FIELD OF THE INVENTION

This invention relates to a traction system that can be used as a power transmission system (e.g. driving machine) which can be used over a large field of applications as an integral device or as a part (e.g. subsystem) of a complex system (such as a transmission gear in a machine).

BACKGROUND OF THE INVENTION

Traction systems of a kind shiftable between a driving state and various traction states are needed in a variety of vehicles. Such a traction system includes wheels (e.g. a main wheel and one or more auxiliary wheels) and a traction belt. Versatile traction systems that permit movement on various different terrains are required for a number of different purposes. These traction systems are not suitable for passing obstacles such as steps and others, moving through staircases or moving over rough terrains.

Some traction systems aimed at solving the problem of obstacles have been developed and are described for example in US 7,334,850 and US 6,422,576 assigned to the assignee of the present application.

GENERAL DESCRIPTION

In accordance with the invention, a new power transmission system that can be used as traction system or as a subsystem of a larger machine is provided. The traction system of the present invention may be used for vehicles of various types such as self-powered vehicles or carriage without self-powering means, e.g. a wheelchair, cars, construction equipment agriculture equipment, recreation vehicles, and toys. The power transmission system may be a power transmission module or a utility in a scraper or cleaning device.

The term "vehicle" refers to any movable platform having one or more wheel structures that moves over a terrain through wheels or track. As will be appreciated from the disclosure below, the traction system provided by the invention is suitable for a wide variety of vehicles such intended for traveling over different terrains.

The traction system in accordance with the invention constitutes a power transmission system (e.g. driving machine) which can be used over a large field of applications as an integral device or as a part (e.g. subsystem) of a complex system (such as a transmission gear in a machine) and especially in the case of the following apparatus:

Driving machine such as moving machines, and all sorts of robots;

Transportation machine apparatuses (a crane, a conveyor belt, a screw conveyor, a conveyor chain, a winch, a hoist, a chain block, an elevator, an escalator, a jack, a container, a pallet for transportation, a bomb for transportation, a pulley, a rail for transportation machines);

Transport vehicles (an engine, a passenger car, a tram, a seat for rail cars, an automobile, a bus, a truck, a dump truck, a loading platform for a truck, a gate plate for tracks, a tractor, a trailer, a fire engine, a garbage collecting vehicle, a truck crane, a snowmobile, a fork lift truck, a headlight for cars, , a steering wheel for cars, a console for cars, a shift lever for cars, a rim for cars, a wheel for cars) as well as vehicle for amusement and recreation such as toys or motor- assisted amusement vehicle;

Washing and cleaning equipments such as automatic cleaning machines or cleaning scraping device;

Devices for lifting, or braking during lowering, with a linear movement, for lifts, lifting trucks, extraction of boring rods, pile-driving equipment;

Sliding control devices, especially for doors, shuttles, machine carriages; Propulsion and braking devices for transport or handling means for passengers, goods or equipment, comprising guided vehicles such as: trains, trolleys, launching catapults for aircraft or rockets;

Actuating motors giving considerable power, for example for driving tools for cold-forging metals by percussion or broaching tools;

Driving motors for alternating machines at relatively low speed and large travel, such as pumps and compressors;

Driving motors for devices of the chain type for caterpillar tractors, conveyors, bucket dredgers, etc;

Driving motors for rotating machines such as crane-driving tables or drilling platforms, vehicle wheels and more generally rotary machines requiring a high torque at low speeds, accurate control, a very wide range of speeds and a rotor having a low weight and low inertia;

Torque or force limiting or transmitting devices; clutches;

Transmission to a distance of angles of rotation or linear displacements, remote recording, remote synchronization, servo-controls.

Further particular characteristics of the invention will be brought out in the description which follows below.

Therefore, there is provided a novel traction system. The traction system comprises a wheel structure, defining a substantially circular circumference; a closed-loop member carried by and engaged with said wheel structure, said closed loop member being operable for expanding its circumference between a substantially circular geometry and various non-circular geometries while remaining engaged with said wheel structure. Preferably, the traction system includes a stretching mechanism configured and operable to apply a force to the closed-loop member to cause the expansion of the closed loop member between the substantially circular geometry and various non-circular geometries. The closed-loop member, when in its circular geometry state, engages the wheel structure all along the circumference - A - of the wheel structure, and when expanded to non-circular states, engages the wheel structure along a part of the wheel structure's circumference.

