb {R} ^{3}} defined by ( s , t ) ↦ ( x ( s ) + t cos ( θ ) , y ( s ) + t sin ( θ ) , t ) {\displaystyle (s,t)\mapsto (x(s)+t\cos(\theta ),y(s)+t\sin(\theta ),t)} where ( x ( s ) , y ( s ) ) {\displaystyle (x(s),y(s))} is the arclength parametrization of the curve, and θ {\displaystyle \theta } is the slope-angle of the curve at the point ( x ( s ) , y ( s ) ) {\displaystyle (x(s),y(s))} . This maps the 2D plane into a surface in 3D space. For example, this maps the circle into the <a href="/facts/Hyperboloid_of_one_sheet/K5mDV2Nw">hyperboloid of one sheet</a>. </p><p>By this map, the involutes are obtained in a three-step process: map R {\displaystyle \mathbb {R} } to R 2 {\displaystyle \mathbb {R} ^{2}} , then to the surface in R 3 {\displaystyle \mathbb {R} ^{3}} , then project it down to R 2 {\displaystyle \mathbb {R} ^{2}} by removing the z-axis: s ↦ ( s , l − s ) ↦ f ( s , l − s ) ↦ ( f ( s , l − s ) x , f ( s , l − s ) y ) {\displaystyle s\mapsto (s,l-s)\mapsto f(s,l-s)\mapsto (f(s,l-s)_{x},f(s,l-s)_{y})} where l {\displaystyle l} is any real constant. </p><p>Since the mapping s ↦ f ( s , l − s ) {\displaystyle s\mapsto f(s,l-s)} has nonzero derivative at all s ∈ R {\displaystyle s\in \mathbb {R} } , cusps of the involute can only occur where the derivative of s ↦ f ( s , l − s ) {\displaystyle s\mapsto f(s,l-s)} is vertical (parallel to the z-axis), which can only occur where the surface in R 3 {\displaystyle \mathbb {R} ^{3}} has a vertical tangent plane. </p><p>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. </p> <h4>cusp of order 3/2</h4> <p>For the first type, one can start by the involute of a circle, with equation X ( t ) = r ( cos t + ( t − a ) sin t ) Y ( t ) = r ( sin t − ( t − a ) cos t ) {\displaystyle {\begin{aligned}X(t)&=r(\cos t+(t-a)\sin t)\\Y(t)&=r(\sin t-(t-a)\cos t)\end{aligned}}} then set a = 0 {\displaystyle a=0} , and expand for small t {\displaystyle t} , to obtain X ( t ) = r + r t 2 / 2 + O ( t 4 ) Y ( t ) = r t 3 / 3 + O ( t 5 ) {\displaystyle {\begin{aligned}X(t)&=r+rt^{2}/2+O(t^{4})\\Y(t)&=rt^{3}/3+O(t^{5})\end{aligned}}} thus giving the order 3/2 curve Y 2 − 8 9 r ( X − r ) 3 + O ( Y 8 / 3 ) = 0 {\displaystyle Y^{2}-{\frac {8}{9r}}(X-r)^{3}+O(Y^{8/3})=0} , a <a href="/facts/Semicubical_parabola/sR3ITc2B">semicubical parabola</a>. </p> <h4>cusp of order 5/2</h4> <p>For the second type, consider the curve y = x 3 {\displaystyle y=x^{3}} . The arc from x = 0 {\displaystyle x=0} to x = s {\displaystyle x=s} is of length ∫ 0 s 1 + ( 3 t 2 ) 2 d t = s + 9 10 s 5 − 9 8 s 9 + O ( s 13 ) {\displaystyle \int _{0}^{s}{\sqrt {1+(3t^{2})^{2}}}dt=s+{\frac {9}{10}}s^{5}-{\frac {9}{8}}s^{9}+O(s^{13})} , and the tangent at x = s {\displaystyle x=s} has angle θ = arctan ( 3 s 2 ) {\displaystyle \theta =\arctan(3s^{2})} . Thus, the involute starting from x = 0 {\displaystyle x=0} at distance L {\displaystyle L} has parametric formula { x ( s ) = s + ( L − s − 9 10 s 5 + ⋯ ) cos θ y ( s ) = s 3 + ( L − s − 9 10 s 5 + ⋯ ) sin θ {\displaystyle {\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 {\displaystyle s^{5}} , we obtain { x ( s ) = L − 9 2 L s 4 + ( 9 2 L − 9 10 ) s 5 + O ( s 6 ) y ( s ) = 3 L s 2 − 2 s 3 + O ( s 6 ) {\displaystyle {\begin{cases}x(s)=L-{\frac {9}{2}}Ls^{4}+({\frac {9}{2}}L-{\frac {9}{10}})s^{5}+O(s^{6})\\y(s)=3Ls^{2}-2s^{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 {\displaystyle x,y} : ( x − L + y 2 2 L ) 2 − ( 9 2 L + 51 10 ) 2 ( y 3 L ) 5 + O ( s 11 ) = 0 {\displaystyle \left(x-L+{\frac {y^{2}}{2L}}\right)^{2}-\left({\frac {9}{2}}L+{\frac {51}{10}}\right)^{2}\left({\frac {y}{3L}}\right)^{5}+O(s^{11})=0} or x = L − y 2 2 L ± ( 9 2 L + 51 10 ) ( y 3 L ) 2.5 + O ( y 2.75 ) , y ≥ 0 {\displaystyle x=L-{\frac {y^{2}}{2L}}\pm \left({\frac {9}{2}}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. </p><p>Setting L = 0 {\displaystyle L=0} , we obtain the involute passing the origin. It is special as it contains no cusp. By serial expansion, it has parametric equation { x ( s ) = 18 5 s 5 − 126 5 s 9 + O ( s 13 ) y ( s ) = − 2 s 3 + 54 5 s 7 − 318 5 s 11 + O ( s 15 ) {\displaystyle {\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 = − 18 5 ⋅ 2 1 / 3 y 5 / 3 + O ( y 3 ) {\displaystyle x=-{\frac {18}{5\cdot 2^{1/3}}}y^{5/3}+O(y^{3})} </p> <h2 id="examples">Examples</h2> <h3>Involutes of a circle</h3> <p>For a circle with parametric representation ( r cos ( t ) , r sin ( t ) ) {\displaystyle (r\cos(t),r\sin(t))} , one has c → ′ ( t ) = ( − r sin t , r cos t ) {\displaystyle {\vec {c}}'(t)=(-r\sin t,r\cos t)} . Hence | c → ′ ( t ) | = r {\displaystyle |{\vec {c}}'(t)|=r} , and the path length is r ( t − a ) {\displaystyle r(t-a)} . </p><p>Evaluating the above given equation of the involute, one gets </p> X ( t ) = r ( cos ( t + a ) + t sin ( t + a ) ) Y ( t ) = r ( sin ( t + a ) − t cos ( t + a ) ) {\displaystyle {\begin{aligned}X(t)&=r(\cos(t+a)+t\sin(t+a))\\Y(t)&=r(\sin(t+a)-t\cos(t+a))\end{aligned}}} <p>for the <a href="/facts/Parametric_equation/5Uvrlgqf">parametric equation</a> of the involute of the circle. </p><p>The a {\displaystyle 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 {\displaystyle a=-0.5} (green), a = 0 {\displaystyle a=0} (red), a = 0.5 {\displaystyle a=0.5} (purple) and a = 1 {\displaystyle a=1} (light blue). The involutes look like <a href="/facts/Archimedean_spiral/wCW7WuyI">Archimedean spirals</a>, but they are actually not. </p><p>The arc length for a = 0 {\displaystyle a=0} and 0 ≤ t ≤ t 2 {\displaystyle 0\leq t\leq t_{2}} of the involute is </p> L = r 2 t 2 2 . {\displaystyle L={\frac {r}{2}}t_{2}^{2}.} <h3>Involutes of a semicubic parabola</h3> <p>The <a href="/facts/Parametric_equation/5Uvrlgqf">parametric equation</a> c → ( t ) = ( t 3 3 , t 2 2 ) {\displaystyle {\vec {c}}(t)=({\tfrac {t^{3}}{3}},{\tfrac {t^{2}}{2}})} describes a <a href="/facts/Semicubical_parabola/sR3ITc2B">semicubical parabola</a>. From c → ′ ( t ) = ( t 2 , t ) {\displaystyle {\vec {c}}'(t)=(t^{2},t)} one gets | c → ′ ( t ) | = t t 2 + 1 {\displaystyle |{\vec {c}}'(t)|=t{\sqrt {t^{2}+1}}} and ∫ 0 t w w 2 + 1 d w = 1 3 t 2 + 1 3 − 1 3 {\displaystyle \int _{0}^{t}w{\sqrt {w^{2}+1}}\,dw={\frac {1}{3}}{\sqrt {t^{2}+1}}^{3}-{\frac {1}{3}}} . Extending the string by l 0 = 1 3 {\displaystyle l_{0}={1 \over 3}} extensively simplifies further calculation, and one gets </p> X ( t ) = − t 3 Y ( t ) = t 2 6 − 1 3 . {\displaystyle {\begin{aligned}X(t)&=-{\frac {t}{3}}\\Y(t)&={\frac {t^{2}}{6}}-{\frac {1}{3}}.