This article describes design aspects of a simple nut and trigger lock commonly seen on medieval crossbows. For practical instructions on how to make the nut, refer to this article.
Parts and the operating principle
The simple nut and trigger lock was the very common in the Medieval times. The lock consists of four parts:
Although this picture showed an axle going through the nut, in Medieval times the nut was usually held in place by simply fitting tightly into a socket carved into the stock. Sometimes the socket was reinforced with horn or steel. If a metal nut is used, a metal (or metal-plated) socket and oil will reduce friction greatly, reducing chances of misfire. Especially in late-medieval models the nut may have had a hole in the center to receive a bolt or a pair of screws. Alternatively cord was passed through the hole and bound to the underside of the stock. These techniques ensured that the nut did not fall off the socket; even in case of steel bolts the socket bore the load of the bowstring.
The nut was often made of horn or antler, in which case the place where trigger contacts the nut (bottom right) was reinforced with steel. The nut can also be made of steel or other durable metal, in which case no extra reinforcement is necessary. The nut has a slot at the top through which the bolt is pushed against the string. The slot is shown clearly here:
The parts of the nut left at the sides are called the nut's fingers. They serve the same purpose as an archer's fingers.
The trigger is forged from steel. An axle goes through it near the front, which gives the trigger considerable leverage. The stock is carved so that the trigger can rotate freely around the axle. When the trigger is pulled, it's front end moves downwards until it slips of the catch in the nut. At this point the pressure exerted by the bowstring rotates the nut and sends the bolt speeding forward. The slight outward curvature of the trigger's long part helps keep the balance point of the trigger as far away from the axle as possible when the crossbow is in upright position. This is important in order to keep pressure on the nut while the crossbow is being cocked with the stirrup. A simple flat spring pushing the trigger away from the stock can be used to achieve the same goal more reliably.
The two lock sideplates are made of metal or horn and riveted, nailed or screwed to the stock. They protect the thin wood at the sides of the stock around the lock. Without the sideplates the wood at the sides might fail when the trigger axle is under stress. Considerable forces affect the trigger axle when the crossbow is cocked and ready to shoot, but also when the trigger is pulled.
Key design aspects
Nut and trigger alignment
There are a few key things to remember when making a nut and trigger lock. Most importantly the nut and the trigger have to be aligned correctly:
If the nut and trigger are aligned as shown on the right, pulling the trigger actually pulls the string backwards. This of course makes the trigger pull very stiff, which translates to poor accuracy. Correct alignment (on the left) actually does the reverse by letting the string move slightly forward when the trigger is pulled. This means the trigger pull is much more comfortable. That said, taking this correct approach too far means the crossbow may shoot spontaneously, making it very dangerous and prone to breakage.
Nut and stock alignment
In addition to aligning the nut and trigger properly, all surfaces touching the string - especially the corners of the "fingers" - have to be rounded and smooth. It also makes sense to make them relatively wide (5mm+) to avoid stressing the string unnecessarily. Also make sure the slot between the fingers is wide enough to accommodate the butt-end of the bolt.
Placing the nut sufficiently deep into the stock is also important:
Make sure the butt of the bolt does not rest on the nut (as on the right). Otherwise the butt-end of the bolt can move freely sideways, causing bolt flight to be unpredictable at best. The forward end is of course kept from moving sideways by the bolt groove. This is probably less of an issue if the bolt fits tightly between the fingers of the nut - unlike in the picture above.
Reducing chances of misfire
There are several reasons for a nut and trigger lock misfires:
- Excessive friction between the nut and the socket: This is fixed by a perfectly rounded nut and socket, a loose but stable fit, and the use of a lubricant (e.g. oil)
- Heavy nut: Usually only an issue with metal nuts: a steel nut can weight anything between 150-350 grams. Fix by using a lighter one or by drilling away excess material as near the edges of the nut as possible.
- Less than optimal nut finger placement/design: Several possible fixes, see below.
If several of these are combined (as in one of my crossbows), there's a high chance of misfire. Using bolts with a large diameter butt-end helps, but does not solve the underlying problem.
Misfires can be effectively removed with a couple of adjustment to the nut finger placement:
Note that the some of the modifications are excessive, just to illustrate the point. There are two key issues that these modifications address:
- Limit the time bowstring acts on the nut fingers: quick release reduces time for unwanted (e.g. upwards) bowstring movement. Lowering the fingers and moving them forward address this issue specifically.
- Reduce the chances of the fingers forming an upwards "slope" for the bowstring: canting the fingers backward and curving them means that when the nut rotates, there's no chance the bowstring can move anywhere but forward. Moving the fingers forward also addresses this issue to some degree.
In general, a nut rotating around an axle is less prone to these issues, as there's inherently very little friction involved.
In this type of lock the contact surfaces of the nut and the trigger are under considerable stress, which can quickly cause them to wear out. The problem is magnified if:
- The poundage of the crossbow is high
- Either of the contact surfaces is significantly harder than the other, i.e. one of hardened steel, and one of mild steel
- There are sharp angles
As an extreme example, take look at this photo:
The upper side of the trigger was originally straight, but it becase significantly canted. The contact surface in the nut was of extremely hard steel; even a good metal file could not bite into it, only a diamond whetstone could. The trigger was of mild steel. The bow had ~300 pounds of draw weight. Within a few hundred shots the trigger had worn out as badly as the photo shows. Even after welding a piece of spring steel to the tip of the trigger and hardening it, the nut contact surface continued to bite into the trigger.
To prevent unnecessary wear when either of the contact surfaces is signicantly harder than the other, it's best to make the harder one rounded. This will prevent the harder one from cutting thin slices from the softer one, although it won't prevent the wear entirely. Making the softer contact surface harder, e.g. making it from a piece of carbon steel also helps.