Whippletree (mechanism)

A whippletree is a mechanical device designed to distribute force evenly through linkages. It is also known by several other names, such as an equalizer, a leader bar, or a double tree. The mechanism relies on a bar that is pivoted at or very near its center. One force is applied to the pivot point from one direction. Other forces are then applied to the tips of the bar from the opposite direction. This setup allows a single point of pressure to be spread across multiple points. Engineers and farmers use this tool to ensure that work or weight is shared equally.

To understand the mechanism, imagine a simple bar acting as a bridge for force. In a tension setup, which involves pulling forces, the center of the bar connects to a load. The ends of the bar, called the tips, connect to the traces of a draught animal. The traces are the chains or straps found on a harness. When the animal pulls, the force travels from the traces to the bar. The bar then transmits that force to the center connection. This process prevents the load from tugging unevenly on the animal. It also stops the traces from pulling inward toward the sides of the animal.

There are different types of whippletrees depending on how they are used. A swingletree, or singletree, is a specific type used for horse-drawn vehicles. When multiple animals work together, the system becomes more complex. For a team of two animals, each animal has its own whippletree. These two are then connected to a further whippletree to balance the combined load. This arrangement is often called a double-tree. For larger teams with three or more animals abreast, even more whippletrees are required. Some of these bars are made asymmetrical to balance odd numbers of animals. This ensures that every animal in the team takes an equal share of the work.

Whippletrees can also operate through compression, which is a pushing force. A common modern example is found in a standard automobile windscreen wiper. The wiper arm applies a single point force to the mechanism. A series of whippletrees in compression then distributes this force along the entire length of the wiper blade. This ensures the blade clears the glass evenly. In modern agriculture, the process is often reversed. Tractors use whippletrees to link several small implements, like mowers or harrows, together. This combines several small loads into one single load at the tractor hitch.

Beyond simple pulling and pushing, whippletrees are used in highly precise scientific instruments. Large telescopes use these mechanisms to support their heavy optical elements. These telescope support cells use three-dimensional whippletree designs. Because they must support multiple points over a wide area, they are more complex than two-dimensional versions. The whippletree provides distributed mechanical support across the mirror. This reduces localized mechanical deflections, which are small bends in the material. By preventing these bends, the device reduces optical distortion in the telescope's view.

Historically, whippletrees played a vital role in the world of mechanical computing. Linkage-type mechanical analog computers used whippletree linkages to perform math. These machines used the straight-line motions of the bars to add and subtract quantities. For example, the IBM Selectric typewriter used these linkages. It summed binary mechanical inputs to rotate and tilt the type ball. This same computing method was applied to naval gunnery. It was used in systems like the Mark 56 Gun Fire Control System to manage sonar and fire control.

The principles of the whippletree connect to many different fields of study and art. In the world of fine art, tension whippletrees are used to create artful hung mobiles, such as those by Alexander Calder. In the field of robotics, the whippletree principle is applied to the rocker-bogie suspension systems used on Mars rovers. Even in modern marine design, catamarans like the Proteus use whippletree suspension. Whether it is used to pull a heavy log, clear a car windshield, or calculate complex math, the whippletree remains a fundamental way to manage how force moves through a system.