TECHNICAL TALKS.
TECHNICAL TALKS.
By F. T. BERSEY, M.I.A.E.
No. 2. ON THE MANUFACTURE OF SPECIAL GEARS. [Following his remarks on engine overhauling in our last issue, Mr. Bersey explains some of the intricate machinery used in the production of special gears often required to secure the maximum performance in various kinds of sporting events.—Epritca.] ANY keen motorists in seeking increased efficiency from their cars imagine that, when
everything has been done to improve engine performance, the only remaining possibility of getting the desired results lies in the direction of lightening the bodywork, or in other means for reducing dead weight. These latter points are, of course, important in their way, but at the same time quite a lot can be done by the judicious alteration of gear ratios, the means for effecting which forms the subject of my present notes. Perhaps the sports car manufacturer is confronted with an almost insurmountable problem in providing gear ratios that will please each and every one of his customers, for the simple reason that drivers vary so much in temperament and in their methods of driving, and as it is obviously impossible to list cars with a wide selection of ratios individuals often prefer to decide upon the ratios for different gears as the result of their own observations. One owner, for example, may desire a top gear ratio that will enable his car to travel all out at the maximum speed on the track, another may desire a specially low first gear to guarantee ultrarapid acceleration, whilst others may pick upon the second or third gear as needing alteration to set up a record for some classic hill climb. In short, the subject of selecting gear ratios is a somewhat complicated one and few sports car owners agree exactly as to what constitutes the ideal in this respect. For the purpose of explaining the methods by which special gears are manufactured, let me take the simple case where the
alteration is effected by changing the ratios of all speeds by the provision of a new crown wheel and pinion in the rear axle.
This may be a simple or complicated matter, dependent upon whether the axle casing gives sufficient room for the alteration or not, for in some instances it is necessary to make a crown wheel with a large amount of ” dish,” in order to throw it over to a sufficient extent to accommodate a different diameter; though, as a rule, the greatest care is taken to avoid any modifications of this kind. Then, again, one has to be careful to avoid any alterations that would interfere with the substitution of the new and old wheels by the owner, because he may desire to use a moderately high gear for touring and only need a lower gear for climbing some of the freak hills to be found in most reliability trials of to-day.
The first step in the process, therefore, is to examine the rear axle very carefully to make sure that the alteration can be accomplished without creating any complications, and having satisfied ourselves on this point we can proceed with the preparation of the gear blanks for the machine on which the teeth are cut. The gear blanks are turned on an ordinary centre lathe and are usually made from 3 per cent, nickel-chrome steel, which can be hardened by cementation. Air-hardening steels are sometimes used, but I am rather inclined to favour the former class of steel, as under heavy duty the air-hardened steels appear to suffer somewhat by spreading at the ends of the teeth. In making the gear blanks the original dimensions are followed closely, thougli in some cases we find it
desirable to allow a little more metal at critical points to provide extra strength, especially where some of the light cars are concerned in which the design is somewhat on the fragile side:
Milled v. Generated Gear Teeth.
Before the introduction of modern gear cutting machines, it was the universal practice to cut the teeth by the aid of form cutters in a milling machine, but this system permits of various inaccuracies. For example, the true form of the gear tooth depends entirely upon the accuracy of the form cutter, and after this had become worn by continued use the teeth of the gears begin to lose their shape and noisy transmission results.
The need for absolute accuracy in the shape of gear teeth has led to the introduction of many ingenious forms of machine tools, among which the Fellows gear shaper occupies a deservedly high place and operates on the principle that a gear tooth can be made to form its conjugate in a blank running in correct relations as to speed, centre distance, etc., with the first wheel. This is accomplished by producing a gear as near perfect as possible and then using it as a generating cutter. The manner of using such a cutter is extremely interesting, for the machine in which it is mounted provides a reciprocating motion similar to that of a shaper, the cutter passing backwards and forwards over the face of the blank parallel to its axis and shapes its way into the latter to the required depth of the pitch to be cut. When this has been accomplished, the feed of the machine causes the cutter and the blank to rotate in unison, the cutter still maintaining its reciprocating motion, the combined result of the rotary and reciprocatory motions being that the cutter generates the teeth in the blank to the precise theoretical shape.
Cutting Bevel Gears.
There are several types of machines operating on the moulding-generating principle, by which bevel gears can be cut with the aid of planing or shaping tools,
the first application of the idea being made by Hugo Bilgram, of Philadelphia, a later example of the same principle being found in the Reinecker machine, illustrated in Fig. 2. This machine does not operate on the principle of completing one side of each tooth before going on to the next, but the rolling action is progressive with the indexing motion so as to finish all the teeth at once. By this method the tool remains stationary in so far as its angular position is concerned, whilst the frame upon which the crown wheel blank is mounted is free to rotate and at the same time rolls the work in its correct relation to the tool, which moves backwards and forwards with a planing action. In addition to cutting straight bevels, the Reinecker machine can form spiral bevel gears, except those of the Gleason variety, which have to be cut on another kind of machine.
Thus it will be seen that by the aid of modern machinery, it is possible to produce new gears with a very great degree of accuracy and the operations being absolutely automatic, to eliminate all possibilities of error.
Treatment of Top Gear Dogs.
Owing to the fact that most sporting drivers cultivate the ” slip ” method of changing gear, we are sometimes called upon to alter the form of top speed dog clutches to facilitate the operation, which in most cases consist in providing a little extra undercutting on the engaging portions of the dogs. This work requires considerable care, as the parts concerned have first to be annealed, then milled without removing an unnecessary amount of metal—to avoid undue backlash—and subsequent rehardening. The illustration reproduced, as Fig. 3, shows the type of machine used for milling the teeth of a top gear dog to give the desired result, the operation being carried out by the aid of a universal milling machine.
The Process of Hardening Gears.
No matter how much care is devoted to the manufacture of special gears, the ultimate results depend
almost entirely upon the manner in which the hardening process is carried out, for there is always a possibility, however remote, that a newly-made gear may warp considerably when submitted to the heat necessary for the carbonising • process. A good deal depends upon the skill of the hardener, but sometimes he cannot avoid a certain amount of distortion, and should this appear likely to take place the gear is firmly clamped between two massive metal plates before it is dipped out. The formation of scale on certain classes of steel is also a point that needs careful attention, for its presence will prevent the proper cooling of the metal, which may, in consequence, suffer from soft patches—a very grave defect in any class of gear. Then, again, in cooling out a large gear wheel, the heat will be so great as to increase the temperature of the cooling water, which is overcome by employing a special form of cooling tank, so arranged as to allow a continuous stream of cold water to pass through whilst the hardening process is going on. To gauge the depth to which the cementation process has extended, or in other words to measure the depth of the hard skin, test pieces are put into the hardening
muffles at the same time as the gear to be hardened. These test pieces are afterwards broken and the fracture will show the demarcation between the soft core of the metal and the hard skin. It follows that if the skin is too deep the teeth will become brittle and liable to break under the driving stresses and an insufficient depth will, on the other hand, impair the wearing qualities of the new gear.
A final test for hardness is made with the Brinell test, by which a hardened steel ball is pressed into the surface of the metal so as to make an indentation that can easily be measured by means of a microscope. The spherical area of the indentation being known, together with the pressure exerted on the ball, gives the basis for calculating out the hardness in accordance with tables provided for the purpose. (To be continued.)