Tímarit Verkfræðingafélags Íslands - 01.06.1950, Page 7

TlMARIT V.F.I. 1950
29
ALLOYS AND SINTERED PRODUCTS.
As a result of metallurgical developnients, many new
alloys, having' greatly improved properties, are now avail-
able to the designer of electrical plant, and many met-
als formerly regarded as laboratory curiosities are in
daily use. By means of the process known as sintering,
in which powdered metals are caused to coalesce at re-
latively low temperatures, it is possible to combine met-
als having widely differing melting points, and this has
naturally opened up new and important possibilities to
the metallurgist.
A mere list of these products would in itself be lengthy,
so reference will be made only to two examples, both
containing copper, but having very different properties
and applications. These are known as beryllium copper,
which is a wrought alloy, and tungsten copper, which
is produced by sintering.
One of the principal uses of beryllium eopper is for
springs which have to carry current and which formerly
had to be made of either phosphor bronze or nickel silv-
er, both of which suffer from several disadvantages.
Beryllium copper is hard and resistant to wear and cor-
rosion. It has high tensile, elastic and fatigue strengths,
and good electrical conductivity. A further great advan-
tage is that its elastic properties can be developed by
heat treatment, like those of steel, as compared to the
cold working required for phosphor bronze. It is thus
possible to form springs or other parts of intricate
shape with the metal in the soft state and then to
modify its elastic properties by heat treatment. With
phosphor bronze, on the other hand, it is possible to make
parts of relatively simple shape only, due to the limita-
tions imposed by the hardness of the material frorn
which the springs are formed.
Beryllium copper can safely be used at temperatures
up to 200 °C without deterioration of its elastic proper-
ties, but it can, nevertheless, be heat treated at a tem-
perature below the melting point of silver solder. This
is a further manufacturing advantage, because it is
possible to silver solder parts together prior to tempering.
The elastic limit of beryllium copper is about 60%
higher than that of phosphor bronze, but not as high
as that of spring steel. Beryllium copper has, however,
the advantage that the permissible strain before the
limit of proportionality is reached is much higher than
for spring steel, thus making beryllium copper parti-
cuiarly suitable for use where great movement is re-
quired. The conductivity of beryllium copper is about
2.5 times that of phosphor bronze and 5 times that of
nickel silver.
Tungsten copper has been produced for use where
great weight in small volume is necessary. Pure tung-
sten is nearly twice as heavy as lead, but it is hard and
brittle and cannot be handled by any of the usual manu-
facturing processes. By combining it with copper, a
material is obtained which still has high density but
'vhich can be formed to any shape required.
SILICONES.
A list of new insulating materials, each with its own
special advantages, would be as lengthy as a list of
aew metals, and attention will therefore be confined to
°ne particular class known as silicones, which are used
for insulating purposes, but which also have many other
applications.
Silicones may be defined as synthetic materials chemic-
ally midway between organic materials and silica. As
a result of their composition, they display in some mea-
sure the heat resistance so characteristic of silica while
still retaining the flexibility of organic materials, and
they may be roughly grouped into resins, synthetic rub-
bers, and oils, although there is really a wide variety
of forms.
Silicone resins have the advantage, when used for in-
sulating purposes in electrical machines — often as
a bonding medium for mica or to impregnate glass
cloth — that they enable the machine to be operated
at a higher temperature than is otherwise possible.
While there is no inherent virtue in running machines
at high temperatures, it is often convenient to be able
to do so when the service conditions are arduous, as for
live-roll motors in steel mills, traction motors used over
tropical deserts and the like. A high permissible tempera-
ture rise also enables the size and weight of the appa-
ratus to be reduced, which is useful in aircraft equip-
ment, and enables a machine to withstand safely oc-
casional severe overloads.
Silicone oils have the advantage that, besides being
resistant to heat, their viscosity changes little with
change of temperature, which makes them particularly
suitable for use in apparatus subjected to a wide tempe-
rature range or which has to operate at very high or
very low temperatures. Silicone oils are not, however,
as good lubricants as hydrocarbon oils.
Synthetic rubbers of the silicone type are particularly
useful as high-temperature gaskets.
Another useful class of silicones comprises water re-
pellant materials. These have a wide field of applica-
tion, one particular use being as a treatment for ceramic
insulators, in order to maintain high surface resistance
under dew-point conditions.
THE POLLOCK COMMUTATOR.
Turning to the actual design of apparatus, three devel-
opments have been selected for comment because they
are all simple in conception yet offer striking advan-
tages over earlier types of construction.
In the orthodox form of commutator, in which the
segments are clamped between vee rings, the copper,
which is a relatively weak material mechanically, has
itself to withstand the bending and twisting moments
caused by centrifugal force, angular acceleration and
varying loads. It is necessary, therefore, to use segments
of considerable depth, and frequently to employ supple-
mentary retaining devices such as shrink rings.
Furthermore, the natural expansion of the copper
with increase of temperature is constrained by the vee
rings; and, where the segments have, for electrical rea-
sons, to be particularly long, it may even be necessary
to divide them and to use a double commutator in order
to avoid expansion difficulties and also to enable the
commutator to withstand the mechanical forces applied
to it.
In the Pollock commutator, so named after its inven-
tor, these difficulties are avoided by means of the in-
genious construction shown diagrammatically in Fig. 4.