# Involute

Involute
Field: Geometry
Image Created By: Xah Lee
Website: Involute-Visual Dictionary of Special Plane Curves

Involute

A colorful illustration of different involutes of a circle obtained by rolling a line around the circle.

# Basic Description

An involute (also known as evolvent) of a curve is the of a selected point on a line that rolls (as a tangent) along the curve.

Alternatively, imagine you have a string attached to a point on a fixed curve. Then, tautly wind the string onto (or unwind from) the curve. The trace of a point on the string gives an involute of the original curve, and the original curve is called the evolute of its involute.

The involute of a circle was first discussed by Huygens in 1693 when he was considering clocks without pendulums for service on ships.[1]

## To Draw an Involute

The involute of a given curve can be approximately drawn following the instructions below:

• Draw a number of tangent lines to the given curve.
• Pick a pair of neighboring tangent lines. With the center at their intersection, draw an arc bounded by the two tangent lines, passing through one of the points of contact between the tangent line and the curve.
• Pick the next pair of neighboring tangents. Set their intersection as the center. Then draw an arc bounded by the two tangents, using a radius that will make the arcs join. Repeat this step for all pairs of neighboring tangents.

 The image on the right is an illustration of the construction procedure above. The black curve that point A, B, C, D, E, F, G lie on is the original curve we have. First, draw the tangent line at each of the seven points respectively. We pick the tangents at point A and B as the first pair of neighbors. They intersect at point X. Then, draw an arc that passes point A, centers at point X and is bounded by the two tangents. We now have $\overset{\frown}{AA_1}$. Pick the next pair of neighboring tangents, namely the tangent lines going through point B and C. Their intersection is point Y. Set Y as the center, draw $\overset{\frown}{A_1A_2}$ that A2 lies on the tangent of point C. Repeat the steps above for the remaining pairs of tangents. Then we get the red curve, which is an involute of the black curve. An illustration of how to draw an involute approximately.

This method does not produce the accurate involute curve because instead of using the the length of arc of the original curve, it uses the sum of the segments of the tangents. However, this error gets smaller as we construct more tangent lines that they are near enough together.

# A More Mathematical Explanation

In this section, we provide various examples of involutes and their equations. We will also introduce [...]

In this section, we provide various examples of involutes and their equations. We will also introduce some properties of involute curves.

## Examples of Involutes

 The involute of a circle resembles an Archimedean spiral. The parametric equation for the involute of a circle with radius a is: $x = a(\cos t + t \cdot \sin t)$ $y = a(\sin t + t \cdot \cos t)$ As shown by the animation, the length of the line segment is equal to the distance traveled by the point on the circle. This is easier to understand if we imagine we are unwinding a string. The line segment we see in the animation is the part of the string that has been unwound. It should have a length that is equal to the distance traveled by the point of contact between the string and the circle. This property holds true for all other types of curves as well. The involute of a catenary through its vertex is a tractrix. For example, given a catenary $x = t$ $y = \cosh t$, the parametric equation for the involute curve is: $x = t - \tanh t$ $y = \frac{1}{\cosh t}$ The involute of an astroid is another astroid that is half of its original size and rotated 1/8 of a turn. For example, if the parametric equation for the astroid is $x = \cos ^3 t$ $y = \sin ^3 t$ The involute of the astorid is $x = -\frac{1}{4}\cos t (-2 + \cos 2t )$ $y = \frac{1}{4} (2 + \cos 2t ) \sin t$ The involute of a cycloid is a shifted copy of the original cycloid. If the cycloid is $x = t - \sin t$ $y = 1 - \cos t$ its involute curve is $x = t + \sin t$ $y = 3 + \cos t$ The involute of a parabola looks like images on the left. For example, if the the parabola is $x = t$ $y = t^2$ its involute curve is $x = \frac{1}{4} (2t - \frac{\operatorname{arcsinh}\ 2t}{\sqrt{1 + 4t^2}})$ $y = -\frac{t \operatorname{arcsinh}\ 2t}{2\sqrt{1+4t^2}}$ On the other hand, if the the parabola is $x = t^2$ $y = t$ its involute curve is $x = -\frac{t \operatorname{arcsinh}\ 2t}{2\sqrt{1+4t^2}}$ $y = \frac{1}{4} (2t - \frac{\operatorname{arcsinh}\ 2t}{\sqrt{1 + 4t^2}})$ The involute of a cardioid is a mirrored, but bigger cardioid. For example, the cardioid is given as $x = (1 + \cos t) \cos t$ $y = (1 + \cos t) \sin t$ Its involute is $x = \frac{1}{2} (1 + 6\cos t - 3\cos 2t)$ $y = 6 \sin \left (\frac{t}{2} \right )^2 \sin t$

