Sicklebar mower draft

The draft requirement for mowers is the sum of a rolling resistance, a mechanical resistance, and a functional resistance related to crop or material flow. Rolling resistance is the force needed to keep a wheeled implement moving at a constant speed while deflecting rubber tires, compressing soil, and overcoming wheel and axle-bearing friction. Mechanical resistance is the power absorbed by the implement under no-load conditions to overcome the friction of moving parts. The functional resistance is related to crop cutting which varies with crop yield, crop moisture and other factors.

In draft measurement work at Tillers International in Scotts, MI we measured the draft components of two ground-driven McCormick-Deering #7 sicklebar mowers. One had a 5-foot cutterbar and weighed 850 pounds. The other had a 6-foot cutterbar and used a hard rubber caster wheel (12×3) to support the front of the mower. The 6-foot mower weighed 900 pounds. Each mower had steel wheels (32×5½). Gearing was such that each rotation of the drive wheels provided twenty-five rotations of the flywheel (fifty cutting strokes).

Draft measurements were made with the 6-foot mower in early June in an alfalfa-grass hay crop yielding about 2½ tons per acre of dry hay. Draft of both mowers was measured in the same field in mid-July in a crop yielding about 1½ tons per acre. At about the same time a friend in Germany, Peter Herold, was making similar tests with a 4.25 ft cutterbar in a 2nd cutting grass-clover crop yielding 1.7 ton/acre.

The average travel speed in each trial was 3mph. Assuming no wheel slip, this travel speed provided about 1,640 cutting strokes per minute. An average draft ranged from 63 to 69 lbf per foot of cutterbar length. Total draft was about 400 lbf for the 6-foot mower, 320 lbf for the 5-ft and 272 lbf for the 4.25 ft mower.

We evaluated the rolling resistance by measuring the pulling force of the mower without cutting. The sum of the rolling resistance and mechanical resistance was measured as the pulling force with the cutterbar engaged but without cutting any hay. The functional or crop resistance was measured as the total draft while mowing minus the rolling and mechanical components.

At Tillers, rolling resistance contributed 94 to 123 lbf (pounds-force) to the mower draft, a load equal to 30% of the total draft. Mechanical resistance added 74 to 93 lbf and accounted for 23% of the total draft. This was the power absorbed by the mower with the cutterbar engaged under no-load to overcome the friction of moving parts. Crop resistance from cutting the hay crop accounted for 47% of the total draft load, adding an average of 152 lbf to the 5-foot mower and as much as 200 lbf to the 6-foot mower. In Germany, Peter evaluated total draft but not the individual draft components.

Based on the results of our work, an average draft of about 65 lbf per foot of cutterbar length is a good estimate of the pulling force needed for mowing. The total draft would be perhaps 50 lbf less with pneumatic rather than steel tires. The caster wheel added to the rolling resistance of the 6-foot mower but reduced the neck load carried by the team. Proper mower adjustment and lubrication will help minimize mechanical resistance and draft. Problems that will increase crop resistance and draft include loose or misaligned guards, dull or loose knife sections and loose guard ledger plates.

Mowing is a challenging task because the weather is often hot, the team needs to maintain a good speed for good mower functioning, and it is best if you do not have to stop part way through a swath. A pulling force for a sled or stoneboat on hay ground equivalent to the pulling force needed for mowing would be a sled loaded to about 700 lbs GVW for the 4.25 ft mower, 800 lbs for the 5 ft mower and 1000 lbs for the 6 ft mower.