CHAPTER VI.
WHEELS.
A wheel for a locomotive vehicle is a circular roller, either cylindrical or conical, the width or thickness of which is considerably less than its diameter. It may be either solid or constructed of various pieces, in which latter case it is called a framed wheel. It may also be made of wood or metal, or a combination of both.
Wheels which were made before the introduction of iron were of course very clumsy in their construction, in order to obtain the requisite strength. Specimens may still be seen in the broad wheels of waggons, technically termed rollers. The naves of these wheels are of enormous size. But when the naves of wheels were reduced for the purposes of elegance, a thin hoop of iron was applied both to the front and back, to prevent them from bursting by the strain on the spokes. When the felloes of wheels were reduced in size, straps of iron, called strakes or streaks, were applied to their convex surfaces covering the joints. But the last improvement was the most important of all, namely, the application of a “hoop-tire” instead of what was called the “strake-tire.” Mr. Felton, in his treatise on coach-building, 1709, says, “Many persons prefer the common sort of wheel on account of their being more easily repaired than the hoop-tyre wheel; but though repairing the latter is more difficult, they are much less subject to need it.”
The earliest form of wheel was no doubt a slice of the trunk of a tree; portions of this being cut out for the purpose of lightening it would be the forerunner of spokes, or we should think the pieces left running from the nave to the felloe would be. The only improvement then effected for a very long period was making them of different pieces of timber instead of all from one piece, though of course the proportions of the parts would be considerably improved, if only for the sake of appearance.
At the end of the seventeenth century, among the wealthier classes, decoration was applied to coaches generally, and wheels in particular, to an extent which would surprise us nowadays. These latter were again ornamented as in the times of the old Roman Empire; the spokes were shaped and carved, the rim moulded, and the naves highly embossed; though, as may be imagined, there was a great want of taste in the application of all this ornament.
Towards the end of the eighteenth century, the extreme height of wheels extended to 5 feet 8 inches, which had but 14 spokes; wheels 5 feet 4 inches high had 12 spokes; wheels 4 feet 6 inches had 10 spokes; and the lowest wheels, 3 feet 2 inches high, had 8 spokes. The naves were of elm, the spokes of oak, and the rims or felloes of ash or beech. The rims of the higher wheels were often of bent timber, in two or more pieces, and were bolted to the tires by one bolt between each pair of spokes. The tire was put on in pieces, until the hoop-tire came into general use, when it superseded the old ones entirely. In consequence of the great height of the wheels it was necessary to make the carriages very long, and the distance from the front to the hind axletree was 9 feet 2 inches in a chariot, and 9 feet 8 inches in a coach, or about 8 inches longer than we should consider necessary now.
These extreme sizes are now very seldom used, except in the case of large dress or state carriages and coaches.
The form of wheel now generally preferred in practice is of the dished or conical kind, and the axle-arm on which it revolves is sloped or bent so far out of the horizontal that the lower spokes are in a vertical position. Undoubtedly the friction is increased by this arrangement, because a wheel on a horizontal axle runs easiest and smoothest; and when the axle-arm is slanted downwards towards the point the wheel has a tendency to bind harder against the shoulder which butts against the nave of the wheel, and the friction between the two is greater than would be the case if the axle-arm were perfectly horizontal. This, however, is a very small objection, inasmuch as this collar is firm and strong, and well fitted to bear any strain that may be thrown on it by the wheel; whereas, if the force acted in the other direction, or against the nuts and linch-pins, there are very few that would last out a day’s journey. The advantage of throwing the strain on the firm and strong shoulder, which is well able to withstand it, is evident; and in this case, in the event of the nuts or linch-pins falling off or giving way, there is not so much danger of the wheel coming off at the moment the nuts go, as its tendency when on a level road is to run upwards towards the shoulder. Besides this, as the lower spokes are in a vertical position, the upper ones spread considerably outwards, and thus afford a greater space for the body between the wheels without the track on the ground being increased; and another advantage is that the mud collected by these conical wheels is thrown off away from the carriage.
The hind wheels of an ordinary carriage vary from 4 feet 3 inches to 4 feet 8 inches; the fore wheels are from 3 feet 4 inches to 3 feet 8 inches. The number of felloes in each circumference varies according to the number of spokes, two spokes being driven into each felloe; 14 to 20 spokes are a usual number for a hind wheel, 12 to 18 for a fore wheel; however, there is no rule to guide one in the matter, experience being the only teacher.
