THE .ROYAL ARTILLERY INSTITUTION. 
375 
M.L.R. field guns and carriages—in which inertia to recoil in the wheels 
is a desideratum, providing their strength admits of their accepting it. 
Further, the height of the shafts and convenience of loading must be 
taken into consideration in determining the diameter of a wheel. In 
the field wheel, and other wheels for ordinary purposes, the height of 
5 ft. has been adopted, as best fulfilling the above conditions; in other 
wheels the height is varied to suit special requirements. 
With regard to the weight of a wheel, it should be at a minimum, 
provided the strength is sufficient, to increase the mobility and diminish 
(in the case of a gun-carriage) the inertia of recoil; but, as before said, 
there are cases in which it is advantageous to increase the inertia. It 
is further convenient, when circumstances admit of it, to have the 
weight not greater than two men can lift. 
Wrought-iron is employed as the material for axletrees, on account of 
its strength and toughness. If for use with wheels having cast-iron 
pipe-boxes, the arms are steeled, to prevent too rapid wear. 
The general dimensions of an axletree depend, naturally, upon the 
weight and nature of the load which it has to support. Its cross 
section is kept at a minimum, consistent with strength, for the sake of 
lightness, and also in the arms, in order that what friction is produced 
between them and the wheels may have as small a leverage in its favour 
as possible. 
In axletrees bedded in wood, the body of the axle is made rectan¬ 
gular in cross section, that there may be better hold or union between 
the axle and the bed. In the later patterns of transport wagons, wood 
beds have been dispensed with, as inconvenient and superfluous. In 
these carriages the whole strain, whether vertical or horizontal, is 
applied close to the shoulder of the axletree, and the body within these 
straining points is relieved; hence a smaller section is admissible for 
the latter, and as it has no strain in any particular direction, as well 
as for convenience, its form is generally circular. A downward in¬ 
clination or “ hollow” is given to the axletree-arm, in order to bring 
the lower spoke vertical when it comes immediately under the arm, 
the wheels being on the level; otherwise, owing to the dish of the 
wheel, it would be in a bad position—in fact, the perpendicular from 
centre of resistance would not pass through centre of pressure in 
the pipe-box. When, under the above conditions, the working spoke 
is not brought quite vertical, but at an angle outwards to the vertical, 
the wheel is said to have a “ strut,” the amount of which is expressed 
in inches, and measured along the axis of the wheel from the point 
where the prolongation of the face of the spoke meets it, to the point 
where the vertical drawn from the extremity of the face at the bosom 
of the felloe meets the same axis. 
Though, on the level, giving a strut to the wheel is slightly disad¬ 
vantageous, on the whole a certain amount of strut is decidedly good, 
as assisting the wheel—or rather its lower or working spoke—under 
the most trying circumstances, when it dips into a rut, &c. Formerly, 
most wheels had more or less strut; for instance, the field wheel had 
about 1 in. At present, no strut is given in the N.P. field wheel, but 
\ in. in the 5-ft. wheel for transport carriages, and to others for the 
same service in proportion. 
