IL307864B2 - A Differential Unit - Google Patents

Description

A DIFFERENTIAL UNIT TECHNICAL FIELDThe invention is in the field of differential units and uses thereof. BACKGROUND OF INVENTIONA differential is a mechanical system which is designed to split power and movement from a source ("input") port to two output ("output") ports. The differential receives power and rotational movement from the vehicle's drive source engine or gearing and splits the power and rotation to the two opposing right and left wheels (in some cases on vehicles with all wheel drive, also to the front-rear). The opposing wheels are configured to rotate separately in the same rotational direction. The difference in rotation between the left and right wheels is required when the vehicle makes a turn, namely when one wheel is required to travel a greater distance than the other. The most common differential unit is an open differential. An exemplary open differential unit of the art is depicted in Fig. 1 . An open differential rotates the wheels where each wheel has traction and provides the required resistance. When one of the wheels loses traction and the other wheel maintains traction, most of the power is transferred to the wheel that has lost traction. In such a situation the vehicle may lose mobility or lose direction. This often occurs where traction is not optimal, such as dirt road, rain, snow, or road hazards, or in vehicles which high power is beyond the ability of the tire to grip the road. Unlike open differential units, a fully locked differential unit is a system with additional components such as disc members ("clutch") or direct mechanical connections that completely prevent the movement of the gears. Limited slip differentials ("LSD") are an intermediate solution. The differential is partially locked and therefore it continues to transfer part of the power and movement to the wheels even when one of the wheels loses traction. There are two main mechanical methods of creating an LSD: a clutch as disclosed in US patent no. 3,211,022, and pressure inducing member that directs friction on the gears through a helical tooth structure. See, US 3,706,2and US 2,859,641.

