5. Double-Chop Forage Harvester

Trailed from an off-set draw bar and power take-off driven, this type of harvester has a feed auger and a flywheel chopper with a set of knives and paddle fan blades. The flywheel runs at 700-1500 rpm depending on the model and the required length of chop.

Plate 15.6 A rotary maize header with cutting discs which gather the crop and convey it to the feed rolls. (John Deere)

Double chop harvesters may either have a pick-up cylinder for collecting previously cut and wilted crops or a flail cutting mechanism for direct harvesting. The crop is cut by the flails which run at a speed of about 1,800 rpm and the lacerated crop is delivered by a cross-conveying auger to the chopper unit. Some models of flywheel chop forage harvester have feed rolls which compress the material to ensure an even feed to the chopping unit. The chopper knives cut the crop against a fixed blade in the opening from the auger to the fly­wheel housing. After chopping, the fan blades blow the cut material up the delivery spout into a trailer.

There is some variation in the range of chop lengths which can be obtained on different harvesters. One model, for example, can chop the crop into lengths varying from 5-22 mm, another has chop lengths ranging from 14-35 mm.

5.1. Controls and adjustments

Length of chop is varied by altering chopper flywheel speed, changing the number of knives or by varying the speed of the feed rolls. A slow cylinder speed with a full set of knives gives the shortest length of chop.

Pick-up or cutting height is adjusted hydrauli-cally or with wheels on the pick-up cylinder. Some trailed machines have a height control ram on the drawbar.

Feed roll reverse Blockages in the feed rolls caused mainly by overload can be cleared by operating the reverse drive mechanism.

Delivery spout This can be adjusted to deliver the chopped material to a trailer pulled along­side or behind the harvester. The deflector plate at the end of the spout can be adjusted to control the discharge angle.

Drawbar Off-set, trailed machines require the drawbar to be moved from the road to the field position before work can start.

5.2. Safety devices

All forage harvesters have mechanical protec­tion devices which are designed to prevent damage to the machine. They include:

• Main drive protection Provided by slip and over-run clutches. The slip clutch allows the power drive to continue turning when there is a blockage in the machine. The over-run clutch allows the machine to slow down at its own pace when the power drive is disengaged.

• Metal detector Forage harvesters are liable to pick up pieces of metal which will do serious damage to the chopping unit. The detector locates ferrous metal objects mag­netically. The entire feed mechanism is stopped instantly and, by engaging the reverse drive, the metal object can be removed after stopping the machine.

• Shear bolts Fitted in the main drive of many forage harvesters.

• Vee-belt drives When used they provide a built-in safety mechanism, since overload will cause belt slip.