Peak Performance and Long Life from Rubber Tyres on Tractors and Implements

New Zealand Journal of Agriculture, Volume 98, Issue 3, 16 March 1959, Page 267

Peak Performance and Long Life from Rubber Tyres on Tractors and Implements

By

J. L. DOUTRE,

Machinery Instructor, Department of Agriculture, Auckland

DUBBER-TYRED machines, particularly tractors, have been adapted to so many ’ ' purposes in New Zealand farming that today's high output per unit of labour is very dependent on them. Such machinery can be operated efficiently and economically only when tyres are correctly fitted and maintained and abuses are avoided. Many tyres find their way prematurely to the scrap heap because of rough treatment or neglect and this article suggests how such tyres could have been saved for years of further wear.

IT is 25 years since rubber tyres were first sold on agricultural tractors. During that time their use has steadily increased, so that now they are . standard fittings on almost all wheel tractors.. Their reduced rolling resistance compared with ' that of steel wheels has led to a general speeding up of tractor work and an increase in power at higher speeds. Rubber tyres have increased fuel economy and performance much more than enough to pay for their extra cost. Certain work now done by modern tractors would be impossible on steel wheels. Advantages of Rubber Tyres over Steel Wheels Rolling resistance consumes much of the traction of the tractor on steel

wheels, ' especially at higher speeds, Tractors that operate well on rubber will scarcely pull themselves in top gear on lugged steel wheels because of the effort consumed in raising the tractor up on to lugs, pushing them into the soil, and pulling them out again. Ground impact is transmitted from the wheel through the tractor and leads to higher maintenance costs and wear. The discomfort of vibration is felt by the operator and leads to fatigue and reduced human efficiency. For these reasons a tractor with steel wheels must be driven more slowly than one on rubber tyres, especially over hard or rough surfaces. • . x j Greater speed is a definite advantage given by rubber tyres, and any machine used for carrying goods on

the farm should have rubber tyres, as they save time and operating costs. The tractor with rubber tyres is not restricted to the farm, but can cover

long distances on roads within a reasonable time; it does not cut up paddocks or damage smooth surfaces, * ~ j x , ~ x x, . Another advantage leading to the almost complete swing to pneumatic tyres has been the need for higher efficiency over a wider range of gears. This greater efficiency has been accompanied by almost parallel fuel economy. Under ideal conditions rubber-tyred tractors use 20 per cent less fuel than steel-wheel tractors on similar work. More gears lead to a better matching of the tractor to its implements and therefore greater mobility. TT , .... . Under some conditions rubber can more traction than steel wheels suJfeS’Thire‘sScmmd g r jp VA/L nll t i r • Where Rubber Tyres are Inferior Rubber tyres can be inferior to steel wheels with a heavy load on wet turf or on grass and clover swards, when rubber tyres must be heavily weighted to get full use of their power without

slipping. Wheel slip wastes power, fuel, and rubber and can be dangerous on slopes or wet areas. Rubber tyres are much less satisfactory than steel wheels on greasy surfaces where the ground beneath is firm and the tyre treads cannot grip. These are also unsuitable for cut-over land, on which sharp stumps are a constant danger to tyres. With rubber tyres there is also the risk of spinning or skidding tyres being cut or split by sharp objects in the soil or grass. Compromise Rubber tyres or steel wheels each have advantages under certain conditions. It is wise to weigh their relative merits, taking, into account efficiency, safety, and speed of working. With heavy loads and on slow work, such as ploughing, wheel slip constitutes a serious loss of power. Because of the relatively slow speed of operation, steel wheels are generally superior for ploughing. Speeding up the work causes an increase in ground friction between plough and soil, only causing further waste of power. Wheel

girdles or chains can be adapted to rubber tyres, but many of the tyres’ original' advantages are lost. If much slow, heavy work is to be done, it is worth while having both steel wheels and rubber tyres. The steel wheels should be stored in the implement shed until required for special jobs. If little ploughing is done, but there is need for extra traction, it is wise to use extendable steel strikes, which can. be easily engaged when needed „ . and act as wheel weights when not in use. With them, operating on steep country or on wet grass becomes much safer, as they prevent slipping back or sideways. However, the added traction can cause a backward somersault if care is not taken when going up steep hills or through ditches or when pulling heavy loads.

Tyre Design ~ , „ .. , , .. + Almost all tb ? load on a Pneumatic tyre is carried by the compressed air within the casing, so it can be said that the weight that a tyre can carry depends mainly on the amount of air in the tyre and the strength of the casing wall to resist the internal air pressure. A small tyre with a strong casing wall can , be . inflated sufficiently to su W or a laad similar wblch F an . be P laced on . a mu " h lar f e * J yre bavm a thm casing and inflated to a lower pressure. Each type of tyre has . Dar f ie . llar advantages and both are lts P artlcular advantages ana corn are use on a tractor - The smaller, higher-pressure tyres fitted in front are designed to reduce rolling friction yet be large enough to carry the load without sinking too deep. In this way they get sufficient penetration to give good steering. In

