One of the slings lines loops over the pin. When the sling rotates far enough, the sling line will come off the pin, causing the sling to release the projectile.
For details on how and why to adjust the release time using the pin, see tuning.
The pin has several names:
- Sling Pin
- Release Pin
- Firing Pin
The most common and most crude pin type is just a piece of round stock permanently placed in the end of the throwing arm. The pin is then bent back and forth to which ever angle is desired. This pin works well on small machines, a test machine that wont see many launches, or when you have built your treb and are too excited to build a more advanced pin before performing your first launches. The biggest advantages of this design are its ease of manufacture, low cost, and is generally the lightest pin type.
Some of the disadvantages include the inability to make accurate and fine adjustments. The bending of the pin can also weaken the pin's attachment to the end of the arm since it is usually just screwed into the end grain of the arm, a less than ideal attachment method in any situation. This can cause the pin to either pull out or rotate to the side. For larger trebs you also run into the problem of having a pin large enough to prevent bending during a launch but also small enough to be able to be bent when adjustment is needed.
In the turnbuckle design the pin is mounted on a wheel that can be rotated. This is usually accomplished by placing one plate on each side of the arm that extend past the end of the arm far enough to mount the wheel. The wheel is then attached to a turnbuckle which can be lengthened or shortened to adjust the angle of the pin.
This design allows for very easy and fine pin adjustments, any angle can be made within the range of the turnbuckle. There is no chance of weakening the end of the arm during adjustment and, since the pin is straight, it wont rotate during a launch.
One disadvantage is the increase complexity of the pin and its assembly. Depending on how the turnbuckle is attached the pin the range of angles available can be limited to maybe about 30-45 degrees. This is probably enough for most trebuchets as long as this range is centered on your ideal pin angle.
Similar to the turnbuckle design the vernier style uses a wheel mounted pin. Instead of using a turnbuckle for adjustment it uses hole alignment between the wheel and the plates to which it is mounted. A holding pin is placed in the aligned holes to prevent the wheel from rotating.
The hole placement and number is very important for this design. If the wheel and plates have four holes in each then the pin angle can be adjusted in 90 degree increments. However, in a vernier system, if you place a different number of holes in the wheel and plates then the number of increments can be multiplied, as long as the two numbers do not have a common multiple. If the plates have five equally spaced holes and the wheel has four, two numbers without a common multiple, then there are 20 pin positions, allowing for 18 degrees of adjustment, eight and nine allows for increments of five degrees. Six and nine, having a common multiple of three, would only allow for six possible positions.
Problems with this method include the locating and drilling of the holes and decreased adjustability of the the pin angle. If the holes are not placed accurately then the holding pin might not fit through. The turnbuckle can be adjusted with almost an infinite fineness. The vernier's adjustment is dependent on the number of holes.
The vernier does allow for a wider range of pin angles, good for a wider range of projectile weights and easier design. It also is usually lighter than the turnbuckle design.
A video explanation can be seen here:
The interchangeable design generally uses a a permanently attached fixture with removable pins. This can allow for quick and easy pin adjustments. Usually the removable pins are taken out and then bent. This allows for a fewer number of pins needed and prevents the weakening associated with the bending of a permanent pin.