Weight, Transfer, Traction, and Safety 423

Transcription

1 Weight, Transfer, Traction, and Safety 423 Figure A tractor front tire. Table Standard industry codes for tire types. [a] Type of Tire Code FRONT TRACTOR Rice tread F-1 Single rib tread F-2 Dual rib tread F-2D Triple rib tread F-3T Industrial tread F-3 DRIVE WHEEL TRACTOR (REAR) Rear wheel, regular tread R-1 Cane and rice, deep tread R-2 [b] Shallow, non-directional tread R-3 Industrial, intermediate tread R-4 IMPLEMENT Rib tread I-1 Traction tread I-3 Plow tail wheel I-4 Smooth tread I-6 OFF-THE-ROAD TIRES (INDUSTRIAL) Rib E-1 Traction E-2 Rock E-3 Rock deep tread E-4 Rock intermediate E-5 Rock maximum E-6 Floatation E-7 [a] Source: Reprinted with Deere & Company, Fundamentals of service tires and tracks. [b] Also includes similar treads for G, L, and ML series codes.

2 424 Chapter 16 construction is similar to that of the belted, biased ply tire, except that the plies are nearly perpendicular to the tire beads (Figure 16.6c). Radial tires can be operated at lower inflation pressures than can biased ply tires and thus have a greater area of contact with the ground. Both front and rear tractor tires are available with either radial or biased ply designs. Radial tires usually are more expensive, but they provide increased traction and a softer ride. The codes listed in Table 16.1 have been developed by the tire industry for tires used on farm equipment. Code R designates a tractor rear tire, and an accompanying digit indicates the type of tread. Likewise, code F indicates a tractor front tire, and a digit indicates the type of tread. Figure Biased and radial ply construction. (Courtesy of Deere & Company.)

3 430 Chapter 16 used to predict the tractive performance of tractors with rear-wheel drive. It is based on Equations 16.11, 16.12, and and on data from numerous drawbar tests in farm fields. Soil strength may vary considerably, depending on soil type, moisture content, and state of tillage. Zoz grouped all agricultural soils into three classes: firm soil, tilled soil, and soft or sandy soil. Because it lacks more precise definition of soil strength, the Zoz chart can provide only an approximate estimate of tractive performance. Other disadvantages of the Zoz chart are that it applies only to 2WD tractors with bias-ply tires. Zoz has developed a computer spreadsheet, which extends the traction predictions to include 4WD and FWA tractors with either bias, or radialply tires. However, the Zoz chart will be used in this textbook for illustrating some important principles of traction. Tractive performance is affected by weight transfer. It would be very difficult to measure the distances Z f and Z r (Figure 16.2) under field conditions, and therefore Equations 16.6 and 16.9 are not of much use in the calculation of weight transfer. Zoz developed the following equation for calculating weight transfer: where R r = weight transfer to rear wheels in kn (lb) DWC = dynamic weight coefficient in decimals F db = drawbar pull in kn (lb) R r DWC F db (16.16) Zoz estimated the following values for DWC: 0.65 for integral implements, such as plows fully mounted on the three-point hitch; 0.45 for semimounted implements, such as semimounted plows; and 0.20 for towed implements, such as plows hitched to the drawbar. Because the weight transfer depends on the dynamic weight coefficient, use of the Zoz chart to predict tractive performance requires that the type of hitch be specified. The Zoz chart can be used with data taken from Nebraska or OECD Tractor Tests. How the chart is used is illustrated in Example Problem Example Problem 16.3 The tractor tested in Nebraska OECD Test No 1765 (see Table 16.2) is to be used with full ballast to pull a semimounted plow on firm soil in eighth gear. Using the Zoz chart, determine the actual travel speed, travel reduction, drawbar pull, and drawbar power. Solution: Before using the Zoz chart, it is necessary to consult the Nebraska test to find values for the static rear axle force (SRAF), travel speed without load (S o ), and axle power (P A ). The Nebraska tractor test shows 6482-kg mass (14,290-lb weight) at the rear axle. The SRAF (same as R ro in Equation 16.8) is equal to: SRAF = [6482 kg] [9.80 m/s 2 ] = 63,245 N (14,290 lb)

4 Weight, Transfer, Traction, and Safety 431 Table Nebraska OECD Tractor Test 1765 Summary 292; Case IH MX 200 Diesel, 18-speed. (continued)

5 432 Chapter 16 Table 16.2 (continued). Nebraska OECD Tractor Test 1765 Summary 292; Case IH MX 200 Diesel, 18-speed. In the Nebraska test on a concrete track, the tractor produced kw (142.3 hp) of drawbar power in eighth gear while traveling 7.94 km/h (4.94 mph) with 2.8% slip. The travel speed without load can be calculated from a modification of Equation 16.11: S o = S a /[1 - TR/100] = 7.94/[ ] = 8.17 km/h