Drawbar
A drawbar is a hitching device that is used in agricultural engineering primarily on tractors and enables various implements and trailers to be "hitched down" . For this it is attached to the rear as the lowest point in the middle of the rear of the tractor.
Historically and depending on the manufacturer, there are different drawbar designs. They differ mainly in the type of attachment to the tractor, in the lengthways structure (short or long, fixed or adjustable in length, one or more parts, straight or curved) and in the coupling points ( shackle , ball head or bolt connection ). What they all have in common, however, is that the drawbar linkage is basically horizontally movable and looks like a pendulum when viewed from behind . Depending on the application, the horizontal play can be released or restricted, and the horizontal position of the coupling point can also be determined to the left or right of the tractor center for off-track work.
In German, pull rods that are only adjustable in length are sometimes referred to as drawbars . Their basic construction resembles the usual drawbars, but thanks to permanent fixation they do not allow any lateral pivoting ("swinging") deviating from the tractor's longitudinal axis.
history
As a historical forerunner of the Zugpendel u. a. swiveling wooden tie rods on carts and other animal-drawn agricultural equipment apply. At the end of the 19th and beginning of the 20th century, locomotives ( steam tractors ) and early tractors only offered very simple, rigidly mounted components for coupling the devices, machines and trailers to be towed. These devices often only consisted of a single large eyelet or a metal plate with a few, then mostly differently sized holes to which a pull chain , pull rod or drawbar could be attached.
With the advent of solid Cross bars with regular holes over almost the entire width of the tractor - first between the rear wheels, and later behind it - was created in the early 20th century, the possibility of equipment not only centrally, but also laterally offset to attach. The use of equipment, which was slightly offset from the tractor lane, was an advantage given the small working widths at that time, for example on slopes or along water ditches. a. reduced the risk of the tractor unit tipping over or slipping. When changing the attachment from one side to the other, however, the often heavy device always had to be lifted out and reconnected.
The first drawbars for tractors were built in the 1920s and then further developed. They reduced the work and time required when changing sides of the attachment compared to the drawbar. Only the bolts in the drawbar frame that limit the side play had to be repositioned; the connection between the tractor and the device remained intact. In addition, when the pendulum play is enabled (free movement to the left and right approximately across the width of a drawbar), drawbars enable significantly narrower turning radii and ensure an almost constant power transmission when cornering under load. The latter was particularly important when tilling the soil in small, non-rectangular fields. In addition, the flexible drawbar attachment compared to rigid drawbar connections made it easier to steer the tractor, especially on heavy and irregular (e.g. stony) soils. This was particularly noticeable when plowing , where the plow turning the ground causes strong one-sided forces (side pull) to act on the tractor, which requires constant counter-steering even when driving straight ahead .
At the same time, the free use of the commuter train initially represented a considerable risk of accidents. Particularly with a short distance between the tractor and the device and a larger device width, it could happen during turning processes that the attached device touches the inner rear wheel (which is always driven in tractors), which almost inevitably leads to the immediate tipping of the tractor to the outside of the curve. Roll-over protection bars (ROPS) and safety cabins were not yet common at that time. Corresponding accidents often resulted in serious injuries or death to the driver.
In the second half of the 20th century, standardization of the commuting was initiated. Among other things, it went hand in hand with a restriction of the pendulum angle, which largely excluded contact between devices and wheels and thus the classic "commuting accidents", and resulted in the first international commuting standard in 1992.
Together with the decline in, among other things, high-pressure balers for the standard tractor, the use of the drawbar has lost its importance towards the end of the 20th century. Due to a variety of technical developments, the former advantages of commuting hardly play a decisive role. - Usual working widths of towed implements often exceed the actual tractor width and classic track-offset work is therefore only rarely found. Strongly increased engine performance and all-wheel drive allow sufficient power transmission (improved traction ) even on heavy soils and in tight bends. Steering systems make it easier to keep in lane and other forms of lower hitching offer greater driving comfort. In addition, the three-point attachment is increasingly becoming the standard for many implements that used to be typically attached below. With automatic hoist controls, especially with EHR and floating position, it offers flexible, robust and powerful technology for the use of work equipment. For towing transport trailers, however, the pin coupling has prevailed.
Normalization
Modern drawbars have been technically described and standardized in ISO standard 6489-3 and DIN 9677 since the early 1990s . Their dimensions and absorbable drawbar loads are regulated depending on the power lift categories in accordance with ISO 730 and the drawbar longitudinal settings. The further a support pendulum is pulled back, the lower the permissible support load.
