SAE TECHNICAL PAPER SERIES 972731 Belt and Tire Tractive Performance Frank M. Zoz John Deere Product Engineering Center Reprinted from: Belt and Tire Traction in Agricultural Vehicles (SP-1291) The Engineering Society For Advancing Mobility Land Sea Air and Space® INTERNATIONAL International Off-Highway & Powerplant Congress & Exposition Milwaukee, Wisconsin September 8-10, 1997 400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (412)776-4841 Fax:(412)776-5760 972731 Belt and Tire Tractive Performance Frank M. Zoz John Deere Product Engineering Center Copyright 1997 Society of Automotive Engineers, Inc. GT / Wd = Gross Traction Ratio, GTR NT /Wd = Net Traction Ratio, NTR MR /Wd = Motion Resistance Ratio = GTR - NTR ABSTRACT Numerous traction tests of tires and belts have been conducted over the last few years. These tests cover a range of tire sizes, weights, and tire pressures as well as a range of belt sizes tested on vehicles of various configurations. Use of wider belts provides the greatest tractive performance improvement under the more adverse tractive conditions. Larger diameter tires with longer footprint operating at correct low pressure nearly equal the performance of belts when tractive conditions are good. The theoretical travel speed, Vt, depends upon the effective radius, r, and the rotational speed. Input power is the product of the theoretical speed, Vt, and the gross thrust, GT. Output power is the product of the actual travel speed, Va, and the net pull, NT. Tractive efficiency, TE, is the ratio of the output power to input power. Tractive Efficiency (ratio) = INTRODUCTION Over the last few years numerous traction performance tests have been conducted on tires and belts. Tires of various size, and pressure have been tested in different soil conditions with various axle weights. More recently belts of various sizes and tread configurations have also been tested on vehicles of various weights and configurations. While full vehicle performance and productivity tests (1) (2) have also been conducted, the objective of this paper is to present specific performance data and general characteristics of the tractive mechanism only, comparing belts and tires. The mechanics of the belt drive mechanism (Fig.2) is similar to the wheel in many respects; but the distribution of the load is dependent upon vehicle parameters. Location of the dynamic load resultant, eh (dynamic balance ratio) (3) depends upon the static distribution and vehicle weight transfer characteristics. TRACTION MECHANICS REVIEW Tractive "inefficiency" is caused by both velocity losses and pull losses. The loss in travel speed is commonly referred to as "slip" although it should more properly be called 'travel reduction". It is the result of the theoretical travel speed not being entirely converted to actual speed due to losses within the soil, between the soil surface and the tractive device (slip?), and within the tractive device (tire windup or belt slippage). The other component of tractive "inefficiency" which is less visible and often overlooked is a loss of pull when motion resistance reduces the amount of gross thrust that is converted to useful output (net traction). This is especially relevant to belts as internal losses within a belt are greater than those within the tire. On soft soils the internal losses are generally compensated for by lower external motion resistance than for tires. A review of traction mechanics is helpful in understanding differences in tractive performance. The basic forces involved in a powered wheel are shown in Figure 1. The torque input, Q, develops a gross thrust, GT, acting at the wheel's loaded radius, Lr. Part of the gross thrust is required to overcome motion resistance, MR, which is the resistance to the motion of the wheel, including internal and external forces. The remainder is equal to the net thrust, NT. Dividing by the dynamic weight on the wheel, Wd, results in dimensionless relationships. 2 Fig.3 Tractor Dynamic Weight Distribution TRACTIVE EFFICIENCY Fig.1 Deformable Wheel on Soft Surface The following is a generalized plot of the traction relationships using radial ply tires and the Brixius (5) traction equations. While traction data has been traditionally plotted with "slip" as the independent variable, it's becoming more evident that pull, that is, net traction is the independent variable. For a properly ballasted and inflated farm tire, tractive efficiency tends to maximize at a net traction ratio of approximately 0.40. This was also recognized by Dwyer (6). Motion resistance tends to be a linear function of either slip or NTR unless (slip) sinkage becomes a factor. Fig.2 Belt Drive In general, the best tractive performance and the most uniform