There’s an old saying: “no crops without fields, no wheat without manure”.
But what is the real nature of this cultivated earth that towed agricultural machines turn, loosen and shape?
From tillage to harvesting, numerous towed machines are used by farmers to help production. These machines, and even more so the bearings they rely on, are in direct contact with an often harsh environment.
What do we mean by a harsh environment? There are two types of pollutants:
- Solid pollutants: earth, sand, stones but also plant residues and hay bale twine.
- Liquid pollutants: mud, manure, fertilisers but also clean water for washing. These pollutants include numerous materials that are abrasive (waste, twine) but also corrosive (manure, ammonium fertilisers).
Let’s focus on earth, but first: what actually is it?
The earth on cultivated land contains solid constituents (mineral and organic material), liquids and gases. Pore space can make up 50% of the total volume, and this is filled with water or gas. Mineral constituents – or the mineral part – come from physical-chemical degradation of the source rock. Organic constituents – the organic part – come from decomposition of plant, animal and microbial life (fungi, bacteria). This part constitutes the humus, which plants require to grow.
If we look at the mineral part in detail, it is made up of elements with different grain sizes, including:
- grit and stones (grain size > 2 mm)
- sands (20 μm-0.2 mm)
- limes (2 μm-20 μm)
- granular clay (< 2 μm)
All of these particles pose a threat to bearings.
How does this affect the bearings?
Grit and stones affect the exterior of bearing units (impact) but also have an abrasive action on external sections of the seals: robust shields are used in front of the seals to protect them.
The smaller the grain size, the easier it is for the substance to get to the seals. Sand will be stopped with multi-lip seals, and for the finest particles that would not be kept out by the external lips of the seals, static grease barriers will be applied within the sealing systems to capture any tiny particles that make it through.
If that is not enough, we keep going: new lips on seals, then grease barriers.
Due to the variety of pollution, and to keep the machine working effectively, the bearings must be equipped with seals specifically developed for this environment. There are many variants, dependent on the position of the bearing in the machine (from single to reinforced triple-lip seals, up to 8-lip seals). In addition, these parts must be fixed on to ductile cast iron bearings to achieve the necessary robustness against solid pollution.
Some machines can have up to 100 bearings, so it is easy to see why the technical characteristics of these parts are so important!