Steer-Driven Reduction Drive System

This invention arose from a search for a better way of steering track-laying vehicles, although the resulting discoveries also apply to many types of wheeled vehicles. Track-laying vehicles, such as tractors, are steered by a clutch and brake system that can unclutch the power to one track and brake one track while another track drives. Such arrangements are expensive and troublesome for many reasons, however.

For example, disconnecting power to the left track of a tractor running on level ground tends to make the tractor turn left because the driving right track outdistances the left, but doing the same thing when the tractor is running downhill tends to make it turn right because the engine holds the right track back while the left rolls free. Braking tracks to slow down tends to straighten the line of travel so drivers must guard against braking on curves. Also, the sharpest turn a track-laying vehicle can make is by braking one track while driving the other, and this stresses the braked track considerably.

In searching for a better solution for these and other problems, we have discovered a way of imposing differential rotation on axle shafts for steer driving both track-laying and wheeled vehicles. Many of these use differential reduction gear drives on opposite sides of the vehicle for turning drive wheels or sprockets, and we have devised an effective way of applying our steer drive system to such reduction gear drives. Our system can simultaneously drive wheels or tracks forward on one side of a vehicle and backward on the other side to allow pivot turns around a central point without overly stressing tracks or wheels. Our system can apply main propulsion drive torque to such pivot turns to accomplish them rapidly, if necessary. Our discovery also provides a steer drive that can avoid slip, even when traction is greatly different on opposite sides of the vehicle, and can apply more power to the side with the greater traction, to get the most from whatever traction is available. Our invention accomplishes these advances by combining inexpensive and well-known components in ways that produce improved results.

Our steer drive system uses a first unlimited slip differential connected for driving a pair of axle shafts and a second unlimited slip differential connected between a pair of control shafts. One of the control shafts is connected to one of the axle shafts for rotation in the same direction, and the other control shaft is connected to the other axle shaft for rotation in opposite directions. An input control gear meshed with a ring gear for the second differential can turn to rotate the control shafts and impose differential rotation on the axle shafts via the shaft connecting means. One wheel or track cannot slip in response to light traction because the axle shafts are geared together through the second differential. The control gear is preferably a worm gear, and the ring gear for the second differential is preferably a worm wheel. Turning the control gear steers the vehicle because of the differential rotation this imposes on the vehicle's axle shafts driving wheels or tracks on opposite sides of the vehicle.

For vehicles using reduction drives, the two differentials can be replaced by the differential capability of a pair of right and left differential reduction gears. Both driving and steering control torque can be supplied to the differential reduction gears on opposite sides of the vehicle, where the torques combine in a reduced drive for wheels or tracks. For reduction drive systems, we also prefer that steering control be input so as to turn, but not be turned by, the differential reduction gears. For making pivot turns more rapidly, we prefer a clutched power take-off system for diverting driving torque to the steering control input, since driving torque is not needed for propelling the vehicle forward or backward during pivot turns.

We have also devised a simple and effective way of inputting steering control torque to differential reduction gears on opposite sides of a vehicle. We do this by making the steering control input element of each reduction drive into a worm wheel so that a pair of worm gears, meshed with the worm wheels, can input the steering torque to the reduction drives. Since the reduction drives cannot rotate the worm gears, steering control is positive, whether the vehicle is turning or not. This arrangement also simplifies the gearing, by taking advantage of input elements already available in the reduction drives. It requires a rotational interconnection between the worm gears, and this can be a single steering control shaft with bevel gear sets on its ends. A single steering motor can rotate the steering control shafting, and a power take-off can help this rotation for propulsion drive-assisted pivot turns. A pair of steering control motors rotating the worm gears can divide the steering load, if desired. A pivot-turning brake, applied to the drive torque train, can prevent rotation of drive inputs during pivot turning, to ensure that pivot turns are precisely made.

FIG. 1 is a partially schematic view of a preferred embodiment of a no-slip, imposed differential according to our invention.