In some embodiments, the traction system comprises an engagement mechanism configured and operable to provide various engagements between the closed loop member and the wheel structure. The engagement mechanism comprises an array of transmission pins arranged in a spaced-apart relationship along the closed loop member and an array of engaging elements provided along the wheel structure's circumference. The transmission pins may be equally spaced from one another defining a certain constant pitch of said array of transmission pins. The engagement mechanism is preferably characterized by a pitch of engagement defined by a number of the engaging elements between two locally adjacent transmission pins. The number of the engaging elements is different for different extended states of the closed loop member.

In some embodiments, the wheel structure and the closed loop member may be configured as gear wheels, the transmission pins being configured to be received by sprockets between teeth of the wheel structure.

Alternatively or additionally, the closed loop member may comprise a flexible continuous belt, which may be formed with an array of transmission pins arranged in spaced-apart relationship along the belt.

In some embodiments, the closed loop member comprises a spring assembly maintaining the engagement between the closed loop member and the wheel structure and operable to cause the shift of the closed loop member between the circular geometry state and various non-circular states.

In other embodiments, the stretching mechanism comprises a roller assembly (e.g. bogy wheel) having at least one roller controllably shiftable between its inoperative retracted state and its operative extended state in which it applies force onto the closed loop member causing its expansion from the normally circular geometry to the non-circular geometry being supported by said wheel structure and said at least one roller. The traction system preferably includes a locking mechanism configured and selectively operable to prevent undesirable expansion of the closed loop member and thus at least partial disengagement between the wheel and the closed loop member.

The traction system may also comprise a disengagement unit controllably operable to hold a part of the closed loop member, thus preventing temporary incorrect engagement between the wheel structure and the closed loop member during the closed loop member shift in between its different states. The disengagement unit may comprise at least one roller controllably movable downwards or upwards during the closed loop member expansion from the circular geometry to non-circular or vice versa respectively.

According to another embodiment of the traction system, the wheel structure comprises two parallel coaxially mounted substantially identical wheels both carrying and engageable with the closed loop member. In this case, the stretching mechanism if any is accommodated in a space between the two wheels.

According to yet another embodiment of the traction system, the wheel structure comprises two substantially identical wheels both supporting and engageable with the closed loop member, the wheel structure being shiftable between axially coinciding relative position of the two wheels corresponding to the circular geometry of the wheel structure and various displaced positions in which the wheels' axes are displaced from one another in a direction perpendicular to said axes thereby defining together a non-circular state of the wheel structure and accordingly of the closed loop member. Preferably, in any of the above embodiments, the closed-loop member comprises a chain of links configured for angular and linear displacement with respect to one another. In case the transmission pins are used, they are mounted on at least some of the links. The closed-loop member is preferably characterized by a certain ratio between the number of the links and the number of the transmission pins.

In some embodiments, the links are connected to one another by a springlike mechanism configured and operable to provide angular and linear displacement of the links. The spring-like mechanism may comprise at least one leaf spring and /or a torsion rod-like member.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Figs. IA and IB show an example of a traction system of the present invention in its driving and traction states respectively;

Figs. 1C and ID show more specifically operation of a stretching mechanism used in the traction system of Figs. IA and IB;

Figs. 2A and 2B show an engagement mechanism between a wheel structure and a closed loop member in the traction system according to an embodiment of the invention;

Fig .2C illustrates an example of the closed loop member configured as a continuous belt; Figs .2D-2E illustrate more detailed view of the closed loop member of Fig. 2C in a closed and expanded state respectively;

Fig. 2F shows the engagement mechanism of Figs. 2A-2B in an expanded non-circular state;

Fig. 2G shows more specifically the engagement mechanism between the transmission pins and the closed loop member;

Figs. 3A to 3C show an example of the configuration of a closed-loop member suitable to be used in the traction system of the invention; Figs. 4A and 4B exemplify the configuration of chain of links with transmission pins suitable for a configuration of the closed loop member for the traction system of the invention;

Figs. 5A and 5B show an example of a locking mechanism for preventing undesirable expansion of the closed loop member;

Figs. 6A-6D exemplify the configuration of a securing mechanism suitable to be used with the locking mechanism of Figs. 5A and 5B to keep the closed loop member in its circular configuration during the movement of the traction system;

Figs. 7A- 7D show another example of the configuration of a closed-loop member suitable to be used in the traction system of the invention;

Figs. 8A and 8B show yet another possible configuration of a closed-loop member which may be used with the configuration of Figs. 7A-7D;

Figs. 9A and 9B show yet another possible configuration of a closed-loop member;

Figs. 1OA and 1OB show another embodiment of the traction system of the invention; and

Figs. 1 IA-I IE show more specifically configuration of a stretching mechanism in the traction system of Figs. 1OA and 1OB.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention provides a novel traction system for use in a vehicle. The traction system is associated with a vehicle's wheel structure, and provides for selectively shifting the wheel structure between its driving state (defining circular geometry) and a traction state (non-circular geometry).

