A crankpin, also known as a crank journal, is a mechanical device in an engine which connects the crankshaft to the connecting rod for each cylinder. It has a cylindrical surface, to allow the crankpin to rotate relative to the "big end" of the connecting rod.
In a reciprocating engine, the crankpin is the part of a crankshaft where the lower end of a connecting rod attaches. In a beam engine, a single crankpin is mounted on the flywheel; In a steam locomotive, crankpins are often mounted directly on the driving wheels.
In a multi-cylinder engine, a crankpin can serve one or many cylinders, for example:
There are three common configurations in crankpin design:
- If a crankpin serves only one cylinder, then the big end (the usually larger end of a connecting rod that attaches to the crankpin, as opposed to the "small end", which is attached to the wrist/gudgeon pin in the piston, or the end of the crosshead in engines so equipped, allowing it to be articulated on both ends) is a relatively simple design, accommodating only one connecting rod. This design is the cheapest to produce, and is used in:
- If a crankpin serves more than one cylinder, then the corresponding cylinders may have an offset, or may be articulated, to simplify the design of the big end bearing. This design is used in most V engines.
- If more than one cylinder is served by a single crankpin but there is no offset, then some or all of the connecting rods must be forked at the big end, or be articulated. This design provides better engine balance than designs with an offset, but requires extra complexity and cost in both design and manufacture, and more weight or closer manufacturing tolerances to achieve the same strength and reliability. Any extra weight added to the big end itself also carries a penalty of adding vibration and reducing balance. As the number of cylinders grows, the effect of the offset on balance becomes less important, and forked connecting rods become less common. They are mainly used in V-twin engines, notably including motorcycle engines, but in the past were found on a number of automobile and aero engines, such as the famous Rolls-Royce Merlin aero engine of the WWII era.
- Articulated rods were used in older or exotic Vee engines, and consist of rods that have a single master connecting rod attached to the crank pin, with the opposing connecting rod (sometimes two) instead using a smaller bearing machined into the big end of the main connecting rod, instead of two dedicated crank journals.
- A similar but more complex design is used in radial engines, which have a master/slave connecting rod, with a single "master" connecting rod attached to the single crankpin (one for each row in multi-row designs), and smaller bearings for each of the corresponding cylinders machined into the big end of the master rod. For more detail see the article on radial engines.