FIG. 2 partially schematic view of our imposed differential applied to a wheel dynamometer.

FIG. 3 is a schematic view of a vehicle pivot turn made possible by our imposed differential.

FIG. 4 is a schematic view of our no-slip, imposed differential applied to a reduction gear drive system.

FIG. 5 is a partially schematic cross-sectional view of a planetary reduction gear system for applying our invention to a sprung vehicle.

FIG. 6 is a partially schematic cross-sectional view of a planetary reduction gear system applying our invention to an unsprung vehicle.

FIGS. 7 and 8 are schematic views of alternative drive arrangements for differential reduction gears systems.

FIG. 9 is a schematic view of a clutch system for a power take-off applying drive torque to the steering control input for powering pivot turns.

FIG. 10 is a partially schematic cross-sectional view of a planetary reduction gear drive, similar to the drive of FIG. 6, and showing worm wheel and worm gear input of steering control torque.

FIGS. 11-14 are schematic views of alternative variations of our steer driven reduction drive system, using pairs of worm gears and worm wheels for input of steering torque.

An explanation of the operation and advantages of our invention follows a description of the basic preferred structure or configuration of our no-slip, imposed differential as best shown in FIGS. 1 and 2.

As applied to a vehicle as shown in FIG. 1, engine power input via shaft 11 turning gear 12 rotates ring gear 13 and case 14 of a conventional, unlimited slip differential 15 connected for driving a pair of axle shafts 16 and 17 on opposite sides of the vehicle. Differential 15 is generally known and well understood in the power transmission art and is suitably sized to the vehicle being driven. This can range from small garden tractors and tillers up to large tractors and earth movers. Differential 15 is preferably a bevel gear differential lacking any limited slip devices so as not to impair differential rotation of axle shafts 16 and 17.

A second differential 20, also of a conventional unlimited slip design, is connected between a pair of control shafts 22 and 23 that are interconnected in a driving relationship with axle shafts 16 and 17. One control shaft 23 and one axle shaft 17 are connected for rotation in the same direction, and another control shaft 22 and another axle shaft 16 are connected for rotation in opposite directions. This causes counter or differential rotation of control shafts 22 and 23 as axle shafts 16 and 17 rotate in the same direction and conversely causes differential rotation of axle shafts 16 and 17 as control shafts 22 and 23 rotate in the same direction.

Gear connections between control shafts and axle shafts as shown in FIG. 1 are preferred for larger and more powerful vehicles. These include axle shaft gears 26 and 27 fixed respectively to axle shafts 16 and 17 and control shaft gears 32 and 33 fixed respectively to control shafts 22 and 23. Meshing axle shaft gear 26 with control shaft gear 32 provides opposite rotation between axle shaft 16 and control shaft 22, and meshing both axle shaft gear 27 and control shaft gear 33 with idler gear 25 provides same direction rotation for axle shaft 17 and control shaft 23.

Gear connections between control shafts and axle shafts are preferably incorporated into an enlarged housing containing both axle differential 15 and control differential 20. For a reason explained below, control differential 20 can be sized to bear half the force borne by axle differential 15 so that the complete assembly can be fitted within a differential housing that is not unduly large.

Smaller or less powerful vehicles can use shaft interconnections such as belts or chains in place of gearing. Also, shaft interconnections need not be limited to the region of the axle differential and can be made toward the outer ends of the axle shafts.

A gear or drive ratio between control shafts and axle shafts is preferably 1:1. This ratio can vary, however, so long as it is the same on opposite sides of the axle and control differentials.

An input control gear 40 meshes with a ring gear 21 of control differential 20 for imposing differential rotation on the system. Gear 40 is preferably a worm gear, and ring gear 21 is preferably a worm wheel so that ring gear 21 turns only when gear 40 turns.

Gear 40 can be turned by several mechanisms, depending on the objective. For steering purposes, gear 40 can be turned by a steering shaft joined to control gear 40 and manually turned by a driver. Steering mechanisms can also use motors for turning gear 40. Alternatives include a DC starter motor 41 electrically turned via a rheostat in a steering system and a hydraulic or pneumatic motor 41 turned by a vehicle's hydraulic or pneumatic system in response to a steering control.