Reference are made to Figs. IA and IB exemplifying the configuration of the traction system 10 in its driving and traction states respectively. The traction system 10 includes a wheel structure 12 having substantially circular circumference and adapted for supporting a closed-loop member 14 configured for engaging with the wheel structure 12. Closed loop member 14 is configured to be expandable in its circumference. This may be implemented using a flexible material of the member 14 and/or spring-like assembly used therein. An example of the configuration of the closed-loop member will be described more specifically further below with reference to Figs. 3A-3C and 4A-4B.

As shown in Fig. IA, member 14 is in a retracted, circular-circumference state in which it fully engages the wheel structure 12 all along the wheel structure's circumferential surface, and as shown in Fig. IB member 14 is expanded to a position in which it has a non-circular geometry while only partially engaging with the wheel structure. The retraction/expansion of the closed loop member 14 is implemented via a stretching mechanism, generally at 16, configured and operable to apply a force to the closed-loop member 14 to cause expansion thereof from its circular-geometry state in which it engages the wheel all along its circumference to an expanded non-circular state in which it engages said wheel only along a part of the wheel's circumference.

The wheel structure may include one or more main wheels. In this specific non-limiting example, the wheel structure includes two main wheels 12A and 12B (see Fig. IB) which have identical circumference and are arranged in a spaced apart parallel relationship and rotatable about a common axis 18.

Also in the present example, the stretching mechanism 16 is accommodated within a space between the two main wheels 12A and 12B, and is configured similar to that disclosed in US 7,334,850 assigned to the assignee of the present application. Figs. 1C and ID show more specifically the operation of the stretching mechanism. Fig. 1C is a cross-sectional view showing the stretching mechanism 16 located in between the two main wheels 12A and 12B. The stretching mechanism includes rollers 20 (e.g. bogy wheels) mounted on a support frame 22 (two arms of the frame being shown in Fig. 1C) attached to wheel shaft 18 and pivotal with respect to wheels 12A and 12B about one or more pivots. In the driving position of the traction system (Fig. IA), the stretching mechanism is in its inoperative position corresponding to the retracted state (circular geometry) of the closed loop member 14, and when the stretching mechanism is being shifted through its operative positions (Figs. IB, 1C and ID) the member 14 expands to various corresponding traction positions. It should be noted that any other suitable stretching mechanism can be used, as well as any other wheel structure configuration, as will described further below with reference to Figs. 10A-10B and 1 IA-I IE.

Reference is made to Fig. 2 A and 2B, illustrating an embodiment of an engagement mechanism between the wheel structure 12 and the closed loop member 14. As shown in the present example, the engagement mechanism utilizes gear wheel configuration of the main wheels 12 A and 12B and an array of transmission pins 24 located in a spaced-apart relationship along the closed loop member 14 and adapted to engage sprocket 25 (constituting engaging elements) between the teeth of the gear wheels. In this example, the closed-loop member 14 includes a stretchable chain of multiple links, generally at 26, axially connected to one another and forming a closed-loop. Transmission pins 24 are mounted on at least some of the links 26 and configured for engaging the closed- loop member 14 with the wheels 12A and 12B.

It should be noted that the chain of links with transmission pins arrangement can be replaced by a flexible/expandable continuous belt formed with spaced-apart transmission pins, e.g. protrusions/grooves which can be coupled with engaging elements at the main wheels, as illustrated by Fig. 2C.

The belt is expandable between its mostly retracted and expanded state, and could be defined by the degree of elasticity of the belt material, or by the geometrical folding (e.g. undulation) of the continuous belt as illustrated in Fig. 2D. In such configuration, the belt comprises a reinforcement element 23 and an arrangement of spring 32 spaced apart by a spacer 30 made of a rigid material (such has steel). The spacer assembly is rigidly fixed to the reinforcement element 23. The transmission pin 24 is made of an elastic material 27. The reinforcement element 23 is made of an elastic material 21 surrounded by a non- elastic material having a fixed length, such that the degree of elasticity of the belt is defined by the elasticity of the springs 32 and of the elastic material 21. In an expanded configuration (non-circular geometry), the reinforcement material 23 is unfolded/straightened, as illustrated in Fig. 2D. In a closed configuration (circular geometry), the reinforcement material 23 is undulated/folded as illustrated in Fig. 2E.

