WHEEL. This is one of the six powers of mechanism ; and without doubt, con tributes more than any of the other five to the general convenience- of mankind, by the wonderful variety of purposes, from a mill to a watch, wherein it is employed. It is our intention, however, in this place, to confine ourselves to the wheel as ap pertaining to vehicles in general, refer ring the readers to the articles MILL work, WATCH work, CLOCK work, &c. for the ap plication of such wheels as come within those branches of the arts.
Of carriage wheels, in general, we shall then treat; observing, that any attempt to prove that a carriage is more easily drawn upon wheels than upon sledges, would be an affront to the understanding of the reader. But whether high, or low, wheels are fitted for the purpose, has been a sud ject of dispute, even among persons of skill. Reason and experience, however, seem perfectly to agree in this, that wheels, whose centres are on a level with the moving power, will be easiest drawn along a level plane ; and that the higher a wheel is the more easily will it get over the obstacles it may meet with, provided the moving power be not below the centre. It seems to follow, therefore, that carriages drawn by horses, or oxen, should have wheels whose centres have the height of the draft. line ; that is, of the shoulders of the hoises, or the yokes of the oxen. This is true, however, only in the case of a horizontal road ; in going up hill the distance of the line of draught from the road is somewhat less; because, when a Alan, or any other, animal, is standing upon the side of a slope, his height is in clined to that slope ; or rather the slope is inclined towards him, where he stands perfectly perpendicular. This being the situation in which cattle labour most, it is necessary to proportion the draft, so as to render it as light as possible while draw ing up hill ; therefore, it is usual, and highly proper so to proportion the height of the axle, especially in carts with two wheels, to the point of draught, that the line drawn from of the wheel to that point should rise at an angle of about twelve or fourteen degrees ; thus, when the horse is labouring up hill he will come nearly to a level with the wheel's centre, and draw to the greatest advan tage. This may serve as a Keneral rule ; but where local circumstances prevail of a different tendency, and also in particular cases, the height of the wheels must be suited to meet such. We reckon that in ordinary work, and where the horses do nut exceed the height of fifteen hands and a lutlf, the wheels should be from four feet eight inches to five feet two. Yet the immense loads drawn in the coal carts at Glasgow, on wheels more than six feet high, and other instances of a like kind, prove that very great powers are gained by using high wheels ; tinder due con struction and application the difference of the wheel's weight will not prove any material drawback. In ascending, high wheels will he found to facilitate the draught in exact ratio with the squares of their diameters; but in descending they are liable to press in the same proportion. An admirable device was produced by Lord Somerville, for throwing the weight behind the centre in going down hill, by *king the fore-part of the body of a cart; so that while the shaft may incline downwards, in proportion to the line of declivity, the bottom of the cart's body should remain horizontal ; this construc tion is now common in Devonshire, So mersetshire, &c.
Wheels are commonly made with what is called a dish ; that is, the spokes are set at an angle into the nave, or centre piece ; so that, when the interior end of the nave is placed on the ground, the wheel may appear to be dished, or hol low, in the centre. Experience has
shown, that when wheels have been made cylindrical, and not with the conical hol low just described, so that the spokes stood at right angles with the centre of the axle, numberless inconveniences arose; the dirt taken up by the wheel used to fall in between the nave and the axle, so as to choak and wear it considerably. Such wheels also required to stand wider apart, and demanded greater roach way; be - sales they were very apt to be wrenched when pressed by any exterior resistance, and the spokes were forced back in the mortices. According to the present plan of dishing wheels, usually to about four inches in five feet of diameter, the -ex terior resistances are avoided ; the axle being so turned down at its ends, as to cause the lower spoke, which bears up the load, to stand perpendicularly under the centre ; thus occasioning the upper parts of the two wheels on the same an gle to spread from each other ; while the lower parts converge in the same propor tion. Cylindrical wheels, that is, such as are not dished, would answer, provided the carriage were always on a perfectly horizontal plane ; but they would subject the nave to he loaded with mud, and pinch the load, when consisting of light articles rising above the body of the carriage.• • The spokes should be set so far from the outer end of the nave, that a perpen dicular from the sole to the under side of the axle may fall, between an inch and two inches, between the hushes. By this, the pressure will be somewhat greater on the outer than on the inward bush, whe» the wheels are on a level. This ought to be so ; for the inner part of the axle arm being much bigger than the outer, it has more friction, therefore should have less pressure ; besides, every sinking of the wheel, more than the other, causes it to pinch the inner bush. The best mode ofplacing spokes in the naves, is to mortice them in two rows, alternate this does not weaken the centre so much as when all the spokes are in one row, or band, and gives a greater degree of resistance outwards. The tire, or iron binding of a wheel, must be so laid on, whether in one or more bands, as to form the frustrum of a cone ; but in heavy wag gons it is usual to make the middle of the tire rise considerably, so as to bear the whole weigh( on hard roads, whereby the carriage will move lighter than if the frus trum were rectilinear ; this form likewise causes stones, &c. to slip aside ; but in soft soils it is apt to occasion much sink ing. The axle arm should be taper, in order that it may give the wheel rather a disposition to slide off; otherwise it would be apt to close inwardly, anfl cre ate excessive friction; hence the necessity fbr good iron wasters exteriorly, and sub stantial linchpins. There is a common practice of setting the wheels fbrward ; that is, giving them a slight inclination towards each other, whereby they are perhaps an inch nearer at their front than at their backs ; this is clone to make the wheel run more even on its sole, or bear ing part, and to prevent its gaping for ward ; but it is evidently a distortion, which prevents the wheel from running exactly at right angles with the transverse section of the carriage. The nave of a heavy wheel, that is, for our ordinary cart for field purposes, need not be more than twelve or fourteen inches in length ; if too short, the wheel will wabble, unless fitted very tight on the axle ; while too long a nave is apt to catch the dirt from the upper part, and to too much beyond tile outer face of the fellies ; the above length is exclusive of the pan at the outer end.