\end{aligned}}} <p>Eliminating t yields Y = 3 2 X 2 − 1 3 , {\displaystyle Y={\frac {3}{2}}X^{2}-{\frac {1}{3}},} showing that this involute is a <a href="/facts/Parabola/Cweo3wgw">parabola</a>. </p><p>The other involutes are thus <a href="/facts/Parallel_curves/37v4lMc6">parallel curves</a> of a parabola, and are not parabolas, as they are curves of degree six (See <a href="/facts/Parallel_curve/37v4lMc6">Parallel curve § Further examples</a>). </p> <h3>Involutes of a catenary</h3> <p>For the <a href="/facts/Catenary/fWVU770E">catenary</a> ( t , cosh t ) {\displaystyle (t,\cosh t)} , the tangent vector is c → ′ ( t ) = ( 1 , sinh t ) {\displaystyle {\vec {c}}'(t)=(1,\sinh t)} , and, as 1 + sinh 2 t = cosh 2 t , {\displaystyle 1+\sinh ^{2}t=\cosh ^{2}t,} its length is | c → ′ ( t ) | = cosh t {\displaystyle |{\vec {c}}'(t)|=\cosh t} . Thus the arc length from the point (0, 1) is ∫ 0 t cosh w d w = sinh t . {\displaystyle \textstyle \int _{0}^{t}\cosh w\,dw=\sinh t.} </p><p>Hence the involute starting from (0, 1) is parametrized by </p> ( t − tanh t , 1 / cosh t ) , {\displaystyle (t-\tanh t,1/\cosh t),} <p>and is thus a <a href="/facts/Tractrix/n42tlXCJ">tractrix</a>. </p><p>The other involutes are not tractrices, as they are parallel curves of a tractrix. </p> <h3>Involutes of a cycloid</h3> <p>The parametric representation c → ( t ) = ( t − sin t , 1 − cos t ) {\displaystyle {\vec {c}}(t)=(t-\sin t,1-\cos t)} describes a <a href="/facts/Cycloid/8Qokqud5">cycloid</a>. From c → ′ ( t ) = ( 1 − cos t , sin t ) {\displaystyle {\vec {c}}'(t)=(1-\cos t,\sin t)} , one gets (after having used some trigonometric formulas) </p> | c → ′ ( t ) | = 2 sin t 2 , {\displaystyle |{\vec {c}}'(t)|=2\sin {\frac {t}{2}},} <p>and </p> ∫ π t 2 sin w 2 d w = − 4 cos t 2 . {\displaystyle \int _{\pi }^{t}2\sin {\frac {w}{2}}\,dw=-4\cos {\frac {t}{2}}.} <p>Hence the equations of the corresponding involute are </p> X ( t ) = t + sin t , {\displaystyle X(t)=t+\sin t,} Y ( t ) = 3 + cos t , {\displaystyle Y(t)=3+\cos t,} <p>which describe the shifted red cycloid of the diagram. Hence </p> <ul><li>The involutes of the cycloid ( t − sin t , 1 − cos t ) {\displaystyle (t-\sin t,1-\cos t)} are parallel curves of the cycloid</li></ul> ( t + sin t , 3 + cos t ) . {\displaystyle (t+\sin t,3+\cos t).} <p>(Parallel curves of a cycloid are not cycloids.) </p> <h2 id="involute-and-evolute">Involute and evolute</h2> <p>The <a href="/facts/Evolute/TmNQdwFY">evolute</a> of a given curve c 0 {\displaystyle c_{0}} consists of the curvature centers of c 0 {\displaystyle c_{0}} . Between involutes and evolutes the following statement holds: <a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a><a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> </p> <i>A curve is the evolute of any of its involutes.