The chain of links may be rigid. In this case, the flexibility/expansion of the closed-loop member is provided by an elastic element (e.g. spring element) appropriately accommodated between the adjacent links, or fixed within the links, for example by casting or with glue welds screw. Alternatively, the closed- loop member may be a continuous belt such as rubber belt (e.g. rubber cushion). Alternatively, the chain of links may be made from an elastic material providing the flexibility of the closed-loop member.

Preferably, the transmission pins are arranged with a certain constant pitch defined by a predetermined number of links 26 in between two locally adjacent transmission pins 24. In the present example of Figs 2A and 2B, this ratio is set to one transmission pin per four links. In some embodiments of the invention, a ratio between the number of transmission pins 24 and the number of links 26 in the closed-loop member 14 is designed, in accordance with the arrangement of sprocket 25 of the gearwheel, the stretchability of the closed loop 14 and the dimensions of the links 26 being such as to enable a variable pitch of engagement between the closed loop member 14 and the main wheels.

It should be understood although not specifically shown includes an appropriate controller preprogrammed for determining and setting an appropriate value of the above ratio for each desired state of the closed loop member and accordingly operates the stretching mechanism and engagement mechanisms. Fig. 2A illustrates an engagement between the closed-loop member 14 and the wheels 12A and 12B while in the traction system driving position. The wheels 12A and 12B operates as sprocket wheel/rim. In this position, the closed- loop member 14 is retracted and engages the wheels along their full circumference. In this case, since the closed-loop member 14 is fully retracted, a high pitch of engagement between the closed-loop member 14 and the gearwheels 12A, 12B is obtained (one transmission pin 24 per four sprockets 25 of the gearwheel).

Fig. 2B illustrates a lower pitch engagement between the wheels 12A and 12B and the closed-loop member 14. It should be understood that the figure shows only a part of the closed-loop member. Comparing the configurations of

Figs. 2A and 2B for the same closed-loop member with a given pitch of the transmission pins arrangement, the engagement of Fig. 2B may corresponded to an expanded position of the closed loop member, corresponding to an operative position of the stretching mechanism defined by one or more traction positions of the traction system. When the closed loop is shifted from its retracted state into an expanded one, a portion of the closed loop member that remains engaging with the wheels is stretched resulting in an increased distance between the locally adjacent transmission pins 24 and in a lower pitch of engagement between the closed loop member 14 and the gear wheels (e.g. one transmission pin per five sprocket).

As further shown in Fig. 2B, in the present example the wheels 12A and 12B are provided with a support structure 28 extending along the wheels' circumference and configured for supporting the closed loop member 14, especially the links in between the transmission pins. The support structure 28 may be a pneumatic element, an elastic element integrally made with an elastic material or made with a combination of rigid and elastic materials.

It should be understood that conventionally, when a chain/belt is laid or stretched around a sprocket wheel, a polygon is created, in which the gap between the teeth sprocket is the imaginary side of that polygon. When the wheel rotates, the rotational forces are carried by the sprocket wheel and are transmitted from the sprockets to the chain. The more teeth the sprockets wheel has, the more sides the polygon has and will operate more similar to a wheel (more smoothly). If a chain is stretched around two sprockets wheels, then while rotating, the tension in a given chain varies in accordance to the sprockets rotation. For example, if the sprockets have three teeth, at one time the belt will be fully stretched and at another time there will be a slack in the belt.

In the traction system of the present invention, the contact area between the wheels 12A and 12B and the closed-loop member 14 is substantially circular.

The transmissions pins 24, which transmit forces from the sprocket 25 to the closed loop member 14, are extended out sideways from the width of the closed loop member as illustrated in Fig. 2F. Therefore, the transmission pins 24 transmit forces only tangentially to the wheels 12A and 12B. All the forces that are built in the direction of the wheel axle, are supported by the support structure

28, therefore, the closed loop is rolling on a true circle.

