Involute

Let ${\displaystyle {\vec {c}}(t),\;t\in [t_{1},t_{2}]}$ be a regular curve in the plane with its curvature nowhere 0 and ${\displaystyle a\in (t_{1},t_{2})}$, then the curve with the parametric representation

${\displaystyle {\vec {C}}_{a}(t)={\vec {c}}(t)-{\frac {{\vec {c}}'(t)}{|{\vec {c}}'(t)|}}\;\int _{a}^{t}|{\vec {c}}'(w)|\;dw}$

is an involute of the given curve.

Proof The string acts as a tangent to the curve ${\displaystyle {\vec {c}}(t)}$. 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 ${\displaystyle [a,t]}$ is given by ${\displaystyle \int _{a}^{t}|{\vec {c}}'(w)|\;dw}$ where ${\displaystyle a}$ 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 ${\displaystyle {\frac {{\vec {c}}'(t)}{|{\vec {c}}'(t)|}}\;\int _{a}^{t}|{\vec {c}}'(w)|\;dw}$ The vector corresponding to the end point of the string (${\displaystyle {\vec {C}}_{a}(t)}$) can be easily calculated using vector addition, and one gets ${\displaystyle {\vec {C}}_{a}(t)={\vec {c}}(t)-{\frac {{\vec {c}}'(t)}{|{\vec {c}}'(t)|}}\;\int _{a}^{t}|{\vec {c}}'(w)|\;dw}$

Adding an arbitrary but fixed number ${\displaystyle l_{0}}$ to the integral ${\displaystyle {\Bigl (}\int _{a}^{t}|{\vec {c}}'(w)|\;dw{\Bigr )}}$ results in an involute corresponding to a string extended by ${\displaystyle l_{0}}$ (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 ${\displaystyle a}$ and/or adding a number to the integral (see Involutes of a semicubic parabola).

If ${\displaystyle {\vec {c}}(t)=(x(t),y(t))^{T}}$ one gets

${\displaystyle {\begin{aligned}X(t)&=x(t)-{\frac {x'(t)}{\sqrt {x'(t)^{2}+y'(t)^{2}}}}\int _{a}^{t}{\sqrt {x'(w)^{2}+y'(w)^{2}}}\,dw\\Y(t)&=y(t)-{\frac {y'(t)}{\sqrt {x'(t)^{2}+y'(t)^{2}}}}\int _{a}^{t}{\sqrt {x'(w)^{2}+y'(w)^{2}}}\,dw\;.\end{aligned}}}$

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

${\displaystyle {\vec {C}}_{a}(s)={\vec {c}}(s)-{\vec {c}}'(s)(s-a)\ }$ and

${\displaystyle {\vec {C}}_{a}'(s)=-{\vec {c}}''(s)(s-a)=-\kappa (s){\vec {n}}(s)(s-a)\;}$

and the statement:

• At point ${\displaystyle {\vec {C}}_{a}(a)}$ the involute is not regular (because ${\displaystyle |{\vec {C}}_{a}'(a)|=0}$ ),

and from ${\displaystyle \;{\vec {C}}_{a}'(s)\cdot {\vec {c}}'(s)=0\;}$ follows:

• The normal of the involute at point ${\displaystyle {\vec {C}}_{a}(s)}$ is the tangent of the given curve at point ${\displaystyle {\vec {c}}(s)}$.
• The involutes are parallel curves, because of ${\displaystyle {\vec {C}}_{a}(s)={\vec {C}}_{0}(s)+a{\vec {c}}'(s)}$ and the fact, that ${\displaystyle {\vec {c}}'(s)}$ is the unit normal at ${\displaystyle {\vec {C}}_{0}(s)}$.

The evolute of a given curve ${\displaystyle c_{0}}$ consists of the curvature centers of ${\displaystyle c_{0}}$. 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.

• Evolute
• Scroll compressor
• Involute gear
• Roulette (curve)

• Involute at MathWorld