Hoof type lug cage wheel for wetland traction

Transcription

1 Chapter 3 Hoof type lug cage wheel for wetland traction The engine power of agricultural tractor (riding tractor) and power tiller (walking tractor) is transmitted to useful work in three ways, viz., power takeoff, hydraulic and drawbar. Out of these three ways, the drawbar is most widely used but least efficient method of power transmission. In off-road vehicles such as tractor and power tiller, transmission of engine power to the drawbar is achieved through traction devices such as wheels and tracks. The basic configurations of wheels and tracks are modified to improve the drawbar output in various parts of the world [1, 2, 3]. The knowledge on the terrain-wheel interaction mechanics is useful to understand the design of traction device. 3.1 Terrain-wheel interaction: theoretical consideration of traction Traction is the driving force developed by the traction device to propel the machine forward. Traction is developed due to interaction of traction device with terrain i.e., soil surfaces. Mohr-Coulomb failure criteria to a soil-plate situation describe the soil shear behaviour under different loading condition [4]. The general relationship of force required for different normal force and the area is expressed by the fundamental relationship as given below. H = A( + p tan ) and H = P + R (3.1a) (3.1b) where, area A=b l (b- width and, l- length of contact surface), p = average normal soil pressure, and are cohesion and internal friction of soil, respectively H is soil thrust, P is drawbar pull, and R is rolling resistance 24

2 Thus, it is seen form the fundamental relationship that depending upon the soil properties, both the vertical load and contact area influence the amount of thrust developed by a traction device. The traction aids for hard soil is mostly standardised with different design of tyre configuration to provide required contact surface. However, a perfect design for wet soil with optimum contact area has not yet evolved. The loose soil trapped between the wheel surface and hard layers beneath the surface deteriorates the traction performance. The details of this aspect are highlighted below. 3.2 Theoretical consideration of wetland traction The high moisture content of wetland soil makes the machinery unsuitable for use with pneumatic tire. Trafficability of the surface soil layer is very poor, being extremely soft with low load-bearing capacity. Cone indices are generally less than 50 Pa [4]. Performance of conventional rubber-tired wheel is not acceptable because of high slippage and adhesion of sticky soil. Farm tractors and other farm vehicles that are operated in these conditions often require special devices used with tires or in place of tires. Some examples of these devices are open lug wheel and float lug wheel. In an open lug wheel, the lug plates are welded to a circular rim. Many types and shapes of lug plates are in use for paddy field operations. In float lug wheel, lugs are connected to the circular ring plate, and the lugs can be folded toward the centre of the wheel when the tractor operates on a hard surface or paved road. Traction prediction models of wetland traction devices are different from the dry land traction models [5] and there are separate models available in literature for wetland traction devices. Fig. 3.1 Forces acting on cage wheel lug interacting with soil 25

3 Operation of a single lug (i th lug) during interaction with soil terrain is used for description of traction mechanics as given by Eq. (3.2) and Fig. 3.1 [6]. The traction developed by a single lug in cage wheel, F i is given below F i = F n Sin (ø-θ) - F t Cos (ø-θ) (3.2) where, F t =tangential component of soil reaction, kn F n = normal component of soil reaction, kn θ = lug angle, degree Ø = rotational angle of cage wheel, degree µ = coefficient of friction of soil on lug Further, the tangential component of the soil reaction is expressed as the product of µ and F n. Thus, Eq. (3.2) is rewritten as F i = F n {Sin (ø-θ) - µ Cos (ø-θ)} (3.3) Again, normal component of soil reaction, F n is related with driving torque T i contributed by i th lug as below [6] Ti F n =..... (3.4) r(cos sin ) where, T i = is the driving torque contributed by i th lug, N-m r = is the working radius, m If there are n numbers of lugs simultaneously interacting with soil then total tractive force is assessed by summation of n individual tractive force. The total traction developed by a cage wheel is given as the sum of those developed by individual lugs interacting with the soil when the cage wheel rotates. The Eq. (3.3) and (3.4) were used to predict tractive force and torque for a specific lugged wheel under wetland conditions. The surface area of interaction and other related 26

4 soil parameters (cohesion coefficient, internal friction) are not appearing in this model and therefore, have limited applications. However, in absence of an appropriate wetland traction models, the understanding of this model provides idea for traction improvement as provided below. The normal component of soil reaction is found important contributor to the tractive force. It is also understood that generation of F n depends upon the ability of soil/terrain to support normal load. A relatively harder pan could contribute better to the generation of traction than a softer soil. A typical wetland comprises of a softer top layer and underlying hard pan. The cone index value reflects this varying condition of soil. The lugs interacting with the upper softer soils fail to generate sufficient traction. Rice production calendar generally includes the period of soil puddling and transplanting of rice seedling processes in which rice field soils are in flooded or slurry-like condition. At this point of time, the wheeled farm vehicles have to struggle with severe loss of their mobility even in the field with appropriate hardpan. Thus, several types of traction and/or flotation devices, such as open-lugged wheel and strakes have been developed and widely used with conventional tires or instead of tires in many rice producing countries in Asia. However, the mechanism of pull and lift generation of a lug of lugged wheel is not sufficiently studied and the design of such wheels is mainly based on trial-and-error experiences without well-developed theories which can predict the performance of lugged wheel even now. The behaviour of soil under lug and the action of lug on soil are complicated as it comprises of rotation as well as translation. The motion of translation is easy to deal with by passive soil resistance theory. The combined motion of translation and rotation of lug is difficult to analyse. The difficulty of prediction increases due to action of multiple lugs. The main purposes of this study are to examine soil behaviour under a lug of lugged wheel through laboratory experiments. As per the studies conducted earlier, the cage wheels are found to be the most effective device for wet rice fields (Fig. 3.2). In a research it was revealed that the cage wheel exert 3 times more pull in comparison with tyres in flooded soil conditions. Research has also indicated that in wet sandy clay, the lug forces are strongly affected by working environment (soil moisture), lug geometry (lug angle, lug width, lug shape) and other operating parameters like travel reduction and sinkage [7]. The lug forces increases due to 27

