The Tractor

Because so many field operations are tractor-powered, special attention must be given to optimizing how tractor engine power is generated and transmitted. For higher horsepower tractors, many tasks use the drawbar to pull tillage and seeding implements through the soil. Efficient transfer of engine power to the ground requires proper attention to ballasting and tire inflation. Other tasks require transfer of engine power through the power-take-off (PTO) shaft (e.g. baling) or hydraulic or electrical systems (e.g., some spray pumps or planter seed metering drives). Taking time to assess how tractor engine power is being transferred and used for field operations can help in developing management strategies that can save fuel. Major areas affecting tractor fuel consumption include ballasting/slip/tire inflation, engine and transmission maintenance, and tractor selection.

Figure 1: Tractive efficiency of transferring axle power to the drawbar as affected by wheel slip for various surface conditions. Source: Mark Hanna

Figure 1: Tractive efficiency of transferring axle power to the drawbar as affected by wheel slip for various surface conditions. Source: Mark Hanna

Ballasting/slip/tire inflation
Excessive wheel slippage during drawbar pull operations creates an obvious waste of labor, fuel, and tractor hours. Conversely, a tractor ballasted so heavily that there is little or no wheel slip sinks too far into the soil, causing rolling resistance as the wheel tries to climb out of the track and extra energy use as tire sidewalls flex. Optimum wheel slip range for maximum tractive efficiency (equal to the ratio of drawbar power to power available at the drive axle) depends on surface conditions (Figure 1). Higher-horsepower tractors often have sensors, allowing drive wheel slip to be monitored from the cab. Slip can be conveniently checked during fieldwork with significant drawbar loads. On tractors without slip measurement, slip can be approximated by measuring the distance a tractor covers during 10 wheel revolutions under drawbar load and comparing this with the distance traveled during 10 wheel revolutions without drawbar load. For example, if loaded wheel distance is 180 feet and unloaded wheel distance is 200 feet, the tractor under load is covering only 90 percent of the unloaded distance, or experiencing a 10 percent wheel slip. As a quick visual check, optimal wheel slip on soil usually occurs when wheel lug marks near the tire centerline are blurred or distorted but lug marks near the outer edge are reasonably distinct (Illustration 1).

Illustration 1: Examples of tractor ballsting. Source: Tractor Ballasting, R.T. Schuler, University of Wisconsin Extension, A3401, 1987

If wheel slip is outside the optimal range of about 9–15 percent (typical for most soil conditions) or if there are questions regarding whether the tractor is ballasted properly, the tractor’s operation manual or various references can be checked for advice on ballasting (see Figure 1) [6]. Specific amounts of total tractor weight per tractor horsepower are generally suggested depending on tractor style (two-wheel drive, front-wheel-assist, four-wheel drive) and operational speed. Tractors using faster field speeds (e.g. 6 to 7 mph instead of 4 or 5 mph) have optimal fuel efficiency using slightly less total weight. Because power is efficiently transferred from engine to drawbar over a range of slip, some variation in weight is allowed. Most tractors are loaded to use only 70–90 percent of rated power available so weight values in Table 2 are near the low side of the appropriate range [4]. Carrying extra ballast for unused horsepower during operations with light drawbar loads (e.g., pull-behind sprayer, mower/conditioner, or baler) results in small amounts of slip. If the tractor is used long periods of time for light drawbar loads but has been optimally ballasted for full drawbar power, consider removing ballast to avoid burning fuel to carry dead weight.

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