The wheels should be made with a due regard to the offices they have to fulfil; but we are inclined to think that this branch of the trade has not received that careful study which it deserves. Coachmakers seem in such a hurry to produce a perfect vehicle all at once, instead of beginning by improving the parts and then applying these improvements to the whole.
The mode of constructing a wheel is as follows:—
The timbers that are to be used should be well and carefully selected. The nave or stock, which is sliced from the limb of a tree, should be as nearly as possible the size required in its natural growth, so that it will require little reduction beyond what it receives in the lathe in bringing it up to the true circular form. The reason of this is that the annual rings which mark the grain of the timber should be as little disturbed as possible, as they are not all of equal strength and durability, the outer rings being pretty strong, but as they get nearer to the centre the wood is much softer. If, then, this outer hard casing is cut away, even only in part, it is signing the death-warrant of the poor nave, for the interior parts of the timber are not nearly so capable of resisting the destructive influences around, and in a very short time they will become completely soft and rotten.
As already remarked, the spokes should be cleft, not cut. The felloes which form the outer periphery of the wheel should also be cut as closely following the grain as possible.
When the wheelwright has carefully selected his timbers, he commences work by turning the stock in the lathe to the size required. Then he marks with a gauge of the same width as the spokes 4 circles as shown at a a a a, Fig. 13. The first and third of these mark the position of the front or face spoke, and the second and fourth mark the position of the back spoke. Two holes are then bored in each mortise in succession, after which they are squared out with proper chisels. Truth of eye and skill of hand are the workmen’s only guide in this operation, though it is evident that it is the most important operation of the whole, as upon it depend the accuracy and solidity of the wheel when finished. The tenons of the spokes are then cut to fit the mortises, parallel in their thickness, but in width they are cut slightly taper-wise, i.e. the extremity of the tenon is made about the same size as the mortise, but at the shoulder it is about one sixteenth of an inch larger, so as to make sure of the tenon filling the mortise when driven home.
Before cutting the mortises the stock should be fixed at some convenient angle, regulated by the amount of dish it is intended to give the wheel. This is particularly necessary, or when the felloes come to be fitted, if the mortise-cutting has been done in a slovenly way, the dish will not exist at all, or if it does it will be in the wrong direction.
Each alternate spoke is now driven in by the blows of a mallet to a perfectly close bearing of the shoulder of the tenon, the workman guiding it as best he can. But it is evident that the position that the spokes will take is by no means certain. Owing to the wedge-like form given to the tenon, the spokes are driven home very tight, and wood not being of a homogeneous texture will yield more in one part than in another; and the mortise, cut in the way that it is, must be to some extent uncertain. Every alternate spoke being driven, or those in the same plane, the remainder are driven in between them in the same manner.
In Fig. 13, b, b, b, are the mortises for the face spoke, and c, c, c, the mortises for the back spoke. Fig. 14 shows a section of an ordinary spoke, the hatched part showing the form of the greater part of its length, and the plain lines completing the rectangle show the extent of the swelling at the shoulder of the tenon at the nave.
Fig. 15 shows a very handy adjunct to the wheelwright’s shop; it is called the “centring square.” It is found extremely useful in marking and setting out the mortises for the spokes. Its construction is very simple, being but a T square, whose stock is the segment of a circle. A is the blade, B the circular stock, the extremities of which should be protected by steel or brass, or, better still, have a steel edge round the whole of the inner surface of the stock, so that it will always keep true; for if it wears at all, of course its true circular form will be destroyed, and it will be rendered useless. And another important thing should be borne in mind, and that is to make the upper edge, C, of the blade in a line with the centre of the curve from which the circular head is struck; the reason of this will be apparent to the most obtuse, for unless the lines radiate from the given centre it is useless for the wheelwright’s purpose.
After the spokes have been driven in they are shaved off by the spokeshave to their proper form, Fig. 14; and the lengths being measured from the nave, the outer tenons are cut, sometimes square, sometimes cylindrical, but leaving the back shoulder square to abut on the felloe with greater firmness. In the manner of tonguing there is a great deal of difference of opinion amongst wheelwrights; that the tongues in size should be slightly in excess of the hole or mortise to receive them, is a generally received idea, but a difference of opinion exists as to the length. But certainly it seems more reasonable to cut the tongues a little shorter than the length of the hole in the felloe to receive them, because when the tyre is put on it shrinks in cooling and draws up all the joints. Now suppose the tongues are a little longer than the holes, the shrinking of the tire causes it to press on these; and as they are firmly fixed at the nave, there is no escape for them, and the result is that the spoke is seriously crippled. It is fair to say that there are many good practical men who work in this latter way, and with to all appearance good results, but it is evident that the principle is not a good one.