ground pressure on a belted tractor can be obtained with a dynamic balance ratio of near 50 % (3). The ratio obtained depends not only upon tractor dimensional parameters and the location and angle of the line of draft but also upon the drawbar pull level. The following chart shows the effect of draft angle and Net Traction Ratio upon the dynamic balance for a tractor with 60% of the static weight at the front. A wheel tractor is shown for simplicity but the weight transfer mechanics are the same for belted and wheel tractors. Note that it only takes a 5 deg draft angle to give a 50% dynamic balance ratio at a typical Net Traction Ratio of 0.40. Figure 3 is for hitching to the drawbar. Even higher weight transfer and hence, higher front weight requirements may result from use of three point hitch equipment. Fig.4 Generalized Traction Relationship The travel and pull losses making up tractive "inefficiency" do not have official terminology. The simplest way to understand them is to look at a "Velocity Ratio" and "Pull Ratio". From Eq 1, The Velocity Ratio is shown as a function of Net Traction Ratio in Fig.5. At zero NTR (zero pull) the actual velocity, Va, is about equal to the theoretical velocity, Vt, depending somewhat on the definition of "zero" slip (7) 3 and the Velocity Ratio is near unity. As pull increases, slip increases and the Velocity Ratio decreases. Velocity Ratio losses depend upon the characteristic shape of the pull-slip curve. Fig.7 Overall Tractive Efficiency with Velocity and Pull Losses The overall tractive efficiency cannot be greater than either the Pull or Velocity Ratio and thus it reaches a maximum value at NTR of about 0.4 with radial ply tires. It will be shown later that a similar value exists for belts. Fig.5 Velocity Ratio Losses Pull Ratio is shown as a function of NTR in Figure 6. At zero pull, the ratio of Net Traction Ratio to Gross Traction Ratio approaches zero (the difference between GTR and NTR is motion resistance which is in the range of 0.05 to 0.15). Due to motion resistance, the Net Traction Ratio can never equal the Gross Traction Ratio and the Pull Ratio approaches but never reaches unity. Pull Ratio, Velocity Ratio, and overall tractive efficiency are shown for real data in Figure 8. Data is for radial ply tires in medium (tilled) tractive conditions. The curves are the result of regression analysis (4) of the field test data. Both velocity (slip) and pull (motion resistance) losses contribute to the overall tractive efficiency. Fig.8 Velocity and Pull Losses with real data Fig.6 Pull Ratio Losses TRACTION DATA Tractive efficiency is defined as output power / input power. It can also be expressed as the product of Pull Ratio and Velocity Ratio. Figure 7 shows how Velocity Ratio and Pull Ratio combine for overall tractive efficiency. Typical traction data plots are shown by Figures 9 and 10. Figure 9 shows an example of the traditional plot with slip as the independent variable. 4 Fig.11 Performance of 20.8R42 Duals on Three Surfaces Fig.9 Traditional Slip Plot of Traction Data Figure 12 is an example of operating at overpressure on the tires. Peak tractive efficiency is reduced as well as the maximum Net Traction Ratio. While the maximum tractive efficiency is only reduced about 5 percentage points, the effect on vehicle performance may be significantly greater depending how implements are matched to the tractor. For example, at an NTR of 0.5, more than 10 percentage points difference results in about 17 % difference in drawbar power and significantly higher slip. Each data point represents the average for 1.5 seconds. This example shows the merging of 3 replications of the test run. Also shown as a smooth curve is the results of a regression of the Net Traction Ratio and Motion Resistance ratio data. These are examples only and all subsequent comparisons are made using the regression coefficients. Fig.10 Net Traction Ratio data plot Fig. 12 Performance at Two Air Pressures TIRE PERFORMANCE COMPARISONS Performance of two size tires at correct pressure is shown by Figure 13. The larger diameter 520/85R46 tire shows significantly higher power efficiency at the same slip. This means the wider smaller OD tire has greater motion resistance loss. Both provide similar maximum pull capability which may give the operator the impression that there is no performance difference between the tires. Figure 11 shows the performance of 20.8R42 Duals on three tractive surfaces. Note that the peak tractive efficiency is reduced as the soil becomes softer and looser but that the peak occurs at a Net Traction Ratio of approximately 0.4 on all soils. Maximum NTR is also reduced as the soil becomes less firm. 5 Figures 15 to 17 show the performance of the 