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Steel Suspension Will Prevail
Page 102
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Manufacture and Characteristics of Steel Springs : Balancing Cost and Strength
ALTHOUGH research into the use of materials other than steel for making vehicle springs is going on. the steel spring will continue to provide good service while the demand for it exists.
This was the summing up of a paper dealing with the manufacture and use of road springs, presented yesterday at the summer meeting of the Institution of Mechanical Engineers, at Sheffield. The authors, all members of the English Steel Corporation, Ltd., were Dr. C. J. Dadswell, Ph. D. B.Sc. (Eng.) director, Mr. J. E. Russell, MA., superintendent of metallurgical research, and Mr. R. Fielding, manager of the spring department.
The steel used for suspension springs in the automobile industry was, they said, silico-manganese. Bar stock for the manufacture of coil springs was ground to a tolerance of 0.002 in. on the diameter and was cut to the required length within a tolerance of plus or minus 1 in. • Approximately 6 in. of each end was anted in a gas-fired furnace and tapered in a roll-forging machine. This was a preparatory step to forming the flat end of the spring.
The tapered bars were reheated to about 800° .C. before being passed to the coiling machine. During coiling, the temperature of the spring dropped god there was a temperature gradient caused by the varying period during which the bar was in contact with the mandrel.
Heated to 920° C.
The springs were next fed into a rotary electric controlled-atmosphere furnace which stabilized and raised the temperature to approximately 920° C. before quenching in oil. Then followed tempering in an air-circulating furnace, when the springs were ready for scragging, grinding and finishing.
Torsion bars were probably the highest-stressed types of spring in service, Occasionally running up to true-surface shear-stresses of 140,000 lb. per sq. in. In their manufacture centreless-ground bar stock was first cut into the correct lengths, and then heated by high-frequency induction before being "upset."
There were three possible methods of spiining the ends, including milling, bobbing or cold-rolling. Cold-rolling had the advantage of speed, hut appeared to be practical only when the splining was fine and shallow. '
In heat treatment, the bars were soaked at 920° C. for an appropriate time and quenched vertically in oil. They were finally tempered at 430-460° C. in forced-air-circulation furnaces and cooled in air.
Black rolled bar was the generally. accepted condition in which steel was supplied and used for laminated springs. The temperatures employed for quenching were similar to those used with coil n44 springs, but slightly higher tempering temperatures were employed to meet the specification of 387-444 Brinell hardness.
After heat treatment, springs were shot-peened. This removed any slight surface scale formed during heat-treatment, as well as putting certain compressive stresses into the surface and work-hardening it.
Although it was held that shotpeening improved the corrosion-fatigue properties of springs, it was undoubtedly true, said the authors, that springs treated in this way were prone to rapid rusting. A protective coating should, therefore, be applied as quickly as possible after treatment.
Scragging Coil Springs
The scragging operation, in the case of a coil spring, consisted of loading it to full closure, and with a laminated spring to a point well past the maximum expected deflection, and releasing it. A torsion bar was twisted well beyond the expected service twist and released.
Before delivery, all springs were subjected to a loading test. In the case of laminated springs it was the usual practice to make the formed camber such that, on application of a deflection calculated from the British Standard test, the spring would return to the required free camber.
It was pointed out that whilst British manufacturers always used silicomanganese steel for springs, it was a queer combination of metallurgy and economics which kept in use a material which had many disadvantages.
Price and Cost Reconciled
Its exclusive use was not because of its inherent metallurgical advantages, but because it possessed, at the moment. the most favourable balance between strength and cheapness. On the other hand, it-was a nuisance in manufacture, because it was prone to many defects to which chromium-vanadium and other spring steels were not.
Service failures of springs were never caused by the large number of relatively small oscillations to which they were subjected; only the occasional large bump contributed towards spring failure. In deflection tests, 300,000 cycles of " full bump" would represent many thousands of miles of normal vehicle usage.
The loss of carbon at the surface of a spring that might occur during the processes which required the spring material to be heated to over 900° C., such as rolling, coiling and hardening,
caused a great reduction In the fatigue limit of the steel.
A drastic reduction of the fatigue properties of a spring could result from damage caused by careless handling during manufacture or service. Increasing hardness alone did not necessarily lead to greater notch sensitivity in fatigue, although there was a widespread impression to this effect. Indeed, notch sensitivity might actually fall with rising hardness.
Early work on the effects of peening had tended to show that the fatigue properties were greatly enhanced after the treatment. Later work, however, had shown 'that when results obtained on carefully prepared and polished specimens were compared with similar specimens shot-pcened, the rise in fatigue limit was seldom more than about 5 per cent.; there might even be a reduction of fatigue limit.
This was particularly true of the harder, high-carbon steels. A greater improvement of properties was obtained on relatively soft steels and on others Of a high work-hardening capacity, such as austenitic steels.
If results on shot-peened springs were compared with those on unpeened, unpolished springs, there was, as one might expect, an enormous improvement in properties.
Shot-peening First
It was the general rule to shot-pecn before scragging, and it had been shown that the fatigue results were much better than when the procedure was reversed. if torsion bars were peened while still under the scragging load, fatigue life was much improved. Springs could, however, be over-peened, as for example, by using too high an intensity
for too great a period. This might cause surface damage; with a drop in fatigue resistance.
The determination of the present camber of a laminated spring was still largely a matter of trial and error, and so far as the authors were aware, no thorough investigation of this point had been made. However, the number of variables in a leaf-spring assembly was so great that it was doubtful whether a really useful purpose would be served by the inevitable complication that a formula would involve.
There was a vast body of evidence to show that when the surface of a spring became rusted and pitted, the fatigue resistance would drop to the extent that there might not be any fatigue limit.
Fatigue tests carried out in vacuum gave distinctly better results than those in air. The rotary-bend fatigue limit of a cold-drawn 0.5 per cent. carbon steel rose from plus or minus 21.4 tons per sq. in. in air, to plus or minus 22.3 tons per sq. in. in vacuum. If a spring were coated with any material that would exclude air, an improvement in fatigue properties might be, expected.