One of the difficulties in making light carriage wheels is to get the spoke tightly into the felloe without splitting it, and the manner of accomplishing this is more successfully done, not by making the tongue on the spoke so large that it will fill the hole in the felloe of itself, but by making the tongue rather smaller and slitting the edges after it has been driven in, and then wedging up with small wedges, just in the same manner that a joiner would wedge up the mortises and tenons of a door.
When the wheel is so far progressed with it is laid on the ground, and the felloes are ranged round it in the order they are to be fitted. It is of the greatest importance that the holes should be bored in the felloe in an exact radial line from the nave; if this is not done, the spoke will have to be strained out of the straight line in order to get it into the hole; this will put an undue pressure upon it, and it is very likely that before the wheel has been long in use the spoke will break off short at the felloe. The exact position of all the mortises and joints should, therefore, be worked out on a full-sized drawing, and this being accurately done, there will not be much danger of going wrong. As the construction of a wheel is somewhat analogous to the arch, it is considered that by giving the felloes a number of joints the strength of the wheel is very much increased. Whether this be so or not must be left to the theorists to determine, as we have no trustworthy results from the various experiments under this heading.
The number of felloes in a wheel is decided by its size and number of spokes, two spokes being driven into each felloe. For an ordinary-sized brougham the felloes should be seven in number for the hind wheels, and six for the front wheels, or fourteen and twelve spokes respectively. For the purpose of connecting the felloes, a dowel or pin is cut on the end of one of them, and a corresponding hole bored on another, and they are fitted together; in common work holes are bored in each felloe, and an independent pin of hard wood or iron fitted into them. There is less time and labour consumed in this latter method; but the felloes constructed on this plan are not very reliable, and their weakness is soon shown by what is known as “dropping,” which is simply caused by the wear and tear to which wheels are subject working the dowels in the holes and enlarging them (the holes), and destroying the truth of the joint, which loss is soon discovered by the play or freedom given to the felloes allowing them to slip out of their place. But it must be borne in mind that this defect is just as liable to take place in felloes put together in the other way if the holes are not truly bored, and the joints are not well fitted.
The following directions as to putting on the tires are given in the “Coachmaker’s Handbook,” an American work:—
“First examine the wheels and see what condition they are in for the tire, so that we can determine what draught to give them. See if the felloes are drawn snug on the shoulder of the spokes, and how much open there is in the rim; for instance, one set we will suppose to be 1½-inch felloe, open ³⁄₁₆ of an inch, give a good ¼-inch draught; 1⅜-inch felloe, ³⁄₁₆ open, just ¼-inch draught; 1¼-inch felloe, ³⁄₁₆ open, ³⁄₁₆-draught, and so on.
“Now in determining what draught to give the above wheels, we supposed them all to be good, sound, hard, hickory felloes; if the felloes are of soft timber just give them a trifle more draught. If the wheels should be above ¼-inch dish, the felloes would want only one-half the opening, but give them the same draught as the above. In running the tire, lay all the above tire in sets on the floor, roll the wheels on them, and allow 1 inch for taking up in bending; then mark the end of the nave with chalk, 1, 2, 3, 4, &c., and the tire with a sharp cold chisel mark I., II., III., IV., &c. Then straighten on a block set up endwise about 2 feet high, a little concave or hollow in the centre, letting the helper strike while the smith manages the tires until the kinks are all taken out of them. Then bend one end a little, so that it can be got into the machine, and take pains to get them as round as possible.
“In running the wheels with a ‘traveller,’ a wedge must be driven in one of the joints of the felloe for the purpose of tightening the other joints in the rim. Then get the length of the felloe, and in running the tire cut it ⅛ inch shorter than the rim measures. In this explanation we are supposed to have steel tire, and we have a kind of steel tire now that is very high and difficult to weld, and there are many smiths that will profit by this lesson if they attend to the precaution we give. This tire steel will not stand as heavy heat as even cast steel, and if it is over-heated in the least it will crack or break in two while hot.
“There is one peculiar fact connected with it that we find in no other kind of steel, and that is this: it is apt to slip, however good the heat, and to obviate this, after scaffing the ends down to a sharp edge, make a rather sharp lap, and while hot take a sharp-pointed punch and punch a hole nearly through both laps, and drive in a sharp pin made of ³⁄₁₆-inch steel wire and ½ inch long. This will not show on the outside of the tire when on the wheel, neither does it weaken the tire like a rivet. We have often seen tires broken where the rivet went through.
