## PSN-L Email List Message

Subject: Re: Knife Edge Discussion
From: ChrisAtUpw@.......
Date: Wed, 14 Nov 2007 11:42:37 EST

```In a message dated 14/11/2007, gpayton880@....... writes:

I am enjoying the discussions regarding the usage of a knife edge pivot
point.  If I understand correctly two points that were made were (1) that  it is
the hardness of the material and (2) a more rounded edge rather than  sharp
edge that is important.

Hi Jerry,

If you use a really sharp knife edge you overload  either the edge
material or the counterface and get automatic failure. The  edge may 1) roll over 2)
distort / compress 3) chip or shatter depending on the  blade angle, the
material and the forces involved.

If you round the edge to get a cylider rolling on a  flat, you can choose
the radius of curvature so that the metal is not at or  close to it's
limiting stress. If you put a cylinder in a sharp V slot and  try to rotate it, it
won't work well. If you put it in a semi circular trough of  larger radius, it
tends to maintain it's position while rolling freely.

This leads me to a question.  What would be the difference between  using a
hard "rounded" edge and using a hard ball bearing?  (Size of  curved surface is
obvious.)  Is it the groove that the edge fits  into?  Ted, told me about a
method that he has used: two parallel roller  bars and a ball fitting between
them.  Orientation of the bars would  dictate whether horizontal of vertical
movement of the  arm.

A ball bearing has a circular contact point  with a flat, semi elliptical
with a single curved surface. A cylinder on a  flat has a line contact with
rounded ends. This greater area and  single curvature allows it to take much
higher loads than a ball of the same  radius, material and heat treatment. You
can use ball bearings in a bicycle  wheel. You have to use roller bearings in a
lorry wheel.

Regards,

Chris Chapman

In a message dated 14/11/2007, gpayton880@....... writes:
<=
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style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
I am enjoying the discussions regarding the usage of a knife edge piv=
ot=20
point.  If I understand correctly two points that were made were (1)=20=
that=20
it is the hardness of the material and (2) a more rounded edge rather than=
=20
sharp edge that is important.
Hi Jerry,

If you use a really sharp knife edge you overlo=
either the edge material or the counterface and get automatic failure.=20=
The=20
edge may 1) roll over 2) distort / compress 3) chip or shatter depending on=20=
the=20
blade angle, the material and the forces involved.

If you round the edge to get a cylider rolling=20=
on a=20
flat, you can choose the radius of curvature so that the metal is not at or=20
close to it's limiting stress. If you put a cylinder in a sharp V slot=20=
and=20
try to rotate it, it won't work well. If you put it in a semi circular troug=
h of=20
larger radius, it tends to maintain it's position while rolling freely.

This leads me to a question.  What would be the difference b=
etween=20
using a hard "rounded" edge and using a hard ball bearing?  (Size of=20
curved surface is obvious.)  Is it the groove that the edge fits=20
into?  Ted, told me about a method that he has used: two parallel rol=
ler=20
bars and a ball fitting between them.  Orientation of the bars w=
ould=20
dictate whether horizontal of vertical movement of the=20
arm.

A ball bearing has a circular contact poin=
t=20
with a flat, semi elliptical with a single curved surface. A cylinder on a=20
flat has a line contact with rounded ends. This greater area and=20
single curvature allows it to take much higher loads than a ball of the same=
=20
radius, material and heat treatment. You can use ball bearings in a bicycle=20
wheel. You have to use roller bearings in a lorry wheel.

Regards,

Chris Chapman
```