3 widths from one manufacturer in comparison to the dual tires on three surfaces. Under firm untilled conditions there is not a lot of performance difference between the four treatments at normal field pulls (NTR approx 0.4 to 0.5). Duals tend to drop off more at the higher pulls and the wider belt provides higher maximum NTR (limited by slip). Fig. 13 Performance of Two Sizes at Correct Pressures Performance of the same tire at different weights, both with correct pressure is shown by Figure 14. Using the correct pressure for the weight keeps the tire operating at it's design deflection ratio where optimum performance is obtained. Maximum Tractive Efficiency comes at about the same Net Traction Ratio for each weight. It should be noted that this means drawbar pull in proportion to the weight. In other words, if only the weight is changed, performance may suffer from not operating at the optimum NTR. Fig.15 Belt Width Comparison on Firm Untilled soil As the tractive conditions become softer and looser, the difference becomes more evident between belts and tires with the belts maintaining their relative position with each other. Fig. 14 Performance at Two Wts with Correct Pressures BELT-TIRE COMPARISONS Fig.16 Belt Width Comparison on Tilled soil. In June of 1996 a series of tests were carried out in Texas to determine the effect of belt width on tractive performance. Three belts of 400, 630, and 810 mm width (16, 25, and 32 in) were available from one manufacturer (A). A 400 mm (16 in) belt was available from a second manufacturer (B). At the same time a set of 20.8R42 dual tires were available for comparison. Tests were to be conducted on untilled and tilled tractive conditions. However, in the process of testing, a subsoiler was used on the load tractor to provide a portion of the load. This then provided a soft subsoiled condition upon which further tractive comparisons were made. Soil was in a good moist condition as rain was experienced shortly before tests were to begin. All tests were carried out over a two day period and test conditions had little time to change. Differences between the belts and tires becomes greatest as the soil becomes soft and loose (Figure 16 and 17). Note that the maximum tractive efficiency for tires still comes at NTR of about 0.4 whereas the belts tend to maximize at a slightly higher pull and demonstrate a wide range of pulls at near maximum efficiency. It should be noted that these tests were all carried out with a minimum of steering. Matching implements at higher NTR on the skid steer belted machine may result in limited steering control under load. 6 Fig.17 Belt Width Comparison on Subsoiled land Fig.20 Belt Manufacturer Comparison on Subsoiled Two manufacturers' 16 inch belts were available for test. Figures 18 to 20 show that there was very little difference in performance between the two belts on the three surfaces tested. It should also be noted that under the firm untilled conditions the dual 20.8R42 tires at 12 psi equaled or outperformed the 16 inch belts. Figures 21 and 22 compare the performance of 25 inch belts and dual 20.8R42 tires on the three surfaces tested. Fig.21 Performance of 25 inch belt on Three Surfaces Fig.18 Belt Manufacturer Comparison on Firm Untilled Fig.22 Performance of 20.8R42 Duals on Three Surfaces Fig.19 Belt Manufacturer Comparison on Tilled Soil 7 CONCLUSIONS Both velocity (slip) and pull (motion resistance) losses are important to the overall tractive efficiency of both belts and tires. Proper tire sizing and pressures are important if optimum tire performance is to be obtained. Wider belts and larger tires provide the greatest tractive performance improvement under the more adverse tractive conditions. Larger diameter tires with longer footprint operating at correct low pressure nearly equal the performance of belts when tractive conditions are good. REFERENCES 1. 2. 3. 4. 5. 6. 7. Shell, L. R., Field Performance of Rubber Belt and MFWD Tractors in Texas Soils. SAE 97F-65 Tumer, R, Field Performance of Rubber Belt and MFWD Tractors in Southern Alberta Soils. SAE 97F66 Corcoran, Paul T and Gove, Daniel S. Understanding The Mechanics of Track Traction._lnt'l Conference On Soil Dynamics, 1985 Upadhyaya, S.K., Chancellor, W. J., Wulfsohn D., Glancey J. L., Sources of Variability in Traction Data. Journal of Terramechanics, Vol. 25, No. 4, pp. 249272, 1988 Brixius, W. W., Traction Prediction Equations For Bias Ply Tires, American Society of Agricultural Engineers, ASAE 87-1622 Dwyer, M. J., The Tractive Performance of Wheeled Vehicles, Journal of Terramechanics, Vol 21, No 1 pp. 19-34, 1984. Brixius, W. W. and Wismer, R. D. The Role of Slip in Traction, American Society of Agricultural Engineers, ASAE 78-1538