Our invention can also be applied to a wheel dynamometer 50 as schematically shown in FIG. 2. For such purpose, the drive axle 51 being tested by the dynamometer provides axle shafts 16 and 17 and axle differential 15, which need not be an unlimited slip differential and can be a no-slip differential such as disclosed in U.S. Pat. No. 2,859,641. Power applied via input shaft 11 turns axle shafts 16 and 17, which can be subjected to varying loads by wheel dynamometer 50.

Previous wheel dynamometers have caused differential rotation of axle shafts 16 and 17 by subjecting them to different loads. This will not work with the differential of U.S. Pat. No. 2,859,641, which applies more power to the more heavily loaded axle shaft.

Our invention can apply differential rotation directly and realistically to axle shafts 16 and 17 via control differential 20 and its control shafts 22 and 23. These are connected to axle shafts 16 and 17 for same and opposite direction rotation as explained above; and when turned by control gear 40 rotated by motor 41, control shafts 22 and 23 impose differential rotation on axle shafts 16 and 17.

Several drive interconnections are possible between the control shafts and the axle shafts of the test axle mounted on wheel dynamometer 50. One preferred arrangement shown in FIG. 2 uses meshed gears 26 and 32 fixed respectively to test axle shaft 16 and control shaft 22 for opposite direction rotation. Sprockets 57 and 53, coupled by a chain 54 and fixed respectively to test axle shaft 17 and control shaft 23, provide same direction rotation. Belts and other gearing arrangements are also possible.

Two important effects occur from the interconnection of control differential 20 and its control shafts 22 and 23 with axle differential 15 and axle shafts 16 and 17. One is a no-slip drive that prevents wheels or tracks from slipping unless slippage occurs on both sides of the vehicle at once. The other effect is imposed differential rotation that can accomplish steering to pivot or turn a vehicle.

The no-slip drive occurs because axle shafts 16 and 17 are geared together via differential 20. Power applied to an axle shaft on a side of the vehicle that has lost traction is transmitted to the connecting control shaft on that side, through differential 20 to the opposite control shaft, and back to the opposite axle shaft where it is added to the side having traction. So if one axle shaft loses traction, the opposite axle shaft drives harder; and the only way slippage can occur is if both axle shafts lose traction simultaneously.

To elaborate on this, consider a vehicle rolling straight ahead with its axle shafts 16 and 17 turning uniformly in the same direction. Control gear 40 is stationary for straight ahead motion; and since control gear 40 is preferably a worm gear, worm wheel 21 of control differential 20 cannot turn. Control shafts 22 and 23, by their driving connections with the axle shafts, rotate differentially in opposite directions, which control differential 20 accommodates.

Axle differential 15 equally divides the power input from shaft 11 and applies one-half of the input power to each axle shaft 16 and 17. If the track or wheel being driven by axle shaft 16 loses traction, it cannot apply the power available on shaft 16 and tends to slip. Actual slippage cannot occur, however, because axle shaft 16 is geared to control shaft 22. So if a wheel or track without traction cannot apply the power on shaft 16, this is transmitted to control shaft 22, which rotates in an opposite direction from axle shaft 16. Since ring gear 21 cannot turn, rotational power on control shaft 22 is transmitted through differential 20 to produce opposite rotation of control shaft 23. This is geared to axle shaft 17 via idler gear 25 so that power on control shaft 23 is applied to axle shaft 17 to urge shaft 17 in a forward direction driving the wheel or track that has traction and can accept the available power. Since only one-half of the full available power can be transmitted from one axle shaft to another via differential 20 and its control shafts, these can be sized to bear one-half the force borne by axle differential 15 and its axle shafts.

