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).
Continued report on the manufacturing process and material specifications for hourglass worms.
| Identifier | ExFiles\Box 153\2\ scan0355 | |
| Date | 12th July 1937 guessed | |
| -9- PRODUCTION OF HOURGLASS WORMS - cont'd.{John DeLooze - Company Secretary} The modified steel of their own specification, used at the Saginaw steering division, is a Nickel-chrome-moly steel, hot rolled and treated by the makers to give an impression of 440 corresponding to a Brinell of 190 ( approximate) and having a fine grain of 7 to 8. The finish cut worms are carburised at 1650°F in a continuous furnace for 16 hours, corresponding to approximate 12 hours in the conventional type, and quenched direct into oil. Drawing is done at 300°F, producing a Rockwell figure on the case of C.58 to C.62 with a core Brinell of 360. The blanks are produced in a conventional manner leaving grinding upon the tracks. The machines used for cutting the threads are dependent upon the degree of accuracy required, the larger and better quality pieces being roughed out upon the thread generator and finally cut on a Fellows hourglass gear shaper. An experimental machine has been devised to approach nearer theoretical conditions for cutting, by modifying a reversed condition hobber to allow a compensated cross slide to operate in conjunction with the angular movement of the cutter, thus attempting to produce pitch line contact during the angular travel of the follower. To facilitate concentricity of the worm and track after grinding, the hardened pieces are placed on a cone arbor and registers ground at each end of the worm track on the parallel portion. The register diameters are then placed into a pot jig and the tracks ground. On typical pieces eccentricity of not exceeding .001 was evident, with a smooth contact surface on the worm. In an attempt to get better effects with the poorer class worms a modified follower was seen at Saginaw. The conventional type follower was split to allow a flat type spring to be inserted between the two halves, with aid of this type follower, slight surface undulations are overcome by allowing the two halves to close slightly and overcome tight spots in the centre steering range position. It should be noted that outside contact is employed by Saginaw, but similar conditions with the springs on the ends of the follower could be tried on our design to eliminate selective assembly, oversize followers, and overcome "tight spots". continued. | ||