Page History: Farm Motors-Steam Traction Engines
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Fundamental Parts of a Traction Engine
A steam or a gas engine, explained in the previous chapters, can be converted into a traction engine by mounting it on trucks and providing additional mechanisms, so that the engine not only will be capable of producing rotation at a shaft, but also will move itself over fields and highways, thus performing the work of many horses in a cheaper, quicker and better manner.
All traction engines must consist of the following fundamental parts:
1.
Power Plant—This, in the case of steam traction engines, consists of a steam engine and boiler. Gas traction engines employ an internal-combustion engine burning gasoline, kerosene, or some heavier oil.
Power-plant accessories include valves and piping from boiler to engine, fuel hopper, water tank, safety valve, water glass and try-cock, steam gage, blow-off, pump or injector or both, a stack and spark arrester. Some steam traction engines have also a feed-water heater which heats with exhaust steam the feed water before it enters the boiler. The accessories of the gas traction engine power plant are fuel tanks, water tanks, batteries and battery boxes, magnetos, carburetors, cooling systems.
2.
Transmission Mechanism—The speed of the engine is too great for direct utilization, and a train of gears must be interposed between the engine and drive wheels.
3.
Reversing Mechanism—Reversing of a steam traction engine is accomplished either by a link similar to that used in locomotive practice, or by some form of single eccentric radial valve gear. It is more difficult to reverse a gas traction engine and a train of gears, similar to that of an automobile, must be employed.
4.
Steering Mechanism
5.
Differential or Compensating Gear—The purpose of this is to allow one drive wheel to revolve independently of the other, this being necessary when turning corners, as is the case with automobiles.
6.
Friction clutch for disengaging engine from propelling gear, so that the power of the engine can be utilized for the driving of separators or other machinery.
7.
Traction-engine frames for supporting the power plant, transmission mechanism and other parts and for keeping all parts in proper alignment. Structural-steel I-beams, angles and channels are employed for frame construction. Cast iron is also used for certain parts.
8.
Traction or drive wheels (Figs. 161 and 162), which must be provided with lugs to give them a firm footing on the ground, and with mud shoes.
9.
Front Wheels—These are made smaller and lighter than the traction wheels, and are provided with smooth tires. To prevent skidding the front wheels are built with a rim in the center (Fig. 170). The front wheels turn upon an axle which is attached to a ball and socket joint, or to some similar mechanism, so as to allow for uneven ground and also to facilitate steering.
10.
Boilers—The boiler of the steam traction engine is internally fired. Some builders utilize the return-flue type (Fig. 158), others the direct-flue or the locomotive type (Fig. 159).
Coal, straw, wood and crude oil are used as fuels for traction engines. In some states lignite is used. Some builders supply traction engines with attachments for converting them from coal-burners into oil-burners.
When using straw for fuel, the furnace is modified as shown in Fig. 160. Slab grates are then substituted for the ordinary coal grates and the straw is fed through a chute S. A hinged trap T is provided to prevent the entrance of air when the straw is not being fed.
To maintain the proper draft, steam traction engines are provided with a blower through which live steam is passed into the smoke stack when starting. When the engine is running it exhausts into the stack through an exhaust nozzle.
In some makes of traction engines, the boiler is mounted upon the truck and is used as the foundation for the engine (Fig. 161). Other types (Fig. 162) have the engine mounted under the boiler, the frame supporting both engine and boiler.
11.
Pumps—Three types of feed pumps are used on steam traction engines: the independent pump which is similar to the types illustrated in Chapter III, the cross-head pump P (Fig. 163), which is driven from the engine cross-head C, and the gear-driven pump (Fig. 164). As in the case of stationary engines, two independent methods should be provided for feeding water into a traction engine boiler, using either two pumps or an injector and a pump.
12.
Feed-water Heaters—Feed-water heaters are used on some traction engines. The type often employed is illustrated in Fig. 165. The feed water passes around the tubes and the exhaust steam passes through the tubes.
13.
Engine Types—The type of engine usually employed is some simple form of steam engine with a slide valve (Fig. 166). Some traction engines have double-cylinder engines. Compound engines (Fig. 167) are also used to some extent.
The details of the engines, governors and accessories do not differ from those described in Chapter IV.
14.
Reversing Mechanisms for Steam Traction Engines—A steam traction engine can be reversed either by a Stephenson link similar to that used on locomotives, or by some form of singleeccentric radial valve gear.
