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KEEPIN HE REAR AXL N TRIM

5th March 1948, Page 30
5th March 1948
Page 30
Page 31
Page 30, 5th March 1948 — KEEPIN HE REAR AXL N TRIM
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By Laurence E. Field, A.M.I.R.T.E. Methods of Locating the Cause of Rear-axle Failures, Precautions to be Taken when Assembling the Unit, and Subsequent Servicing to Obtain the Best Performance

BEFORE delving into inherent rear-axle troubles and their remedies, it would be well to classify the various types in common use on heavy vehicles to-day.

Taking the hypoid type first, the name designates a special gear-tooth formation that is spiral in shape. With hYpoid gears the drive-pinion shaft can be placed near the bottom of the axle casing, permitting a lowslung propeller shaft with consequently lower body flooring.

It should be noted that hypoid gears, as they mesh and demesh, have a wiping action that differs from most other types of gear. This wiping action increases the rate of wear between the mating surfaces, and special hypoid-gear lubricants have been evolved to counteract this wear. It is imperative that the correct grade of oil be used in these units. and that topping-up is made with the same grade of oil

Hypoid-gear Lubrication To increase the lubricating properties, certain mineral chemicals are added to these extreme-pressure oils, and various manufacturers use different formulm. If such differ ently constituted oils be mixed, certain chemical reactions occur which may result in a breakdown of the lubricating wall. Likewise, it is sound policy to drain and flush these axle cases every 5,000 miles as condensation, resulting from various heat ranges, causes a deterioration of the protective properties of these

special oils. .

The most popular type of rear axle with both designer and operator is that in which a spiral bevel and crown wheel are used. Its chief advantages are simplicity of design and manufacture, accessibility for adjustment and replacement, and relatively low cost of installation. A rear axle, however, has a hard job of work to perform, and this type of design has its limitation with regard to heavy loading and arduous operating conditions.

Spiral-bevel Maintenance ' This is not an attempt to decry the spiral bevel-type .of rear axle; if careful and regular maintenance be carried out—bolts kept tight, nuts fully split-pinned or wired, universal joints kept tight, pinion drivingflange secured firmly on the pinion, and frequent draining and flushing Of the differential case—then this type of final drive can hold its own.

The worm-drive design was basically the forerunner of the modern rear axle, and for extra-heavy duty it stands supreme, the two adverse factors being its higher cost, both in material and in manufacture. and

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its much heavier overall weight. This. design is split into two types—the overhead and underslung worm drives. The fatter permits of a low propeller shaft and finds favour with operators of passenger and commercial vehicles needing low body. flooring.

The removal of the driving and differential unit is facilitated in this type because of its ease of withdrawal from, the axle casing. This is usually carried out by disconnecting the rear end of the propeller shaft, withdrawing the axle shafts and undoing the differential case nuts, then lowering the unit down to a suitable jack or stand.

In addition to the foregoing popular designs, there are several types of double reduction final drives; special types of two-speed rear axle were evolved for use in the Services, and it is likely that this design will become more commonly employed by commercial vehicle manufacturers in the near future.

There are usually .unmistakable signs which herald the approach of rear axle trouble, noise usually starting with a hum on the over-run which develops into a growl indicating that all is not well with the differential assembly.

Worn roller bearings in the differential case and piniort drive usually start the expensive ailment of worn teeth on the pinion and crown wheel. Emphasis must be placed on correct lubrication, but the care with which the differential assembly is built up has a pronounced effect on the service life of the unit. Taking the worm drive, the average end-play on the worm shaft should not exceed 0.005 in., and this amount of clearance is usually sufficient for the entire differential assembly; it is absolutely essential that end float be left on the bearings.

The important point in the correct setting of a worm gear is that just sufficient clearance must be present between the worm threads and the teeth of the worm wheel at the entering side to allow oil to be carried in as the gear revolves. When the assembly is running under load a certain amount of deflection takes place in the various components, consequently, when the unit is assembled on the bench under lightload conditions, special attention must be paid to the markings obtained during adjusting.

The area of contact of the worm with the driven faces of the wormwheel teeth must be definitely towards the leaving side of the worm wheel, because, when running pressure is exerted, the area of contact moves across towards the centre line of the wheel. This movement must not extend to the entering side, otherwise the mating faces will be deprived of their proper lubrication and rapid wear and premature failure of the gear will result!

Looking Back

It is imperative to remember that in a commercial vehicle with overhead-worm drive, the driving face of the worm wheel is that face which looks towards the rear of the vehicle when at the top. Points of note when assembling any type of differential unit are the correct adjustment of the side-bearing nuts. It is impossible to obtain the requisite amount of preload unless the proper tools are used; a hammer and punch are useless for this job and there is the danger of pieces of metal being chipped off the lugs and finding their way into the bearings with disastrous results to follow.

The pinion locking-ring should be tightened with the appropriate " C " spanner and the locking tab should fit snug in the lugs securely to hold it in position. These locking tabs are usually made reversible so as to provide a fine adjustment, and the necessity to tile the tab should never arise.

If undue force be applied to the pinion-shaft nut in order to insert the split pin it is possible that the pinion and gear teeth may be damaged. It is always best to file the ,face of the castellated nut so that when it is tightened solid on the pinion shaft the split-pin holes line up, the right size pin fits tightly and the ends are properly turned over.

Before remeshing the pinion, it is important that the crown wheel runs truly; it is worth while checking the run-out with a dial-gauge This should not exceed .005 in., and while this most useful of measuring instruments is on the job the back-lash can be measured by suitably mounting the gauge firmly and checking the movement of the crown wheel, while holding the pinion shaft still.

Pre-loading Pinion Bearings

Some manufacturers specify a definite amount of pre-load that must be applied to the pinion bearing so that from 15 to 30-in -lb will be required to rotate the pinion shaft. Of course, there is no gear load on the pinion at this time, and for the differential assembly pre-load on the bearings should be such that a slight drag is felt when rotating the assembly by hand. It must not be forgotten that clearance must exist on the unit between the crown wheel . and differential case and the bearing

pedestals each side.

The manufacturers' service manuals should, at all times, be consulted for accurate information regarding fits and clearances, and where several different makes are handled in one workshop it is essential that the information relative to a specific type be taken from the correct manual. If the foregoing points are borne in mind they may do much to lighten your "rear end" troubles and keep the wheels turning

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