Trunion

A trunnion (from Old French "trognon", trunk) is a cylindrical protrusion used as a mounting or pivoting point. First associated with cannons, they are an important military development.

Alternatively, a trunnion is a shaft that positions and supports a tilting plate. This is a misnomer, as in reality it is a cradle for the true trunnion.

In mechanical engineering (see the trunnion bearing section below), it is one part of a rotating joint where a shaft (the trunnion) is inserted into (and turns inside) a full or partial cylinder.

In a cannon, the trunnions are two projections cast just forward of the center of mass of the cannon and fixed to a two-wheeled movable gun carriage. As they allowed the muzzle to be raised and lowered easily, the integral casting of trunnions is seen by military historians as one of the most important advances in early field artillery.

With the creation of larger and more powerful siege guns in the early 15th century, a new way of mounting them had to be specially designed. Stouter gun carriages were created with reinforced wheels, axles, and “trails” which extended behind the gun. Guns were now as long as eight feet in length and they were capable of shooting iron projectiles weighing from twenty-five to fifty pounds. When discharged, these wrought iron balls were comparable in range and accuracy with stone-firing bombards.

Trunnions were mounted near the center of mass to allow the barrel to be elevated to any desired angle, without having to dismount it from the carriage upon which it rested. Some guns had a second set of trunnions placed several feet back from the first pair, which could be used to allow for easier transportation. The gun would recoil causing the carriage to move backwards several feet but men or a team of horses could put it back into firing position. It became easier to rapidly transport these large siege guns, maneuver them from transportation mode to firing position, and they could go wherever a team of men or horses could pull them.

Due to its capabilities, the French- and Burgundy-designed siege gun, equipped with its trunnions, required little significant modification from around 1465 to the 1840s.

King Charles VIII and the French army used this new gun in the 1494 invasion of Italy. Although deemed masters of war and artillery at that time, Italians had not anticipated the innovations in French siege weaponry. Prior to this, field artillery guns were huge, large-caliber bombards: superguns that, along with enormous stones or other projectiles, were dragged from destination to destination. These behemoths could only be used effectively in sieges, and more often than not provided just a psychological effect on the battlefield; owning these giant mortars did not guarantee any army a victory. The French saw the limitations of these massive weapons and focused their efforts on improving their smaller and lighter guns, which used smaller, more manageable projectiles combined with larger amounts of gunpowder. Equipping them with trunnions was key for two reasons: teams of horses could now move these cannons fast enough to keep up with their armies, without having to stop and dismount them from their carriages to achieve the proper range before firing. Francesco Guicciardini, an Italian historian and statesman, sometimes referred to as the “Father of History,” wrote that the cannons were placed against town walls so quickly, spaced together so closely and shot so rapidly and with such force that the time for a significant amount of damage to be inflicted went from a matter of days (as with bombards) to a matter of hours. For the first time in history, as seen in the 1512 battle of Ravenna and the 1515 Battle of Marignano, artillery weaponry played a very decisive part in the victory of the invading army over the city under siege. Cities that had proudly withstood sieges for up to seven years fell swiftly with the advent of these new weapons.

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