AN HYBRID, EXTREMELY SIMPLE, STEERING SOLUTION FOR TRACTORS WITH CONTINUOUSLY VARIABLE TRANSMISSION RATIO

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1 VOL., NO. 5, MAY 5 ISSN Asian Research Publishing Network (ARPN). All rights reserved. AN HYBRID, EXTREMELY SIMPLE, STEERING SOLUTION FOR TRATORS WITH ONTINUOUSLY VARIABLE TRANSMISSION RATIO Luca Piancastelli and Leonardo Friiero Deartment of Industrial Engineering, Alma Mater Studiorum University of Bologna, viale Risorgimento, Bologna, Italy leonardo.friiero@unibo.it ABSTRAT The imlementation of an electrically controlled dual drive inherently stable heavy duty tracked vehicle is introduced in this aer. The roosed solution makes it ossible to obtain a VT (ontinuously Variable Transmission) by using two single stage lanetary gearings for each srocket. An additional motor is added to the annular gear. By controlling the motor seed, it is ossible to vary the transmission ratio continuously and to obtain also continuous reverse seed. The torque of the electric motor and the engine are added. It is then ossible to obtain extremely high torque at very low seed. This track steering mechanism is simle, continuous, efficient and additive; this means that, when transitioning to a turn, the mean track velocity remains the same. It is also additive, since keeing both tracks moving hels revent the tracked vehicle from getting stuck. Driving in a straight line is easy. Neutral steering can be achieved. Finally, the energy from the slowed track can be transferred to the sed u one in a regenerative way. An examle is roosed in this aer to clarify the concet. Keywords: ontinuously Variable Transmission (VT), steering, tractors. INTRODUTION An examle is roosed to exlain the VT solution. This is the best way to introduce the concet. The data are taken from existing heavy duty vehicles. However the examle suffers from many shortcogs. For examle the torque converter is not included in the model. The mass of the vehicle and the many data needed for the design of a transmission and steering device are not included. The focus of this aer is on the transmission concet, not on its design. In general it is a good hilosohy to kee the seeds as high as ossible down to the final reduction. This is due to the fact that the mass of the transmission goes with the torque, while the ower goes with the torque multilied by the seed. It is a good olicy to use the torque curve fully, from the seed where torque begins to be "reasonable" u to the maximum seed. A good design may reduce transmission mass and volume by a factor 4 from an "commercial", aarently "low cost" choice. Transmission design requires secific knowledge about transmission comonents, design solutions and it should include a simulation of the dynamics. In general, it is convenient to design the vehicle around the engine and the transmission and not vice-versa. The common ractical aroach to design the vehicle and then to install the transmission generally oututs very slow, underowered machines. This fact is articularly common in heavy duty tracked vehicles, with masses that often run out of designer control. This aer introduces the VT concet and the equations that can be used for its calculation. The numerical examle demonstrates the feasibility of this VT solution. Tracked vehicles steering solutions Steering a tracked vehicle is very different from steering a wheeled vehicle. In a tyical four wheeled car, the front two wheels can be ivoted to oint the vehicle. The ower is delivered to the traction wheels via differential gear (s). This tye of transmission allows the radial velocity difference between the wheels to be accounted for, and the wheeled vehicle can theoretically turn without any sliage or skidding. With a few excetions, like the universal carrier and tetrarch, tracked vehicles have to rely on skid steering. If one track is moving faster than the other, the forward and rearward sections of the tracks sli laterally over the ground and the vehicle turns. The ideal track steering mechanism is simle, continuous, efficient and additive; this means that, when transitioning to a turn, the mean track velocity remains the same. Tests have shown that additive steering works better when the vehicle is in dee mud or sand, since keeing both tracks moving hels revent the tracked vehicle from getting stuck. In addition the ideal tracked vehicle steering mechanism should make driving in a straight line easy. Neutral steering should also be allowed, that is, that the tracked vehicle can turn within its own length. Additionally, the energy from the slowed track should be transferred to the sed u one in a regenerative way. Traditional track steering The most obvious way to steer a tracked vehicle is to have a single engine-transmission driving both tracks, and to slow down one side or the other with brakes. A clutch system is added so that the ower is first disconnected from the inside track and then the brake is alied. This is called "clutch and brake" steering. lutch and brake steering is subtractive and non-regenerative. A tracked vehicle that is driving downhill turns the oosite 79

