Tyres for greenery maintenance
From the transmission of traction force to the support of the machine load, to the reduction of vibrations caused by irregularities in the ground, tyres perform an important function for all moving machines, even those for greenery maintenance
Tyres play an important role for all moving machines and therefore also for those intended for the maintenance of green areas. In fact, they perform various functions, such as: supporting the load of the machine and discharging it onto the ground; transmit the traction force thanks to adhesion and friction with the ground; dampen vibrations due to irregularities in the ground; give the forward direction to the machine. The invention dates back to 1845 by the Scotsman R.W. Thomson, with a subsequent refinement brought by the British J.B. Dunlop, founder of the company of the same name which still produces tyres today. Since then there has been a strong evolution which has led to the production of different types of tyres suitable for specific types of machines and for different operating, soil and turf conditions. Some tyres mounted on machines for green areas are comparable with those used for agricultural machinery, others are specific for green areas and aimed at respecting the turf and reducing compaction.
General constitution of a tyre. Tyres are today made with advanced technologies and consist of natural rubber, mainly obtained from latex coming from the rubber tree (hevea brasiliensis) and synthetic elastomers (polymers derived from petroleum). These different elements, correctly mixed with synthetic filaments of nylon, polyester and metal, form the material for the construction of tyres. In essence, not only rubber but also textile, synthetic and metal fibers are used. The different technical components of a tyre can be summarised as shown and described below.
The tread is the part of the tyre that comes into contact with the surface. It is normally made of synthetic rubber generally consisting of the styrene-butadiene copolymer, indicated with the acronym SBR (Styrene Butadiene Rubber). It has good impact and abrasion resistance. It is characterised, in the various types of tyres, by different groove designs on the rolling surface. In the case of machinery for green areas, some designs are similar to those of tractor and agricultural machinery tyres, others are similar to those of cartyres. This depends on the type of machine and the conditions of the contact surface. There are also completely smooth tyres, i.e. without tread, called racing tyres or slick tyres. In the field of gardening machinery, these are often used for machines intended for the maintenance of golf courses and, in particular, for green lawn mowers, in order to guarantee maximum respect for the turf. In fact, it should be kept in mind that on the green the grass is kept at a height of 3-6 mm and this entails a frequency of 5-6 cuts per week in the spring-summer period and 3 cuts per week in the autumn-winter period. It follows that it is important to use tyres that reduce compaction and friction with the turf to a minimum. On these smooth tyres there is the abbreviation NHS (not for highway service), to indicate that they are not approved for public roads. In fact, according to the Highway Code, tyres must have a tread with a minimum thickness of 1.6 mm.
The belt is the component that is placed between the rolling band and the carcass. It is a multiple layer made up of the intersection of textile fibres and/or steel wires which create an almost non-deformable structure. It has the function of increasing the tyre's resistance to impacts and cuts. It affects the tyre's footprint.
The carcass is the element that supports the load of the machine and allows the transmission of all the load forces between the wheel and the ground. It is made up of synthetic textile filaments and rubber that form canvases of different thicknesses that are overlapped, forming more than one layer.
The sidewall or shoulder is made up of a mixture of special rubbers designed to give resistance to the tyre as it is the most vulnerable part that resists bending during exercise. The sidewall ends with the bead which is the area that comes into contact with the rim and which guarantees the friction necessary to preventthe tyre from rolling on the rim. Inside there is a steel wire inserted which gives the necessary resistance.