In addition to the tractor or to the trailer hitch (Clutch bearing block mounted) support ( support frame , the support plate ) are normalized drawbar essentially of a Zugpendelrahmen ( draft frame , the pendulum frame ) and two superposed flat steel bars ( tie rod , pendulum rod ) having an open to the rear hitch ( Mouth ). The drawbar frame specifies the horizontal position and the maximum side play. With the manufacturer and tractor-specific support frame, it can also form a structural unit as a drawbar support bracket . When coupling the hitch of the pendulum rod is guided over the towing eye , which is located as a counterpart on the device to be attached or on the drawbar of the trailer. To attach the drawbar eye in the hitch, the two drawbar arms (flat steel ) are provided with holes that accommodate a vertical socket pin , which in turn z. B. to be secured with a spring pin .
In the case of standard-compliant drawbars, drawbar eyes with an inner diameter of 40 or 50 mm, which correspond to the following standards, may be attached: ISO 1102 (DIN 74053-1), ISO 8755 (DIN 74054-1 / 2), ISO 5692-1 (also ISO 20019; DIN 9678-2), ISO 5692-2 (DIN 11026) and DIN 11043.
Since technical standards are only indirectly legally binding, various manufacturers continue to offer non-standardized drawbars, especially for old tractors. Non-standard drawbars for new tractors are mostly roughly similar in terms of their dimensions and structure to the standardized versions.
Usage today
Nowadays, at the beginning of the 21st century, drawbars are mainly used to pull simple tillage equipment such as harrows and rollers or balers , rakes and other machines where the drawbar has a rather low vertical load. Outside of road traffic, this is possible in Germany without a type approval .
According to international regulations, however, drawbars are type-approved and may therefore also be used to connect tractors with transport trailers in road traffic with the existing approval (if necessary after individual acceptance) . However, due to the lack of angular mobility (rotatability around the tractor's longitudinal and transverse axis), this is not recommended or - in connection with certain towing eyes - only to a limited extent. Compared to other types of attachment that also work with towing eyes, such as bolt , hitch or pitch-fix couplings, they also have a large length and side play, which leads to uncomfortable driving behavior.
A drawbar is only part of the standard equipment of new standard tractors in a few cases, but it is often available as special equipment. Occasionally, system and large tractors such as Claas - Xerion - or Case-IH- STX models are equipped with it in order to provide a stable turning circle-reducing towing device for very wide implements or system carriers that require a type of bottom hitch. Standardized drawbars as well as simple drawbar forms with only one flat steel are still installed in some small and compact tractors as inexpensive and flexible (swiveling or at least adjustable in length) hitching devices, because in their areas of application (e.g. for lawn care) track-offset work plays a greater role and rather low tensile and support loads occur.
Web links
Individual evidence
- ↑ cf. the naming of pulling devices of various small tractors, u. a. on the part of John Deere Germany
- ↑ see e.g. B. the drawbars on Letter-series tractors from John Deere from the 1920s and 1930s and u. a. US patents 1715682 ( Drawbar hitch for tractors , George Starks, 1929) and 2473357 ( Swinging drawbar , Edward G. Blunier, 1949), both with detailed explanations
- ↑ The first draft and drawbar designs in North America may have existed as early as the 1910s; so far, however, no evidence has been found for this.
- ↑ a b Current: ISO 6489-3: 2004 (Agricultural vehicles - Mechanical connections between towed and towing vehicles - Part 3: Tractor drawbar) , German: Agricultural vehicles - Mechanical connections between towed and towing vehicles - Part 3: Tractor drawbar ; outdated previous standard: ISO 6489-3: 1992
- ↑ DIN 9677: 1993-12 (agricultural machinery and tractors; drawbars) , identical to ISO 6489-3: 1992
- ↑ Current: ISO 730: 2009 (Agricultural wheeled tractors - Rear-mounted three-point linkage - Categories 1N, 1, 2N, 2, 3N, 3, 4N and 4) , German: Agricultural wheeled tractors - Rear-mounted three-point linkage - Categories 1N , 1, 2N, 2, 3N, 3, 4N and 4 ; Replaced previous standards: ISO 730-1 to ISO 730-3 (from 1977)
- ↑ a b c d DLG leaflet 387: Hitching devices on tractors. (PDF; 1.8 MB) (No longer available online.) Deutsche Landwirtschafts-Gesellschaft , April 2013, pp. 19–22 , archived from the original on February 13, 2017 ; accessed on February 13, 2017 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
