The floating arm trebuchet is a

counterweight A counterweight is a weight that, by applying an opposite force, provides balance and stability of a mechanical system. The purpose of a counterweight is to make lifting the load faster and more efficient, which saves energy and causes less wear ...

siege weapon which is a modern variation of the medieval

trebuchet A trebuchet (french: trébuchet) is a type of catapult that uses a long arm to throw a projectile. It was a common powerful siege engine until the advent of gunpowder. The design of a trebuchet allows it to launch projectiles of greater weigh ...

. Their defining feature is free movement of an axle on the throwing arm, and a linear drop channel for the counterweight. The design and construction of such machines is a popular project assignment in post-secondary engineering classes.

Basic design

A traditional trebuchet, which is simple to engineer, consists of an arm resting off-centre on an

axle An axle or axletree is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In the former case, bearing ...

with a hinged

at the short end of the arm.> The difference in the lengths of the two ends of the arm provides

mechanical advantage Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. The device trades off input forces against movement to obtain a desired amplification in the output force. The model for t ...

. In contrast, a floating arm trebuchet features an axle on the arm freely moving along a horizontal glide track. This allows for a higher drop height for the counterweight, which is allowed to fall down a sturdy linear vertical "drop channel". The arm is on a moving axle and the

torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...

D, volume=26, doi=10.1088/0143-0807/26/4/002, issue=4 The forward motion of the axle as the arm rotates adds speed to the projectile end of the arm. This

linear Linearity is the property of a mathematical relationship (''function'') that can be graphically represented as a straight line. Linearity is closely related to '' proportionality''. Examples in physics include rectilinear motion, the linear r ...

drop of the counterweight is more efficient in transferring

potential energy In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors. Common types of potential energy include the gravitational potentia ...

kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...

than the

pendulum A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the ...

counterweight motion of a traditional trebuchet. The higher the drop channel, the more energy is released, but this puts more strain on the mechanism. The dropping of the counterweight forces the arm to swing to one side of the glide track and quickly jerk in the opposite direction to launch a projectile. This pulls the counterweight upwards slightly after a launch. The projectile can be launched at a high velocity.

Operation

The floating arm trebuchet uses a counterweight held on top of a tall drop channel to transfer potential energy into kinetic energy. A projectile is loaded into a sling at the end of the throwing arm. Upon releasing the trigger, the counterweight is allowed to drop through the channel. This causes the wheel on the axle of the arm to impact the glide track, which is usually held steady with strong support beams. While this is happening, the wheel is moved backwards by the torque acting on the arm from the counterweight. The arm then jerks forward and releases the projectile from the sling. To reset the device, the counterweight must be raised back to the original height and locked in with the trigger mechanism. While the floating arm design can have a lighter frame than a traditional trebuchet because the overall forces produces are less, the impact of the arm on the glide track will cause eventual damage and may need maintenance. Reducing the drop distance of the wheel axle to the glide track would reduce damage, but also reduce the range of the trebuchet.

Trigger mechanism

The most common of several trigger mechanisms is two

eye bolt An eye bolt is a bolt with a loop at one end. They are used to firmly attach a securing eye to a structure, so that ropes or cables may then be tied to it. Eye bolts Machinery eye bolts are fully threaded and may have a collar, making them sui ...

s and a peg through both bolts. The eye bolts are usually placed at the bottom of the frame, locking above the arm to prevent accidental release. In larger designs, the trigger system is applied at the top of the drop channel. Large scale designs require special trigger mechanisms that can hold the weight of the throwing arm.

The sling / pouch

The sling or pouch, which contributes to acceleration of the projectile and then releases it at the desired launch angle, is often made using

polyester Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...

rope or other durable thin material. The release mechanism is a hook with a specific bend in its tip to determine when centripetal force will release one end of the sling, freeing the projectile to continue on its path.

Friction reduction

A floating arm trebuchet has three points of contact where friction can interfere with efficiency. Friction when the counterweight is dropping down to the glide strike can be reduced by using

silicone grease Silicone grease, sometimes called dielectric grease, is a waterproof grease made by combining a silicone oil with a thickener. Most commonly, the silicone oil is polydimethylsiloxane (PDMS) and the thickener is amorphous fumed silica. Using this ...

or a thin film of plastic on the drop channel. Friction of the wheel on the arm axle against the glide track should be managed in a similar way. To reduce friction of the sling on the launch track, the track should have a smooth surface.

Potential uses

The floating arm trebuchet loses the range advantage over regular trebuchet at very large scales, because the difficulty in construction of the drop channel and axle increases with size. A regular trebuchet is a better realistic war machine. However, a floating arm trebuchet is more efficient when built to a size of between one meter and five meters, making it suitable for hobbyists and model builders. These machines and their designers are frequently to be found competing in various recreational events, for example, pumpkin chucking contests."Pumpkins fly at chuckin’ festival"
''Stowe Today''.

References

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External links

Engines of War
Trebuchets