rough going with heavy pulls some of the weight is transferred to the rear wheels. The rear wheels work on the other principle of using large-section, largediameter, lower-pressure tyres. The casing walls are comparatively thin in relation to tyre size, and support for the load depends on the greater volume of air. This system has four advantages: 1. A heavy load can be carried on a tyre inflated to a relatively low pressure. The larger, softer tyre deflects .to spread the tread wider, giving greater ground contact. 2. The large-diameter tyre has a long area of ground contact. 3. A large tyre makes fewer turns and is therefore subject to less wear. 4. When it is climbing over an obstruction a large tyre has a lower angle of rise and the tyre being softer also absorbs part of the obstruction, so that vertical lift and consequently rolling resistance are reduced. Effect of Tread Design The open, angled tread bars on modern tyres give maximum traction combined with good cleaning for field work. The tyres should be fitted so that the apex of the tread design faces forward. Soil then moves outward from the centre, giving wider ground contact as the tyre digs in and assists the cleaning action. Tyres used on hard roads should have a continuous central rib. Trailed equipment such as drills and topdressers which use ground drive to operate the machines should have the

tyres fitted with tread reversed so that the apex faces backward.

Tractors working regularly in sand, where cleaning is not a problem and the tyres tend to dig in, often have the rear tyres reversed so that the tread actually gathers the sand beneath the wheel. Tyres should be changed back when the tractor is not working on sand. Special button tread designs have . been developed for tractors working extensively on sand. Front tractor tyres or equipment not dependent on ground drive for opera-

tion do not need traction tread. A straight rib design prevents side slip and assists steering. Tyres with block treads on the side walls have been designed for improving ability, to climb out of furrows. High central skid ring tyres are available for use in soft fields. They act as a rudder to make sharp turns or maintain a straight course under loads. Weighting Tractor Tyres Wheel slip is one of the main causes of waste of power with farm tractors. Improper weighting of the tractor, poor matching of machinery to the tractor to use the power efficiently, and over-inflation of tyres on soft going lead to considerable loss of power, fuel, and rubber. A tyre should not slip for more than 5 per cent of its circumference on the road or more than 15 per cent in the paddock. Excess tyre wear will occur if this is exceeded. To use the full power available from the motor and get traction a modern rubber-tyred tractor must be heavily weighted. For every 2 lb. added, about 1 lb. more drawbar pull can be obtained, up to the capacity of the engine. For each 100 lb. added to the tractor, the drawbar pull is increased in accordance with ground surface conditions as follows: lb. Green, clovery sward .. . . 36 Sand . . .. .. .. 36 Sandy loam .. . . .. 50 Dry clay .. .. .. .. 55 Concrete or bitumen roadways 66

Three ways of loading the tractor are:

1. Transferring the weight of the implement and its load to the tractor by design and hitch arrangement.

2. Adding cast iron or concrete weights to the tractor wheels or framework.

3. Adding weight to the wheels by filling the tyres partly or completely with water. The type of surface used by the tyres determines the drawbar efficiency of the tractor. Correct Inflation Pressures It is disturbing that probably more tyres are thrown on the scrap heap because of misuse than ever actually wear out. Correct inflation pressure is the most important factor in the satisfactory performance of tractor or implement tyres. The volume of air in the tyre carries the load and at the recommended pressure the tyre is able to flex normally without injury. Air is free; tyres are dear. A little time spent in servicing tyres can save costly replacement. Under-inflation causes damage to the tyre by repeated folding and buckling of the side walls; the result is a series of breaks and cord separations. Local heating and bending cause cracks to form at the base of each lug. Misuse can consume years of tyre life in a few hours: Grit, stones, and vegetation working in between the

tyre and rim, causing damage to the tyre beading; rims bruising the side walls of tyres when they go over a bump or obstacle; and tyres “creeping” on the rims or wheels actually spinning in the tyres, ripping the valve stems off the tubes. When an under-inflated tyre is used on hard roads the tread bars wipe

across the road as the tyre squirms under load and again as it resumes its natural shape. This scraping action causes the trailing edge of the tread bars to wear quickly. The centre of the tyre casing buckles upward, causing rapid wear of the shoulders of the tyre. Pressures of 10 to 12 lb. can be used safely in soft ground where the tread bars penetrate easily. On hard ground or dry turf the tyre pressure should be increased to prevent distortion of the casing between the lugs. When a tractor is changed from field work to road work tyres should be re-inflated to about 20 lb. Some traction may be lost, but tyre life is extended. When one tyre is to be continuously in the plough furrow or on the lower side of a hill the tilt of the tractor subjects the side wall of that tyre to severe thrust. This side-wall pressure, combined with the load of the implement, causes the inside of the tyre to wrinkle, giving similar effects to under-inflation. To correct it the pressure in the tyre should be increased by about 4 lb. When both tyres are being subjected to heavier loads or hillside work inflation should be increased accordingly, but the pressure should never exceed the maximum recommended. A hand pump, power-take-off pump, or spark plug pump can be used to add a few pounds of air needed to bring tyres up to the recommended pressure. Correct pressure in rear tractor tyres is critical, as they carry a heavy load and transmit terrific drawbar pull. Rear tyre pressures should be checked at least twice a week when the tractor is being used. They should still be

