« НазадПродовжити »
Q. What are the advantages of the two piece crankcases ?
A. When a two piece crank case is employed the lower portion serves merely as an oil container or cover and the crankshaft is supported by the upper half. When it is desired to adjust the main bearings or the connecting rod big ends the lower portion or oil pan of the crank case may be removed and the entire interior of the crank case is readily accessible. (Fig. 55.)
Q. How can interior parts of barrel type crankcases be reached?
A. Most forms of barrel type crankcases are provided with large openings or hand holes at the side which are closed by easily removable plates to permit of ready access to the crankcase interior.
Q. What is the function of the flywheel?
A. The flywheel is a heavy cast iron member attached to the crank shaft and is utilized to keep the crank shaft turning during the idle strokes by virtue of power stored in its rim during the working strokes of the motor. The one and two cylinder forms of engines require much heavier flywheels than four and six cylinder forms need. (Fig. 57.) Q. What determines flywheel diameter?
The flywheel diameter is determined by the materials of which it is composed and by the speed of the crankshaft. A certain peripheral speed limit has been established for cast iron, which it is not desirable to exceed.
Q. Define peripheral speed.
A. The peripheral speed of the flywheel is the distance that any one point on its circumference covers in a given unit of time. Peripheral speed depends directly on the size of the flywheel and the number of revolutions at which it turns. For a given diameter the flywheel that turns the fastest has the greatest peripheral speed.
Q. What is safe peripheral speed for cast iron flywheels?
A. The safe peripheral speed for cast iron flywheels of ordinary construction is approximately 5000 feet per minute. A flywheel 5 feet in circumference could not turn more than 1000 R. P. M. without exceeding this limit.
Fig. 55.-Top Half of Horizontally Divided Two-Piece Crank Case with Crankshaft, Camshaft, Operat
ing Gears and Connecting Rods in Place.
Q. What can be done to increase Aywheel speed without danger?
A. The flywheel may be made of a stronger material, such as steel, if it is desirable to retain the same diameter, or it may be made smaller in diameter if made of cast iron.
Q. Describe simplest type of flywheel.
A. The simplest form of flywheel consists of a web having a heavy rim or a simple cast hub joined to a heavy rim member by a series of radiating arms or spokes which are cast integral with the hub and the rim.
Q. What is the advantage of web wheels?
A. The web wheel is a safer method of construction and is also stronger than the spoke type. When a flywheel of the spoke type is turning at a high rate of speed it will do considerable injury to any object inadvertently thrust between the spokes. As the web construction makes practically a solid flywheel it is not possible to catch anything in the wheel.
Q. Why are flywheel spokes sometimes shaped like fan blades?
A. Fan blades are often incorporated in flywheels so that member acts as a fan or blower to induce a draft of air through the radiator of a water cooled engine or around the cylinders of an air cooled type.
Q. How are clutches attached to flywheels?
A. Cone clutches have the female members recessed in the fly. wheel interior, while multiple disc clutches or those of the expanding band type are housed in a drum or casing either machined with the flywheel casting or attached thereto by some method of mechanical fastening. Three and five plate clutches are carried on simple stud members screwed into the flywheel.
Q. How are flywheels attached to crankshafts?
A. Flywheels are attached to crankshafts either by means of a taper shaft and key retention or by means of bolting it when of the web type to a flange formed integral with the crankshaft. The latter is the preferred construction.
Q. Why must flywheels be securely fastened?
A. It is imperative that flywheels be attached firmly to the crankshaft, because any looseness between the flywheel and the shaft will produce a pronounced knocking sound, and the methods of fastening, such as keys or bolts, may be entirely sheared off.
Q. What is the meaning of marks on flywheel rims?
A. The marks placed on flywheel rims, as indicated at Fig. 39, are placed thereon to assist in timing the valves and ignition spark. The abbreviation E. 0. means “Exhaust Opens,” while E. C. means “Exhaust Closes." The letters I. 0. mean “Inlet Opens, while I. C. means “Inlet Closed." The letter S. indicates the sparking point, while U. C. means “Upper Center,” which means that the piston is at the top of its stroke, and L. C. means “Lower Center," which corresponds to the crank shaft position when the piston in the cylinder being timed is at the bottom of its stroke.
Q. What influence does flywheel diameter have on spacing of timing marks?
A. For a given number of degrees crank travel the marks are farther apart on large diameter flywheels than they are on those of lesser radius.
Q. What are the manifolds?
A. The manifolds are the built-up members or pipes that convey the fresh gas from the carbureting device to the valve chambers or which convey the inert products of combustion from the exhaust valve chambers to the muffling device.
Q. How are they secured to cylinders?
A. Manifolds are usually attached to cylinders by means of flange couplings bolted to the cylinders or by stirrups or retention bars which hold them securely in place.
Q. What precautions must be observed when designing manifolds ?
A. Inlet or fresh gas manifolds should always be proportioned so that the gas from the carburetor does not have to turn any sharp corners and so the distance between each valve chamber of a multiple cylinder engine and the carburetor will be approximately the