“In welding, first have your fire perfectly clean, the coal pretty well charred, and the fire hot, but rather small, for the smaller the fire, if hot, the less it will waste your tire each side of the weld; have the borax charred; put a little on the weld while hot, pull the fire open with the poker and place the lap in the hottest part; roll a few pieces of coked coal on the weld, blow steadily, carrying your tire back and forward through the fire, or stop the blast a moment until the lap is heated alike all through; take it out and weld with hammer and sledge. With this precaution you will never fail getting a good welding heat, and need not upset your tire before welding. If your tire is upset before welding it makes the lap so much thicker that before it can be heated through alike there is a liability to over-heat, and waste away the tire on each side of the weld.
“In laying the tires down the heaviest should be laid at the bottom, and levelled with brick, so that the tire will rest permanently on every brick or bearing, and the rest laid on top the way they will fit best, to prevent warping the tire in the fire. A level stone should be used to lay the wheel on when the tire is put on. If the tires do not get warped in the fire do not hammer them at all, without there are some kinks left in the tires in fitting them; avoid hammering if possible, for it marks the tire; cool off, gradually pouring the water on out of the spout of a tea-kettle until it shrinks enough so it can be taken up; then roll it in soap-water to prevent it from hardening, until it is so cool that it will not burn the felloes, truing up while the helper is rolling it in the water with a mallet covered with thick leather at both ends; let the third person take the wheel and finish truing the tire with a leather-covered mallet; while it is so hot that you cannot bear your hand on it the felloes move easily under the tire, and it should not be moved after it has cooled off if it can be otherwise avoided, for this reason, when the tire gets cold all its roughness and imperfections become embedded in the felloe.”
“The tire once moved will move the easier next time. After the tires are all on examine the wheels, and see if there are any crooked spots in the tire that do not set down to the rim; should there be any, heat a short piece of iron and lay on the tire, it will soon heat it enough to burn the felloe, but take it off before that time, and rap it down with a hammer. It is a bad practice to heat the tires on a forge as some do, for in truing them in fitting we have to bend them cold, and if heated on the forge and one place red hot, you will often find there a short crook edgewise. If some of the wheels are dished more than the others, put them on the off-side of the carriage. Never take a tire off if it can be avoided, without it is so loose or tight as to spoil the wheel when run.”
When the tire has got sufficiently cold it is riveted to the felloe by countersunk rivets, one on each side of the felloe joints.
The strength or weakness of wheels plays an important part in the durability of the vehicle, for in whatever manner the various forces are mechanically met they at last concentrate themselves on the wheel; it is highly necessary, therefore, that great pains should be taken in constructing them. The stock is not necessarily the foundation on which to build a wheel, and, further, there are many objections to its being so constituted. In the first place, when its centre is all scooped out for the reception of the axle-box, and its sides are mortised out to receive the ends of the spokes, it is nothing but a mere shell. Every mortise hole is more or less a receptacle for water, which the best workmanship cannot wholly exclude; and as one part of the stock is always more porous than another, that is the part that will soonest absorb wet and begin to decay. If, therefore, the stock could be dispensed with greater durability would be insured.
A thoughtful inventor, turning over these things in his mind, has, during the last few years, produced a wheel of novel construction, which is found practically to be superior to the one in common use. All the spokes, instead of being shouldered down to enter the stock, are made wedge-shaped at the end, and instead of the wheel being constructed from the centre to the felloe, it is constructed from the felloe to the centre. Every felloe is made and fitted with its two spokes, which, as they converge towards the centre, press upon each other in such a manner that when the whole periphery is put together a solid centre is produced by the spokes themselves, as shown in Figs. 16 and 17; so that instead of being dependent on wooden stocks the spokes are dependent upon each other, and by being tightly wedged together create a mutual support and resistance. The whole are secured by two metal flanges, one at the back and one at the front of the centre of the wheel, which are tightly screwed up, by which means the greatest amount of solidity is obtained for the entire structure of the wheel.
This invention is due to the Messrs. McNeile Brothers, of the Patent Steam Wheel and Axle Company, and it is a significant fact that wheels similarly constructed have for a considerable time been adopted by the Royal Artillery; moreover, they have been extensively used on street cabs, heavy carts, more particularly the latter, and have invariably maintained their character for superiority. For ourselves, we can see that a wheel so constructed must possess peculiar advantages. There is no stock to rot, and the wheel cannot in any sense be spoke-bound, as is frequently the case with wheels of ordinary construction, by the mortise in the stock and the bore in the felloe not ranging in a true line with the spoke. In the growing desire to produce wheels of light construction, great efforts have been made to reduce the size of the centre, and the inventors of these wheels have been very successful in obtaining this object. At the centre their wheels are exceedingly light and ornamental in appearance, and to render them still more uniform they have shortened the arm of their axles, and consequently curtailed the length of the axle-box, so that there is the smallest possible projection at the centre of the wheel. At the same time all the advantages and peculiarities of Collinge’s principle are retained. In the ordinary Collinge axle the bearing is not upon the whole length of the arm, and practically speaking Messrs. McNeile have in their axles cut out all that part which is useless in this respect, so that although their axle-arm is considerably shorter, the bearing is the same as in the Collinge axle of ordinary construction.
