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P O W E R T R A N S M I S S I O N P R O D U C T S G E N E R A L I N F O R M A T I O N
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G E N E R A L I N F O R M A T I O N
S tat i c C on d u c t i v e B e lt s
There is always a demand for belts and other rubber products
to be used in the presence of explosive gases, liquids, powders,
dusts, etc., where the possibility of static sparks must be kept to a
minimum.
Below, we hope to outline, in nontechnical terms, a basic over-
view of static conductivity.
The ordinary manifestations of static electricity are present in
everyone’s daily life: in combing one’s hair, walking across a dry
carpet, separating two sheets of paper, etc.
The differences between a static spark and the current from a
lighting or power circuit are differences in duration, voltage,
and amperage. Usually the sparks are very short in duration
since there is no continuous source of current. The voltage of
a static spark is very high. About 20,000 volts are required to
produce a spark which will jump a one-inch gap in dry air. The
amperage and the energy, however, are usually very small.
There are many ways in which static may be generated: by
friction between two unlike materials, by the breaking up of a
liquid into a spray or mist, etc.
Any material can be electrified to some extent. If the material is
a conductor, however, it may be discharged by connecting any
point with the ground. If it is a nonconductor, the charge must
be removed at the point where it is generated.
In distinguishing between conductors and insulators for static
charges, they must not be confused with the actions of similar
materials when used with ordinary electric current. The
conductivity required to dissipate a static charge is so small that
materials which are satisfactory “insulators” for ordinary electric
current may act as “conductors” for static charges.
The term “resistivity” applies to the specific resistance of the
substance of which the conductor is made. It is numerically
equal to the resistance between the opposite faces of a cube of the
substance whose edge is one centimeter. The unit of resistivity
is the Ohm-Centimeter.
The specific resistivity of most rubber compounds is
approximately 1015 (10 followed by 14 zeros) ohm-cm. For all
practical purposes, it is sufficient to know that the resistivity
of rubber is very, very high and that it is a good insulator. It is
possible, however, to make a rubber compound having a
resistivity of 100 ohm-cm or less. Thus compared to ordinary
rubber compounds, these stocks may be classed as conductors.
However, when compared to copper, which has a resistivity
of 0.0000017 ohm-cm, the very best of conducting rubber
compounds, would still be classed as insulators.
Six mega-ohms is the maximum limit recommended by RMA
and industry for all Static Conductive Belts. Belts produced and
designated by Veyance Technologies as static conductive meet
this RMA recommendation. If special customers insist on tight-
er static conductive limits than required by RMA, such limits
should be carefully noted and emphasized on the order so that
these belt orders can be specially processed through the plant.
However, merely using a conductive belt does not eliminate the
static problem entirely. The entire system must be grounded
since, if no ground is provided, the belt or other parts of the
system may by charged either by conduction or induction from
some outside source.
It is, of course, necessary to see that belt and pulley surfaces are
kept free of foreign substances, such as dirt, dust, belt dress-
ing, etc., which are not themselves conductors. The pulleys,
of course, must be a conductive material which rules out most
nonmetallic materials unless they are specially designed and
treated.
Where the explosion hazards are severe, we strongly recom-
mend that the user periodically check, not only the belts,
but all other possible sources of static sparks. Often the
material itself, as in the case of smokeless powder, may
be a source of static charges. Likewise, the clothes of the
operators will generate static. It is essential that all, and not just
part, of the static sources be eliminated if the danger of static
discharge is to be averted.