The crankshaft is the backbone of the reciprocating engine. It  is subjected to most of the forces developed by the engine. Its  main purpose is to transform the reciprocating motion of the  piston and connecting rod into rotary motion for rotation of  the propeller. The crankshaft, as the name implies, is a shaft  composed of one or more cranks located at specified points  along its length. The cranks, or throws, are formed by forging  offsets into a shaft before it is machined. Since crankshafts  must be very strong, they generally are forged from a very  strong alloy, such as chromium-nickel-molybdenum steel. A crankshaft may be of single-piece or multipiece  construction.  Figure 1-7  shows two representative types of  solid crankshafts used in aircraft engines. The four-throw  construction may be used either on four-cylinder horizontal  opposed or four-cylinder inline engines. The six-throw shaft  is used on six-cylinder inline engines, 12-cylinder V-type  engines, and six-cylinder opposed engines. Crankshafts  of radial engines may be the single-throw, two-throw, or  four-throw type, depending on whether the engine is the  single-row, twin-row, or four-row type. A single-throw radial  engine crankshaft is shown in  Figure 1-8 . No matter how  The crankshaft is carried in a position parallel to the  longitudinal axis of the crankcase and is generally supported  by a main bearing between each throw. The crankshaft main  bearings must be supported rigidly in the crankcase. This  usually is accomplished by means of transverse webs in the  crankcase, one for each main bearing. The webs form an  integral part of the structure and, in addition to supporting  the main bearings, add to the strength of the entire case.  The crankcase is divided into two sections in a longitudinal  plane. This division may be in the plane of the crankshaft so  that one-half of the main bearing (and sometimes camshaft  bearings) are carried in one section of the case and the other  half in the opposite section.  [Figure 1-6]  Another method  is to divide the case in such a manner that the main bearings  are secured to only one section of the case on which the  cylinders are attached, thereby providing means of removing  a section of the crankcase for inspection without disturbing  the bearing adjustment. Many of these engines use simple gear trains to drive the  engine’s accessories at the proper speeds. Crankshafts Figure 1-6. Typical opposed engine exploded into component assemblies.  1-8 Magneto Oil pump Accessory case assembly Prop shaft Generator Crankcase (left half) Tachometer generator Starter Oil sump Camshaft Connecting rod Cylinder Crankcase (right half) Induction system Crankshaft Main bearing surface Parting surface Transverse webs Camshaft bearings