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:
<= FONT=20 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= ad=20 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

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