2 VOL., NO. 5, MAY 5 ISSN Asian Research Publishing Network (ARPN). All rights reserved. direction of the driver's intent as the steering clutch is engaged. In fact the track on the side of the turn seeds u due to the loss of engine braking. The situation will be rectified once the brake engages. The clutch and brake systems can achieve wide turn radii only by engaging the clutch instead of the clutch+brake, with imrecise resulting turn. Finally, neutral steer is not ossible. A few tracked vehicles use a single engine a searate transmission for each track. To turn the vehicle one track is ut in a higher gear ratio than the other. Since this solution is too bulky and comlex, there is usually a single gearbox for both tracks, but each final drive has a double eicyclic reduction gearbox. By engaging one of the eicyclic elements with a band brake the final reduction ratio of the drive srocket is reduced, roducing a turn. By disengaging all the brakes, the steering mechanism goes into neutral. This geared steering is almost always used with an auxiliary clutch and brake system. There are negligible efficiency (<%) and weight enalties (-%) for having and additional eicycle into the final drives. This system is not additive. As long as the clutch and brake system is not activated, it is regenerative. It is continuous as it uses band brakes and eicyclic gearing. In its most common form, it only rovides a single radius of wide turns in addition to the auxiliary clutch and brake system for very tight turns. Normally the single radius of turn rovides a comromise between "high" and low seed driving; being too tight for the former and too narrow for the latter. There is no smooth transition between turning and not-turning. In ontrolled Differential Steering (DF or letrac) the outut from the transmission runs through a differential gear. The steering brakes are still resent. When one of the brakes is engaged, the differential diverts the ower to one track or the other for steering. In letrac systems the ower is diverted through a series of idler gears. DF has a small efficiency enalty (-%) on the system because of the additional gearing. DF is additive and regenerative, as one track seeds u while the other slows down. Both tracks are energied at all times during a turn, so the system is also continuous. At to efficiency this system roduces a turn of a single radius. However, with reduced efficiency, turns of greater radius can be achieved by sliing the brake. An imortant roblem of DF comes from the differential. As the tracked vehicles the tank hits a atch of uneven ground, more ower will be diverted to the track with less resistance on it. For this reason, a DF equied tracked vehicle has the tendency to "self-steer" in off-road conditions. This behavior requires continuous corrections by the driver. A differential locking system may be added to reduce this tendency. Double (Wilson, Maybach) and trile (Merrith- Brown, Merrith-Maybach) differential systems are an evolution on the controlled differential system. In these systems there are two inut shafts leading to the steering unit. One inut shaft is from the transmission, while the other comes "directly" and indeendently off the engine. The ower is then rovided to the steering mechanism even if the tracked vehicle is in neutral. This gives the tracked vehicle, the ability to turn about on its own axis (neutral steer). Double and trile differential systems also outut different turn radii deending on which gear the vehicle is in. The steering brakes are still added to achieve different turn radii. Double and trile differential systems are still very efficient, but less than DF or geared steering due to the friction from all the additional gears. These systems have a tendency to self-steer when they hit uneven terrain, however a differential locking system may be added to reduce this tendency. In Double Differential with Hydrostatic/Electric Steering Drive there are two inuts to the steering mechanism; one leading from the gearbox, and the other leading from a variable-dislacement hydrostatic um or a variable-seed electric motor. This system offers all the advantages of the double/trile differential system, excet instead of having one or two discrete turn radii er gear ratio, it rovides a smoothly variable range of turn radii based on the dislacement ratio of the hydrostatic um or on the seed of the electric motor. In addition, the um/motor torque reduces the tendency of the vehicle to self-steer on uneven terrain through a cross-action resistance in the differential. Due to the low efficiency of the hydrostatic motor, the hydrostatic steering drives are substantially less efficient than urely mechanical double or trile differentials. In the case of the electric motor, it deends on the efficiency of the electric motor, which is always a comromise with dimensions of motor and controller. This solution offers the best control and it is widely used in modern tracked vehicles esecially in the double differentials version. These highly secialied transmission systems are roduced in small numbers and their availability on the market is consequently reduced. The first vehicle with a double differential with hydrostatic steering drive was the har B (95). Figure-. T Double differential with Hydrostatic Steering Drive [9]. The ower slit system The slit device consists of a lanetary gearbox (Figure-). The lanetary gear is a system that consists of several outer gears (lanet gears ()) revolving around a central one, (sun gear ()). The lanets are mounted on a movable carrier (P) which rotates relatively to the sun gear. There is an outer ring gear (annular gear ()), which meshes with the lanet gears. The advantages of lanetary 8