checked when it is not working or the tyres should be blocked up. A lowpressure gauge is needed to get accurate readings. It should be checked occasionally for. accuracy against another one. Special gauges are also available for testing waterfilled tyres. Air pressures should be adjusted when the tyres are cold. Air pressure builds up as tyres get warm and tyres seem to be over-inflated. Tyre construction allows for this extra pressure, and if it were bled off when a tyre was warm, the tyre would be under-inflated when it cooled off. Valve caps should be used, as they keep dust and dirt from entering the valve stems. These cause damage to the valve and consequent loss of air. Over-inflation of tyres does not compensate for overloading, because it does not add strength to the tyre; actually it weakens the tyre by reducing its ability to absorb shocks. The casing is subjected to strain on the inside, the cords are stretched, and they lose much of their tensile strength, making them too weak to resist impact breaks. When rubber is under tension it is susceptible to cuts. The tread contact with the ground is reduced, causing wheel spin, side slips, and consequently increased tread wear, especially on the centre line of the tyre tread and the leading edge of the tread bars. Less ground contact through overinflation gives less flotation, poor traction, and consequently reduced power. Ground shock is not absorbed by the tyre; it is transmitted to the tractor and to the operator, increasing fatigue of both metal and man. When a tractor is to be heavily loaded larger tyres should be selected to support the weight and should be operated at their recommended pressure. Effect of Water on Tyres As tyres cut much more easily when the rubber is wet, spinning of the wheels in water or mud should be avoided. Water also quickly weakens and destroys the cord fabric in tyres. When a puncture is repaired the tyre and tube should be wiped clean and dried before being mounted. Special processes of gum dipping and various types of fabric material have been developed to reduce the damage by water, but once moisture or farmyard acids have access to the fabric of the tyre its life will be rapidly reduced unless it is repaired by vulcanising to seal the fabric away from such corrosion.

Operational Care Most objects that damage tyres can be avoided by careful driving. Reduced speed over rough areas and avoidance of sudden rises, ditches, stones, or stakes will pay handsomely. Brakes should be used intelligently; their sudden application both wastes rubber and creates a strain on tread bars and tyre casings. Continued pivoting on one wheel causes distortion of the tyre body, which may loosen cords and separate the ply. Sharp turns also tend to twist objects into the tread. Rubber tyres should not be run against banks, stakes, or obstructions that might damage the side wall or cause a diagonal break across the tyre as it strikes a barrier. Oil, Grease, and Sprays Mineral oil in any formvapour, liquid, or hard greaseis destructive to rubber. If oil spills on to a tyre or the tyre runs through a pool of oil, it should be wiped off quickly before the oil has a chance to start deterioration of the rubber. Petrol or kerosene

should never be used to clean a tyre. A rag, sawdust, or sand will absorb the oil and assist in cleaning. Sprays containing, copper or solvents should be kept off the tyres. After the tractor has been used for spray work the tyres should be hosed to clean off any spray residue.

PROPER INFLATION

OVER-INFLATION

UNDER-INFLATION

Inspection Tyres should be inspected regularly for possible injury, which will need specialised attention. When such damage is found the tyre should be taken to a competent tyre serviceman for vulcanising. Most tyres can be repaired economically. Scuffs, cuts, and breaks that expose the fabric or enter the cords of the tyre should be repaired as soon as possible before moisture and foreign matter enter to cause deterioration that makes reconditioning of the tyre impossible. When a cut or snag extends into a rubber lug or tread bar but has not reached the tyre cords, bevelling any loose material and rounding the base of the tear will prevent its tearing further or collecting other matter which might work into the cords. When tyres are removed for any reason the inside should be inspected for imbedded tacks, nails, or other metal that may have been picked up. Wafer-filled Tyre Tubes There is probably no better way of weighting a tractor than by using, water ballast. In cold climates sufficient calcium chloride should be added to prevent the water from freezing; 2| lb. of calcium chloride per gallon of water will prevent slush forming

in tyres at 12 degrees F. (20 degrees of frost) and keep from freezing hard up to degrees F. When ice forms in a tyre the expansion creates very great strain on the cords, causing them to stretch and break. The tyre will then be useless. A tyre three-quarters filled with water and one-quarter with air will require inflation more often to maintain safe working. Tyres completely filled with water do not have to be checked as regularly and can be run at slightly lower pressures than normal; the added weight reduces wheel slip. Usually these tyres must be filled by tractor servicemen, because the farmer seldom has the necessary equipment. They must be run more slowly and with greater care. If they strike an obstruction, they have no cushion of air to absorb a serious shock and are more likely to be damaged. Belt Work As static electricity can build up when the pulley on a tractor engine is used to drive a belt (because of the insulating effect of the tyres), the frame should have an earthing wire attached. A further precaution is to watch that the belt does not cut into a front tyre. This can be prevented