</i> Involute and evolute <h2 id="application">Application</h2> <p>The most common profiles of modern <a href="/facts/Gear/9KVHRfUR">gear</a> teeth are involutes of a circle. In an <a href="/facts/Involute_gear/3kawpyMv">involute gear</a> 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. </p><p>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 <a href="/facts/Cycloidal_gear/pPxK5hGA">cycloidal gear</a> system.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> </p> <p>The involute of a circle is also an important shape in <a href="/facts/Gas_compressor/I0vV0Tx2">gas compressing</a>, as a <a href="/facts/Scroll_compressor/87reCFut">scroll compressor</a> can be built based on this shape. Scroll compressors make less sound than conventional compressors and have proven to be quite <a href="/facts/Mechanical_efficiency/EqAKnryR">efficient</a>. </p><p>The <a href="/facts/High_Flux_Isotope_Reactor/BmAqsqgq">High Flux Isotope Reactor</a> uses involute-shaped fuel elements, since these allow a constant-width channel between them for coolant. </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Envelope_(mathematics)/jr5DkCGD">Envelope (mathematics)</a></li> <li><a href="/facts/Evolute/TmNQdwFY">Evolute</a></li> <li><a href="/facts/Goat_grazing_problem/ufIkvx1w">Goat grazing problem</a></li> <li><a href="/facts/Involute_gear/3kawpyMv">Involute gear</a></li> <li><a href="/facts/Roulette_(curve)/GDXynj3Y">Roulette (curve)</a></li> <li><a href="/facts/Scroll_compressor/87reCFut">Scroll compressor</a></li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="http://mathworld.wolfram.com/Involute.html">Involute</a> at <a href="/facts/MathWorld/yc1jodbq">MathWorld</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Rutter, J.W. (2000). Geometry of Curves. CRC Press. pp. 204. ISBN 9781584881667. <a href="9781584881667" target="_blank">9781584881667</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>McCleary, John (2013). Geometry from a Differentiable Viewpoint. Cambridge University Press. pp. 89. ISBN 9780521116077. <a href="9780521116077" target="_blank">9780521116077</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Arnolʹd, V. I. (1990). Huygens and Barrow, Newton and Hooke : pioneers in mathematical analysis and catastrophe theory from evolvents to quasicrystals. Basel: Birkhaüser Verlag. ISBN 0-8176-2383-3. OCLC 21873606. <a href="0-8176-2383-3" target="_blank">0-8176-2383-3</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>K. Burg, H. Haf, F. Wille, A. Meister: Vektoranalysis: Höhere Mathematik für Ingenieure, Naturwissenschaftler und ..., Springer-Verlag, 2012,ISBN 3834883468, S. 30. <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>R. Courant:Vorlesungen über Differential- und Integralrechnung, 1. Band, Springer-Verlag, 1955, S. 267. <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>V. G. A. Goss (2013) "Application of analytical geometry to the shape of gear teeth", Resonance 18(9): 817 to 31 Springerlink (subscription required). <a href="/wiki/Resonance_(journal)" target="_blank">/wiki/Resonance_(journal)</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> </ol>