From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Detailed process for manufacturing and treating springs, including specifications for heat treatment, shotblasting, and stress limits.
| Identifier | ExFiles\Box 178\2\ img064 | |
| Date | 22th March 1940 | |
| -2- OY 9/D/MAR.22.40 . (a) The spring is wound with a liberal excess length. (b) Ends are ground after winding before heattreat to save expense of grinding the hardened wire. (c) Springs would then be heattreated and quenched in oil from about 1600°F.{Mr Friese} (d) He would then pass the springs thru between two steel plates in a lead bath at around 750°F.{Mr Friese} Exact separation between plates would be adjusted by trial to give correct final load in compressed condition. It might be 1/16" less than the final free length. Springs would be in the lead only 2 or 3 minutes and would cool in air. This will give them length uniformity and straightness and leave them about 51-52 Rockwell C. (e) Springs will then be tempered in an air furnace at around 825° and cooled in air. This will get hardness down to around 47-49 Rockwell. (f) Springs would then be shotblasted. For a wire size of around .187, shot should not be less than .030 to .040. If done in a closed wheelabrator of the tumbling barrel type, the shotblasting should take about 12 minutes. In a Tilghman type shotblast with around 80 lbs. pressure or 65 lbs. at the nozzle, the job will take, he thinks, 3 times as long, perhaps 40 minutes. It is important to shoot between the coils and really do some work on the inside of the coils where the stress is highest. Equal uniformity with wheelabrator results cannot be obtained. He emphasizes that thoroughness of shotblasting is of first importance. Uniformity is also important to prevent bending the springs. The spring will increase in length .010" or .015" in shotblasting. (g) Stress release at 425-450°F.{Mr Friese} (h) Press springs solid and release several times. If properly done, the setting will not exceed .010 - .015". 6) To judge by results on Swedish wire, the endurance limit should be about:- 20000 to 95000 lbs/sq.in. without shotblasting. 20000 to 130000 " " after " or 135000 " " " But only 20000 to 115000 " " " if stress released at 660°F.{Mr Friese} Stress released at 100, 200, 300, 400 deg.F., there is no reduction of endurance limit. But after 500° it falls off fast. These stresses are figured by Zimmerli including the Wahl correction. They may, therefore, not be actual stresses. | ||