Traction and related terms.pptx

Traction in a tractor tire

1. Lecture on Traction and related definitions, traction theory , tyre types, water ballasting, wheel slippage, mechanics of a rigid wheel (traction and towed) By - Er Kishan Kumar Patel PhD 1 st year (FMPE) SV COLLEGE OF AGRICULTURAL ENGINEERING AND TECHNOLOGY & RESARCH STATION INDIRA GANDHI KRISHI VISHWAVIDYALAYA, RAIPUR 2019-20

2. Traction Traction . The process by which a tractor develops tractive force and overcomes motion resistance to produce desired motion. Traction is the force in the direction of travel, developed by the traction device medium (soil) and transferred to the vehicle. The power developed in engine, finally goes to wheels or tracks, which move the tractor with or without an attached load. During the movement of wheels, some slip occurs, which causes reduction in speed. The slippage also occurs with increasing load.

3. Traction can be increased by: 1. Using rubber tyres with grooves 2. Placing tyre chains 3. Using ribbed treads on track links and 4. Putting lugs, cleats or grousers on the wheel rim.

4. Some important definitions related to traction It is the ratio of the total force output of the traction device in the direction of travel to the dynamic weight on the traction device. It is the force required in the direction of travel to overcome the resistance of motion.  The force developed on the tractor interface by the traction device as a result of applied torque from the power source. Tractive force Coefficient of Traction Rolling Resistance

5. The efficiency of the tractive device is  converting the axle input power into output power, the term tractive efficiency (TE) has been defined as TE = Output power/ Input power x100 Basically tractive efficiency is converting the axle torque into net traction It is the ratio between rolling resistance and dynamic weight. Any force imposing resistance against desired motion. The depth to which the traction device penetrate into the soil measured normal to the original, undisturbed surface. Several factors lower the tractive efficiency; among these are steering, rolling resistance, slip, and friction in and deflection of the traction device. Traction efficiency Coefficient of Rolling Resistance Sinkage Motion resistance

6. Rim pull is a term which is  – used to designate the tractive force between the rubber tyres of the driving wheel and the surface on which they travel. The coefficient of traction is high enough to eliminate tyre slippage. The maximum rim pull is a function of the power of the engine and the gear ratios between the engine and the driving wheels. It is the relative movement of the wheel or track in the direction of travel for a given distance under load and at no load condition. It can be calculated by the formula: Wheel slip, percent = N 1  – N  / N 1  × 100 Where, N 1  = number of revolutions of driving wheels or sprockets for a given distance under load and; N  = number of revolutions of the driving wheels or sprockets for the same distance at no load Rim Pull Wheel Slip or Track Slip

7. Mechanics of a rigid wheel

8. Above figure illustrates one of several alternative methods of describing the forces acting on a wheel. The figure is divided into three distinct force states: braked, driven, and driving. The transition point between the braked and driven force states is the towed wheel condition. A towed wheel is unpowered: axle torque is zero neglecting bearing friction. The transition point between the driven and driving force states is the self-propelled wheel condition. For a self-propelled wheel, pull is zero with the applied torque simply overcoming the motion resistance of the wheel.

9. The curves presented in Figure represent a given soil strength, tire size, and load. As soil strength increases, the curves move upward to the left, as soil strength decreases, they move downward to the right. In Figure, both the axle torque and pull are plotted as functions of wheel slip. These reactions develop from soil stresses resulting from slippage (motion loss) of the wheel. Slip is defined as Where,  S - wheel slip  V a  - actual travel speed  V t  - theoretical wheel speed = r   r - rolling radius of wheel on hard surface    -  angular velocity of wheel

10. A towed wheel is an unpowered wheel and  axle torque is considered to be  zero . The towed force of a towed pneumatic tire is generally dependent upon load, size and inflation pressure, as well as soil strength. For soils that are not very soft and tires that are operated at nominal tire inflation pressures, the towed force can be predicted from- Where F T   = towed force of wheel W= dynamic wheel load B = unloaded tire  width D = unloaded  tire diameter. C n = wheel numeric C n = B D CI/W CI = cone index TOWED WHEEL

