A whippletree (or whiffletree) is a mechanism to distribute force evenly through linkages. It consists of a bar pivoted at or near the center, with force applied from one direction to the pivot, and from the other direction to the tips. Several whippletrees may be used in series to distribute the force further. Whippletrees may be used either in compression or tension.
Draught whippletreestraces are the chains or straps on each side of the harness, on which the animal pulls). For these, the whippletree consists of a loose horizontal bar between the draught animal and its load. The center of the bar is connected to the load, and the traces attach to its ends. Whippletrees are used especially when pulling a dragged load such as a plough, harrow, log or canal boat or for pulling a vehicle (by the leaders in a team with more than one row of animals).
A whippletree balances the pull from each side of the animal, preventing the load from tugging alternately on each side. It also keeps a point load from pulling the traces in onto the sides of the animal.
If several animals are used abreast, further whippletrees may be used behind the first. Thus, with two animals, each will have its own whippletree, then a further whippletree will balance the loads from their two whippletrees – this arrangement is sometimes known as a double-tree, or for the leaders in a larger team, leader-bars. With three or more animals abreast, even more whippletrees are needed; some may be made asymmetrical to balance odd numbers of animals. Multiple whippletrees balance the pulls from the different animals, ensuring that each takes a fair share of the work.
Other agricultural whippletrees
Whippletrees may also be used in modern agriculture, for example to link several ganged agricultural implements such as harrows, mowers or rollers to a tractor. In this case the effect is to combine several small loads to a single load at the tractor hitch (the reverse of the use for draught animals).
Windscreen (windshield) wipers
A series of whippletrees are used in compression in a standard windscreen wiper, to distribute the point force of the sprung wiper arm evenly along the wiper blade.
Some designs for large telescopes use whiffletrees, attributed to Grubb, to support the optical elements. The tree provides for distributed mechanical support, thereby reduces localised mechanical deflections, which in turn reduces optical distortion. Unlike the applications described above, which are two dimensional, these whiffletrees are 3D designs since the tree has to support a circular area. Other variants of the term are whiffle-tree and whiffle tree.