Reactionless drives Vs Stick-Slip drives by JL Naudin ( 03-03-01 )

All informations in this page are published free and are intended for private/educational purposes and not for commercial applications

WARNING, you may be fooled by some false reactionless drives experiments....

Some of them use the well known "STICK-SLIP" technique for moving itself on a surface. This method is commonly used in nanorobotics....

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Stick-Slip Friction :   http://entropy.davidson.edu/faculty/dmb/PY430/Friction/stick-slip.html
In any case where the coefficient of kinetic friction is less than the coefficient of static friction there will exist a tendency for the motion to be intermittent rather than smooth.  The two contact surfaces will stick until the sliding force reaches the value of the static friction.  The surfaces will then slip over one another with a small-valued kinetic friction until the two surfaces stick again.  The most simple model for explaining this mechanism of friction, known as 'stick-slip,' is the case of a spring with a mass attached. In this setup there is a mass attached to a coiled spring being pulled by a tension force so that the spring moves at a constant velocity.  The surface upon which this setup rests has a coefficient of kinetic friction that is much less than the coefficient of static friction. When the spring is pulled one unit of distance, the tension is enough to overcome the force of static friction, and the block begins to move.
Because the kinetic friction is far less than the static friction the block moves at a velocity faster than that of thespring, rapidly restoring the spring to its unstretched length causing the block to once again come to rest to start the entire process over again.  The body will again remain at rest until the tension exceeds the static friction causing the block to move forward another unit of distance until the mass stops because of the compression of the spring back to its unstretched length.
By performing this run at numerous spring velocities and making plots of position versus time, the trend we begin to see is that the faster the spring velocity,
the motion of the mass becomes less jerky.  Also, the motion of the mass becomes less jerky if the two coefficients of friction approach the same value.
Now the case of the mass and spring may seem like a very localized case of 'stick-slip' friction, but the fact is that even a stiff rod has some amount of elasticity and will stretch when it is pulled.  Although the 'stick-slip' mechanism may not be as visible to the eye, it will still occur on a reduced microscopic level.
Furthermore, elastic deformation occurs in all driving mechanisms, such as the transmission of an automobile, the chain on a bicycle, or in various cutting tools.
All of these examples are cases wher the static friction is higher than the kinetic friction and may be prone to intermittent 'stick-slip' motion.

Finally, what about the velocity dependence of this and other types of friction?  In the case of 'stick-slip' friction the velocity dependence is easily seen in the case of the mass and spring.  This same type idea applies to kinetic friction in general.  The coefficient of kinetic friction for all materials shows some dependece on velocity to a marked degree.  At very slow speeds mu for kinetic friction increases with speed until it reaches some localized maximum, at which point mu begins to decrease with increasing velocity.  In fact very low values of mu are found for metals moving at very high speeds (several hundred meters per second).  Thus, kinetic friction and 'stick-slip' friction are, for the most part, velocity dependent forces.
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A good "Stick-Slip drive" sample can be found in this patent from the US Navy :

http://l2.espacenet.com/dips/bnsviewer?CY=ch&LG=fr&DB=EPD&PN=US3916704&ID=US+++3916704A1+I+

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US3916704:Vibratory locomotion means

Inventor(s):    Gaberson; Howard A. , Oxnard, CA
Applicant(s):
The United States of America as represented by the Secretary of the Navy, Washington, DC
Issued/Filed Dates:
Nov. 4, 1975 / April 23, 1973

A vibratory locomotion device which consists of one or more skids resting the ground supporting a payload, and a vibrating mass attached to the skid. The mass, operated by a motor which creates a sinusoidal oscillation, alternatively pulls and pushes on the skid at a predetermined angle to the ground as it vibrates. When the mass is at the top of its stroke, it lifts and pulls forward on the skid and, when it is at the bottom of its stroke, it pushes downward and backward on the skid. Since the downward force increases the normal and, hence, the available friction force, the angle of the mass to the ground can be adjusted so that no backward slide results. As a result of the vibration of the mass, the skid incremently shuffles forward along the ground. >>