5 increase in sinkage and slip and decreases as the soil moisture increases. During a study on performance evaluation of a cage wheel in different soils with different design condition of the cage wheel, it was found that cm lug spacing and 60 0 lug angle performed better in sandy soil for a slip range of 20±30% than other combinations tried in paddy field conditions [8]. This indicates that along with lug geometric parameters, the soil parameters also have significant effect on cage wheel performance. Many studies have been conducted in designing and testing of cage wheels by modifying their shape and size for power tillers in puddled soft soils [9, 10]. These studies have concluded that a cage wheel design suitable in one soil condition might not perform well in other conditions. In conclusion, cage wheels have proved to be one of the best traction aids for wet land cultivation. The pull and lift forces developed by cage wheel lugs depend upon the number of lugs in contact with the soil. By increasing the number of lugs, these forces can be increased up to a certain limit, after which there is expected to be a decrease in lug forces due to lug interference. The conventional cage wheel fitted with pentagonal steel plate type lugs having 45 0 lug spacing has tendency of more sinkage, especially in loose soil with more depth. In this case the loss of power is more and finally the use of power tiller becomes economically less viable. The orientation of lugs on cage wheel also affects the performance by affecting the pull and lift forces of the cage wheel. Due to more sinkage and slip, the tractive efficiency is reduced affecting the overall economy of use of machinery in particular wetland. Fig.3.2 Schematic diagram of cage wheel showing different parameters 28

6 Attempts were made to improve the performance by increasing the number of lugs, but blocking of cage wheels due to tendency of filling of the space between the lugs by soil is the limiting factor for the number of lugs on cage wheel. In the above designs of lugs there is no scope for shifting the loose soil from below the lug so that lug gets support on firm surface and develop more pull. There is need to design and develop a lug capable of auto flushing of loose soil. 3.3 Considerations for traction improvement and concept for hoof type lug design The draft animals like oxen having split hoof are capable of developing better pull in wet loose soil as compared with those having non-split hoof. This is possible because when animals apply pressure on the soil surface to move forward the loose soil move out through the split portion and hoofs get support on firm soil, thus can develop better pull. This concept has been used in designing of new lug. The shape of the hoof of draft animals is nearly elliptical with length more than width. The traction theory also suggests that for better traction the length of contact should be more than the width of contact in a traction wheel or track. Considering the above the elliptical shape was considered for design of new lugs. The maximum length and width of draft animals found in the study region was measured and on an average found to be in the ratio of 1.2:1.0 having split area of about 10% of the total surface area of hoof. Based on above the new lugs of elliptical shape having maximum length and width in the ratio of 1.2:1.0 are designed as shown in the Fig Two sets of cast iron lugs are developed; i.e. without split (A) and with v shaped split (B) keeping effective area of contact identical. The area of split is kept at 10% of the gross surface area to enable easy flow of loose soil through it. A B Fig. 3.3 Hoof type lug design A-Non split, B- Split lug 29

7 Four sets of both the designed lugs (split and non-split) having surface area 8000, 12000, and mm 2 were fabricated and mounted on test cage wheel. The orientation of lug mounting is shown in Fig Lug spacing Fig. 3.4 Mounting of lugs on cage wheel and its components The concept of wetland traction used for design of split lug with the limitations of mechanistic design procedure are highlighted in this Chapter. The comparisons of split lug with non-split lugs and identification of the area of individual lug based on the traction performance are detailed in next Chapters. 30

8 References [1] Sirohi, NPS. Tractive performance of power tiller with traction aids in unsaturated soil conditions. Indian J Rural Technology, 3; 33-38, [2] Hendriadi, A., & Salokhe, V.M. Improvement of a power tiller cage wheel for use in swampy peat soils. Journal of Terramechanics, 39; 55-70, [3] J, Liu., et al. Effect of straight Grousers Parameters on Motion Performance of small rigid wheel on Loose Sand. Information Technology Journal, 7(8); , [4] Liljedahal, J.B., et al. Tractors and their power units, CBS Publishers and Distributers, New Delhi, [5] Senatore, C., & Sandu, C. Torque distribution influence on tractive efficiency and mobility of off-road wheeled vehicles, Journal of Terramechanics, 48; , [6] Wu, Y., et al. Effectiveness of a cage wheel as a traction aid, Transactions of the ASAE, 47(4); , [7] Watyotha, C., Gee-Clough, D., & Salokhe, V.M. Effect of circumferential angle, lug spacing and slip on lug wheel forces, Journal of Terramechanics, 38; 1-14, [8] Gholkar, S., & Keen. The effect of axle load and tyre inflation pressure on the Tractive Performance of a two wheel drive tractor on soft clay paddy field, Soil and Tillage Research, 60; , [9] Hermawan, W., Yamazaki, M, & Oida, A. Theoretical analysis of soil reaction on a lug of the movable lug cage wheel. Journal of Terramechanics, 37; 65-86, [10] Watyotha, C., & Salokhe, V.M. Pull, lift and side force characteristics of cage wheels with opposing circumferential lugs, Soil and Tillage Research, 60; ,