Due to the variable pitch of engagement of the closed-loop member on the wheel, the number and the distribution on the sprocket on the wheel have been appropriately selected to fit the closed loop member both in its circular geometry (i.e. wheel mode) and in its non-circular geometry wheel (i.e. track mode). However, by simply selecting a common denominator (expansion ratio corresponding to the two modes) between the two modes, the number of teeth would increase and each sprocket would have a small size, which is problematic to support a substantially high load (in the order of tones). The present invention overcomes the above-mentioned problem by appropriately configuring the transmission pins 24 (number of transmission pins and the distance between them) to transmit the rotational forces from the sprocket 25 to the closed-loop member 14. As illustrated in Fig. 2G, the links 26 and in particular, the transmission pins 24 are not lying on the sprockets 25 but on the support structure 28.

Reference is made to Figs. 3A-3C showing more specifically an example of the configuration of closed-loop member 14 suitable to be used in the invention. Member 14 comprises a chain of links 26 connected to one another in a manner allowing relative displacement between them up to a certain predefined maximal distance, and an array of spaced-apart transmission pins 24 provided on some of the links, e.g. with a certain constant pitch. Fig. 3A shows the retracted state of the closed-loop member (partially seen in the figure), corresponding to a minimal distance between the adjacent links. Fig. 3B shows the expanded state of the closed loop member 14, corresponding to a higher distance between the adjacent links 26 spaced from one another by space 30. It should be understood that a variable distance between the links 26 results in variable distance between transmission pins 24 corresponding to different states of the closed loop member 14. This in turn results in various pitch of engagement between the closed loop member 14 and engaging elements (resets) 25. As shown, that the chain of links is configured for both pivotal (Fig. 3C) and linear (Fig. 3B) displacement with respect to one another.

The full stretching ability of member 14 (i.e. stretching all along its circumference until the largest distance between the links) may be exploited for expanding the closed loop member 14 for the traction movement of the vehicle.

In this connection, it should be noted although not specifically shown that the stretching mechanism 16 preferably also includes one or more additional rollers. Such roller(s) is/are appropriately mounted with respect to the wheel structure and operable for controllable movement downwards or upwards towards lowermost (ground) or uppermost portion of the closed-loop member to be operative during an intermediate state of the traction system while being shifted during the vehicle's movement (main wheels' rotation) from the driving to traction state or vice versa. This is in order to keep a respective portion of the closed-loop member slightly spaced and thus disengaged from the main wheels for a certain (short) time during the intermediate state of the traction system. The time during which the additional roller(s) is/are operated is defined by time needed for the closed loop member to be brought to the desired engagement pitch defined by the corresponding traction state. In the operative position of the additional roller(s), the roller(s) form(s) a part of the entire circumferential surface of the traction system. It should be understood that such additional roller(s) actually constitute a temporarily disengagement mechanism allowing for holding a portion of the closed loop member while disengaged from the wheel structure and thus allowing sliding movement of that portion of the closed loop member with respect to the main wheels.

Figs. 4A and 4B show more specifically an example of the configuration of the chain of links 26 with transmission pins 24. Link 26 is a two-part integral unit, one part 26B being shorter than the other part 26A, and part 26A is shaped like a bracket defining the groove-like space 30. The arrangement is such that part 26B of one link is engageable by said space 30 of part 26A of the adjacent link. This engagement allows varying angular and linear positions of one link with respect to the other thus defining an axial connection between the links and a change of the circumferential length of the closed loop member. The engagement may be implemented against a tension of a spring element appropriately accommodated between the adjacent links. In this specific example, a spring element 32 is provided being located in a slot made in part 26B of the link and is retracted and extracted towards and away from the slot. Thus, each spring holds the coupling between two adjacent links, such that the movement of the links with respect to one another induces retraction/contraction of the spring. Springs are in their normally closed (retracted) positions maintaining the links to be tightly coupled to one another (i.e. minimal distance between them) and thus the entire closed loop member to be tightly engaged with the main wheels 12A and 12B. In the present example, the spring element 32 is configured as a leaf spring or semi-elliptical spring. As also shown in the figures, link part 26A are attached to the adjacent link (its part 26B) via a rod-like connector 29 mounted by its opposite ends in slots 27.

It should be understood that the above configuration is a non-limiting example and the invention can utilize any other configuration of the closed-loop member as well as that of the stretching mechanism.

