Tractors frequently serve as the power source for field machines. A hitch connects the implement to the tractor. The tractor provides the tractive force to move the implement through the field. Thus, this chapter deals with tractor hitching and traction.
Hitching Systems
Principles of hitching
Most agricultural operations involve hitching some type of implement to a tractor. The forces transmitted through a hitch can affect the performance of both the tractor and the implement. Modern hitches include feedback for automatic control of pull and/or depth of tillage implements. In addition to transmitting forces, the hitch may also be required to carry the implement for transport.
Types of hitches
Early tractors included only a drawbar hitch, which permitted pulling but not carrying an attached implement. The three-point hitch has now become standard equipment on most tractors. The tractor of Figure 7.1 is equipped with both a drawbar and a three-point hitch. Terminology of a three-point hitch is illustrated in Figures 7.2 and 7.3. The points of attachment of the hitch links to the tractor are called link points, while the links are attached to the implement at the hitch points. Quick-attaching couplers (Figure 7.4) have been developed to allow faster attachment of three-point hitches to implements. Dimensions of three-point hitches have been standardized by ASAE (now ASABE) since 1959. Table 7.1 shows four of the hitch categories that have been standardized for different sized tractors; in addition, there is a category 0 hitch standard for garden tractors. The hitch dimensions that are standardized include hitch pin diameters, mast height, and lower hitch point spread. Exact dimensions can be found by referring to ASAE Standard S217. There are also ASAE standards relating to the drawbar; Standard S207 specifies minimum vertical loads that drawbars must withstand, while Standard S203 specifies the location of the drawbar hitch point relative to the PTO shaft.
Figure 7.1 – A tractor with a drawbar and three-point hitch.
Figure 7.2 – A three-point hitch for a tractor
Figure 7.3 – Three-point hitch connections on an implement
Figure 7.4 – Three-point hitch with quick-attaching coupler
Table 7.1. Three-point hitch categories (ASAE Standard S217.10).
An implement with a mast (Figure 7.3) can be carried entirely by the tractor and such implements are said to be fully mounted. Semi-mounted implements are attached to the tractor by only the two lower links and the ground must provide part of the implement support. Semi-mounted moldboard plows are the best known example of this type of hitch. Thus, the three types of hitching include towed (for implements hitched to the drawbar), semi-mounted, and fully mounted implements.
