Composition Of Transmission-gear Steel





If the nickel content of this steel is eliminated, and the percentage

of chromium raised slightly, an ideal transmission-gear material is

obtained. This would, therefore, be of the following composition:

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, 0.800 to 1.100 per cent.



The important criterion in connection with the use of this material

is that the steel be properly deoxidized, either through the use

of ferrovanadium or its equivalent. Approximately 2,500 sets of

transmission gears are being made daily from material of this analysis

and are giving entirely satisfactory results in service. The heat

treatment of the above material for transmission gears is as follows:

Normalize forgings at a temperature of from 1,5.50 to 1,600 deg.F.

Cool from this temperature to a temperature of 1,100 deg.F. at the

rate of 50 deg. per hour. Cool from 1,100 deg.F., either in air or quench

in water.



Forgings so treated will show a Brinell hardness of from 177 to

217, which is the proper range for the best machineability. The

heat treatment of the finished gears consists of quenching in oil

from a temperature of 1,500 to 1,540 deg.F., followed by tempering

in oil at a temperature of from 375 to 425 deg.F. Gears so treated

will show a Brinell hardness of from 512 to 560, or a scleroscope

hardness of from 72 to 80. One tractor builder has placed in service

20,000 sets of gears of this type of material and has never had to

replace a gear. Taking into consideration the fact that a tractor

transmission is subjected to the worst possible service conditions,

and that it is under high stress 90 per cent of the time, it seems

inconceivable that any appreciable transmission trouble would be

experienced when material of this type is used on an automobile,

where the full load is applied not over 1 per cent of the time,

or on trucks where the full load is applied not over 50 per cent

of the time.



The gear hardness specified is necessary to reduce to a minimum

the pitting or surface fatigue of the teeth. If gears having a

Brinell hardness of over 560 are used, danger is encountered, due

to low shock-resisting properties. If the Brinell hardness is under

512, trouble is experienced due to wear and surface fatigue of

the teeth.



For ring gears and pinions material of the following chemical

composition is recommended: Carbon, 0.100 to 0.200 per cent; manganese,

0.350 to 0.650 per cent; phosphorus, 0.040 maximum per cent; sulphur,

0.045 maximum per cent; chromium, 0.550 to 0.750 per cent; nickel,

0.400 to 0.600 per cent.



Care should be taken to see that this material is properly deoxidized

either by the use of ferrovanadium or its equivalent. The advantage

of using a material of the above type lies in the fact that it will

produce a satisfactory finished part with a very simple treatment.

The heat treatment of ring gears and pinions is as follows: Carburize

at a temperature of from 1,650 to 1,700 deg.F. for a sufficient length

of time to secure a depth of case of from 1/32 to 3/64 in., and

quench directly from carburizing heat in oil. Reheat to a temperature

of from 1,430 to 1,460 deg.F. and quench in oil. Temper in oil at a

temperature of from 375 to 425 deg.F. The final quenching operation

on a ring gear should be made on a fixture similar to the Gleason

press to reduce distortion to a minimum.



One of the largest producers of ring gears and pinions in the automotive

industry has been using this material and treatment for the last 2

years, and is of the opinion that he is now producing the highest

quality product ever turned out by that plant.



On some designs of automobiles a large amount of trouble is experienced

with the driving pinion. If the material and heat treatment specified

will not give satisfaction, rather than to change the design it is

possible to use the following analysis material, which will raise

the cost of the finished part but will give excellent service:

Carbon, 0.100 to 0.200 per cent; manganese, 0.350 to 0.650 per

cent; phosphorus, 0.040 maximum per cent; sulphur, 0.045 maximum

per cent; nickel, 4.750 to 5.250 per cent.



The heat treatment of pinions produced from this material consists

in carburizing at a temperature of from 1,600 to 1,650 deg.F. for a

sufficient length of time to secure a depth of case from 1/32 to

3/64 in. The pinions are then quenched in oil from a temperature

of 1,500 to 1,525 deg.F. to refine the grain of the core and quenched

in oil from a temperature of from 1,340 to 1,360 deg.F. To refine and

harden the case. The use of this material however, is recommended

only in an emergency, as high-nickel steel is very susceptible

to seams, secondary pipe and laminations.



The main criterion on rear-axle and pinion shafts, steering knuckles

and arms and parts of this general type is resistance to fatigue and

torsion. The material recommended for parts of this character is

either S. A. E. No. 6135 or No. 3135 steel, which have the chemical

composition given in Tables 9 and 7.





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