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The
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 five different hitch sizes, called categories,
or classes.
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 |
Typical Tractor HP |
Top Link
Pin Diameter |
Lift Arm
Pin Diameter |
|
0 |
Up to 20 |
16mm (5/8 in) |
16mm |
|
1 |
20 to 55 |
19mm |
22mm |
|
2 |
40 to 100 |
25mm |
29mm |
|
3 |
80 to 225 |
32mm |
37mm |
|
4 |
180 to 400 and up |
45mm |
51mm |
Harry Ferguson
patented the three-point 'linkage' for agricultural tractors in
Britain in 1926. His credit does not lie in invention of the
device, but in realisation of the importance of rigid attachment
of the plough to the tractor. He is also attributed with several
innovations to this device (e.g. hydraulic lift) which made this
system workable, effective, and desirable to the point of using
it on mass marketed tractors (e.g. the Ford-Ferguson 9N). The
hydraulically operated and controlled three point hitch utilized
the draft of the mounted tool to moderate the depth of the tool
and therefore the load on the tractor.
Before the 1940s, each manufacturer used their own systems for
hitching, or attaching their implements to their tractors.
Commonplace was the two-point hitch system which could not
effectively be used for lifting many implements. At this time,
farmers would have to purchase the same brand implements as
their tractor to be able to correctly hook up the implement. If
a farmer needed to use a different brand implement with the
tractor an adaptation kit - which were typically clumsy,
ill-fitting, or unsafe - had to be installed.
In the 1960s, tractor and implement manufacturers would
eventually agree on the three-point hitch as the one standard
system to hitch implements to tractors. As patents on technology
expired, the manufacturers were able to refine the system and
create useful modifications. Now, nearly all manufacturers have
adopted some standardised[1] form of the modern three-point
hitch system; many companies also offer safe adaptation kits for
converting the non-standard hitch systems to the three-point
hitch system.
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