A whippletree, or whiffletree, is a mechanism to distribute force evenly through linkages. It is also referred to as an equalizer, leader bar, or double tree. It consists of a bar pivoted at or near the centre, 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, such as to simulate pressure over an area as when applying loading to test airplane wings. Whippletrees may be used either in compression or tension. They were also used for subtraction and addition calculations in mechanical computers. Tension whippletrees are used in artful hung mobiles, such as those by artist Alexander Calder.

Draught whippletrees

Whippletrees are used in tension to distribute forces from a point load to the traces of draught animals (the traces 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 centre 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 A plough or plow ( US; both ) is a farm tool for loosening or turning the soil before sowing seed or planting. Ploughs were traditionally drawn by oxen and horses, but in modern farms are drawn by tractors. A plough may have a wooden, iron or ...

harrow Harrow may refer to: Places * Harrow, Victoria, Australia * Harrow, Ontario, Canada * The Harrow, County Wexford, a village in Ireland * London Borough of Harrow, England ** Harrow, London, a town in London ** Harrow (UK Parliament constituency) ...

, log or canal boat or for pulling a vehicle (by the leaders in a team with more than one row of animals). A '' swingletree'', or ''singletree'', is a special kind of whippletree used for a

horse-drawn vehicle A horse-drawn vehicle is a mechanized piece of equipment pulled by one horse or by a team of horses. These vehicles typically had two or four wheels and were used to carry passengers and/or a load. They were once common worldwide, but they have m ...

. The term ''swingletree'' is sometimes used for draught whippletrees. 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 has its own whippletree, and a further one balances the loads from their two whippletrees—an arrangement 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 an equal share of the work.

Other agricultural whippletrees

Whippletrees are also used in modern agriculture—for example, to link several ganged agricultural implements such as

s, mowers or

roller Roller may refer to: Birds *Roller, a bird of the family Coraciidae * Roller (pigeon), a domesticated breed or variety of pigeon Devices * Roller (agricultural tool), a non-powered tool for flattening ground * Road roller, a vehicle for compa ...

s to a tractor. This combines several small loads into a single load at the tractor hitch (the reverse of the use for draught animals).

Windshield (windscreen) wipers

A series of whippletrees is used in compression in a standard windshield wiper to distribute the point force of the sprung wiper arm evenly along the wiper blade.

Telescopes

Some designs for large telescopes use whippletrees to support the optical elements. The tree provides distributed mechanical support, reducing localised mechanical deflections, which in turn reduces optical distortion. Unlike the applications described above, which are two-dimensional, the whippletrees in telescope mirror support cells are three-dimensional designs, since the tree must support multiple points over an area.

Mechanical analogue computers

Linkage-type mechanical analog computers use whippletree linkages to add and subtract quantities represented by straight-line motions.Svoboda, Antonín, ''Computing Mechanisms and Linkages'', M.I.T. Radiation Laboratory Series, New York, 1948, McGraw-Hill Book Company (also New York, Dover Publications, 1965) Section 2-6, Fig 2-12, p 37. The illustration here of whippletrees for a three-animal team is very similar to a group of linkage adders and subtracters: "load" is the equivalent of the output sum/difference of the individual inputs. Inside the computer, cylinders on the knob shafts have thin metal tapes wrapped around them to convert rotary to linear motion. One widely used application was in the

IBM Selectric The IBM Selectric typewriter was a highly successful line of electric typewriters introduced by IBM on 31 July 1961. Instead of the "basket" of individual typebars that swung up to strike the ribbon and page in a typical typewriter of the period ...

typewriter (and the IBM 2741 derived from it), where the linkages summed binary mechanical inputs to rotate and tilt the type ball. This type of computing method was also used for

naval gunnery Naval artillery is artillery mounted on a warship, originally used only for naval warfare and then subsequently used for shore bombardment and anti-aircraft roles. The term generally refers to tube-launched projectile-firing weapons and excludes ...

, such as the MK 56 Gun Fire Control System and sonar

fire-control system A fire-control system (FCS) is a number of components working together, usually a gun data computer, a director, and radar, which is designed to assist a ranged weapon system to target, track, and hit a target. It performs the same task as a ...

References

External links

Photo of whippletree for draught horses by Beig Alert on Flickr
(Note: this photo shows three muddled sets of whippletrees: one single, one double and one triple.)
Photo InTheHead of a windshield wiper's whippletree by Barefoot on Flickr

Major telescopes from Lord Rosse to about 1980
* tp://ftp.aao.gov.au/pub/local/will/pilot/PG_notes/pilot%20m1%20support.doc/ Support of PILOT primary mirror by Peter Gillingham 2 March 2007
Mirror Edge Support Calculator

{{DEFAULTSORT:Whippletree (Mechanism) Horse driving Mechanics Simple machines Horse harness