Of course, unusable power available on axle shaft 17 because of a loss of traction on that side of the vehicle is transmitted through the same control shaft and control differential route to opposite axle shaft 16. This arrangement applies the most power to the wheel or track having the best traction, which is ideal for advancing the vehicle. The wheel or track that has lost traction will maintain rolling engagement with the ground while the other wheel or track drives. The only time wheels or tracks can slip is when they both lose traction simultaneously.

To impose differential rotation on axle shafts 16 and 17 for pivoting or turning the vehicle, it is only necessary to rotate control gear 40. This differentially rotates axle shafts to turn or pivot the vehicle because of the different distances traveled by the differentially rotating wheels or tracks on opposite sides of the vehicle. Steering controlled by rotating gear 40 can be applied to track-laying vehicles and wheeled vehicles that steer without using turning wheels. It can also be used for differentially rotating drive wheels or tracks to provide power-assisted steering for a vehicle that also has turning wheels.

Whenever control gear 40 turns, it rotates ring gear 21, which turns the casing of differential 20 to rotate control shafts 22 and 23 in the same direction. The connection of control shafts 22 and 23 with axle shafts 16 and 17 converts the same direction rotation of control shafts 22 and 23 to opposite differential rotation of axle shafts 16 and 17, as accommodated by axle differential 15. This drives wheels or tracks forward on one side of the vehicle and rearward on the other side of the vehicle, depending on the direction of rotation of control gear 40.

Such differential rotation is added to whatever forward or rearward rotation of the axle shafts is occurring at the time. So if a vehicle is moving forward or backward when control gear 40 turns, the differential rotation advances and retards opposite axle shafts and makes the vehicle turn.

If a vehicle is not otherwise moving when control gear 40 turns, wheels or tracks go forward on one side and backward on the other side so that the vehicle pivots on a central point. This is schematically illustrated in FIG. 3 for a vehicle having a pair of tracks 75 and 76. Both tracks can have a rolling engagement with the ground as the vehicle rotates around a center point 77 by driving right track 76 forward and left track 75 rearward. The tracks experience some heel and toe scuffing, but this is less stress than if one track were braked still and the other driven. The pivot turn also spins the vehicle on one point 77, without requiring motion in any direction as must occur when one track is braked and another is driven.

Pivot turns can also be made with wheeled vehicles, such as tractors, loaders, forklifts, etc. Many of these now have turning wheels that could advantageously be replaced by non-turning wheels steered by imposed differential rotation according to our invention. To accommodate such vehicles, our invention can be applied to more than one drive axle powering more than one pair of drive wheels. Evidence indicates that pivot-turning capability can save considerable time for loaders, tractors, and forklifts compared to turning patterns requiring forward and rearward motion.

Our no-slip steer drive can also be applied to vehicles with reduction gear drives where it can take advantage of differential capability of a reduction gear system to eliminate conventional differentials. One preferred way of doing this is shown schematically in FIG. 4.

By using a pair of reduction gear systems 60 and 61 that also have differential capacity, as explained more fully below, both driving and steering control torque can be input directly into differential reduction gears 60 and 61. These differentially combine the driving and steering torques to rotate reduction drive members 64 and 65 respectively turning drive elements 62 and 63, which can be wheels or track sprockets that advance and steer a vehicle in the way described above.

Driving torque can input same direction rotation into differential reduction gears 60 and 61 directly from drive shaft 11, gear 12, and gear 18 turning shaft 48 as shown by the arrows from moving the vehicle either forward or reverse. Steering control torque, derived from steering control motor 41 turning worm 40 and worm wheel 42, can be input via gears 43-45 to produce counterrotation of steering control shafts 46 and 47 oppositely turning elements of differential reduction gear systems 60 and 61 to produce differential rotation for steering purposes. Gear systems 60 and 61 differentially combine and reduce the driving and steering control torques and apply the result to drive elements 62 and 63. There are also many other ways that same direction driving torque and opposite direction steering control torque can be input to differential reduction gears 60 and 61. Among them is a solid shaft inputting steering torque to one differential reduction gear directly and to another differential reduction gear via a torque reversing idler gear.