3 VOL., NO. 5, MAY 5 ISSN Asian Research Publishing Network (ARPN). All rights reserved. gearings are high ower density, larger gear reduction in a small volume and multile kinematic combinations. This lanetary gear system is used to achieve our ontinuous Variable Transmission (VT). The concet of VT was conceived in 49 by Leonardo Da Vinci. The idea was then develoed in the last two centuries, due to a growing need of comfort and economic efficiency. VT makes it ossible vary continuously the rotational seed of the inut shaft indeendently from the rotation seed of the outut one. The largest resulting advantage is the otimiation of the oerating oint of the engine and the consequent otimiation of fuel consumtion and torque. Figure-. Planetary gearing. In our case the VT transmission is obtained by a lanetary gearing. In this system the reduction ratio is achieved by owering the shaft () and connecting the carrier (P) to the rear wheels through reduction gears. As the seed of the motor shaft increases the transmission ratio is reduced u to maximum value. Table-. Heavy vehicle data. Symbol Descrition Value Unit D Srocket diameter.67 m V max Maximum seed 8.56 m/s V Minimum seed.6 m/s rm max, n,max Maximum ower engine seed, rm rm torque=rm Maximum torque engine seed 9 rm rm Min engine seed 9 rm P Maximum engine ower, kw T Maximum engine Torque,754 Nm i final Final reduction gear ratio.68 - Since the shaft is connected to the traction wheels and the engine moves shaft, the transmission ratio will be (): ( ) () () From the Willis equation: This value (4) can be inserted in equation (). (4) The ordinary gearing relative to this lanetary one has and as inut and outut shaft. () (5) The heavy vehicle data are summaried in Table-. It is then ossible to calculate the traction wheel average rotational seed at V max and V (6) (7). n D 6 V.[ rm] (6) 8

4 VOL., NO. 5, MAY 5 ISSN Asian Research Publishing Network (ARPN). All rights reserved. n max 6Vmax D 66.[ rm] (7) In order to achieve V and V max at rm max, the following gear ratios should be obtained i =99.69 and i max=8.. Since the differential gearing has a reduction ratio i final=.68, the VT should achieve i VT= i VT= i =75.76 and i maxvt = i max=.4. The lanetary gearing will be calculated with i =.4 that will be obtained with the annular gear locked (ω =). The first ste is to calculate i =.7 and i =.4 (8) (9). i i i ( ) i ( ) i i mn mn mn i (9) r r r i Then it is ossible to calculate =.6 and =9.7, by assug k= (). (8) ( )sin i i k () ( )sin ( ) i By assug that, for manufacturing urose, it is better to have >7, the final choice is calculate =9, = and =7. It is then ossible to calculate i with ω (). This can be obtained from equation (5) (Figure-) () i With i =75.76 and the engine at rm max, it is ossible to calculate ω = ω,lim=-7.8 [rad/s] = - rm. It is also useful to calculate the angular velocity of the lanetary gearing at the maximum torque (rm Tmax=9 rm), with i. The result is ω = ω limtmax=- 8 [rad/s] = -66 rm. Figure-. n (annular gear seed) with gear ratio i and engine rm (n ) (). As it can be seen n has the maximum value at {i,rm max} n max=, rm. The imum value is at {i max,rm } n max= rm. The comlete lanetary gearing may have an efficiency of around 97% (η tot=.97). It is then ossible to write the system of equations (). Pdiss tot P P P i P P P Pdiss () Where the first equation evaluates the dissiated energy, the last is the equilibrium of the system around the rotation axis of the lanetary gearing and the second-tolast is the energy conservation equation. The remaining equations are the ower, torque seed relations of the four shafts. Shaft goes to the wheels through the differential gearings. It is then ossible to evaluate (n,n ) (), (n,n )(4) and the total outut torque total (5). total toti () i tot i i toti tot i i i (4) (5) It is then ossible to calculate and at maximum ower and at maximum torque. The heavy duty internal engine has a torque that follows the linear equation (6) 8

5 VOL., NO. 5, MAY 5 ISSN Asian Research Publishing Network (ARPN). All rights reserved. ice rm rm (6) torque max rmmax rmtorque From equation (), it is then ossible to choose the electric motor (or the internal combustion engine) that acts on the annular gearing At maximum ower (,kw@,rm) we will have at i=i max =8, Nm, n = and =8,6 Nm, n =-. rm at i=i (4). At maximum torque (, 754Nm@9rm) we will have at i=i max =8,685 Nm, n = and =8,794 Nm, n =-6 rm at i=i (4). The second engine or motor should then have a stall torque stall=8, 685 Nm a max torque max=8, 794 Nm at 6 rm and a max ower of P mmax=8, 66*,.