where a smaller propeller gear step-down is desired. In the  case of gas turbine turboprop engines, more than one stage  of reduction gearing is used do to the high output speeds of  the engine. Several types of lower powered engines can use  the spur and pinion reduction gear arrangement. main bearings. They are also used in gas turbine applications  where radial loads are high. Generally, a rotating shaft in a  gas turbine engine is supported by a deep-groove ball bearing  (radial and thrust loads) on one end and a straight roller  bearing (radial loads only) on the other end. The increased brake horsepower delivered by a high  horsepower engine results partly from increased crankshaft  rpm. It is therefore necessary to provide reduction gears to  limit the propeller rotation speed to a value at which efficient  operation is obtained. Whenever the speed of the blade tips  approaches the speed of sound, the efficiency of the propeller  decreases rapidly. Reduction gearing for engines allows the  engine to operate at a higher rpm, developing more power  while slowing down the propeller rpm. This prevents the  propeller efficiency from decreasing. Since reduction gearing  must withstand extremely high stresses, the gears are machined  from steel forgings. Many types of reduction gearing systems  are in use. The three types most commonly used are spur  planetary, bevel planetary, and spur and pinion. [ Figure 1-34 ] Propeller shafts may be of three major types: tapered, splined,  or flanged. Tapered shafts are identified by taper numbers.  Splined and flanged shafts are identified by SAE numbers.  The propeller shaft of most low power output engines is  forged as part of the crankshaft. It is tapered, and a milled slot  is provided so that the propeller hub can be keyed to the shaft.  The keyway and key index of the propeller are in relation  to the No. 1 cylinder top dead center. The end of the shaft  is threaded to receive the propeller retaining nut. Tapered  propeller shafts are common on older and smaller engines. The propeller shaft of high-output radial engines is generally  splined. It is threaded on one end for a propeller hub nut.  The thrust bearing, which absorbs propeller thrust, is located  around the shaft and transmits the thrust to the nose section  housing. The shaft is threaded for attaching the thrust-bearing  retaining nut. On the portion protruding from the housing  (between the two sets of threads), splines are located to  receive the splined propeller hub. The shaft is generally  machined from a steel-alloy forging throughout its length.  The propeller shaft may be connected by reduction gearing  to the engine crankshaft, but in smaller engines the propeller  shaft is simply an extension of the engine crankshaft. To  turn the propeller shaft, the engine crankshaft must revolve. The spur planetary reduction gearing consists of a large  driving gear or sun gear splined (and sometimes shrunk)  to the crankshaft, a large stationary gear, called a bell gear,  and a set of small spur planetary pinion gears mounted on a  carrier ring. The ring is fastened to the propeller shaft and the  planetary gears mesh with both the sun gear and the stationary  bell or ring gear. The stationary gear is bolted or splined to  the front section housing. When the engine is operating, the  sun gear rotates. Because the planetary gears are meshed with  this ring, they also must rotate. Since they also mesh with the  stationary gear, they walk or roll around it as they rotate, and  the ring in which they are mounted rotates the propeller shaft  in the same direction as the crankshaft but at a reduced speed. In some engines, the bell gear is mounted on the propeller  shaft, and the planetary pinion gear cage is held stationary.  The sun gear is splined to the crankshaft and acts as a driving  gear. In such an arrangement, the propeller travels at a  reduced speed but in opposite direction to the crankshaft. In the bevel planetary reduction gearing system, the driving  gear is machined with beveled external teeth and is attached to  the crankshaft. A set of mating bevel pinion gears is mounted  in a cage attached to the end of the propeller shaft. The  pinion gears are driven by the drive gear and walk around the  stationary gear, which is bolted or splined to the front section  housing. The thrust of the bevel pinion gears is absorbed by a  thrust ball bearing of special design. The drive and the fixed  gears are generally supported by heavy-duty ball bearings.  This type of planetary reduction assembly is more compact  than the other one described and, therefore, can be used  The relationships between pressure, volume, and temperature  of gases are the basic principles of engine operation. An  internal combustion engine is a device for converting heat  energy into mechanical energy. Gasoline is vaporized and  mixed with air, forced or drawn into a cylinder, compressed  by a piston, and then ignited by an electric spark. The  conversion of the resultant heat energy into mechanical  energy and then into work is accomplished in the cylinder.  Figure 1-35  illustrates the various engine components  necessary to accomplish this conversion and also presents  the principal terms used to indicate engine operation. Flanged propeller shafts are used on most modern  reciprocating and turboprop engines. One end of the shaft is  flanged with drilled holes to accept the propeller mounting  bolts. The installation may be a short shaft with internal  threading to accept the distributor valve to be used with a  controllable propeller. The flanged propeller shaft is a very  common installation on most propeller driven aircraft.  Reciprocating Engine Operating  Principles Propeller Shafts Propeller Reduction Gearing 1-24