By using worm 40 and worm wheel 42, shafts 46 and 47 can be rotated by steering control input to turn, but not be turned by elements of reduction gear systems 60 and 61. Another way to accomplish the same effect is to make steering motor 41 a hydraulic motor driving gear 43, which then cannot turn motor 41. Either way, it is important to hold steering control shafting against rotation except when differential rotation is required for turning and is supplied by a steering control input.

Differential reduction gear systems 60 and 61 can have many different forms as shown in FIGS. 5-8. Both planetary and non-planetary differential reduction gears are possible, and inputs and outputs for these can be arranged in various ways.

FIG. 5 schematically shows one preferred way of applying our invention to a planetary reduction drive suitable for a sprung vehicle. Drive shaft 48 extends through axle housing 49 and drives sun gear 66. Steering shaft 46 extends through a brake assembly 70 secured to housing 49 and turns a gear 71 meshed with a gear 69 keyed to planetary ring gear 68. This applies steering control torque to planetary ring gear 68 surrounding driving torque sun gear 66. In between these is an array of preferably three planet gears 67 (one of which is shown) that turn a housing hub 64 at a reduced speed applied to drive element 62 in the form of a wheel (only the inner portion of which is shown). A brake 72 in brake assembly 70 can brake wheel 62, and the planetary reduction gear system is arranged well outboard into the wheel to allow a wide stance for springs supporting the vehicle.

A similar planetary reduction gear arrangement is shown in FIG. 6 for an unsprung vehicle having a main support housing 80 suitable for a track-laying vehicle. Again, driving torque is applied via shaft 48 to sun gear 66 and steering control torque is applied via shaft 46 to gear 71 meshed with a gear 69 that turns planetary ring gear 68. Planet gears 67, rotating at a reduced speed between sun gear 66 and ring gear 68, turn rotor 73 and shaft 64, which can mount a drive sprocket (not shown) for advancing an endless track. A brake 72 can brake rotor 73.

As schematically suggested in FIG. 7, steering control torque via shaft 46, gear 71, and ring gear 69 can also be applied to planet gears 67 of a planetary reduction gear system receiving driving torque from shaft 48 to sun gear 66. This produces reduced rotation of planetary ring gear 68 which turns a drive element in the form of a sprocket or wheel 62. It is also possible to input driving torque to the planets or ring of a planetary reduction gear, while inputting steering control torque to an element not receiving driving torque, and have the remaining element combine the two torques differentially and apply the result to a wheel or sprocket.

Bevel and spur gear differentials can also be used in reduction drive systems according to our invention, as schematically suggested in FIG. 8. Driving torque on shaft 48 can be input to side gear 82, and steering control torque on shaft 46 can be input via gear 71 and ring gear 69 to opposite side gear 83. Differential rotation of side gears 82 and 83 provides a reduced drive output at the differential hub casing 84 for turning a sprocket or wheel 85. Spur gear differentials can also be used in place of a bevel gear differential such as shown in FIG. 8, and the two drive and steering control torques can be input to different elements to cause differential and reduced rotation of an output.

Pivot turns can be power assisted or powered totally by driving torque to be executed more rapidly. Since a vehicle is not using driving torque for forward or rearward movement when pivot turning occurs, driving torque is available for powering pivot turns; and FIG. 9 schematically shows a preferred way of accomplishing this.

A power take-off 92 from a transmission 91 or main propulsion engine 90 rotates clutch parts 93 and 94. Either of these can be engaged with its counterpart 95 and 96, each of which are meshed with bevel gear 97 rotating with worm gear 40. To apply driving torque to a pivot turn in one direction, clutch part 93 is meshed with clutch part 95 to turn bevel gear 97 in the desired direction for rotating worm gear 40 and worm wheel 42. Steering control motor 41 can be unclutched for engine powered pivot turns or can combine its torque with the torque provided via bevel gear 97. For applying propulsion torque to power a pivot turn in an opposite direction, clutch part 94 meshes with its counterpart 96, driving bevel gear 97 in an opposite direction and turning worm 40 and worm wheel 42 in an opposite direction.

