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The
Sheedy Bearing Revolution (cont'd)
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Chemical
Resistance
The
ability of a bearing to resist attack by a wide range
of chemicals is a new consideration in bearing selection.
Much time, care and cost, has been expended over many
years in designing and fitting complex sealing mechanisms
and housings to keep reagents away from bearings. In
the vast majority of cases, this has been to protect
the bearing, not the material, fluid or process.
The
Sheedy Bearing introduces a new concept, the concept
of not protecting the bearing from the environment.
Intrinsic
chemical resistance leads to several new thoughts. Can
sulphuric acid be used as a lubricant? Will the acids
in animal fats affect the bearing? - or - can they act
as lubricants? Can I run the bearing in salt or sugar
water without protect? In all cases the answer is YES,
if you use a Sheedy Bearing.
What
then are the limits? What will the bearing withstand?
What options are there in difficult areas? The answer
is that we have yet to find an environment in industry
where we cannot come forward with a solution. In rare
cases it has taken some months of research, in conjunction
with our client, to arrive at an acceptable result;
however, we have always been able to arrive at a solution.
General
Principles In Applying Sheedy Bearings
Rolling
Bearing have been with us for years. They rust, need
constant lubrication, can break and expel components
and if run dry, are noisy. By comparison, the Sheedy
Bearing is extremely quiet, rarely requires lubrication,
it does not rust, all of the internal components are
completely retained and it is fully resistant to cleaning
with hot water, steam, caustic solutions, soap, many
solvents, hot water, etc.
In
essence, the Sheedy Bearing is a plain bearing. This
is a type of bearing which has not been widely used
in general industry for many years and may not be well
understood by users. Engineering plastics are used extensively
in the more common types of Sheedy Bearing and these
materials are also poorly understood in many areas.
And finally, poor selection of plastics in bearing applications
in the past has resulted in some trepidation in using
"plastic bearings".
The
Sheedy Bearing has changed all of this. The unique materials
developed for and used in , the Sheedy Bearings, coupled
with the expert engineering advice and experience Sheedy
offers, provide an exceptional world class product.
Sheedy Bearings can solve many of the intractable bearing
problems in industry, but you must understand the unique
aspects of the bearing to take full advantage of the
many benefits it provides.
Running
Clearance
It
has already been said that this is a "plain"
bearing, that is, it is formed from a series of cylindrical
surfaces running on one another. The unique selection
of materials and pre-pack lubricant, when used, minimise
the friction between surfaces and the wear rate experienced
through rubbing. Great care is taken with the surface
finish on all contacting surfaces and all possible steps
are taken in the design of the bearing to minimise heat
build-up. Unique tribological conditions are generated
within the bearing and highly specialised surface treatments
are utilised to protect and enhance performance.
Plain
bearings require space within the bearing that is not
necessary in rolling element types. This space results
in a bearing with more play than a ball or roller race,
and if this is not understood, it can result in both
poor selection and inappropriate installation of the
bearing.
Plastics,
being resistant, absorb and damp vibration, without
generating any noise. Surface finishes are more controllable
and as well as generally providing lower coefficients
of friction, the wear products are far less abrasive
than those in metallic bearings. These advantages result
in plastic bearings being more wear tolerant than the
more common metallic plain bearings.
Loading
Characteristics - The PV Factor
PV
is the product of Surface Velocity and Pressure per
unit area, between the two surfaces in contact. A high
PV indicates a high bearing performance capability and
a low PV indicates a lesser performance.
Considerable
confusion can occur in comparing PV figures, as several
different measurement methods are used to determine
the PV value. Sheedy Bearings apply standard metric
units to define the PV value, that is, Newton per Millimetre
Second (N/mm Sec) and all PV figures quoted in this
material are in these units.
As
PV is the product of Pressure and Velocity, it follows
that increasing the speed of a plastic bearing will
reduce its load bearing capability. Conversely, decreasing
the speed of the bearing increases the load bearing
capability, the practical load limit being the maximum
load the bearing can carry at rest, the Static Load,
without deforming to a point where it cannot function.
The
load bearing capability of all Sheedy Bearings is therefore
a mathematical relationship. This relationship depends
upon the internal geometry of the bearing, the strength
of the various bearing components and peripheral factors
such as lubrication, temperature, the characteristics
of the load. The relationship is unique for each bearing
type and is represented as a series of Speed/Load curves
or tables. More generalised methods are being formulated
as a natural development from our ongoing research and
quality testing programme.
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P.O. Box 1192, Echuca VIC 3564, AUSTRALIA