Types of tyres and their identification. Tyres can be classified into different types. A first distinction is between radial tyres and diagonal tyres. The radial ones are characterised by having a belt with crossed plies independent of the carcass. The carcass ply threads are arranged normally to the centre line ofthe tyre. The tread is rigid while the tyre is deformable and the sidewall is flexible. In tyres with a diagonal carcass, the ply threads are arranged diagonally forming an angle of 30°-60° with respect to the centre line of the tyre. The sidewalls are stiff and resistant and this makes the tyre less flexible. Inthe radial tyre the footprint is quadrangular in shape and, when it is under load, the footprint lengthens but the load remains uniformly distributed over the entire surface. Inthe diagonal tyre the footprint is ovoid in shape and, when it is under load, it also lengthens but the load is not uniformly distributed on its surface. Another distinction is between tube-type tyres and tubeless tyres. For the latter, the sealing between the bead and the rim is very accurate in order to avoid air leaks. Another typology is represented by solidtyres, without air inside them, but deformable, and also between conventional tyres and low pressure tyres, and again the already mentioned difference between smooth and treaded tyres.
The measurements that characterise a tyre are represented by: width or chord (C); side height (H); shape or aspect ratio, i.e. the percentage ratio between the height and the chord (H/C); rim diameter, i.e. the distance between two opposite stops of the rim; speed code expressed by a letter; load index, i.e. the bearable load expressed by a number; PR (ply rating) number, i.e. the number of layered canvases; operating pressure. The aspect ratio is used to define the deformability of a tyre. In conventional ones the ratio is around 75-80%; in deformable ones it is equal to or less than 60%. The latter allow to increase the footprint surface and, therefore, reduce the load on the ground. The speed code found for vehicles intended for green areas is expressed by the letter A followed by a number: from A1, which indicates that the maximum speed is 5 km/h, up to A8 for the maximum speed of 40 km/h. For higher speeds other letters are used, for example: B for 50 km/h, G for 90 km/h, etc. A total of 23 letters.
Standardisation has therefore led to the definition of a standard symbology, used worldwide, which expresses the sizes that identify the tyre type, be it for cars, agricultural machinery and also for landscaping machinery. This symbol is marked on the side of thetyre and reported in the registration document. Thus, for example, the identification with the writing 185/65R15 88H means: 185 width in mm, 65 aspect ratio (the height of the sidewall is 65% of the width), R radial, 15 rim diameter in inches, 88 load index, H maximum speed in km/h (specifically 210 km /h). If the tyre is diagonal, there is a dash instead of the R. There may be other elements that are marked on the sidewall, such as: driving wheel, steering wheel, direction of rotation of thetyre, maximum inflation pressure, tubeless tyre, NHS (not road approved), etc.
Other types of marking can be adopted and, in the case of specific tyres for gardening machinery, you can find a marking with two or three numbers followed by the PR number. In the identification with the two numbers, for example 3.50-8 4PR: 3.50 indicates the width of thetyre in inches, while 8 expresses the diameter of the rim again in inches, 4 the number of plies. The three-number classification, for example 13x5.00-6 4PR, indicates: 13 the diameter ofthe tyre when not under load, 5.00 the width ofthe tyre, 6 the width of the rim. All values are expressed in inches.
Choice of tyre. Making the right choice is important not only because the correct use ofthe tyre allows you to respect the turf and reduce soil compaction, but also to obtain an economy in fuel consumption and to increase operator comfort by virtue of the reduction of the stresses to which it is subjected. When choosing, both the size and the type must essentially be taken into consideration, as well as obviously the price. The size must be consistent with the weight of the machine, the engine power and the traction force that is exerted. When choosing the type of tyre, radial or diagonal, conventional or flexible, etc., the operating conditions must be considered: type of turf, type of operation, state of the surface with which the tyre comes into contact, etc. In general terms it can be stated that, for turf, especially if valuable, flexible radials with a low aspect ratio are generally preferred, while for forestry areas diagonal tyres offer good performance, by virtue of their rigid and very resistant sidewalls. For the near future, as electronics and digitalisation increasingly affect landscaping machines, we are moving, as already happens for other types of machines, towards tyres "connected" with the on-board computer. This makes it possible to best adapt the inflation pressure to operating conditions, thus reducing both compaction (very important in fine carpets) and fuel consumption and at the same time increasing comfort and productivity.
