evaporating 2

Dear Mike,

Thank you for the interest in our work. Yes – I am interested in cooperation.

Let us try to fix something during the Summer.

Best wishes,

jerzy wojewoda

 http://www.abdn.ac.uk/~eng373/JW_summary.html

Dear Prof.

I’m sorry that I have no much experience in
this field. But I have some points it may help you.

1-     In Dry curves, I think the Hysteretic loops (Friction force - vs -
sliding velocity) is expectable, it should be compared to the sliding curve of
the reciprocating counterface (sliding velocity - vs – time) because the velocity
in reciprocating rigs usually unsteady.  

2-     It is very difficult to study the transient region from lubricated to dry friction. In our
previous study, In order to compensate the amount of evaporated
lubricant during test period (specially at high speed), simple drop feeding
technique was added to the tribometer. Not that the rubbing material may absorb
the lubricant (Polymers absorbs water).

3-     About the Sofware, you may try The LabviewTM
Virtual Instrument (VI).

Good Luck.

Ahmed

Dr. Ahmed Abdelbary
Mechanical Engineering Department
Faculty of Engineering
Alexandria University
Alexandria, Egypt

Michele Ciavarella, Politecnico di BARI - Italy, Rector's delegate.
http://poliba.academia.edu/micheleciavarella

Although, I do
not know the nature of the serum, it is obvious that tribological
contacts at higher sliding velocities experiences higher temperatures
which evaporate the serum inducing dry coulomb friction as mentioned. In
that case, the serum quantity around the contact is very
small as contrary to fully immersed condition in other industrial
machineries. Usually in biological contacts, the fluid flows rather than
static condition. I still do not know the experimental set up here. I
generally
used continuously flowing solution.

Another
possibility is that, if there is no much evaporation, then high
friction could be due to hydrodynamic lubrication where viscous serum
causes the drag of sliding members. If the serum has the nature of
increasing the viscosity with temperature, then this is a possibility.

Another
possibility for higher friction deposition of serum contents as layers
between sliding members also due to protein denaturation. So it is no
necessary to state high friction is coulomb friction. A closer observation of contact can confirm this fact.

Tribological phenomena under lubrication are very difficult to predict
or simulate. I do not know any such standard package in matlab or so to
predict any such phenomena. Experiments are the best.

Dr.Geetha Manivasagam

Professor

School of
Mechanical and Building Sciences

VIT University

Vellore 632 014

Michele Ciavarella, Politecnico di BARI - Italy, Rector's delegate.
http://poliba.academia.edu/micheleciavarella

Dear Prof Ciavarella,

Thank you for your e-mail and the hysteresis
loops. I assume that the loop is obtained through the actuation of the
device from rest to a certain position and then released? The problem is
that I'm not familiar with the device that you're referring to and thus
cannot make very specific comments. However, if you should choose to
let me know how the device is operated and the hysteresis curve obtained I would be happy to elaborate on my view.

The loop is very similar to the characteristics of a pressure metered
dose inhaler (please see the attachment) which we were working with in
recent times.

We have given some explanation of the loop in that case in the paper.

In general viscous friction is directly proportional to the sliding
speed, thus more or less a straight line of the slope: eta*v/h  would be
expected after static friction is overcome. Any deviations, where this
proportionality doesn't exist means that mixed or boundary regime of
lubrication is prevalent.

Please let me know whether these information are of any use to you.

Best wishes,

Homer

Chair of Dynamics
Dynamics Research Group
Wolfson School of Mechanical & Manufacturing Engineering
Loughborough University
Loughborough, UK
Editor: Proceedings of Institution of Mechanical Engineers, Journal of Multi-body Dynamics

Michele Ciavarella, Politecnico di BARI - Italy, Rector's delegate.
http://poliba.academia.edu/micheleciavarella

Well you have a period where the velocity is still positive, but the
friction force has gone negative, implying that friction is doing work
rather than absorbing it. That is certainly impossible, so there must be
some energy storage device (probably a spring) in the loop somewhere
that is giving back energy during unloading.

I presume the experiments are done with constant amplitude and
frequency, so the dependence on velocity might be completely illusory. I
would suggest creating a model with (i) Coulomb friction, (ii) a spring
and (iii) a viscous damper, and seeing what kind of curve this would
predict if the amplitude of oscillation is big enough to cause the
friction element to have a period of sliding.

The comparatively similar (and flat) loading and unloading curves when
velocity is high, suggests that the damping component is fairly small.

You can get a force velocity curve like that in your hip joint .ppt from
a simple quasi-static model. Just take a massless block sliding on a
plane with Coulomb friction. Connect it to a spring and move the other
end of the spring with a sinusoidal motion u = cos(t), so that the
velocity of the end of the spring is v = -sin(t).

Assuming the limiting friction force F0 is less than unity, there will
be periods of slip and periods of stick. In the two attached figures,
the first plots (i) u(t) and (ii) F(t) as functions of time for F0 =
0.2, and the second plots F(t) against v(t). This second has very much
the shape you are observing.

If you think mass is also important (i.e. if the loop changes shape at
different frequencies (does it?)), you could add mass into this model.

An alternative approach here is to take the raw experimental data and
try to do a system identification job on it, using a combination of
linear elements and hysteretic elements.

Michele Ciavarella, Politecnico di BARI - Italy, Rector's delegate.
http://poliba.academia.edu/micheleciavarella

Many thanks to all, but here I am using Force - speed cycles, I am not sure the area enclosed has any meaning.

Michele Ciavarella, Politecnico di BARI - Italy, Rector's delegate.
http://poliba.academia.edu/micheleciavarella