Crankshaft Design Principles

This page lays out some of the crankshaft design principles along with descriptions of the main features. That part of the engine which transmits the reciprocating motion of the pistons to the driven unit in the form of rotary motion. That part to which the connecting rods are attached.

In the case of many of the older stationary engines the crankcase was open.

The three main components of a crankshaft:

  1. Big end – small shaft that locates the conrod and reacts the force transmitted from the piston.
  2. Main journal – this carries the crankshaft in the main bearings and reacts all of the forces.
  3. Crank webs – connect the big end to the main journal.
Crankshaft for a 4 cylinder in-line engine.

Machining even a single throw crankshaft can be difficult, a 4 cylinder in-line crankshaft is up there as a difficult item to machine at any scale.

Big End – The connection between the conrod and crankshaft in an internal combustion engine. Also known as a Crankpin.

Counterbalance – Series of weights attached to or forged integrally with crankshaft & placed to offset reciprocating weight of each piston and rod assembly.

Crank – That part of the crankshaft, which is in the form of a crank and crank pin. The purpose of the crank is to convert reciprocating motion into rotary motion.

Crank Angle – Angle that can be measured between the crankshaft and top dead centre.

Crankshaft Gear – A gear mounted on the front of the crankshaft that is used to drive the camshaft gear.

Crankshaft Journal – The journals running in the main bearings as opposed to those for the big-end bearings.

Crankshaft Pulley – A wheel attached to the front end of the crankshaft which is connected by fan belts to the fan, the alternator, and other devices so that the rotating crankshaft can drive these other parts as well.

Crank Rumble

This is an amplitude modulation of engine noise perceived inside a car with a modulation frequency of 0.5 order. Some of the factors affecting crank rumble are:

  • Bearing clearances
  • Powertrain mount bracket stiffness
  • Ancillary bracket stiffness
  • Crankshaft counter balance weight
  • Flywheel-end bearing housing stiffness
  • Bed-plate stiffness
  • Crankshaft pin diameter
  • Crankshaft web stiffness
  • Flywheel bending stiffness
  • Flywheel mass and inertia
  • Oil pressure and viscosity

Some additions that improve crank rumble are:

  • Bending damper on crankshaft nose
  • Structural sump
  • Ladder frame

Crankshaft Sprocket – A chain-sprocket mounted on the nose of the crankshaft which drives the camshaft by means of a timing chain.

Crank Web – One of the pair of arms which carry the big-end journal.

Position Sensor – A sensor which sends information concerning the precise position of the crankshaft so that accurate ignition timing can be achieved.

Runout – A term used to describe how much a crankshaft is bent.

Throw of Crankshaft – The distance between the center of the crankpins and the center of the journals of the crankshaft. It is equal to half the stroke of the engine.

Top Dead Centre – The position of the crank when the piston is in its farthest position from the crankshaft, in its nearest position to the cylinder head. Abbreviated tdc.

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