A power take-off can be derived from many points along the main propulsion drive train, including engine 90, transmission 91, and other points. A power take-off can be made to turn continuously or be operated only when needed for pivot turns. The engagement of clutch parts 93 and 94 can be made responsive to full turn of a steering wheel, calling for a pivot turn; and any engagement of clutch parts 93 and 94 can be locked out during forward or rearward movement of the vehicle, if desired. Propulsion assisted pivot turning can also be applied to worm wheel 42 by a worm gear separate from steering control input worm gear 40, and different clutch arrangements can be used for engaging and disengaging the diversion of drive torque for pivot turning. Applying drive torque to the steering control input allows pivot turns to be accomplished more rapidly than would be possible with a small sized steering control motor 41, adequate for forward and rearward steering.

We have discovered several advantages from using worm inputs for applying steering control torque to differential reduction gears. Worm gears inputting steering control torque to worm wheels of the differential reduction gears accomplish a substantial reduction that allows smaller steering control shafting to extend from a steering control motor to the reduction drives. Also, since worm wheels cannot rotate worm gears, the reduction drives cannot rotate the steering control shafting and are held against rotation, except when steering torque is input for turning purposes. The steering control shafting can be simplified; and since input gearing is necessary to apply steering torque to reduction gear drives anyway, a significant economy is achieved by making such an input in the form of worm gears and worm wheels.

FIG. 10 shows how a simple modification can be made to the embodiment of FIG. 6 to convert the reduction drives to receive steering torque from worm gears. Instead of ring gear 69 meshing with spur gear 71, as shown in FIG. 6, ring gear 69 is converted to worm wheel 111 and is meshed with worm gear 110, which is rotated by steering control shaft 46, via a meshed set of bevel gears 112 and 113. These accommodate the angle necessary between shaft 46 and the axis of worm gear 110, which is normally perpendicular to the axis of worm wheel 111.

The embodiments of FIGS. 7 and 8 can be converted to worm gear and worm wheel input, in a way similar to that shown in FIG. 10, for the embodiment of FIG. 6. In each case, ring gear 69 is converted to worm wheel 111 and spur gear 71 on shaft 46 is converted to worm gear 110, driven by bevel gears 112 and 113 that rotate with shaft 46. As these alternatives make clear, any element to a differential reduction gear or planetary reduction gear can be combined with a worm wheel to receive steering control torque via a worm gear. In each case, worm and worm wheel inputs are provided to each of the differential reduction gears, on each side of the vehicle; and the gearing is arranged so that steering torque counterrotates input elements of opposite reduction drives.

One preferred form of worm input of steering control torque to opposite differential reduction drives is schematically shown in the embodiment of FIG. 11. An input element of each of the reduction drives is combined with a worm wheel 111, and each of these is meshed with a worm gear 110. Bevel gears 112 and 113 rotate worm gears 110, when steering control shaft 46 rotates, and this produces counterrotation of worm wheels 111 for equal and opposite steering input to the reduction drives, as explained above. With a single steering control shaft 46 turning bevel gears 113 arranged inside of bevel gears 112, as shown in solid lines in FIG. 11, bevel gears 112 counterrotate. Then if each worm gear 110 is right hand threaded, worm wheels 111 counterrotate, as desired. Right and left hand worm gears can also be used in pairs, and counterrotation of worm wheels 111 can be established by moving one of the bevel gears 113 to the opposite side of one of the bevel gears 112, as shown in broken lines at the right side of FIG. 11.

Steering control motor 41 turns steering control shaft 46 via bevel gear 114 to accomplish differential rotation of the reduction drives, for steering the vehicle. When steering motor 41 is not turning, and the vehicle is proceeding straight ahead, steering control shaft 46 does not rotate, and the steering input worm gears 110 are held still. This locks worm wheels 111 against differential rotation so that the reduction drives respond only to driving torque and advance equally, moving the vehicle straight ahead.

