Piston Engine Crankshaft

The purpose of this component is to change the reciprocating motion(up and down movement) of the piston into rotary motion.  Crankshafts are usually alloy steel forgings with their journals and crankpins hardened to resist wear.  The crankpins and journals are usually hollow, to reduce weight, these spaces being interconnected by drillings in the crank webs to provide passages for lubricating oil.

A shaft is classified according to the number of ‘throws’ or cranks, for instance a ’six throw’ shaft has six crankpins.  The crankwebs are sometimes extended, the extra metal providing a means of balancing the assembly or provide provisions for attachment of damping weights. Suitable drives at each end of the crankshaft transmit the torque to the reduction gear and the accessory drives and In direct drive engines the crankshaft is connected to the propeller with or without the propeller governor.

The simplest crankshaft is the single-throw or 360° type.  This type is used in a single-row radial engine. It can be constructed in one or two pieces.  Two main bearings (one on each end) are provided when this type of crankshaft is used.

The double-throw or 180° crankshaft is used on double-row or 180° crankshaft is used on double-row radial engines.  In the radial-type engine, one throw is provided for each row of cylinders.

Piston Engines Crankcase

This is the name given to that part of the engine that houses the crankshaft and connecting rods.  It provides mounting faces for the cylinders or cylinder blacks, reduction gear, wheel case and other units.  It may be a single casing or build-up of several sections depending on the type of engine.  It will contain the main bearings which are usually plain metal bearings for in-line engines and roller bearings for radial engines.  The engine mountings for in-line engines take the form of our feet and a steel ring is usually used for radial engines.  Provision is made at the lowest point of the crankcase for collection of engine oil for recirculation which is known as the engine oil sump

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The crankcase is subjected to many variations of vibrational and other forces.  Since the cylinders are fastened to the crankcase, the tremendous expansion forces tend to pull the cylinder off the crankcase.  The unbalanced centrifugal and inertia forces of the crankshaft acting through the main bearing subject the crankcase to bending moments which change continuously in direction and magnitude.  The crankcase must have sufficient stiffness to withstand these bending moments without deflections or deformations.  If the engine is equipped with a propeller reduction gear, the front or drive end will be subjected to additional forces causing engine case to be more stressful.