The mirror., 1922-04-27
ROLLING TEETH IN GEARS
Continued from page y

10 assume tnat rolled ShrOuded gears OF
the same pitch, permitting the use of rolled
gears of half the pitch for the same pur-
pose.
As the actual operation of molding the
plastic gear blank ceases when the heated
blank has cooled below its critical tenn
perature, and before there is any appre-
ciable increase in the temperature of the
die roll, it is of course necessary to design
and proportion the die rolls for molding
gears at that temperature. Consequently
the dies must all be somewhat oversize as
compared with the finished gears at nor
NAL temperatures. Die rolls properly made
and proportioned should as rule permit
of the production of least 500 accurately
formed gears before any redressing be-
comes necessary, and should also permit of
a considerable number of redressings dur
ing their lives.
All types and sizes of die rolls having
the masterform gear teeth can be very
easily redressed whenever wear or dulling
makes it necessary. 1t might at first ap-
pear that this refnishing, especially in the
case of gears of the spur type, would i'm
mediately introduce appreciable mac
curacies in the product due to the inev-
liable slight reduction in the diameter of
the die roll, which must result from each
re6nishing. 1t must be remembered, how
ever that both the gear blank and the die
roll are mounted on shafts which are pos
itively and independently driven and
whose rotations are exactly synchronized
by separate timing gears. Thus the angu-
lar distance between the teeth nodded on
the blank is controlled and maintained
constant, regardless of any slight reduction
in the diameter of the die, or the size of
the die teeth. The only effect of this
reduction in diameter, if it is appreciable
at all, would be a very slight creep be-
tween the die and blank when in motion.
This creep would be held uniform and
constant by the timing gears and, as the
thickness of the nodded teeth is dependent
on, and controlled by the width of the
tooth spaces of the die, even a considerable
amount of creep may be readily counter
acted by simply reducing the thickness of
the die roll teeth that slight degree neces-
sary to maintain constant the width of the
die roll tooth spaces. This modification
of die roll proportions, however, is entirely
unnecessary in the case of the crown or
fiat bevel type of dies used in the molding
of any type of bevel gears. With these
forms the grinding of the teeth does not
affect the diameter of the die rolls.
The greater inherent strength of rolled
gears is due to the effect on the structure
of the metal by rolling under high pres
sure. This pressure and the thorough
kneading action of the die teeth on the
metal while in the plastic state, in addi-
ton to building up and COMPRISING the
metal into the formed teeth, gradually
breaks up the coarser crystalline structure
and produces dense, thoroughly worked
and more fbrous structure. The minute
crystals are rearranged in trussed forma-
tion, thus tying the teeth to the body and
equalizing structural strains. The elimi-
nation of internal structural strains is so
marked that no distortion whatever occurs
in subsequent heat-treatment, and it has
even been found entirely unnecessary to
use any clamps to hold the gear true in
quenching. Exacting laboratory tests am
ply confirmed by service tests under the
most severe operating conditions, have
shown a superiority for rolled gears over
the best of cut gears, averaging IS per
cent in strength and 20 per cent in hard
ness.
The hot-rolling process makes possible
quantity production of gears at far lower
unit cost than by the cutting method and