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SPHERICAL CYCLOID
Curve studied by Jean Bernoulli in 1732, and later by Hachette in 1811, and by Reuleaux. |
Cartesian parametrization: .
Spherical curve, algebraic iff q is rational (degree = 2(numerator + denominator of q)). |
A spherical cycloid is the locus of a point on
a circle rolling without slipping on a fixed circle, the angle between
the two circles remaining constant equal to ;
here, a is the radius of the fixed circle,
that of the moving circle, and xOy the plane where lies the fixed
circle.
When =
0, we get the
hypocycloid,
and when = ,
the epicycloid;
apart from these two cases, the cycloid is traced on the sphere corresponding
to both the base and the rolling circles, hence its name of spherical
cycloid. The center W of this sphere is the
point on Oz at height
and its radius is .
Therefore, the spherical cycloid is a roulette of the motion of a sphere over a sphere. |
Therefore, except in degenerate cases, the spherical cycloid is also the locus of a point on a cone of revolution that is rolling without slipping on a cone of revolution with the same vertex: these two cones are the cones with vertex W that contain the fixed circle and the moving circle, respectively. |
If q is rational, then the spherical cycloids
are composed of a number of isometric arches equal to the numerator of
q.
If q is irrational, then they are composed of an infinite number
of isometric arches.
The arches meet at cuspidal points, obtained for , located on the rolling circle, at altitude 0, and their vertices are located on the circle at altitude , with radius . When
and , i.e. ,
the radius of the circle passing through the vertices is less than or equal
to the radius of the base circle. In this case, the curve is referred to
as a spherical hypocycloid.
In the intermediate case , these two circles have the same radius and the curve is a spherical helix. In this case, the vertices are also cusps, the center of the moving circle is W (it is therefore a great circle of the sphere) and rolls without slipping on the circle passing through the vertices, and the above rolling cone degenerates into a plane. If we change q into q/(q + 1) and into , then the curves look the same and the circles passing through the vertices have the same altitude, but are not equal: contrary to the case of plane epi- and hypocycloid, there is no double generation.
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In this animation, the circles passing by the vertices of the two cycloids have the same radius, but not the same altitude.
Special case q
= 1 (base and rolling circles with equal radii) :
Cartesian parametrization: |
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The curve is the intersection between the sphere
and the cone of revolution tangent at its vertex to the sphere ;
therefore, it is a spherical
biquadratic.
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The superb models below show the generation of spherical cycloids by the rolling motion of a cone on another cone.
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Each point of the wheels of these bikes describes a spherical epicycloid. |
The relative movements of the conical gears describe spherical epicycloids. |
See the generalization to spherical
trochoids.
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© Robert FERRÉOL, Alain ESCULIER 2018