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From Engine to Axle.

27th April 1916, Page 13
27th April 1916
Page 13
Page 14
Page 13, 27th April 1916 — From Engine to Axle.
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Following the recent papers read before the Institution of Automobile Engineers, which have dealt respectively with rear axles and engines, a third has now been read dealing with those parts of the transmission which were not included in the previous two reviews. Major Stinson, the author, disclaims any intention to 4' present any new features or enunciate any new theories," but he has dealt with his. subject in an informative manner, and we therefore reproduce the paper in instal ments as opportunity occurs. This instalment follows that on page 156 of last issue.

Breached Castellations.

With the introduction of broaching, the early method of using square shafts has now been almost entirely replaced by castellated shafts, which are cheaper to produce, avoid bursting of the gear and increase the stiffness of the shaft for a given diameter of material. Proportions of the eastellations which have been adopted as standard in America are given in the Appendix, and also a list of size,s and fits recommended by the author for shafts suitable for gearboxes. These dimensions have a shallower key than those given by the American standard. Many firms • also use, the castellated shaft for driving the fixed gears, and the dimensions of the shaft are kept the same whether the gears are sliding or fixed, the difference being made in the female. The most satisfactory method is to lot the gears fit on the bore of the gear and to have a eleararice on the outside diameter of the shaft, since the gear can be ground out to size after hardening, and the shaft ground down to give the requisite clearance in the event of excessive distortion of the gear in hardening ; the grooves are milled with a taper-sided and radial-faced cutter" whilst the shaft is supported on its centres from which also the journals of the .shaft are ground. By using as large a number of caste,llations as possible, the stress on the solid key is reduced, and the depth of castellation can therefore be decreased, so that less material has to be removed in broaching. For the fixing of their worms to the worm shaft, the Lanchester Co. have a standard of 14 eastellations, the top and bottom diameters of which are respectively 1.1 in., 1.2 in., and 1.2 in., 1.3 in. for their 20 and 30 h.p. worms. The parallel-sided castellation, although not so theoretically correct as the radial-faced castellation, is cheaper arid more convenient to manufacture, and is quite satisfactory in service.

Gears Fixed on Castellated Shaft.

When it is requiredto fix a gear on a castellated shaft, which is also used for sliding gears, the neatest arrangement is to turn down the castellation to the bottom of the groove, and to insert a rectangularsectioned split ring, as shown in Fig. IO, the castella

tion being turned out of the wheel hub to take it. This is better than having an increased diameter of shaft, as it could be made out of smaller stock, and does not encroach on the length of the box.

Separate Gears Joined Together.

. Where it is necessary for any reason to make two gears separately and . join them together, this can conveniently be done by screwing the two parts respectively, male and female, and allowing the castellations or keysto form the locking against unscrewing. This is shown in Fig. 31, and is a particularly neat arrangement, which the author believes was fast introduced by the Coventry Chain Co.

The question of lubrication of the change-speed box is very important. Owing to the. difficulty of keeping the lubricant from leaking through the bearings, it was fiormerly common practice to employ a grease, but this means loss of power, and when the grease solidifies a -path is churned for the gear to rotate in and the lubricant does not reach the places which it is most important that it should.

Gland Design for Gearbox Shafts.

It has taken designers a long time to realise the necessity for a substantial air vent to the box to allow for the expansion and contraction of the air in the box due to the rise in temperature, but when a vent is fitted in conjunction with a suitable gland, a comparatively thin gear oil can be employed. Probably the commonest form of gland is the felt washer, but. this frequently takes the form of a felt ring pressed into a tapered annular groove, and unless correctly proportioned, gives rise to a pressure on the shaft which acts as a brake, and many such glands appreciably add to the lost work of the chassis.

An excellent form of gland is shown in Fig. 28, in which a plate is held in position by the outer ring of the ball bearing, and a felt washer is held in place by the screwed collar fixing the ball race, and it would appear that the influence of the plate is very important in preventing leakage. A very satisfactory plan is to have a reverse thread turned on the shaft or in the casing and allow a bare clearance between these two, as shown in Fig. it, but this does not prevent a leakage if the oil level is too high.

A Design which Has Proved Useful.

