On

the five-bearing shafts as depicted in Fig. 50-B are used on motors having individual cylinder castings. medium and heavy-duty power plants, the five-bearing crank shaft is considered better because it is supported by a bearing at the sides of each crank throw. The crank shaft shown has a flange forged integral at one end to which the fly-wheel is attached by bolts.

A

B

FIG. 50.-Crankshafts for Multi-cylinder Motors. A-Two Throw Crankshaft With Timing Gear Attached. BFour Throw, Five Bearing Shaft.

Utility of Fly-wheel.-The gasoline engine of the one or two-cylinder type would not be a practical power producer without some means of equalizing the uneven power generation. Energy must be stored up to carry the crank shaft and other moving parts through the idle strokes and this is usually accomplished by providing a heavy wheel which will continue to revolve after an initial impulse given it no longer exists. A fly-wheel is usually a heavy cast-iron member, having the rim, spokes and hub cast integral. The larger diameter flywheels do not need as heavy rims as do those where the weight is carried nearer the center of the shaft. The fly

wheel weight depends upon the number of cylinders and power of the motor. Assuming conditions where motors would have the same power, those having the least number of cylinders would require the heaviest fly-wheel.

As a general rule about ninety to one hundred pounds fly-wheel weight is allowed to each horse-power of moderate speed one and two-cylinder engines. As the number of cylinders increases, the number of explosions and the torque become more uniform and the fly-wheel weight can be reduced. If a single-cylinder engine of a certain power required a fly-wheel of four hundred pounds weight, a double-cylinder engine would operate satisfactorily with one weighing three hundred and twenty pounds and a four-cylinder motor would require but two hundred pounds of fly-wheel weight. Some of the important conditions that determine fly-wheel dimensions are bore of the cylinder, compression in pounds per square inch, speed of crank-shaft rotation, and method of power transmission.

Usually multiple-cylinder engines of large bore have fly-wheels heavier than are actually needed in order that the engine may be more easily started. Heavyduty engines require heavier fly-wheels than mediumduty types and engines which must run steadily, such as those furnishing power to drive a dynamo for electric lighting, require heavier fly-wheels than do motors intended merely for traction purposes. Fly-wheels used on tractor engines are generally simple-spoked forms when made for single-cylinder engines though the forms used for multiple-cylinder motors often dispense with spokes and have a solid web of metal joining the fly-wheel rim and hub.

It is important that the fly-wheel be held to the shaft in a positive manner. The simplest method consists of

boring out the fly-wheel hub so it is a tight fit on the crank shaft and then driving in a key between the shaft and hub into suitable keyways machined in these members to prevent the fly-wheel from turning. A better method of fly-wheel retention is outlined at Fig. 51. In this a flange is formed integral with the crank shaft and the fly-wheel member is firmly secured to this by means of substantial retaining bolts. When these are properly fitted it is practically impossible for the fly-wheel to become loose, as sometimes happens in the simple-keyed construction.

FIG. 51.-Typical Fly-Wheel and Method of Attaching to Crankshaft Flange by Bolts.

The diameter of a fly-wheel must be held to certain limits and the weight one can put at the rim is restricted by the limits imposed on circumferential speed. It is stated that a safe speed for a cast-iron pulley is about a mile a minute for any point on its circumference. The

slower the speed of the engine, the larger the diameter of the fly-wheel one can use and the more effective it becomes as a balancing member. A fly-wheel 3 feet in diameter would have a circumference of approximately 10 feet. As any point on the rim would turn this distance in a revolution, in order not to exceed the safe peripheral speed the fly-wheel should not revolve faster than 500 revolutions per minute. If a fly-wheel was 6 feet in diameter its circumference would be twice as great as the small member previously mentioned and its speed would be limited to 250 revolutions per minute.

Engine Base and Bearings.-All gasoline engines require a substantial base member to which the cylinder or cylinders are attached and which supports the crank shaft as well. The engine base must be of substantial construction in order to keep all parts of the motor in alinement and will vary widely in form depending upon the type of engine. Engine bases used on the simpler tractor power plants are made of cast iron, though some of the engine bases used with the multiple-cylinder engines of the automobile type are made of aluminum. Cast iron is a suitable material for this purpose, and while it is somewhat brittle and unreliable in nature when made in light sections, there is not the need for saving weight in a tractor engine that exists in automobile practice, so strength is obtained by using a little more iron.

A simple engine bed such as used for a single-cylinder stationary type power plant is shown at Fig. 52, and it will be noticed that the lower portion of the crank-shaft bearings are formed integral with the engine bed. On the multiple-cylinder engines the form of the crank case will depend upon the number of cylinders and their dis

position. They are often approximately cylindrical. members, which may be divided horizontally along the crank-shaft center line or which may be in one piece, having end plates to support the main bearings and large openings through which the interior mechanism can be inspected. It is obvious that the diameter of the crank case must be large enough to permit the crank shaft and connecting rod big ends to turn inside of it, and its length is controlled by the number of cylinders and their arrangement.

FIG. 52.-Base for Single Cylinder Heavy Duty Motor.

The crank case of a single-cylinder or double-opposed cylinder, horizontal type, would be about the same in length. That of a four-cylinder engine will depend upon the method of casting the cylinders. A shorter crank case is utilized when the cylinders are cast in pairs than when individual cylinder castings are employed. The crank case shown at Fig. 53 is a type used on a threecylinder engine and is somewhat shorter than would be necessary with four cylinders. As will be evident, it consists of two parts. The upper portion serves as the