A power take-off 115 from the main propulsion drive is preferably clutched and unclutched for rotating bevel gear 114 and steering control shaft 46, for propulsion drive-assisted pivot turning, as explained above. To ensure that pivot turns are precisely made, pivot turn brake 120 can be applied to transverse drive shaft 48, to ensure that drive torque inputs to the reduction drives are held still as steering torque is applied for pivot turning. There are several locations along the drive train where pivot turn brake 120 can be applied.

Another embodiment of worm input of steering control torque to differential reduction drives is shown in FIG. 12. Since a worm input of steering control torque naturally spaces transverse drive shaft 48 and steering control shaft 46 well apart, the drive train can take advantage of the well-known expedient of arranging a drive transmission on a hollow shaft 101 so that transmission 91 and shaft 101 are concentric with transverse drive shaft 48. Main propulsion engine 90 can also be arranged transversely, as is common in front wheel drive vehicles, so that transmission 91 is alongside engine 90 on drive shaft 48 and is connected to engine 90 by suitable gears 102 and 103.

Power take-off 115 can conveniently be derived from transmission 91, for applying drive torque to steering control shaft 46, which steering motor 41 normally turns. Power take-off 115 is preferably clutched and unclutched for pivot turning purposes, as explained above; and suitable gears 104, 105, and 106 can apply either driving torque from power take-off 115 or steering torque from motor 41 to rotation steering control shaft 46. This is input to worm wheels 111 by worm gears 110 rotating with meshed bevel gears 112 and 113, as explained above.

The arrangement of FIG. 12 takes advantage of the familiar arrangement of engine 90 and transmission 91 relative to a transverse drive shaft 48, which can be accomplished in our steer drive system, because steering control shaft 46 is well spaced from transverse drive shaft 48. If shafts 48 and 46 need to be elevated for ground clearance purposes, gearing from the differential reduction drives can be extended downward to wheels or sprockets. There are many other ways that engine 90, transmission 91, power take-off 115, and steering motor 41 can be interconnected for rotating drive shaft 48 and steering control shaft 46 in our steer-driven reduction drive system, and the illustrated gearing shows only one example.

The arrangement of FIG. 13 uses counterrotating steering control shafts 46 and 47, similar to the ones illustrated in FIG. 4. These are rotated through bevel gears 114 by steering control motor 41, for normal turning, and by power take-off 115, for power-assisted pivot turns. The counterrotation of shafts 46 and 47 turns bevel gears 112 in the same direction so that right hand and left hand worm gears 110 are needed to counterrotate worm wheels 111. An alternative possibility is to make both worm gears the same and to mesh one of the bevel gears 113 on the opposite side of one of the bevel gears 112, as shown in broken lines, to establish counterrotation of worm wheels 111.

In the embodiment of FIG. 14, a pair of steering control motors 141 are arranged coaxially with worm gears 110 for sharing the steering control load. Worm gears 110 are rotationally interconnected by steering control shaft 46 and sets of meshed bevel gears 112 and 113, at end regions of control shafting 46. Drive torque from power take-off 115 is used to rotate steering control shaft 46, via bevel gear 114, for propulsion drive-assisted pivot turning.

In addition to a pair of steering control motors 141, directly rotating worm gears 110, as shown in FIG. 14, it is also possible for a single steering control motor to have rotatable shafting extending from each end and spanning the distance between worm gears 110. Such a steering control motor would then be concentric with steering control shaft 46, like transmission 91 is concentric with transverse drive shaft 48, in the embodiment of FIG. 12.

In any of the above embodiments, it is possible to use variations. For example, it is possible to change gearing angles, to substitute hypoid gears for bevel gears, to use different forms of gearing, to arrange pivot turn brake 120 at various places along the drive torque train, to derive torque for power take-off 115 from various places along the drive torque train, and to establish the desired direction of rotation by varying the gearing.