The three-point hitch (British English: three-point linkage) most often refers to the way ploughs and other implements are attached to an agricultural tractor. The three points resemble either a triangle, or the letter A. Three point attachment is the simplest and the only statically determinate way of joining two bodies in engineering.
Implements may be hooked onto the hitches of the tractor, so that the tractor carries their weight. Wheeled trailers that carry their own weight may also be pulled by the hitch, usually through an intermediate drawbar with a central swivel.
Ferguson's invention was that where an implement such as a plough exerts a drag force, the linkage may use this to increase the downward force on the rear wheels, and thus the traction available.
The hitch's utility and simplicity has made it an industry standard.
The three-point hitch is made up of several components working together. These include the tractor's hydraulic system, attaching points, the lifting arms, and stabilizers.
Three-point hitches are composed of three movable arms. The two outer arms - the hitch lifting arms - are controlled by the hydraulic system, and provide lifting, lowering, and even tilting to the arms. The center arm - called the top link - is movable, but is usually not powered by the tractor's hydraulic system. Each arm has an attachment device to connect implements to the hitch.
Each hitch has attachment holes for attaching implements, and the implement has posts that fit through the holes. The implement is secured by placing a pin on the ends of the posts.
The hitch lifting arms are powered by the tractor's own hydraulic system. The hydraulic system is controlled by the operator, and usually a variety of settings are available.
The primary benefit of the three point hitch system is to transfer the weight and stress of an implement to the rear wheels of a tractor.
There are four different hitch sizes, called categories. The higher category hitches have sturdier lift arms and larger connector pins. There is some flexibility in the tractor hp at which one category hitch ends and the next begins.
|Category||Tractor HP||Top Link Pin Diameter||Lift Arm Pin Diameter|
|0||Up to 20||5/8 in||5/8 in|
|1||20 to 45-55||3/4 in||7/8 in|
|2||45-55 to 90-95||1 in||1⅛ in|
|3||90-95 and up||1¼ in||1 7/16 in|