Agriculture Reference
In-Depth Information
load and inflation pressures. In fact, because of the soil-tire interface, common practice
dictates that tire inflation pressures be reduced to the absolute minimum to achieve the
best possible performance and fuel efficiency. As tractor size increases above the current
upper limits, one or more of the following limitations must be overcome: (1) allowable
tire loads must increase for existing section sizes; (2) tires must be added to axles
(i.e., duals and triples); (3) tire diameters must increase; or (4) drive trains must be
reconfigured to include more than two axles. The dilemma in European countries
is that tractor manufacturers must work within the 3.0 and 3.5 m transport widths
thereby limiting tire spacing and/or section widths. By today's standards it is imprac-
tical to achieve axle loads in excess of 15,000 kg. The two viable options that remain
are larger diameter tires, or more axles.
When matching tillage tools and seeding equipment with available power, it
is common to see a fully loaded no-till planter develop draft forces approaching
2000 N/drill row from ASABE (2011). Assuming a seeding speed of 10.0 km/h, this
implement requires tractor engine power approaching 6.25 kW/row. Putting this in
perspective, a 36-row no-till planter will require a 325-kW tractor assuming a trac-
tive efficiency of 77% and a transmission efficiency of 90% for a four-wheel drive
(4WD) tractor. It is the combination of implement width, ground speed, draft, and
tractive efficiency that mandates the minimum tractor size. The tractor must be bal-
lasted to take full advantage of the engine power. Typically, ballasted tractor mass
ranges from 65 to 70 kg/engine kW for a minimum total tractor mass of about 21,000
kg. With a 60/40 static weight split between the front and rear axles, as is typical of
properly ballasted 4WD tractors, and assuming row-crop dual tires, each tire must
support a load of up to 3150 kg/tire. From manufacturer specifications, the minimal
acceptable tire is 480/80R42 at an inflated pressure of 48 kPa. When going to single
tires, the minimal acceptable tire size is a 900/50R42, again inflated to 48 kPa. For
row-crop tires the minimal tractor width is 3.53 m, whereas for single tires the mini-
mum width is 2.84 m (9.32 ft). The latter case is what most European producers are
required to accept.
Although this discussion is focused on tractors, similar situations have arisen
for other field machinery. Table 5.1 summarizes some of the equipment parameters
becoming commonplace in the United States. Of major concern is the continual
TABLE 5.1
Summary Statistics for Modern Field Machinery Power and Mass
Unballasted
Mass (kg)
Ballasted/Loaded
Mass (kg)
Engine Power
(kW)
Equipment
4WD Tractor
22,900
27,200
464
Class IX Combine with 16 Row
Corn Head
21,500
31,600
390
55 T Grain Cart
14,800
69,300
-
High Clearance Self-Propelled
Sprayer (4000 L tank)
13,700
17,860
323
