Connecting Rods

: The Working Of Steel

The material used for all connecting rods on the Liberty engine

was selected at the option of the manufacturer from one of two

standard S. A. E. steels, the composition of which are given in

Table 13.


Steel No. X-3,335 6,135

Carbon, minimum 0.300 0.300

Carbon, maximum 0.400 0.400
r /> Manganese, minimum 0.450 0.500

Manganese, maximum 0.750 0.800

Phosphorus, maximum 0.040 0.040

Sulphur, maximum 0.045 0.045

Nickel, minimum 2.750

Nickel, maximum 3.250

Chromium, minimum 0.700 0.800

Chromium, maximum 0.950 1.100

Vanadium minimum 0.150

All connecting rods were heat-treated to show the following minimum

physical properties; Elastic limit, 105,000 lb. per square inch:

elongation in 2 in., 17.5; per cent, reduction of area 50.0; per

cent., Brinell hardness, 241 to 277.

The heat treatment used to secure these physical properties consisted

in normalizing the forgings at a temperature of from 1,550 to 1,600 deg.F.,

followed by cooling in the furnace or in air. The forgings were then

quenched in oil from a temperature of from 1,420 to 1,440 deg.F. for the

No. X-3,335 steel, or from a temperature of from 1,500 to 1,525 deg.F.

for No. 6,135 steel, followed by tempering at a temperature of from

1,075 to 1,150 deg.F. At the option of the manufacturer, the normalizing

treatment could be substituted by quenching the forgings from a

temperature of from 1,550 to 1,600 deg.F., in oil, and annealing for

the best machineability at a temperature of from 1,300 to 1,350 deg.F.

The double quench, however, did not prove satisfactory on No. X-3,335

steel, due to the fact that it was necessary to remove forgings

from the quenching bath while still at a temperature of from 300

to 500 deg.F. to eliminate any possibility of cracking. In view of the

fact that this practice is difficult to carry out in the average

heat-treating plant, considerable trouble was experienced.

The most important criterion in the production of aviation engine

connecting rods is the elimination of burned or severely overheated

forgings. Due to the particular design of the forked rod, considerable

trouble was experienced in this respect because of the necessity

of reheating the forgings before they are completely forged. As

a means of elimination of burned forgings, test lugs were forged

on the channel section as well as on the top end of fork. After

the finish heat treatment, these test lugs were nicked and broken

and the fracture of the steel carefully examined. This precaution

made it possible to eliminate burned forgings as the test lugs were

placed on sections which would be most likely to become burned.

There is a great difference of opinion among engineers as to what

physical properties an aviation engine connecting rod should have.

Many of the most prominent engineers contend that a connecting rod

should be as stiff as possible. To produce rods in this manner in

any quantity, it is necessary for the final heat treatment to be made

on the semi-machined rod. This practice would make it necessary for a

larger percentage of the semi-machined rods to be cold-straightened

after the finish heat treatment. The cold-straightening operation

on a part having important functions to perform as a connecting

rod is extremely dangerous.

In view of the fact that a connecting rod functions as a strut,

it is considered that this part should be only stiff enough to

prevent any whipping action during the running of the engine. The

greater the fatigue-resisting property that one can put into the

rod after this stiffness is reached, the longer the life of the

rod will be. This is the reason for the Brinell limits mentioned

being specified.

In connection with the connecting rod, emphasis must be laid on the

importance of proper radii at all changes of section. The connecting

rods for the first few Liberty engines were machined with sharp

corners at the point where the connecting-rod bolt-head fits on

assembly. On the first long endurance test of a Liberty engine

equipped with rods of this type, failure resulted from fatigue

starting at this point. It is interesting to note that every rod on

the engine which did not completely fail at this point had started

to crack. The adoption of a 1/32-in. radius at this point completely

eliminated fatigue failures on Liberty rods.