Preferably, the traction system has an appropriate locking mechanism adapted for maintaining the circular-geometry engagement between the closed loop member and the wheel structure. This is more important for high-speed movement of the vehicle in which centrifugal forces typically apply radial force onto the closed-loop member in a direction of disengagement thereof from the wheel. The locking mechanism may utilize any suitable electrical or mechanical mechanism. For example, a certain load may be provided being attached to the spring 32 to keep it in its retracted position.

In this connection, reference is made to Figs. 5A and 5B showing an example of a locking mechanism 50. Mechanism 50 is configured to define a sliding path for rod member 29 with respect to link 26. More specifically, slot 27 has an elongated curved geometry such that rod 29 moves along a first part 27A of the slot toward a second part 27B thereof where the rod is fastened and from which it can be released to slide back along part 27A by applying a higher force. Using such a curved slot 27 prevents the undesirable expansion of the closed member 14 by requiring a specific application of the higher force to rods 29, which is critical during the movement of the traction system when centrifugal forces act on the rods 29 urging them for linear movement along the slot. Also, this arrangement facilitates controlling of the application of force to expand the closed loop member when needed. It should be understood that an appropriate mechanism is provided to unlock the mechanism 50.

Figs. 6A-6D exemplify the configuration of a securing assembly 60 suitable to be used with the locking mechanism of Figs. 5A and 5B to keep the closed loop member in its circular configuration during the movement of the traction system (i.e. corresponding to locked position of mechanism 50) and allow unlocking of mechanism 50 when needed to thereby enabling expansion of the closed loop member through non-circular states. The link 26 has an upper part 26C (seen also in Fig. 4A) containing the securing element/assembly 60. As shown in the figures, the securing element 60 is pressed downward towards the slot 27, and thus can apply a radial force onto the rod 29. The rod 29 is normally urged by the leaf spring element (not shown here) to slide from slot part 27B to and along slot part 27A. Thus, when the rod 29 is in its inoperative position being retained in the slot part 27B, and when the speed of the traction mechanism exceeds a predetermined threshold, in which centrifugal forces typically apply radial force onto the closed-loop member in a direction of disengagement thereof from the slot part 27B and thus from the wheel structure, the securing member 60 is brought into its lowermost position thus applying to the rod an appropriate force to facilitate keeping the spring 32 in its extracted position maintaining the rod in slot 27B. The securing element 60 is configured to unlock rod 29 from the curved slot 27B to slide towards and along slot part 27A. To unlock the rod 29, the securing element 60 is operated to exert a radial force one the rod 29 higher than the tension force of the spring element 32 thus acting against the spring and assisting the rod movement along slot 27A in a direction from slot 27B. Thus, as shown through Figs. 6A-6D, the locking and securing mechanisms 50 and 60 operate together to selectively keep the rod inside the slot part 27B when maintaining the circular geometry of the traction system is needed (Fig. 6A) and to controllably release the rod enabling various expended states of the closed loop member 14 (Figs. 6B, 6C) until creation of the mostly expanded state thereof (Fig. 6D).

As indicated above, the full stretching ability of the closed loop member is obtained by the stretching connection (i.e. spring-like connection) between adjacent links. The arrangement may be such that a part of one link is engageable with a corresponding part of the adjacent link. This engagement allows varying angular and linear positions of one link with respect to the other, thus defining an axial connection between the links and a change of the circumferential length of the closed loop member. The engagement may be implemented against a tension of a spring element appropriately accommodated between the adjacent links. The links may be made stretchable one with respect to the other by spring arrangements. In the above-described specific but not limiting examples, the leaf spring assembly was used.

Reference is now made to Figs. 7A-7D showing another example of the configuration of a closed-loop member suitable to be used in the traction system of the invention. In this example, the adjacent links are engaged one to another via a spring-like structure utilizing an elastic connection mechanism 70. In this specific not limiting example, a link 260 is a two-part unit, formed by link-parts 26A and 26B. Link parts 26A are connected to one another via parts 26B, where link part 26B is pivotal with respect to its associated part 26A, such that the pivotal movement results in varying a distance between the adjacent links 26 A, thus expanding and retracting the closed loop member. The pivotal movement is enabled by elastic connection mechanism 70 comprising elongated torsion member 72.

As shown more specifically in Fig. 7B, such torsion member 72 is formed by rod 29 enclosed within an elastic tubular case 74. Rod 29 is attached to link 26A. As for its elastic coating 74, it has a connection portion 76 (seen in Fig. 7C) by which it is attached to link 26B. Thus, when a twisting effect is applied to rod 29, it causes the pivotal movement of the link 26B with respect to its respective link 26A. In this specific example, the elastic connection mechanism 70 is an integral part of the link 260 by which the strechability of the closed loop member is obtained.

