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U.S.A. War Department Specifications for Motor Transport Chassis.

7th June 1917, Page 6
7th June 1917
Page 6
Page 7
Page 6, 7th June 1917 — U.S.A. War Department Specifications for Motor Transport Chassis.
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Forecast of the British After-the-Peace W.D. Model. Freedom of Choice for Final Drive. Emancipation from Fussy Insistence on Adherence to Detailed Dimensions, Twelve Good Points and True.

_Ellis concludes the list of the twelve main requirements. Rigid adherence to these points is required. There are one or two less vital but 'nevertheless equally interesting features of the specification upon which we may now touch.

Engine.

As regards the engine, in view of American practice generally, it is of interest to note that the gudgeon pin is to be secured in the piston, the connecting rod being fitted with a phosphor-bronze hush to take a bearing on the pin. The unit power plant type of transmission is not to be accepted. The gearbOx must be three-point suspended, and must be a separate

unit. The test pressure for the cylinder water jackets is 100 lb. per ,square inch, which appears to be rather high. The British specification only calls for half that amount, and some surprise was expressed even at that requirement at the time when the British specification was drawn up.

Onepiece construction is insisted upon for the valves ; this, also, is interesting in the light of American practice, as a considerable use has been made over there of valves haying heads and stems of different materials. Any arrangement of valves is permitted, that is to say, L head, T head. or even overhead valve engines will he accepted. Whatever the arrangement, however, it is insisted that the whole of the valve gear be suitably enclosed, and if of the overhead type, suitable felt washers to be fitted to the rocker arm bearings to prevent the entrance of dust or dirt. The capacity of the oil reservoir must be such that the vehicle can be run fully loaded for a distance of not less than 200 miles over ordinary

roads without replenishment. The engine oiling sys i tem is by pressure throughout, 30 lb. per square inch being desired as a minimum with the engine running

at 800 revolutions per minute. This pressure oiling extends to the camshaft bearings, as well as to the big ends and main -crankshaft bearings. Both the main bearings and big-end bearings are to be so arranged that any wear can readily be taken up. In order to provide for this, laminated metallic shims, made up of sheets of equal thickness, which thickness must not exceed .0015 in., must be provided.

Fuel Supply.

In camection with the petrol supply system, it is notable that no location is fixed for the petrol tank, but it is asked.that the carburetter be fitted as•high as possible on the engine, and it is insisted that with the sinallest possible supply of fuel in the tank, and while ascending the steepest negotiable gradient,

there must be no possibility of a failure of fuel supply to the carburetter. It will be gathered that this means that the petrol tank will either have to be carried on the dash, or that some form of pressure or vacuum feed will have to be installed.

Dust Collectors.

An important requirement, which evidently reflects experiences gained in the Mexican campaign, is that dust collectors of suitable design are to be fitted both to the carburetter intakes and also to the crankcase breather tube. If desired, the same collector may serve both purposes.

Radiator Well Protected.

Provision is made for a radiator protector, which mast extend the full height and width of the radiator. It is to be constructed of heavy structural steel, or steel castings, and must be firmly bolted to the chassis frame. It must be so installed as not to interfere with the starting handle.

Low Load Line.

The construction of the chassis must be such that the load line of the vehicle must not be higher than. 45 ins. from the ground when fully loaded.

No Fabric Discs as Universal Joints.

Two universal joints are to be provided between clutch and gearbox, and two between engine and differehtial. Fabric universal joints are not to be permitted.

Freedom of Choice for Final Drive.

With regard to the final drive, either internal gear drive, chain drive, worm gear drive, or double reduction gear in a central housing is permitted. The driving axle may be either of the full floating or semi-floating type. A clause stating that preference will be given to that type of final drive which provided the greatest amount of ground clearance, in conjunction with provision for retaining grease or . oil and exeluding dirt, would seem to exclude exposed chains as a means of transmission.

For the wheel and axle bearings, steel taper roller or two-point ball bearings must be provided. All thrusts must be compensated for by adjustable ball or roller bearings. The plain type of thrust bearing or three-point ball bearings will not be permitted.

• Brake Gear.