To reverse an engine by means of the Stephenson link, it must be provided with two eccentrics, each being connected by an eccentric rod to the end of a link. A block connected to the valve slides along a groove in the link.
This type of reversing link as applied to a traction engine is illustrated in Fig. 168. The two eccentrics shown at E are attached to the curved link L by means of the eccentric rods A and B. The position of the link is varied by the reverse lever through the reach rod. In one position of the link the motion to the valve is given by one eccentric, driving the shaft in one direction. This direction of rotation is reversed by raising the link, so that the valve receives motion from the other eccentric. If the reverse lever is moved so that the block is in the middle of the link, the motion given by both eccentrics will be equal and opposite, and the valve will have no motion.
Most traction engines employ a single-eccentric radial valve gear (Fig. 169). This reversing gear consists of an eccentric fastened on the crankshaft with an eccentric strap which has an extended arm, pivoted in a sliding block. The block slides up and down in a guide and gives motion to the eccentric rod, which is transmitted to the valve through the rocker arm and valve stem. The block guide is hung on a trunnion and it can be tilted in any direction by the reverse lever acting through the reach rod. The angle at which the guide is set determines the direction in which the engine is to run. The reverse quadrant is usually provided with three notches. When the reverse lever is in the central notch, no motion is given by the sliding block to the valve stem. In the position shown, the block sliding up and down in the block guide moves the valve in one direction. Placing the reverse lever in the notch at the extreme right reverses the engine.
15.
Steering—Steering is accomplished by turning the front axle. This is done by chains C (Fig. 170) which wind upon a spool. The spool is operated by hand through a worm W and pinion P (Fig. 170). Another method is to operate a screw by the worm and pinion, the screw moving a nut which is connected by a system of levers to the front axle. Some traction engines employ steering mechanisms similar to those of automobiles. On large traction engines steering is accomplished by power furnished by the engine through a friction disc.
16.
Transmission Systems and Differentials—A friction clutch, the function of which is to disengage the engine from the propelling gear, is illustrated in Fig. 171. The flywheel W is fixed to the engine shaft, and, when used as a belt wheel, it is not connected to the arm C, and thus does not transmit motion to the pinion F which is rigidly connected with the arms C. When the clutch is thrown in, pressure is applied at E which rests in a groove in the piece D. This results in B crowding the shoe A against the inner rim of the flywheel. The friction clutch has two shoes made of wood or of some other yielding material AA, which press against the inner rim of the flywheel when the clutch is thrown in, and this transmits the motion of the engine through the arms C and pinion F to the transmission. Means are provided for taking up the wear on the shoes so as to keep the clutch effective at all times.
The transmission mechanism delivers the power from the engine to the traction wheels which must revolve slower than the engine crankshaft. The transmission system of a steam traction engine is very simple and consists of a train of spur gears (Fig. 172). The gear A receives motion from the engine and delivers this through the train of gears to the gear B, which is connected to the traction wheel.
When a traction engine turns a corner, the drive wheel on the S. outside of the curve must turn faster than that on the inside. If the two drive wheels were rigidly connected, one would have to skid or slip, when turning a corner, and this would throw a great strain on the front wheels and axles. The differential, sometimes called a compensating gear, allows, if occasion demands, one drive wheel to move faster than the other.
In principle the traction-engine differential is similar to the automobile differentials (Figs. 141 and 142).
The differential can be placed between the two drive wheels on the rear axle. A more common method is to have the differential on a separate shaft, the traction wheels being driven from that shaft by means of pinions.
The principle of differentials as applied to steam and gas traction engines is illustrated in Figs. 173 and 174. The differential shaft S consists of two parts, each being connected either directly or through gears to the drive wheels. Two bevel gears are keyed to these two differential shafts and engage several bevel pinions, marked B, which turn freely on their respective shafts. The power from the engine is transmitted through the pinion P to the large spur gear A. When the engine is going ahead on a level road and both drive wheels are rotating at the same speed, the two bevel gears will also revolve at the same speed and the small pinions marked B will remain stationary. In turning a corner or in meeting some obstruction, if the drive wheel connected to one bevel gear moves slower than that connected to the other, D, the pinions B will revolve on the bevel gear D. In other words, the difference in motion between the two drive wheels is compensated for by the revolution of the pinions B.
Another traction-engine differential, as applied to gas traction engines, is shown in Fig. 175, the letters designating the same parts as in Figs. 173 and 174. The two pinions E and F connect the differential with the two drive wheels. W is a brake wheel.
Information Sources
- Farm Motors by Andrey A. Potter 1917 pages 168-180