The main sections of the museum are listed below;
Tools of the trade, some ways to investigate problems;
Dye penetrant testing
of materials engineering terms
with replacement gears
A heavy commercial vehicle company announced that it would no
longer supply spare parts for one of its vehicles. This was a
problem for customers, as the two largest gears in the gearbox
tended to wear faster than the others and it became impossible
to replace them even when the other parts still had a great deal
of useful life. A specialist firm set out to manufacture spare
parts and soon had orders for dozens of sets of these two particular
This firm had measured the gears and manufactured new ones of identical
dimensions using one of the strongest steels available. It machined
the gears and sent them out for heat treatment as in addition to hardening
and tempering the whole gear the teeth had to be surface hardened to
match the hardness of the original.
All seemed to be well until a pair of gears was returned, having failed
by teeth breaking off in just over three weeks' service. The failure
was assumed to be due to poor driving. So another pair was put in the
gearbox, but a similar failure occurred in less than one week.
The problem was then easily identified by comparing the new gears to
The gears were made of steel that started off as cast billets. In the
as-cast state the steel has no directionality to its microstructure
but, as it is worked down to billet or bar, directionality appears
as grains and inclusions are extended in the direction of working.
The directionality is revealed as 'flow lines' which resemble the grain
in timber. For any given quality of steel, the strength is much better
across the flow lines than in parallel with them (the wood analogy
Figure 1 shows the new and old gears,left and right respectively, it
also shows the inner upper ring of teeth which were breaking off. A
radial section was cut from the original gear and an unused new gear.
These were polished and the flow lines revealed by etching. Figure
2 shows the old gear on the left and the new gear at the right. The
teeth which were breaking off were the small ones which are at the
bottom left in the sections in Figure 2. On the old gear they are worn
down, which is why the gears needed replacing. The unused new gear
shows the teeth in side profile. The section has been cut so as to
include the full section of the large teeth around the outside.
Click on images below to see larger versions.
What difference is there in the flow
lines in these two sections?
In the new gear they all run parallel to the axis and if this were made
of wood you would expect them to break off easily. In the old gear the
flow lines are in a looped radial pattern tending to run at right angles
to the gear's axis. If you look carefully at the small teeth you will see
that they run pretty well at right angles to the direction of the flow
lines in the new gear.
The original gears had been forged before the teeth were cut whereas the
replacements had been machined directly from round bar. The flow lines
tell us that the old gears had started off as a length of round bar about
twice as long as the gear is thick but of smaller diameter. This was hot
forged to squash it down, causing the outside to spread out so that the
shape finished up with a much larger diameter and shorter length than when
it started. This was done specifically to develop the flow line pattern
you can observe. When the teeth were cut the grain flow was oriented at
right angles to the applied bending stresses in service. The teeth were
thus as tough as they could be for this type of steel and the flow lines
were oriented in directions which imparted the maximum resistance to failure.
In contrast the new gears had been machined from stock of the full diameter
the gear required. There was no forging, so the flow lines remained parallel
to the gear axis.
The result was that the flow lines in the teeth were in the most unfavourable
orientation to resist fatigue and brittle fracture. This is why they broke
off so quickly under service loads, despite having the same hardness (and
tensile strength) as the original gears.
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