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Re: Cylinder cylinder intersection: Design of a starter crank.

Hi Matthias,

if I understand correctly, you want to calculate the intersection line 

of 2 cylindrical surfaces. This can be done as follows.

Assume we have a vertical cylinder along the z axis of radius: 1. The 

second cylinder is horizontal along the y axis of radius 1/2. This gives 

the equations:

r1 = 1; r2 = 1/2;

eq = {

    x^2 + y^2 == r1,

    y^2 + z^2 == r2


We may solve this equations for x using e.g. Reduce:

res = Reduce[eq, {x, y, z}]

this gives:

(y == -Sqrt[1 - x^2] || y == Sqrt[1 - x^2]) &&

  (z == -(Sqrt[-1 + 2*x^2]/Sqrt[2]) || z == Sqrt[-1 + 2*x^2]/Sqrt[2])

You see, there are 2 possibilities for y and two for z, giving a total 

of 4 functions. We may Plot this functions using ParametricPlot. For 

convenience, we change the result res from equations to rules using: 


ParametricPlot3D[{x, y, z} /. {ToRules[res]}, {x, 0, 1},

  AspectRatio -> Automatic, AxesLabel -> {"x", "y", "z"}]

The four curves join smoothly as expected.

For wall thickness you would evaluate the curves at specific points. 

E.g. the maximal z value is optained from x=1 and z=Sqrt[-1 + 

2*x^2]/Sqrt[2]. The min. x value from: z==0 and z=Sqrt[-1 + 2*x^2]/Sqrt[2].


Matthias Bode wrote:

> Dear Groupmembers:


> I have made futile attempts at designing a "simple" starter crank to start an internal combustion engine.


> The starter crank has a short transversal rod that pierces the crank perpendicularly and engages in the starter ratchet mounted on the engine's cranks haft. The crank's longitudinal axis intersects the rod's longitudinal axis.


> The crank's lever measures 320 mm in length, the transversal rod measures 40 mm in length which is equal to ratchet's outer diameter.


> The diameter of the crank must not exceed 18 mm (min. 16 mm), the diameter of the transversal rod must not exceed 9 mm (min. 6 mm). - (A thinner crank disengages more easily from the ratchet when the engine starts.)


> I estimate the maximum force to be exerted on the crank's handle (just before the engine's crankshaft will start to rotate) to be ~ 700 N.


> I studied  to find out how much material of the crank would be left given the bore for the transversal rod.


> I got completely stuck at equation (12) - which I originally intended to solve using MATHEMATICA.


> Given the tensile strengths of the crank and the rod (some high tensile str ength steels) the task is to determine the diameter of the transversal rod in such a way that crank and rod will withstand about the same forces. - Obviously a very thin rod will yield to the shear strain whilst too thick a rod will leave too little material of the crank, and alter the lever ratio rod to crank to the detriment of the latter.


> (I already made such a starter crank using mild steel: The rod (diameter 6 mm) was shorn off AND the transversal hole in the crank (diameter 16 mm) be came more or less elliptic. - Torsion and bend of the crank itself are minimal and pose no problem.)


> Solutions or references to (engineering) litterature will be highly appreciated.


> Best regards,




> S 17.35775=B0, W 066.14577=B0




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