A Criticism of Wheel and Tire Practice.
Page 13
If you've noticed an error in this article please click here to report it so we can fix it.
(By An Engineering Contributor.)
Steel-wheel Design.
It is now generally acknowledged that wheels either of cast or pressed steel are practically the only types suitable for all classes of Overseas work ; there are, however, a few composite wheels on the market embodying a steel centre and wooden felloe, and these, owing to the means of adjustment for taking up shrinkage of the felloe, are also quite suitable. For much heavy work cast-steel wheels are prefer • able to other types, but, of course, welded pressed
steel or With steel wheels score in certain directions. With the pressed-steel class it is impossible to vary the thickness of the metal in accordance with the changing stresses to be borne at different points of the structure. For this reason the material has to be of such a gauge as will be strong enough for the most highly-stressed portions of the wheel. On the other hand, for light work these wheels have much to recommend them.
Wheels built up from either pressings or stampings, riv eted together, whilst they are, as a rule, on the heavy side, are very often cheap for certain classes of employment. Care must be taken that the rivets do .not. work loose.
To produce a really good cast-steel wheel requires very careful forethought. and design. Masses of cast metal ere always likely to cause trouble through shrinkage, and, although actual breaks may not occur at the time of manufacture, the metal may be in a serious state of tension and win probably be porous, the resulting risk of future collapse being thus greatly alsentuated. Sections, therefore, should be kept as uniform as possible.
Many cast-steel wheels have a habit of cracking at the places where the spokes enter the rim : this is due to the slight rocking action of the rim, using the spokes as fulcrums ; the risk of breakage is, of course, ieereased if the spokes are in tension. To prevent any risk of this cracking, the London General Omnibus Co., Ltd., has designed a wheel in which the spokes taper from the sin) to the hub, instead of in the usual fashion ; the spokes spread out at the rim and form a buttress for it. This wheel has been in use for some considerable time and has proved entirely satisfactory. It is the most recent effort in steelwheel design.
An important point with cast-steel wheels is the number of spokes ; if the number he large they will be very fragile, while if it be small the rim will have to be unduly heavy.
The balancing of wheels is of far more importance than is generally appreciated, as a wheel which is a few ounces out of balance may cause an uncomfortable and deteriorative amount of vibration.
Resilient and Spring Wheels. tTp to the present there have been very few really satisfactory and reliable spring or resilient wheels exploited in this country. Wheels of the pneumatic type have achieved little success on any but the lightest, classes of commercial vehicle, as the conditions under which they have to work are far too severe, while the air containers cannot stand up4to the heavier loads, driving and braking strains to which they are subjected. Twin pneumatics have been little used in this eountry_ No resilient or spring wheel of any type can hope to imitate exactly the action and damping properties of a pneumatic tire. The success of the pneumatic tire has been greatly due to its power of absorbing and ejecting small obstacles, without disturbing the wheel, and nothing else is likely effectively to incorporate this property.
Any pressure on a pneumatic tire is distributed equally all over it by reason of the air compression, while on the other hand hardly more than one spring wheel attempts to incorporate this feature.
Many spring-wheel crevices are inclined to disintegrate or become noisy, owing to the combined effects of wear, vibration, and damp. There are, however, a few quite successful spring wheels on the market, and at least one make of these has been used satisfactorily on the heaviest steam vehicles'and, be it noted, with steel tires. Another type of spring wheel has run 16,000 miles without loss of temper or resiliency in the springs; both these designs have been very carefully worked out, dirt and damp being as far as possible excluded by the use of guard plates and well-made joints.
Many designs which have already appeared seem to have no provision for taking the driving or braking strains, the springs, cushions, or air containers being relied upon to do this work in addition to their own. With many spring wheels which have been produced in the past, a slip of the vehicle into the kerb ,might quite easily have the effect of forcing the rim off the wheel, or even the rest of the wheel off its hub. In other types, in order to fit a new tire, it has been necessary to take a great part. of the wheel to pieces, and usually the tires could only be obtained from the makers of the wheel—a considerable disadvantage.
Demountable Tires Considered.
The great importance of using easily demountable tires for Overseas work cannot be too fully emphasized. Many vehicles, of course, may have to carry out much of their work, possibly hundreds of miles from any place where a hydraulic or other press is installed, and the delay caused by waiting while wheels are being re-tired may be expensive. Up to quite recently quickly-detachable solid tires were viewed by users with a great deal of suspicion ; this was chiefly due to the failure of designers fully to understand the severe conditions under which a solid tire has to work. Many of the devices put on the market: were very liable to bind through rusting, amid in such a case the use of a powerful press often became a necessity.
With reference to the stresses which may be encountered by a tire, it must be remembered that the greatest stress is, as a rule, caused by sudden application of the brakes. With a heavy vehicle the drag on the tire may easily, under certain circumstances, rise to one of several tons, and it must be noted that any such pull may be exerted in either direction, according to whether the vehicle be on an up or a.down grade,-or be checked when running either backwards or forwards.
Detachable tires should be locked by some method of wedging, though the use of a large number of separate wedges has not much to recommend it except as a makeshift ; keys, to assist the friction grip, may be incorporated with advantage. An important point to be remembered is that very few of the detachable tire fittings are suitable for more than one type of tire, ..and, if that particular tire happen to be non-obtainable, the vehicle may be held up.
One very good form of detachable tire has lately been brought out ; a demountable rim, to which the solid tire is directly vulcanized, is. bolted on to a skeleton wheel centre; there are thus only two parts other than the bolts. By using rims of different diameters, almost any size of tire may be used, as the bolts pass through the lugs on the rims, and these lugs can he long or short to suit the spokes of the when centre.