Referring to Figs. 8A and 8B, the configuration of a closed-loop member 14 is shown which may for example be used with the configuration of Figs. 7A- 7D. In the example of Figs. 8A-8B, adjacent links 260 and 262 are stretchably connected to one another via an elastic connection mechanism 70. The elastic connection mechanism 70 includes a first rod 29 rigidly connected (as indicated in Fig. 8A by circles) to a first elastic tubular case 80 and to the link 260, and a second rod 84 rigidly connected to a second elastic tubular case 82 at one side and to the link 262 at the other side. The tubular cases 80 and 82 are connected to one another. Using this configuration, the elastic connection mechanism 70 is not necessary a part of the links but an elastic connection between them. The first and second tubular cases 80 and 82 enable to obtain a high degree of strechability by providing a long bended/twisted rod. Linear and angular/pivotal movement of the links with respect to one another is obtained.

Referring to Figs. 9 A and 9B, the configuration of a closed- loop member

14 is shown. In the example of Figs. 9A-9B, adjacent links 260 and 262 are stretchably connected to one another via an elastic connection mechanism 70. The elastic connection mechanism 70 includes a pair of leaf springs 9OA and 9OB exerting forces one against the other and interconnecting the adjacent links. The springs 9OA and 9OB are connected to the links via a pair of double rod 92. The pair of leaf springs enables to obtain a high degree of strechability. Linear and angular/pivotal movement of the links with respect to one another is obtained.

Fig. 9A illustrates the more expanded state (non-circular) of the closed loop member 14 in which the leaf springs 9OA and 9OB are adjacent and the lower part of the double rod 92A and 92B are spaced -apart. Fig. 9B illustrates the more closed state (circular) of the closed loop member 14 in which the leaf springs 9OA and 9OB are spaced apart and the lower part of the double rod 92A and 92B are adjacent.

It should be understood that typically when the closed-loop member is expanded, the elastic connection mechanism exerts force on the closed loop member to be closed back to its normal closed state. However, by using this configuration, the closed loop member does not tend to be back in its normal closed state, because the amount of forces exerting on the closed-loop member decreases as the closed-loop member expands. This is due to the direction and the distance between the forces applied to the spring relatively to the axle of the pin. Typically, when the distance between the applied forces increases, the torque also increases. In the configuration of the present invention, as the closed loop member expands, the distance between the applied forces and the axle of the pin decreases, reducing the torque and therefore the tension applied to the closed loop member is smaller.

Reference is made to Figs. 10A-10B showing another example of a traction system 100 of the present invention. To facilitate understanding, the same reference numbers are used for identifying components common for all the examples of the invention. Traction system 100 includes a wheel structure 12 and a closed loop member 14. Wheel structure includes two substantially identical wheels 12A and 12B having parallel axes of rotation 18A and 18B. The closed loop member 14 is supported by both wheels 12A and 12B. Wheels are arranged in a spaced-apart relationship (Fig. 10B). The configuration is such that in the circular geometry state of the wheel structure 12 the wheels 12A and 12B are in their axially coinciding relative position (Figs. 1OA and 10B) and in various displaced positions their axes are displaced from one another in a direction perpendicular to said axes such that the two wheels define together a non-circular state of the wheel structure and accordingly of the closed loop member (Figs. HA and HE).

Displacement positions of the wheel structure are achieved by a movement of at least one of the wheels 12A and 12B with respect to the other. Such movement effects linear and angular orientations of the wheels' circumferences with respect to one another. This is shown in a self-explanatory manner in Figs. 1 IA-I IE, these displaced positions of the wheel structure result in various non-circular states of the closed loop member. It should be understood although not specifically shown in the figures that a stretching mechanism suitable to be used in the traction system 100 may include a drive unit (e.g. electrical drive) associated with a rotation shaft of at least one the wheels 12A and 12B to effect the wheel's movement with respect to the other, and a need for auxiliary rollers can be eliminated, because the same main wheels 12A and 12B support the closed loop member in all the states thereof.

It should also be noted although not specifically shown that the above- described examples regarding the engagement mechanism (transmission pins and engaging elements) on the closed loop member and wheel structure respectively as well as above exemplified configuration of the closed loop member can optionally be used in the traction system 100.