11. The variations of driving wheel performance is based on the consideration that the  normal tire inflation pressures in agricultural tyres produce tire deflections of approximately 20 per cent. Therefore,  traction prediction equation can be given as for net pull, slip, and load - Where, F P   = wheel pull W = dynamic wheel load e = base of natural logarithms Cn = CIBD/W CI, B,D and S as stated before. DRIVING WHEEL

12. Ballasting Ballasting The normal load on traction wheels can be increased by putting water or air inside the pneumatic tyre-tube. Tyre ballasting is where the tubeless tractor tyre or inner tube is filled with water in order to provide extra weight to stabilise a tractor. Filling water in the tube is water ballasting, and filling sand in the tube is sand ballasting. Generally the tube is filled up to maximum of 75% by the ballasting material and rest with the air at the rated air pressure. It can also be used to lower the centre of gravity of the machine which, in turn, makes it more stable in situations where the ground is uneven.

13. Water ballasting procedure Agricultural tyre valves are usually of the ‘air and water type’. This is done by using the correct air/water adapter on the end of the water hosepipe and airline. You need to position the tractor tyre with the air/water valve at the top of the tyre, and then fill the tyre or inner tube with water. Once the tyre has been fitted on the tractor or machine, the tyre is deflated to a low level of pressure, around 7psi or 0.5 bars. It’s a good idea to add anti-freeze to the water in case temperature drop below freezing, and then only ballast the tyre up to a maximum of 75% of the tyre volume, which is usually to the level of the tyre valve and thus which allows the air to escape.

14. Finally, finish inflating the tyre with air and adjust the pressure as required. As only 25% of the tractor tyre or inner tube is filled with air, you should regularly check the air pressure in the tyre, we suggest once per month.

15. Tractor tyres A tire forms a torus of a complex flexible reinforced composite material surrounding a steel hub and filled up with air under pressure.

16. The agricultural tire must perform the following functions: Support the vehicle and associated loads at some low level of ground pressure, Absorb shock loads and cushion the vehicle against minor surface irregularities, Provide traction (and braking),    . Provide for steering and directional stability, Resist the abrasive action of the various surfaces on which it is expected to operate. TIRE FUNCTION

17. Tyre types (a) Hard surfaces such as roads (b) Normal agricultural work, dry soil (c) Soft, wet agricultural soils (d) Lawns, low sinkage is required (e) Dry soil, heavy loads as in earth moving (f) Saturated puddle soils

18. The bias ply tyre construction utilizing rubber-coated layers known as plies composed of textile cords, usually nylon and sometimes Kevlar. The plies layered diagonal from one bead to the other bead at about a 30 degree angle. One ply is set on a bias in one direction as succeeding plies are set alternately in opposing directions as they cross each other and the ends are wrapped around the bead wires, anchoring them to the rim of the wheel. The layers of plies are them covered with more rubber to form the tread of the tyre. Bias ply tyres are sometimes called cross-ply tyres. Bias /cross ply tyres

19. The radial is a type of tyre that is constructed with rubber coated, reinforcing steel wire belts that are assembled parallel and run from side to side, bead to bead at an angle of 90 degrees to the circumferential centreline of the tyre. They constrict the radial ply cords and stabilize the tread area. This makes the tyre more flexible which reduces rolling resistance to improve fuel economy. Then numerous rubber coated steel belts are then constructed into the "crown" of the tyre under the tread to form a strong stable two-stage unit. Radial tyres

20. Tractor tyres

21. References and suggested readings http://ecoursesonline.iasri.res.in/mod/page/view.php?id=126223 http://www.agricultureinindia.net/machinery/farm-tractor/traction-and-traction-theory-used-in-tractors-agriculture/16191 https://www.bigtyres.co.uk/blog/tractor-tyre-ballasting.html#:~:text=Tyre%20ballasting%20is%20where%20the,where%20the%20ground%20is%20uneven. Tractors and their power units by John B. Liljedahl , Paul K. Turnquist, David W. Smith and Makoto Hoki