*π/,=,48 kw. These value are easily obtainable with an electric motor, but not with an internal combustion engine. In this case the seed of the annular gear should never arrive at ero. The torque available at the final reductions will be the sum of the torque of the two engines or of the electric motor+engine (5) (Figure-4). Figure-4. Torque available at the final reduction. As it can be seen the torque is extremely effective, since its maximum is available at low rm, were it is more needed. The new steering mechanism The new steering mechanism is deicted in Figure-5. In this solution we have internal combustion engines: engines ower directly the srockets, while engine generates the electric ower for the two motors M and M. By varying the seeds of M and M it is ossible to vary continuously the seed ratio of each srocket. It is also ossible to reverse the seed always by varying the relative seed of engine and motors and. In this case the steering system is obtained by different seed ratio of the left (S) and right (S) srockets. These seed ratios are obtained with different rotation seeds of the motor M and M. In the examle of Figure-6 a single generator G is used. Several other configurations are ossible. Note the extreme simlicity of the transmission of Figure-5, when comared with the one of Figure-. The final reductions and the torque converter(s) are not included in Figure-5. This later is not considered in the calculations. The reverse system should be dimensioned from a dynamic model of the vehicle. Additional reverser (s) can be added for more flexibility and erformance. This track steering mechanism is simle, continuous, efficient and additive; this means that, when transitioning to a turn, the mean track velocity remains the same. It is also additive, since keeing both tracks moving hels revent the tracked vehicle from getting stuck. Driving in a straight line is easy. Neutral steering can be achieved. Finally, the energy from the slowed track can be transferred to the sed u one in a regenerative way. Several other solutions based on the same concet are ossible. With accurate design it is also ossible to use an internal combustion engine in lace of the electric motor. ONLUSIONS The imlementation of an electrically controlled dual drive inherently stable heavy duty tracked vehicle is introduced in this aer. This vehicle has to use a comuteried control system to achieve the directional stability. This solution makes it ossible to obtain a VT by using two single stage lanetary gearings for each srocket. An additional motor is added to the annular gear. By controlling the motor seed it is ossible to vary the transmission ratio continuously and to obtain also continuous reverse seed. The torque of the electric motor and the engine are added. It is then ossible to obtain extremely high torque at very low seed. This track steering mechanism is simle, continuous, efficient and additive; this means that, when transitioning to a turn, the mean track velocity remains the same. It is also additive, since keeing both tracks moving hels revent the tracked vehicle from getting stuck. Driving in a straight line is easy. Neutral steering can be achieved. Finally, the energy from the slowed track can be transferred to the sed u one in a regenerative way. An examle is roosed in this aer to clarify the concet. REFERENES [] Stuart J. McGuigan and Peter J. Moss. A Review of Transmission Systems for Tracked Military Vehicles. Journal of Battlefield Technology. (). Figure-5. a ossible solution for the roosed VT. [] Differentials, the Theory and Practice by Philli Edwards. onstructor Quarterly, No., Setember

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7 VOL., NO. 5, MAY 5 ISSN Asian Research Publishing Network (ARPN). All rights reserved. 9(5): 87-88, EBSO Publishing, Estes Street, P.O. Box 68, Iswich, MA 98, USA. [] L. Piancastelli, L. Friiero, G. Donnici. 4. A highly constrained geometric roblem: The insideouthuman-based aroach for the automotive vehicles design. Asian Research Publishing Network (ARPN). Journal of Engineering and Alied Sciences. ISSN , 9(6): 9-96, EBSO Publishing, Estes Street, P.O. Box 68, Iswich, MA 98, USA. [4] L. Friiero, F. R. urbastro. 4. Innovative methodologies in mechanical design: QFD vs TRIZ to develo an innovative ressure control system. Asian Engineering and Alied Sciences. ISSN , 9(6): , EBSO Publishing, Estes Street, P.O. Box 68, Iswich, MA 98, USA. [5] L. Piancastelli, L. Friiero. 4. How to adot innovative design in a sortscar factory. Asian Engineering and Alied Sciences. ISSN , 9(6): , EBSO Publishing, Estes Street, P.O. Box 68, Iswich, MA 98, USA. [6] L. Piancastelli, L. Friiero, I. Rocchi: 4. A lowcost, mass-roducible, wheeled wind turbine for easy roduction of renewableenergy. Published by Pusha Publishing House. Far East Journal of Electronics and ommunications. ISSN: 97-76, (): 9-7, Allahabad, India. [7] L. Piancastelli, L. Friiero, E. Morganti, A. anaaro:. Fuy control system for aircraft diesel engines ediioni ETS. International journal of heat and technology. ISSN , (): - 5. [8] L. Piancastelli, L. Friiero, T. Bombardi. 4. Béier based shae arameteriation in high seed mandrel design. International Journal of Heat and Technology. (-): [9] John J. Moskwa, PI, David E. Foster, co-pi, Gordon Wright, co-pi, Scott A. Munns, Zachary J. Rubin, Joseh W. Anthony, Wenbo Wang. University of Wisconsin-Madison Powertrain ontrol Research Lab - Engine Research enter, htt://arc.engin.umich.edu/events/archive/annual/conf 98/foster_aft.df. 85