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What Is a Crank Pin?

Crank pins were used in classic steam locomotives.
Various nuts, bolts and screws.
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  • Written By: Pauline H. Gill
  • Edited By: C. Wilborn
  • Last Modified Date: 01 September 2014
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Mechanical cranks convert linear motion into rotary motion or vice versa. In general, cranks consist of a main shaft which rotates, a crank pin which revolves like a planet around it, and a crank throw to solidly connect them. Handles or knobs on hand-driven cranks may either comprise the crank pins or spin freely on the crank pins. Crank pins are most often parallel to their main shafts. For cranks that can turn through a full 360 degrees, their crank pins must be offset to the side of the main shaft.

Cranks can be found at work in every area of life wherever manual motion is converted to rotary motion. Fishing reels, manual winches, meat grinders, and garden hose reels all use cranks to allow people to easily create continuous rotary motion. Bicycle pedals function as crank pins between the rider’s foot and the pedal crank that drives the chain.

There are two types of cranks. The first type is the continuously rotating crank, such as in engine crankshafts or on bicycles, where the crank can continuously turn more than 360 degrees without having to reverse. The second is the partial circle crank, where the entire rotary motion of the main shaft may be 90 degrees or less, as with with steering linkages or ventilation damper adjustments.

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Sometimes the crank pin function is not a round pin or bolt at all. It may instead be a small pedal. Such is the case on some automotive clutch, brake, or accelerator pedals, where the bottom of the driver’s foot rolls across the face of the pedal as it is depressed.

Gasoline and diesel engine crankshafts usually consist of crank pin journals each surrounded along the length of the crankshaft by a main bearing journal. A journal is simply part of a shaft, while a bearing is that piece which supports the journal and allows it to move smoothly. Crankshafts are usually cast or forged as one piece.

Crank pins in automotive engines may be from 1.5 to 3 inches in diameter, (38 to 76 mm) and bear loads of a ton or more (990 kg) when the piston is pushed down by the combustion cycle. They are ground to be absolutely smooth to present a long-wearing surface to the piston connecting rod and bearing attached to them. Crank pin applications in small instruments may require only 1 ounce (28.3 grams) or less of force, and may have jeweled bearings to increase their precision and longevity.

A crank pin may be a pin, bolt, or screw, or it may be machined into the crank as one piece. On the classic steam locomotives that ruled the tracks of the early to mid-1900s, the crank pins were part of the large main drive wheels. Long steel rods connected the steam cylinders on each side of the locomotive to one of these wheels. The remainder of the drive wheel crank pins were connected by a single long beam that kept the wheels on each side turning in unison. Crank pins on the engines of ocean going steam powered liners can be up to several feet in diameter (approximately .5 meters) and help turn 20 foot (6 meter) diameter propellers.

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