A form of gland which the author has used with success is shown in Fig. 12, which he thinks was first introduced by the apier Co. In this type, a sliding washer is driven with the shaft by three pins equally spaced, and is kept up to its work by three springs spaced in between the driving pins, and it is therefore necessary to have the working surfaces pound smooth. When these are dry, the frictiiin being greater than the pressure of the, spring, releases the sliding washer and allows sufficient 'lubricant to get through to reduce the friction, and as soon as ,the oil .has lubricated these surfaces the springs then come into play; and prevent. leakage, but owing, to the smooth surfaces combined With a very thin film of oil. they are held tokether by atmospheric pressure, as in the case of two dead smooth surface plates ; whilst the gearbox can be run without leakage when filled with paraffin, the friction set up by the atmospheric

pressure referred to is probably a great disadvantage to its use.

The lubricant employed should be-a mineral oil without any chalk or other body, since a vegetable oil, on decomposition, gives rise to rust and so destroys the ball bearings and therefore qiiiekiy affects the box as a whole, since rust is one of the greatest enemies to ball bearings, and if pitted even• to so small an extent that it can only be discerned by a microscope, the days of the bearing are numbered.

Ball Bearings Must• be Kept Clean.

In parenthesis, the author would mention the fact that far too 'little care is given to ball bearings in the shops. It is admitted that these bearings are responsible for a lot of noise, and it has been found that even the fluff held in anspension in the atmosphere' tnakes a difference between a quiet and a noisy bearing, and for this reason the makers as a last operatift wash out the bearings in paraffin and pack them with grease in a room supplied With filtered air, no that if a door or window of the 100/11 is opened air rushes out and not in ; but in spite of this very great precaution it is a common sightto see bearings lying unprotected on a bench covered with filings and. ehippings, and manufacturers are then surprised if the bearing gives trouble, and straightaway blame the ball-bearing maker.

'Practicable Oil Vents and Mud-holes Wanted.

Another feature in connection with the lubrication of the gearbox is that the inlet and outlet for the oil are as a rule very much too small ; if the draughtsman had to fill up or empty a box himself, he would very soon see that they were correctly proportioned. In the case of high-class cars, the oil filler is now brought out to the side of the frame and situated at such a level that the box cannot be over-filled, v,iiich does not cost much, but is a great convenience. A substantial mud trap shourld be cast in the bottom of the box so that any sand left in the casting after pickling and which may be shaken out by vibration, or any sealing from the hardened gears, may gravitate to the bottom and not again get into circulation. Some years ago, the Vauxhall Co. fitted a valve in the bottom of the box itself, so that when the plug of the mud sump was opened only the contents of the trap could be released. This is also being done on one or two commercial vehicles, and in Fig. 13 is shown the arrangement adopted on the Hanford box.

Shalt Supports in Gearboxes.

In the case of the engagement for direct drive,,it is very necessary that the dogs or internal teeth should be truly concentric, otherwise there is a radial thrust set lip which will be detrimental to the spigot bearing and which 'accounts in Some cases for the rapid wear of this 'part when a plain bearing is used. A' small' diameter spigot bearing on a high-speed shaft is at all times difficult to lubricate, but in the case of this particular one the difficult' can be overcome by pumping the lubrication to the bush through a holedrilled betWeen two of the teeth, as shown in Fig. 31! This shows the method adopted in the case of a chain drive, and here it is necessary to insert a small capper • pipe for the purpose. Owing to the construction of the chain, there,is little tendency for the oil to escape outwards, but in the ease of spur gears it may be necessary to,have a -flange on dach side of the pinion,' and where the pinion is supported by a bearing on both.Sides this is quite a simple matter. If the box is short., asin Fig.,23, the extra support for the constant mesh pinion is':not necessary, but if the bearings are at all far apart this is highly desirable, particularly in the case of heavy duty boxes such as those for COM, mereial vehicles. With the bearings far apart on the lay shaft, the shaft is stiffened by ,allowing the constant mesh wheel to be overhung, as shown in Fig. 34.. but owing to the slightly higher duty on this bearing it may then be necessary to increase its 'capacity.

Solid Stamped Gears and Riveted Assembly.; As far as possible, wheels should be stamped solid with their bosses, as the cost of the separate arrangement outweighs the advantages of cheaper replace. ment, and less distortion in hardening, and, moreover, it is questionable if the total cost ofthe replacement of the separate piece is actually cheaper when the fit. ting costs are taken into account. Where it is neces-: ary to fix a wheel to a centre or to fix two wheels to; gether, the author advocates the use ofrivets, rather Than bolts and nuts ; this is cheaper in construction, has fewer parts, and there is less likelihood of bits • getting into the oil, while the riveting swells out the rivet, and so &wares a good fit, whereas with a bolt the stress in some designs occurs at the end of the screwing, which is admittedly the weakest part of the bolt...

(To be continued.)


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