MECHANICAL TYRE INFLATION.
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How the Problem of Avoiding the Labour of Inflating Large Tyres Has Been Overcome. By Henry Sturmey.
WITH THE ADVENT of the giant tyre for large load wagons and the more general recognition of the practicability of pneumatic tyres for commercial vehicle work, since the perfection of the cord type of air tyre, the question of tyre inflation calls for attention; because, upon the correct inflation of an air tyre depends 13.0th its efficiency and its life, and because, moreover, larger loads call for greater air pressure. In this latter 'respect, in particular, there is yet a. large field for developmont, because high pressures call for improved methods of inflation, as work with the hand-pump is not practical beyond the pressures used in passenger car tyres. As a .matter of act, tyre pumping with a hand-pump-is -one of the most
s .exhausting operations, and there are very many men owning cars to-day who would drive and fend their own ears instead of employing a chauffeur, but for the bugbear of the possibility of having to change a cover and pump up a tyre. . To meet this difficulty, many devices have been
employed. Thus, a, series th very efficient footpumps is now on the market, in which leg power instead of arm power is employed, and in which the principle of the toggle joint is introduced, all of which give superior results, i.e., lessened fatigue to the operator, as compared with hand-pumps. But. these pumps, as well as the hand-pumps, .depend upon physical effort for their operation, and, seeing that the very nature of a Inotor vehicleAupon which only the pneumatic tyre is used, implies -the existence, ready. to hand, of an engine, it has for years been recognized by many that the logical development would be to employ the engine to pump the tyres and so save the driver the whole of this
exhausting work. . . To that end numerous devices have, at various times, been introduced. Thus, according to one development, small pumps are inserted in the place of a spark plug and operated by the force of compression, exerted by the return of the piston to the top of the cylinder, acting on a larger-area piston than the one employed to inflate the tyre. These pumps have two inter-connected barrels of different diameters, with inter-conneetc.d pistons, and will pump a 4 in. tyre up to 80 lb. or so in about three minutes. A further development of this idea is a. simple check-valve device, replacing the spark plug, which enables the engine piston itself to act as an air pump, the compression air being forced directly into the tyre. This device, however, is only suitable for light cars with small tyres requiring not more than 45 or 50 lb. inflation, so it hardly comes into the consideration of the commercial vehicle user.
In America, for a. number of years, mechanical engine-operated tyre pumps have been fitted to motorcars, worked usually by a gearwheel in mesh with one of the gearwheels of the timing gear, and these have proved very satisfactory. Other mechanical pumps have been operated by a, secondary belt from the fan Pulley, and I remember, some time ago, one of this 'type was marketed, especially designed for use on Fords.
Before the war, too, this question of mechanical tyre_ inflation also took another form, as the pump was combined with an air tank, in which air was maintained constantly at the full pressure required, so that a, tyre could be instantly inflated to the regal ired pressure; or, rather, it could be inflated to that pressure as quickly as the air could get through the connecting tubing after it, was connected up. This system" however, was usually installed in connection with a device by which the air in the reservoir, operating in the reverse direction on the pump, converted it into a motor for starting the engine ; but the general adoption of the electric self-starter has caused that type of installation to pass into oblivion.
Naturally, where light commercial motor vehicles are conoerned, using air tyres of no larger diameter than those used en touring cars, and calling fm no higher pressure, either of these devices can be equally well employed, but, _with the introduction of giant tfres of 7 in., Sin., 9 in., 10 in., and even 12 in. diameters, designed for use under loaded vehicles carrying as much as eight or ten tons, very much higher tyre pressures are called for, the pressures running to from 120 to 160 lb: .presiure, or not far off double the pressures used in touring car tyres, and some very different and more efficient system of inflation becomes imperative.
As everyone who has used a, hand-pump to inflate a tyre knows, the labour, as well as the time taken, increases rapidly as the higher pressures are reached. It takes only about a minute, as a rule, to bring the pressure in a car tyre up to 45 lb., but the work then becomes harder and slower, until it. is five or six minutes before 90 lb. is readied, and tten the operator is pretty, nigh exhausted. Seeing, therefore, that it takes 15 to 20 minutes to inflate a big tyre to 140 lb. with a, hand-pump and the work increases all the time, it is easily seen that, with giant tyres, hand inflation is quite impracticable and mechanical inflation becomes a necessity, and, necessity being the mother of invention, the large use of air tyres on heavy duty trucks in America has. resulted in the development of several types of specially doigned inflators for this purpose.
These pumps are all air-cooled, their cylinders being provided with flanges like those on the cylinder of a motor-bicycle, and I illustrate, one—the Wizard—which shows the development in design which has taken place, not only in regard to the attainable pressures, but also in regard to simplicity, as, in this model, a sample of which will be shown at the Commercial Vehicle Show, there are no valves, the only working parts being the piston, connecting rod pin, and crankshaft. This is an important point, as much of the hard work and loss of time entailed with the ordinary type of pump, when the higher pressures are approached, is caused by valve leakage, so that only a small portion of the air at each stroke reaches the tyre, the rest simply coming ba,ck into the pump. In the Wizard design, the piston goes Aght to the top of the cylinder, filling the entire cylinder space, on. the top of its stroke, the constructional detail and operation being as follows:— The crankshaft gives the piston an oscillatory motion. When the piston is at the highest point, air intake holes uncovered by the piston admit air into the crankcase, and here it receives its first compression. On the down stroke, a groove in the piston registers with a larger, air intake, and the space above the piston is filled. Further dawn the stroke, file groove in the piston registers with a groove in the cylinder, admitting the compressed air in the crankcase into the cylinder above, thus giving an overcharge of air. On the up stroke, the air is compressed until a groove on the piston on the side opposite to the one already referred to registers with the outlet port, the piston thus being able to travel to the top of the cylinder and all valves being dispensed with.
This pump, as are most others designed for commercial motor tyre inflation, is not operated off the timing gear of the engine, but by a gearwheel which engages with, and is driven by, one of the gearwheels in the transmission case' to which the pumps are designed to be attached. So fully has this system of working been recognized in
America as being correct, that the Cotta Gear Co. and other firms making commercial vehicle transmission for the trade over there, are now standardizing gearboxes with an aperture in the cover for the entry of the gearwheel, and bolt holes and mountings for the attachment of the pump. With pumps of this type, large tyres can be inflated to required pressures in five or six minutes, and it may be mentioned here that correct inflation with these large tyres, under,' heavy loads, is even more important than in the ease of touring cars, if satisfactory tyre service is to be secured and troubles are to be minimized.