Particular attention has been paid to the drawing up of that part of the specification which deal S with the brakes. The usual two sets of brake. geararc to be fitted; they may both operate on the rear wheels, or may be fitted one set to the rear wheels and the other on the transmission gear if desired. Either, however, Must be sufficiently powerful to skid the wheels or to stop the vehicle within a distance of not more than 50 ft., with the vehicle running on a level, dry macadam road, at a speed of 15 miles per hour. The brake adjustment must be readily accessible, and any adjustment must be possible without the necessity for tools. The brake pins, joints, or other bearings, must be fitted with bronze bushes.

Springs.

_ In regard to springs, we have already made mention of these to the extent of stating the definition of the steel which is to be used. The deflection of the rear springs must come between limits of 1000 lb. per in. and 800 lb. per in. ; the front springs to be not more than 800 and not less than 500 lb. per in. The maximum deflection which must be limited by permanent stops, muse be stated by the maker when tendering. These stops must be so arranged that they will: prevent the springs from being deflected to a point at which the ,stress will be greater than 75 per cent, of the elastic limit of the material from which they are made, Chassis Weight.

The maximum weight of the chassis without body., tools, or equipment, and without petrol, water, or is to be 6000 lb. The load to be borne by the chassis ia to be made up as follows :—

Net useful load 3000 lb.

Body 1050 Lb. Three men ... 525 lb.

Extra, equipment 200 lb.

Total ... 4775 lb.,

which is equivalent to a gross weight of chassis, body, load and equipment‘410,47:5 lb. As already stated, it is recommended that these chassis should be designed as though for a net useful load of 4000 lb. The corresponding weights in the case of the British one-and-ahalf-ton B type subsidy model are :

Gross weight : 5 tons (11,200 lb.).

Useful load : 14, tons (3360 lb.). Weight of chassis, body, men and equipment: 1 ton 5 cwt. (2800 lb.).

Net weight of chassis as 2 tons 5 cwt. (5040 lb.).

The Three-ton Chassis. •

As regards the three-ton vehicle, the weight of the chassis is not to exceed 8000 lb. This is without body, tools, equipment, petrol, water or oil. The load will be made up as follows : Useful load ... '... 6000 lb.

Body ... ... 1500 lb. Three men ... .. 515 lb.

Equipment ... 200 lb.

8225 lb.

The gross weight, therefore, of chassis, body, load and equipment is 16,225 lb. The corresponding figures asked for in the British specification are: Gross weight ... 7 ton 10 cwt. (16,800 lb.) Net useful load ... 4 ton10 cwt.(10,080 lb.) Body, 3'men, and equipment... 1 ton 10 cwt. (3,360 lb.) Net weight of chassis, 3 tons (6720 lb.).

The wheelbase must be suitable for the accommodation of a body 12 ft. long ; it shall not be less than 10 ft. 4 ins., or more than 13 ft. The British specification for a corresponding size chassis asks for a wheelbase of not less than 13 ft. The wheel track may be from 591ins. to 64i ins, on the 5-in. single-tired front wheels ; it must be 644ins, in the case of the 5-in. twin-tired rear wheels.

Engine.

The engine must have a capacity of not less than 411 cubic ins., and must develop a, torque of not less than 2475 inch-pounds at a normal piston speed of 1000 ft. per minute. Torque at half this speed must be 90 per cent. of the maximum.

The British specification calls for an engine, minimum here 41 ins. ; the stroke is not stated ; the horsepower is to be 30. If we presume a stroke of 54j

the capacity would be 362 cubic inches. The American specification, therefore, calls for an engine 14 per cent: greater than the British. As regards torque, 30 h.p., at the speed given and with a stroke engine, is equivalent, in round figures, to 1100 revolutions per minute, the torque being 1720 inch-lb. This, again, is considerably less, size for size, than is asked for by the American specification.

The low gear ratio must not be less than 50 to 1, as comparing with 40 to 1 on the smaller chassis, and 42 to 1 in the corresponding British vehicle.

Springs.

The deflection of the rear springs shall be not more than 1500 lb. per inch, and not less than 1000 per inch. The deflection of .the front springs shall be not more than 900 and not less than 650 lb. per inch.

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Organisations: U.S.A. War Department

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