POINTERS ON WORM GEARING:
Page 51
If you've noticed an error in this article please click here to report it so we can fix it.
Some Promising Rivals to Phosphor-bronze for Worm Wheels.
S03,IE extremely Interesting facts regarding the development of worm gearing in the United States of America were divulged at a meeting of the Indianapolis Section of the Society of Automotive Engineers.
The chief engineer of the Cleveland Worm and Gear Co., Cleveland, in referring to the materials used for worm gearing, said that there are three kinds. of bronze now in common use for motor work. .Aluminiumbronze has proved satisfactory, but is somewhat difficult to cast, owing to the rapid rate of oxidization of the aluminium, and is, therefore, less favoured than the so-called phosphor-bronze, known also as English Fear-bronze; nor is if preferred to S.A.E. No. 65 phosphor gear-bronze. On the other; hand, aluminiumbronze has the advantage that it can be die-cast.
Recently, a nickel-bronze has been placed on the market, and experimental work indicates that it Is much to be preferred to the other three. It is a copper-tin bronze, with the addition of a slight percentage of nickel, the exact amount varying amongst different producers. The physical-test properties are higher than the standard bronze, and it is greatly superior in ductility. •
The requirements for steel from which worms are to be made are that it should be fairly free from warpage owing to heat treatment, and that, whilst being easy to machine in its normal state, it should be capable of being case-hardened to a 75 to 85 scleroscope hardness on the threads and yet retain a tough core, The ease should have a depth of about .,16 in.
The general engineer of the Baush Machine Tool Co., Springfield, Mass., also made interesting remarks regarding the new bronzes for worm gearing. Ile gave a specification of an alloy which has came into use rapidly as being: 88.5 per cent. copper, 10 per cent. tin, 1 per cent. nickel, .25 per cent, lead, and .25 per cent. phosphorus. The ultimate tensile strength of this material is over 40,000 lb. per sq. in., and it has an elongation of approximately 4 per cent, and a Brinell hardness of 90 to 110 when cast in a three-sided chill.
He pointed out that Duralumin has also been forged, heat-treated, and made into worm gears which have stood up in a very satisfactory manner under hard tests on the Fifth Avenue buses in New /urk, The cost of Duralumin worm gears is approximately the same as that of the bronze type, whilst the saving in weight is about two-thirds. The minimum tensile strength is approximately 55,000 lb.
In the discussion which followed It was mentioned by Mr. Calkins that under normal loads Duralumin stood up as well as bronze, but that this did not hold good when the load was increased, and, in his opinion, it is a satisfactory material for worm wheels where the load is not too great.
A representative of the TimkenDefroit Axle Co. referred to the fact that in petrol-electric buses employed in Philadelphia the worm gearing has a ratio of 11 to 1, the worm turns at 5,500 r.p.m., and, so far as could be ascertained, there is no definite speed limitation. At that speed the worms are transmitting approximately 100 h.p. This should dispel any doubts as to their load-carrying capacity.