Trend of Motor-Truck Design Toward Worm Drive
MORE than sixty per cent of the American motor-trucks listed on the market at the present time are worm-driven. Last year twenty-two per cent of the trucks listed were wormdriven, thus showing that the popularity of this form of drive has increased.
To understand the reason for this great increase, one must first know the cause for any form of gearing for transferring the power of the truck motor to the rear wheels in order to make the truck move. The average gasoline motor of the truck of today revolves at the rate of from 1,000 to 2,000 revolutions per minute. It is out of the question for the rear wheels to revolve at any such speed because they would simply spin around and not secure enough traction between the tires and the ground to make the vehicle move. The necessary reduction between the speed of the motor and that of the wheels under varying conditions of roads is secured through some form of changespeed mechanism and the form of gearing used between the motor-shaft extended and the axle of the driving-wheels.
The latter type of gearing may be divided into four main classes as shown in the accompanying illustration, although there are some few other types used on special vehicles. The four most common types are: i, Bevel drive; 2,
Worm drive; 3, Double-chain drive, and 4, Internal-gear drive.
In the bevel drive the power of the motor is transmitted through the clutch and change-gear mechanism to a longitudinal shaft at the rear end of which there is mounted a bevel-pinion. This meshes with a larger bevel - wheel to which are connected the ends of the rear wheel axles. When the bevel-pinion is made to revolve, the bevel-wheel revolves, which in turn sets the wheels in motion and causes the vehicle to move. This construction is shown in Fig. 1.
The method of worm drive shown in No. 2 is exactly the same except that a worm-gear and worm-wheel are used instead of a bevel-pinion and wheel.
The double-chain-drive method is based upon the same principle as the former methods except that instead of the extended motor-shaft reaching the rear axle of the truck, it ends at a rearaxle unit or jackshaft attached to the frame forward of a stationary axle on which the rear wheels are mounted. The jackshaft is the same as the rear-axle unit shown in No. 1 except that instead of having wheels mounted on the ends of its shaft, it has sprockets. Endless chains are passed around these sprockets to two other larger sprockets, one bolted to each of the rear wheels of the truck. The motor power is thus transmitted to the jackshaft, to the jackshaft sprockets and thence through the chains to the rear wheel sprockets.
The internal-gear drive method shown in No. 4 also employs a jackshaft but this, instead of being mounted on the truck frame, is made into a unit with a stationary axle carrying the rear wheels. The wheels are revolved through small pinions positioned on the ends of the jackshaft which mesh with large internal gears bolted to the rear wheel spokes.
The bevel drive is used mostly on trucks of one-ton capacitv and under. It is not used on larger trucks because the greater gear reduction necessary due to the greater weight of the truck and its load would make the bevelpinion too small or the bevel-wheel too large for practical purposes. This necessarily larger gear reduction is secured by means of the doublechain drive by making the driving-wheel sprockets much larger than those on the jackshaft.
On the other hand, the necessary gear reduction for larger than one-ton trucks can be secured by means of the worm and wormwheel due to the design of the wormteeth and its rubbing instead of rolling action on the worm-wheel. It is also more efficient than the bevel or double-chain methods and delivers more power to the rear wheels because of the elimination of much of the friction of the
bevel-pinions and chains. It also has the advantage over the double-chain method in being an enclosed drive, like the internal-gear type, thus preventing dirt and grit getting on to the driving members, and causing loss of power and excessive wear.
Another advantage of the worm drive over both the chain and internalgear types is that the aggregate parts weigh less than either. This lightens the truck, permitting more of the motor power to be used for hauling the load to be carried instead of moving the heavy vehicle itself.
Measuring Rainfall on the Farm
N excellent equipment for collecting and measuring either rain or snow consists of a simple pail or bucket. The location selected for setting out the pail should be in some open lot or field unobstructed by large trees or buildings. If the diameter of the pail is just io^ ins. at the upper edge, each ounce of water collected represents 0.02 in. of rainfall. The pail should hold twelve quarts, in fact, most twelve-quart pails are exactly io^ ins. at their upper edge. The depth of the rainfall, as shown by the water caught, may be found by weighing the contents of the pail. ^ An ordinary small balance which reads in ounces and half-ounces is suitable for the purpose. In hot weather, when water evaporates quickly, the record should be made as soon as the rain has stopped, if possible.