Forage harvester knife sharpening system: prevent play and uneven grinding
Uneven cutting and increased fuel consumption can result when worn sharpening-system guides cause the knives on the chopping drum to be ground unevenly. Keeping the bearings free of play and setting the rear guide correctly helps the knives run close to the shear bar with a consistent clearance.
The basics: drum shape and knife sharpness
The sharpening system does more than sharpen knives. It also determines the shape of the chopping drum. As the drum rotates, the grinding stone travels from left to right. The aim is an even drum shape so the knives pass as closely as possible to the shear bar across the full width.
Why engine speed changes during sharpening
Vibration can affect grinding quality. To avoid resonance between the engine and the number of knives on the drum, many forage harvesters use an adjusted engine speed during sharpening. The original article lists the following CLAAS JAGUAR engine speeds:
- 20-knife drum: 840 rpm
- 24-knife drum: 1,370 rpm
- 28-knife drum: 1,180 rpm
- 36-knife drum: 920 rpm
Front and rear guides
The grinding-stone holder uses two guides:
- Front guide: The holder runs on a straight, chrome-plated steel rod using linear bearings.
- Rear guide: A bearing runs along a steel strip, which is straight on most machines. The bearing must roll smoothly without play. An old, rough-running or uneven bearing will leave visible waves or notches in the ground knife surface.
Play and adjustment of the rear strip
The front and rear guides and the grinding-stone threaded bushing must have virtually no play. The original article specifies a maximum of 0.1 mm. Excessive play may require replacing bearings or the threaded bushing, or adjusting shims under the upper guide.
The rear strip can be adjusted on both sides through slotted holes. It is aligned at the factory but must be reset after removal, with no more than 0.1 mm variation in the stone-to-knife distance according to the source. If new knives grind correctly on one side but not the other, the rear strip may need adjustment.
Note from the original article: From approximately model year 2016, CLAAS chose to bend the strip by 0.4 mm so the stone grinds slightly lower in the middle than at the ends.
How sharp should the knives be?
- Grass: Razor-sharp edges are not essential, and sharpness wears off quickly.
- Maize: Very sharp knives tend to retain their edge for longer.
Modern forage harvesters with forward-grinding drums use a self-sharpening principle, with knives passing extremely close to the shear bar. Bas' rule of thumb: Adjust the shear bar regularly, check that the drum is true, and use those observations to decide when to sharpen. The original article suggests an average of around ten sharpening passes per 30 hectares of maize.
In brief
- Grind the drum evenly so the knives pass close to the shear bar.
- Check the front linear bearings and the rear guide roller bearing.
- Keep the guides and threaded bushing free of play, with a maximum of 0.1 mm as specified in the original article.
- Set the shear bar regularly and assess sharpening needs based on the cutting result.