Tímarit Verkfræðingafélags Íslands - 01.06.1950, Síða 5

TÍMARIT V.F.l. 1950
27
Fig 1.
Typical Fabricated Frames for small A.C. and D.C.
Motors.
fabrication offers the further advantage that the metal,
being rolled steel, is known to be sound and homogeneous.
With a casting there is always the possibility that blow-
holes or other flaws will be revealed by X-rays or during
machining, and it may then be necessary to scrap the
casting and start afresh, which will naturaliy delay
the completion of the job by many months, apart from the
financial loss involved.
In the early days of fabrication it was usual to
produce the necessary shapes by welding together many
small pieces, but nowadays the number of separate pieces
and the number of welds are kept to a minimum by cold-
bending the metal to the required contours. This can be
done without leaving locked-up stresses in the metal,
because it is now general practice to anneal all fabricated
structures after manufacture.
By the use of a suitable welding sequence, it is possi-
ble to reduce distortion to such an extent that parts
can be pre-machined before welding. This is naturally
a great advantage, since machining after assembly
involves the use of heavy machine tools, whereas it is
eonvenient and cheap to handle and machine the indi-
vidual parts before they are welded together. In Fig. 2
various flanges, bosses and seatings, all of which were
pre-machined, can be clearly seen.
SUPERSONIC TESTING
For specially shaped parts in which the metal is highiy
stressed, such as machine shafts, rotor bodies for high-
speed alternators and the like, forgings have to be used,
and steps must naturally be taken, as for castings, to
check that there are no hidden flaws in the heart of
the metal. Where the thickness is not too great, inspection
by X-rays gives the desired information, but this method
suffers from the disadvantage that complicated, immo-
bile equipment is required, thus making it necessary to
bring the part to be examined, which may weigh many
tons, to the X-ray room. The development of an alternative
method of examination, known as supersonic testing,
which requires only light, portable equipment, is there-
fore arousing much interest.
The principle of supersonic testing is the same as that
of radar: pulses of waves are transmitted through the
medium under examination and the required informa-
tion is given by the reflection of the waves from objects
which interrupt the uniformity of the medium.
Instead of electro-magnetic waves, a supersonic test-
ing set uses mechanical waves having a frequency of a
few megacycles, these waves being generated by the
piezo-electric effect in a quartz crystal. The crystal is
applied to the object to be examined, with an oil film
in between to give good contact, and the reflections of
the waves from cavities, slag inclusions or the like are
picked up by another crystal and displayed on a cathode
ray oscilloscope.
The interpretation of the patterns on the cathode ray
screen is a subject requiring a great deal of skill and
experience, and at the present time no test of this nature
Fabricated Exhaust Casings for
60.000 kW Steam Turbines.