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Science Forum Index » Mechanics Forum » which friction coefficient to apply ?
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| Guest |
 Posted: Tue Apr 29, 2008 9:06 pm |
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Hello,
If looking up a friction coefficient in friction tables, they vary
sometimes, or a friction interval is given.
So, I am wondering which friction coefficient I have to apply ? Are
there any rules of thumb therefore ? Should I take an average or ...
friendly greetings |
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| Guest |
| Posted: Wed Apr 30, 2008 6:19 am |
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On Apr 30, 3:06 am, de.bruy...@gmail.com wrote:
Quote: Hello,
If looking up a friction coefficient in friction tables, they vary
sometimes, or a friction interval is given.
So, I am wondering which friction coefficient I have to apply ? Are
there any rules of thumb therefore ? Should I take an average or ...
friendly greetings
When I started my career a wise senior design engineer told me to
never count on friction to help you and always assume it will hurt
you. In other words, never use the average friction values, use the
min or max depending which is most conservative for the problem you
are working on. For example, in designing a bolted joint, use the max
friction value to determine the minimum pre-load for a given bolt
torque. Then use the min friction to determine the maximum tensile
stress in the bolt at the same torque. If you can't live with both
extremes redesign the joint.
Dave |
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| N:dlzc D:aol T:com (dlzc) |
| Posted: Wed Apr 30, 2008 8:18 am |
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Guest
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Dear de.bruyn.b:
<de.bruyn.b@gmail.com> wrote in message
news:f23d6d0f-4666-4612-988c-3f7502364031@m36g2000hse.googlegroups.com...
....
Quote: If looking up a friction coefficient in friction tables,
they vary sometimes, or a friction interval is given.
So, I am wondering which friction coefficient I
have to apply ? Are there any rules of thumb
therefore ? Should I take an average or ...
If you need a typical value, say for a homework assignment, take
the average.
If a surface is being freshly used for "braking" use the higher
value, wearing into the lower value.
If it is a life-critical application take the value that will
cause the most harm.
They are empirical relations, established by Nature, without
anything in the way of theoretical support. She likes wiggle
room.
David A. Smith |
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| Brian Whatcott |
| Posted: Wed Apr 30, 2008 7:26 pm |
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Guest
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On Wed, 30 Apr 2008 00:06:52 -0700 (PDT), de.bruyn.b@gmail.com wrote:
Quote: Hello,
If looking up a friction coefficient in friction tables, they vary
sometimes, or a friction interval is given.
So, I am wondering which friction coefficient I have to apply ? Are
there any rules of thumb therefore ? Should I take an average or ...
friendly greetings
Friction varies. It would be well to take the coefficient most adverse
to the purpose, then apply a design factor in the usual way.
Brian W |
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| Guest |
| Posted: Thu May 01, 2008 10:35 pm |
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I thank you for the information. So I always have to pick the most
harmfull value ... Also thank you verry much for the example, Dave.
Another question : if friction is a critical value in an application,
and I can't test it in the application itself, should I rely on a
tribometer test ? Or is this a little bit 'exagerated' (is
tribotesting just meant for scientific resource ?). In other words,
can friction value be that critical that an exact value is needed
(since we all use design factors, etc ...) ? |
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| N:dlzc D:aol T:com (dlzc) |
| Posted: Fri May 02, 2008 8:22 am |
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Guest
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Dear de.bruyn.b:
<de.bruyn.b@gmail.com> wrote in message
news:a1d4b155-3ad3-4fd7-be80-831ca84a9770@m73g2000hsh.googlegroups.com...
Quote: I thank you for the information. So I always have
to pick the most harmfull value ... Also thank you
verry much for the example, Dave.
Another question : if friction is a critical value
in an application, and I can't test it in the
application itself, should I rely on a tribometer
test ?
Any investigation you can do into the exact circumstances will
pay off in added assurance. But bosses rarely tolerate infinite
testing. Since every component in the final assembly, with its
full range of surface finishes and manufacturing tolerances,
added to all of its neighbor's variations in the assembly, added
to every possible contaminant on surfaces, constitute the
Universe that you will be investigating.
Quote: Or is this a little bit 'exagerated' (is tribotesting
just meant for scientific resource ?). In other
words, can friction value be that critical that an
exact value is needed (since we all use design
factors, etc ...) ?
Where friction is that critical, safety wire is used, or constant
supervision (such as applying brakes, or routine re-tightening)
is instituted.
David A. Smith |
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