A wheellock, wheel-lock, or wheel lock is a friction-wheel mechanism which creates a spark that causes a firearm to fire. It was the next major development in firearms technology after the matchlock, and the first self-igniting firearm. Its name is from its rotating steel wheel to provide ignition. Developed in Europe around 1500, it was used alongside the matchlock (c. 1410s), the snaplock (c. 1540s), the snaphance (c. 1560s), and the flintlock (c. 1610s).
Design The wheellock works by spinning a spring-loaded steel wheel against a piece of pyrite to generate intense sparks, which ignite gunpowder in a pan, which flashes through a small touchhole to ignite the main charge in the firearm's barrel. The pyrite is clamped in vise jaws on a spring-loaded arm (or 'dog'), which rests on the pan cover. When the trigger is pulled, the pan cover is opened, and the wheel is rotated, with the pyrite pressed into contact. A close modern analogy of the wheellock mechanism is the operation of a lighter, where a toothed steel wheel is spun in contact with a piece of sparking material to ignite the liquid or gaseous fuel. A wheellock firearm had the advantage that it could be instantly readied and fired even with one hand, in contrast to common matchlock firearms, which required a burning cord of slow match ready if the gun might be needed and demanded the operator's full attention and two hands to operate. On the other hand, wheellock mechanisms were complex to make, making them relatively costly.
The "dog" The dog is a spring-loaded arm pivoted on the outside of the lock plate. A sparking material, usually a small piece of iron pyrite, is clamped and held by vise-like jaws at the swinging end of the arm. The dog has two possible positions to which it can be pivoted by hand: a "safe" position, in which the dog is pushed towards the muzzle of the firearm, and an "operating" position, where the dog is pulled towards the operator so that the pyrite in its jaws can engage either the top of the pan cover (see below), or (in the absence of the pan cover) the edge of a steel wheel bearing longitudinal grooves around its circumference. Flint is not suitable as a sparking material in the wheellock because it is too hard and would quickly wear away the wheel grooves.
The wheel The upper segment of the grooved wheel, made of hardened steel, projects through a slot cut to its precise dimensions in the base of the priming pan. The wheel is grooved on its outside circumference with three or more V-shaped grooves with transverse cuts at intervals to provide a friction surface for the iron pyrite. The wheel is fixed to a shaft, one end of which projects outside the lockplate. The outside projection is of square section to permit a spanner (wrench) to be engaged for subsequent tensioning of the lock. The other end of the shaft fits through a hole in the lockplate, and on this end is forged a cam, or eccentric. One end of a short, robust chain (made of three or four flat, parallel links like a short piece of bicycle chain) is fixed to the cam, while the other end of the chain is held in a groove at the end of the longer branch of a large heavy V-spring, which is generally retained by a screw and a headed bracket through upstands inside the lockplate.
The pan As in all muzzle-loading firearms (prior to the introduction of the percussion cap), the pan transmits the fire to the main charge of gunpowder inside the breech of the barrel, via a small hole (or "vent") in the side of the breech, that gives on to the pan. The priming pan of all wheellocks is provided with a sliding cover that has two purposes, the first of which is to contain the priming powder and afford it some protection from the elements (the second is examined below, under 'Operation'). The pan cover may be slid open and closed by hand, but it is also attached to an arm inside the lock plate, which is acted upon by the eccentric on the shaft of the wheel.
The sear or trigger mechanism The trigger engages one arm of a "z"-shaped sear pivoting in its centre between two upstanding brackets riveted or brazed to the inside of the lockplate. The other arm of the sear passes through a hole in the lockplate, and engages in a blind hole on the inner side of the wheel, thus effectively locking it and preventing any rotation but only because of a secondary sear or wedge that is pressed under the rear arm of the sear—that is between the lockplate and the sear—when the forward part of it engages into the recess in the wheel. When the trigger is pulled, the secondary lever is withdrawn from its position and the strong pull of the mainspring pushes the unsupported main sear back into the lock and the wheel is free to rotate. The mechanism may seem overconstructed, but it prevents the trigger mechanism from working against the very powerful mainspring as it is the case with all vertical acting sears in flint and percussion locks or even modern firearms that still have cocks (revolvers).
Preparing to fire First, the dog is rotated forward to the "safe" position, and the priming pan is pushed open (if it is not already so). After loading a powder charge and ball through the muzzle in the usual way, the operator takes his "spanner", slips it onto the square section of the wheel shaft, and turns it until a click is heard (about one-half to three-quarters of a revolution), and the wheel is felt to lock in place, whereupon the spanner is withdrawn. What occurs is that when the wheel is turned, the mainspring is tensioned via the chain, which is wound partially around the shaft. The click is the sound of one end of the sear engaging in the blind hole on the inside of the wheel, thus immobilising it. The pan is then primed with powder, and the pan cover pulled shut. Finally the dog is pulled back so that the pyrite in its jaws is resting on the top of the pan cover, under some pressure from the spring at the toe of its arm.
Operation
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