bessemer screw stock. It is sufficiently uniform to be used for
unimportant carburized parts, as well as for non-heat-treated
screw-machine parts. A number of the large automobile manufacturers
are now specifying this material in preference to the regular bessemer
grades.

As second choice for carbon-steel screw-machine parts we recommend
ordinary bessemer screw stock, purchased in accordance with S. A.
E. specification No. 1114. The advantage of using No. 1114 steel
lies in the fact that the majority of warehouses carry standard
sizes of this material in stock at all times. The disadvantage
of using this material is due to its lack of uniformity.

The important criterion for transmission gears is resistance to
wear. To secure proper resistance to wear a Brinell hardness of
from 512 to 560 must be obtained. The material selected to obtain
this hardness should be one which can be made most nearly uniform,
will undergo forging operations the easiest, will be the hardest
to overheat or burn, will machine best and will respond to a good
commercial range of heat treatment.

It is a well-known fact that the element chromium, when in the form
of chromium carbide in alloy steel, offers the greatest resistance to
wear of any combination yet developed. It is also a well-known fact that
the element nickel in steel gives excellent shock-resisting properties
as well as resistance to wear but not nearly as great a resistance
to wear as chromium. It has been standard practice for a number of
years for many manufacturers to use a high nickel-chromium steel
for transmission gears. A typical nickel-chromium gear specification
is as follows: Carbon, 0.470 to 0.520 per cent; manganese, 0.500
to 0.800 per cent; phosphorus, 0.040 maximum per cent; sulphur,
0.045 maximum per cent; chromium, 0.700 to 0.950 per cent.

There is no question but that a gear made from material of such an
analysis will give excellent service. However, it is possible to
obtain the same quality of service and at the same time appreciably
reduce the cost of the finished part. The gear steel specified is
of the air-hardening type. It is extremely sensitive to secondary
pipe, as well as seams, and is extremely difficult to forge and
very easy to overheat. The heat-treatment range is very wide, but
the danger from quenching cracks is very great. In regard to the
machineability, this material is the hardest to machine of any
alloy steel known.

COMPOSITION OF TRANSM