Information on the mechanization of agriculture, gardening, components and multifunctionality.

Shredding of lawn grass

The maintenance of lawns presents different technical needs depending on the quality and intended use of the surfaces. The mechanical industry creates specialized machines, with various types of engines, able to carry out targeted operations, optimizing consumption and maintenances

by Pietro Piccarolo
January - February 2017 | Back

The grass cut from the lawns can be: expelled and collected later; directly collected in a basket while cutting; shredded inside the housing and expelled on the turf. In the latter case, we can distinguish, albeit schematically, the two limit situations, that is: working on lawns with a good ornamental value, and working of low value lawns such as high grass areas and lawns infested with weeds, as well as slopes, escarpments, river banks, roadsides, or even forest undergrowth.

The first type of operations lead to the so-called mulch mowing, while the second type of operations are more generally referred to as grass shredding. Their implementation requires the use of different types of machines.


Mulching of high-value lawns

In mowers that perform mulch mowing, also called grass-cycling, instead of ejecting it after the cut, the grass is sent towards the upper part of the housing and while falling back down it is cut further, thereby obtaining a fine shredding. The shredding fragments are then thrown onto the turf. The cutting-shredding section is constituted by sharp blades along its entire length, while in normal mowers the blades are only sharp at their end. To facilitate the shredding, the housing has a particular shape, and it may include special deflectors that direct the cut grass towards the blades.

A necessary condition to carry out grass-cycling is high mowing frequency, approximately every 4-6 days. This is because the biomass to be cut and shred should not exceed 1.5-2.5 cm, i.e. no more than a third compared with the 5-8 cm of grass height. At this stage, the grass has a high moisture content which promotes its rapid degradation, reducing the risk of the felt becoming thicker. The disadvantage of the mulching-effect mowing, in addition to the higher frequency, is represented by a risk of increased formation of felt that, beyond a certain thickness, involves carrying out defelting interventions. Conversely, the benefits are especially represented by the lower execution time, since the cut grass disposal operation is gone. An added benefit is the recovery of nutrients, water and the organic substance contained in the fragments of grass removed and put back into the turf growth cycle. It is as if you were performing a fertilizing operation with natural elements. This technique should not be used when the grass is too high or when it is wet. In case of wet grass, visible heaps are formed on the turf, with a slower degradation. When instead the height is excessive, the limit is represented by the high biomass of the portion that is to be removed and which is difficult to shred and has a slower degradation. In this case, it is necessary to perform catcher mowing at a certain height and, only subsequently carrying out the mulch mowing. In the period of greatest grass growth which, in our climate, generally lasts until June, we recommend carrying out catcher mowing. Starting in summer, the grass growth rate is reduced, creating the best conditions for mulch mowing. In any case, if the mulching is managed well, it can be performed throughout the year. The ideal is to be able to combine both catcher mowing and mulch mowing on one mower.

Precisely to offer the user the possibility to use the same machine to carry out both catcher mowing and mulch mowing, depending on the conditions of the vegetation, many manufacturers have created machines with versatile cutting apparatus. In practice, to switch from catcher mowing to mulch mowing, it is simply a matter of inserting a stopper into the mouth of the discharge channel.

If there is an adequate structure and shape of the housing and the blades, all types of lawnmower, from those with a walk-behind operator to those with the driver on board, can perform mulch mowing. On machines with walk-behind operators, the choice is between an electric motor (with cord or battery) and an internal combustion engine. The electric power supply allows a reduction of the cost per hour of operation, reaching more than 80% compared to petrol. By contrast, for the same performance, the purchase price of a cordless lawnmower is higher, at about 30% more than a petrol-powered lawnmower. Considering the environmental and operator comfort aspects, the choice, in purely economic terms, is therefore linked to the yearly hours of operation of the machine. In professional lawnmowers with driver on board, i.e. designed to operate on surfaces exceeding 2500-3000 m2, the choice of power, for equal performance, is between petrol engine and diesel engine. The maintenance of a petrol engine is less expensive and the purchase price is lower. Diesel engines, by contrast, have a lower fuel consumption, but the purchase price of the lawnmower is higher. It is generally believed that using motorized equipment powered by diesel is justified for applications greater than 300 hours a year.

Even as regards robot lawnmowers, there has been an evolution implemented by some manufacturers, with the introduction of cutting blades which enable grass-cycling with excellent results. On the market, there are also disc plates with blades capable of mulch mowing, which can be applied, front or rear, to compact tractors, or even to traditional professional mowers with driver on board. However, this solution is more widespread for shredding on non-prized turf.


Shredding and brushcutting of non-prized turf

On non-prized turf where the cut grass is not collected, sometimes because of difficult accessibility, the grass shredding in general is not as extreme as in the mulch mowing described above, and the operation is often due not only to the mowing but also to brushcutting. This is because the grass is high and sometimes mixed with shrubs and thin brushwood. The machines that are employed are self-propelled grass flail mowers and flail mowers mounted on walking tractors or self-propelled units, or even on tractors. The shredding apparatus may be of the type with horizontal rotating blades, or of the type with knives (Y-shaped), or hammers, hinged on the horizontal rotating axis. The horizontal blade equipment is suitable only for operations on grassy surfaces, while the second type is best suited for operations on more demanding surfaces, where in addition to the grass there are bushes and brushwood residues.

The self-propelled flail mowers can be either walk-behind and with the driver on board. They are often used on sloping areas, and are characterized by a very low centre of gravity. The propulsion elements can be wheeled or tracked. The shredding apparatus, in general, is made of knives or hammers hinged on the horizontal rotating axis. The powers involved range from 8-10 kW, with working lengths between 50 cm and 120 cm. The transmission is mechanical or, more often, hydraulic. When areas very sloping, radio-controlled machines are used.

The walking tractors on which the flail device are mounted have powers between 7-8 kW and 12-15 kW, and the working widths range from 70-80 cm to 120-130 cm. The coupling of the flail device with the self-propelled units can also be achieved with single-axis tool-carrying self-propelled units.

The flail device coupling also relates to compact tractors with power ranging from 15 kW to 25 kW, as well as the higher power tractors, up to 100-120 kW. The coupling is done with the three-point linkage, and the drive is by means of the PTO. The flail device can be mounted on the front, rear or even sideways. The working widths reach 2 metres and more. The choice of working width is related to the size and shape of the areas to be worked. A special case is that of flail devices carried by telescopic arms, whose use is mainly aimed at shredding on roadsides, on slopes and on river banks. Thanks to the particular kinematics of the telescopic arms, it is possible to operate very close to the road edges. The horizontal outreach is a few metres (sometimes greater than 6 metres), as is the case for the height in vertical position.


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