For more examples of involutes, you can visit WolframMathWorld -- Involutes and Evolutes

## Properties of Involutes

• The normal of the involute is tangent to the original curve.
From the procedure of drawing an involute, we know that the involute curve is joined by arcs that are bounded by tangent lines of the original curve and centered at their intersections. Therefore, the pairs of neighboring tangent lines can be considered the radii of the circle which the arc between them belongs to. If we draw the involute curve accurately, we can know for sure that at each point on the involute, its normal is tangent to the original curve.
Alternatively, we can imagine we are unwinding a string from a fixed curve. Because the string is held taut, the string is perpendicular to the path of the ball attached to its end. At any time, the string is tangent to the fixed curve. Therefore, we can conclude that the normal of the involute is tangent to the original curve.
Therefore, we can think of involute as a curve orthogonal to all the tangents to a given curve.
• A parallel of an involute is also an involute and any two involutes are parallel.
Every curve has many involutes because the initial point, where the involute intersects the original curve, can be chosen arbitrarily. The various involute curves obtained by choosing different initial points are parallel to each other. In other words, any two of them are a constant distance apart at all points (this distance can be measured along their common normal).
You might realize that the starting point for the line to roll along the original curve is not unique. This may result in involute curves to intersect each other (as found in the main image). However, this does not contradict the parallel property. For example, we have a circle and unwind two strings from the circle. The string at attached to the circle at point A and B respectively. After some unwinding, the line that is attached at point A will pass point B, and starting at this point, the involute curves would be parallel again, because the case becomes a long string and a short string that unwind simultaneously from the circle, and both of them are connected to the circle at point B.

## General Formula for Involutes

 Since we can think of the involute as the path of the end of a string that is unwounded from a fixed curve, the length of string that has been unwound equals the distance traveled by the contact point between the string and the original curve. Therefore, if the point on the original curve is represented as (f(t ), g(t )), the corresponding point on the involute is: $\vec r_{inv} = \vec r_{cur} - s\vec T$ where $\vec r_{cur}$ represents a point on the original curve (the contact point between the string and the curve), $\vec T$ is the unit tangent vector to the original curve at this point (the current direction of the string), s is the distance traveled by this point so far (how much of the string has been unwound), and $\vec r_{inv}$ is the corresponding point on the involute curve (the position of the end of the string). The image on the right explains these variables visually. The distance traveled by this point is: $s = \int\,ds = \int \frac{ds}{dt}\,dt = \int\sqrt{f'^2+g'^2}\,dt$ The unit tangent vector is: $\vec T = \frac{\frac{d\vec r_{cur}}{dt}}{\left\vert\frac{d\vec r_{cur}}{dt}\right\vert}$ If we write the equation parametrically, we will get $x = f - \frac{sf'}{\sqrt{f'^2+g'^2}}$ $y = g - \frac{sg'}{\sqrt{f'^2+g'^2}}$

# Why It's Interesting

 One of the most commonly used gearing system today is the involute gear. The image on the right is an example of such a gear. In an involute gear system, it is desired that the two wheels should revolve as if the two pitch-circles were rolling against each other. This effect can be achieved if the teeth profiles are drawn as involutes of the base-circles and the tops of the teeth are arcs of circles that are concentric with, and bigger than the pitch circles. To see why involute gear can revolve as if the pitch-circles are rolling against each other, consider two points, Q′ and R′, which will rotate to points Q and R in the same time interval. If Q′Y and R′Z are the tangent lines to the base-circles at Q′ and R′, respectively, then $Q'Y + ZR' = QP + PR$ Due to how involute is constructed (unwinding the string), we know that $Q'Y = \overset{\frown}{Q'Q} + QP$ and $ZR' = PR - \overset{\frown}{R'R}$ Therefore, $\overset{\frown}{Q'Q} = \overset{\frown}{R'R}$ Because Q′ and R′ move the same distance in the same time interval, we can conclude that Q and R move with equal velocities. Therefore, points on the pitch-circles will also move with same velocities. In the animation to the right, we can see that each pair of gear teeth has an instant contact point. This point is called the pitch-point, and it moves along one single line as the gears rotate. This line is called the line of action and it is the common internal tangent of the two circles. In other words, the involutes of the two circles are always tangent to each other at a point on their common internal tangent. Because of this design, a constant velocity ratio is transmitted and the fundamental law of gearing[2] is satisfied. Involute gear has the advantage that it is easy to manufacture since all the teeth are uniform. Also, the velocity ratio is not affected by changing distance between centers. Lastly, the pressure on the axes is constant.

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# References

1. Yates, Robert C.(1952). A Handbook on Curves and Their Properties. Edwards Brothers, Inc.
2. Wikiversity (Gears). (n.d.). Gears. Retrieved from http://en.wikiversity.org/wiki/Gears