### Re: how does kinetic friction relate to velocity of movement

Date: Tue Aug 1 22:33:51 2000
Posted By: Arnold Anderson, Staff, Tribology/Friction systems, retired (Ford Scientific Laboratory)
Area of science: Physics
ID: 964788053.Ph
Message:
```
Sorry, Steve.  You did not provide enough information for a guaranteed
answer to your quandary.  But there still may be hope.  Let us assume a
situation.  Say we are talking about a bicycle wheel, spun by hand.
Unless you have faster hands than I, the air resistance will be
negligible, so the friction will result primarily from the bearings and
seals.  Under these assumed conditions, friction will be nearly constant,
at least over the range of motion that one can detect with normal
instrumentation.

Classical lubrication theory describes three regimes of lubrication.  This
example falls in the first category, called boundary lubrication, where
friction is nearly constant with the lubrication parameter.  This
lubrication parameter is linear with speed and lubricant viscosity, but is
inversely related to the applied load.  Since the lubricant temperature is
nearly constant, its viscosity also is nearly constant.  The applied load
in our assumed case is simply the weight of the wheel.  Thus, for our
assumed case, the lubrication parameter varies simply and linearly with
speed.

Higher operating speeds could lead to a second regime, called thin film
lubrication.  Here, friction drops with increasing speed as a full
hydrodynamic lubricating film develops.

Even higher operating speeds would lead to the third regime, called thick
film lubrication.  Here, the friction rises with increasing speed.

What you were told was correct.  Dynamic friction does depend on
velocity.  However, that does not necessarily mean that the dependence is
large--at least over moderate speed ranges.

The transition from static to dynamic friction is a bit stickier (pardon
the pun). In the presence of some lubricants, static friction can be lower
than low speed dynamic friction.  With dry, unlubricated frictional
contacts, static friction normally is higher.  The last time I tried to
study static-to-dynamic transition behavior, my patience was severely
tested.  At room temperature, some fatty acids (chemisorption agents) were
found to be effective friction modifiers at speeds below 0.001 rpm.
Several days of testing were required to map frictional transitions (from
rest to 0.1 rpm) for these potent boundary friction modifiers.

I doubt that your test would have detected this transition from dynamic to
static friction.

You may wish to look up a reference on friction, lubrication, and wear.
Volume 18 of the ASM Handbook, Lubrication Regimes, is a good starting
point. Not much was found on the Internet, but you might try the following
site (watch out for their non-linear scales):
http://squid.ucsb.edu/~sfalab/ProjectFrictionPhDia.html

```

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