One of the greatest disadvantages in the manufacture of wheels is the want of uniformity between one another. Scarcely any two wheels are alike. Scarcely any spokes in a wheel radiate alike; some are as much as an inch apart more than others at the felloe; and as the shrinking of the tire varies, some wheels, as a consequence, get more dish than others, the spokes either compressing in the nave mortises, or yielding by elasticity in the direction of their length. To get them at all accurate, it is necessary to employ very skilful workmen, and as skilful workmen are not so numerous as they might be, the cost of wheels is very much increased. Another disadvantage attends them: a workman may put his work badly together, and there is no means of detecting it till the wheel is in actual use. A badly framed wheel will show as well to the eye as a good one, and until it breaks down, no one, whether maker or customer, can detect the inaccuracy. Unless the master watches every wheel while the spokes are driving he can only depend on the good faith of his workmen.
There is no remedy for this evil except substituting machines for men’s hands. The machine, if it cuts true once, will cut true always. Every piece of wood in a wheel ought to be shaped by machinery. The felloes should be sawn to their exact size, curve, and length by machine saws; they should be bored by machine augurs, and rounded by machine shavers. The spokes should be tenoned by machine saws, and shaped by machine lathes. The naves should be cut by a machine lathe, and the mortises in the same cut by a machine chisel. The spokes should not be driven in by the irregular strokes of a mallet, but be forced into their places by the regular pressure of a machine. And when the tire is put on the wheel should be fixed in a frame, in order to preserve an exact size and shape. When all these things are done, we may hope to procure wooden wheels alike in form and quality, and moreover, accurately circular, which very often they are not at present. All the machines should be worked by a steam engine. There is scarcely any article of manufacture for which there is so large a demand, and there is no great variation in their mode of construction. Coachmakers generally seem to cling to the old traditions of their craft with great tenacity; possibly they think it savours of sacrilege to let progress enter their workshops too rapidly.
The above remarks may be qualified by stating that some of the largest manufacturers have introduced machinery, generally, into the departments in which it is applicable, and more particularly in the wheelwrights’ department. Some years since Messrs. Holmes, of Derby, had mechanical appliances worked by steam power for the following purposes:—Cutting tenons on the spokes, squaring the ends of the felloes, also regulating their length according to the size of wheel required; a narrow upright saw for cutting curved timber; machine for cutting felloes of the required size and curve; machine for boring felloes for the ends of spokes, and many other appliances for lightening hand-labour and insuring greater accuracy in the manufacture. But workshops filled up in this way are not yet the rule, though their number is increasing, and so are the inventions for application of mechanical power to the various processes.
It seems rather paradoxical to state that the dished or conical wheel is the strongest. But the fact is, its strength arises from the solid hoop-tyre; with a strake tyre the upright wheel would be the strongest. When running, the great lateral strain on the wheel is from the outside. Consequently, if the wheel be dished in an opposite direction, the thrust will be in the direction of the greatest resistance. The spokes cannot yield, because in yielding they would increase the area of the circle, and this the tyre will not permit. Upon the same principle in carpentry, which constitutes the curved or cambered beam the strongest, the dished wheel is stronger than the straight one.
Here is one very important item which must not be overlooked in the wheelwright department, and that is, the size of the axle-box. The axle-box is a lining of cast iron, on which the axle-arm takes its bearing. Two forms of these are given in Figs. 18 and 19.
Fig. 20 shows an improved form of stock. It will be seen it is to be applied to straight wheels, and requires no further description beyond noting that the stock is not weakened so much as in the ordinary way by mortising, an iron band, A, circumscribing the nave and forming a hold for the spokes.
Wheels should be made with a sufficient number of spokes to properly divide the space at the felloes, and afford sufficient support to prevent sinking in between the spokes, and at the same time avoid too many to weaken the stock. The less the number of spokes, the stronger the hub and the weaker the felloe. Judgment should be used in dividing the difference, so as to make each part of the wheel strong in proportion.