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The material most widely
employed for shaft construction is
steel. In the sliding area it is at any rate desirable to perform
a surface hardening treatment. As regards the sealing action,
materials such as special and/or spheroidal cast irons
are also acceptable
but, in these cases, the installation of
a sliding bushing made of treated steel is preferred. As regards
the sealing action against water at low peripheral speeds,
non-ferrous metals such as bronze or special sintered materials may be
used. These are however generally not
recommended as being poor heat conductors. Stainless
steel insert bushings may be used in certain cases.
Even the degree of finishing and hardness of the shaft
are of considerable relevance for the life of the shaft seal.
6.1 Hardness
As a basic principle, the acceptable hardness of the shaft could
be of 45 HRC,
and should increase with increasing peripheral
speed. The standard DIN 3760 guidelines specify
that for speeds exceeding 4 m/sec. the shaft must
have a hardness of
about 55 HRC.
The choice of an appropriate
hardness depends not merely on
the peripheral speed, but also on such factors as lubrication and the
presence of abrasive particles in the fluid to be
retained or in the outer environment. Poor lubrication and
adverse environmental conditions may in fact demand shafts
of greater hardness.
6.2 Roughness
The roughness is an index
of the shaft's degree of finishing. It can be expressed in various ways:
- Average roughness amplitude (Rz) is the arithmetic mean of
the individual roughness amplitudes of 5 consecutive gauge
lengths.
- Maximum
roughness amplitude (Rmax) is the largest of the
partial roughness amplitudes found in the overall gauge
length Lm section measured.
- Average roughness (Ra) is the arithmetic mean of the absolute
values of all the distances of the roughness profile R within the
overall gauge Lm length.
If the roughness is expressed in Rmax or Rz, the degree of the
shaft's surface finishing must be within the following limits:
- Rmax <=
6,3
µm
- Rz from 1 to 4 µm
(As regards the surface, the most significant roughness
index
is Rmax). Experimental tests aimed at measuring the behaviour
of shaft seals in relation to various sliding surfaces have evidenced
that the best results obtainable on a large
scale production level (not merely from a viewpoint of top
performance, but also of uniformity, repeatability of results
and ease of execution) can be achieved on sliding surfaces
finished by plunge grinding (in this case the roughness Ra must be in
the range of 0.2 to 0.63 µm).
This type of grinding
is in fact essential in order to avoid generating micro-grooves
of a helical shape (typical of multipass grinding) which are, due to
their "pumping action", very harmful from the pont of view of
seal efficiency. In
all those cases where it is not possible, due to any particular
construction demands, to create a surface with the desired
characteristics, or in case of any corrosive actions on
the shaft, it will be necessary to provide a sliding track in
the sealing zone.
Thanks to this solution, in
case of wear only the track need be replaced.
6.3 Chamfer
To avoid
damaging the sealing lip during installation, a chamfer
or rounded access need be provided,
in the direction of installation. Grooves, machining
burrs, corrosions or other defects present on the shaft sealing
surface cause leaks. It is therefore advisable to protect the shafts up
to the moment of their installation, to avoid
accidental damages.
6.4 Machining
tolerances
As
relates to the machining tolerances of the shaft in the regions
of the seal, a minimum machining tolerance corresponding
to the ISO h 11
standard is required.
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