SUMMARY OF INVENTIONThe invention disclosed herein concerns a differential unit configured as a locked or a limited slip differential with self-opening and/or limited slip capabilities for assembling on a variety of vehicles. A " differential unit " or assembly is understood to mean a planetary arrangement of a plurality of gears that are configured to transfer input torque (e.g., from a driveline, a motor shaft, and so forth), generally laterally, to axle shafts that can rotate in a different speed from one another (a case may or may not be present depending on the structure of the differential needed). Each of the gears is a rotating circular element having a plurality of cut teeth or cogs, which mesh with another compatible gear to transmit torque and speed. The gears in a differential unit of the invention are typically gears of different sizes (diameters), or having different numbers of teeth, when the same gear module is used, and which produce a change in torque through their gear ratio. The rotational speeds and torques of any two meshing (interacting) gears differ in proportion to their diameters. The gear sets may include gears that differ in their sizes and teeth pattern, i.e., helix angle. Spur gears having a zero-helix angle and helical gears having a helix angle of 10 to degrees may be used. Independent of the type of gear used, bearings of different forms may also be used. Such include needle bearings and tapered roller bearings or any other bearing known in the art, including the gear itself that may function also as the bearing. Differential units of the invention are structured of at least three (or three or more) different gear sets, typically three gear sets, which are coaxially and parallelly aligned. Each gear set has a planetary arrangement and comprises of a plurality of gear members, including a plurality of planet gears (assembled into a multiplanet gear assembly), which may be of same or different diameters (e.g., primary and secondary planet gears, as disclosed herein) and optionally one or both of a sun gear member and a ring gear member. A carrier is used to hold the three sets. For example, a carrier may be a casing or a housing internalizing one of the three gear sets, whereby each of the other gear sets is mounted on each side of that carrier. The at least three gear sets may have a common sun gear or may have three independent and coaxially arranged sun gears. All planet gears within a given gear set are meshed such that their pitch circles roll without slipping. Each of the gear sets is typically different in at least the gears used, the gears diameters, the gears’ ratio, the teeth pattern, the gear material or composition, etc. Each of the sun gears, the planet gears and the ring gears is as known in the field. The " sun gear " refers to a gear mounted for rotation about the same axis as the rotational axis of a carrier and meshes with a number of "planet" gears arrayed around its periphery. By use of the term " planet gear " it is not intended to imply that the planet gears necessarily rotate about a sun gear (in cases where a sun gear is not needed). Yet, where a sun gear is present, the planet gears rotate about the sun gear. Where a sun gear is not present, each of the plurality of planet gears is mounted on a face of the carrier in a way that may allow rotation of the planet gears about an imaginary sun gear. A " ring gear " or an internal gear has internal teeth that mesh with all planet gears in a given gear set. The ring gear is positioned coaxially to the sun gear and to the center of the carrier, and is further provided around the planet gears in a given gear set. In such a way, the ring gear is operable to drive the planet gears, or to be driven by them. In some cases, the ring gear may be provided also with outwardly oriented teeth (external teeth) to mesh with an external unit, e.g., input or an external planetary unit. In a first aspect of the invention, there is provided a differential unit comprising at least three coaxially and parallelly aligned gear sets, each of said gear sets comprising a plurality of meshing primary planet gears and one or both of a sun gear member and a ring gear member, wherein a diameter of said planet ring gear to a diameter of the sun gear member (in some configuration the diameter of the sun gear or ring gear to the diameter of primary planet gear) in a given gear set, defines a gear ratio, such that each gear set in said at least three gear sets having a gear ratio that is different from that of another of the at least three gear set in the unit. In some configurations of a differential unit of the invention, the differential unit is configured and provided in a so-called "full configuration". In other configurations, the differential unit is configured and provided in a so-called "partial configuration". In the so-called "full configuration", each of the three gear sets comprises a sun gear, a plurality of primary planet gears and a ring gear. The three sets of gears are coaxially assembled on a carrier (typically made of aluminum, cast iron, steel or any other material that can maintain a strong structure). The sun gear is provided at the center of each gear set and configured to mesh with the teeth of all primary planet gears in a given gear set. The sun gear may be smaller or larger (in diameter or have less or more teeth) than the other sun gears of the other gear sets and mounted on the carrier frame. The carrier has a shaft for each of the planet gears that surround the sun gear and provided on a face of the carrier based on the gears’ relative sizes. The planet gears are surrounded by the ring gear. The full configuration may be structured in two different ways: 1. The sun gears of the three gear sets (three or more in total, depending on the number of gear sets) are coaxially connected as one unit and the three ring gears are separate from each other; or 2. The ring gears of the three gear sets (three or more in total, depending in the number of gear sets) are coaxially connected as one unit and the three sun gears are separated from each other. In the so-called "partial configuration", the sun gear or the ring gear may not be present. In this configuration, the planet gears of the three or more gear sets are connected coaxially as one unit and mounted on the carrier frame, such that two or more of the united planet gears mesh with the teeth of the sun gear or the ring gear in a given gear set, depending on whether the sun gear or the ring gear is present. A second group of planet gears, referred to herein as secondary planet gears, may be included and mounted on the carrier, such that the ring gear or the sun gear meshes with them but not with the united planet gears. The planet gears from the united group mesh with the secondary planet gears in a given gear set. In such a configuration, it is possible to combine a gear set that is absent of a ring gear with a gear set that is absent of a sun gear, in cases the ring gear and the sun gear that exist mesh a different group of planet gears. In a differential unit, the power and movement are transferred to each of two output ports which are associated with the differential unit. The transfer of power and the resulting movement overcome the resistance of both output ports and thus becomes possible when forces are applied to them in opposite directions. This is achieved by an assembly of three (or more) gear sets that are of different gear ratios. The term " gear ratio ", as used herein, generally means a size ratio or a diameter ratio between different gear members in a given gear set. When used in connection with a "full configuration", the gear ratio is a ratio between the diameter of the ring gear to the diameter of the sun gear in a given gear set. The gear ratio is different between the three or more gear sets. When used in connection with a "partial configuration" the term gear ratio is the diameter ratio between the ring gear to a planet gear (assuming all planet gears are of the same diameter) in a given gear set or the diameter ratio between the sun gear to a planet gear (assuming all planet gears are of the same diameter) in a given gear set. This ratio as well is different for each of the three or more gear sets. In cases where the planet gears comprise both primary planet gears (or united planet gears) and secondary planet gears (not united and potentially of a diameter different than that of the primary planet gears), the gear ratio determined for a differential in the partial configuration is based on the primary (or united) planet gears.

As the same gear type has to be used in a given gear set, the gear ratio can alternatively refer to a ratio of gear teeth rather than to a ratio of the diameters. As stated herein, the differential units of the invention are characterized by three or more gear sets, wherein each of the gear sets has a different gear ratio. For example, in a differential unit having three gear sets which are coaxially and parallelly aligned around a common axis, the gear ratio for each gear set may be designated R1, R2 and R3, wherein R1, R2 and R3 are different. The order in which the three gear sets are arranged may be different. The gear set having the largest gear ratio may be positioned in the middle between the two other gear sets (each of a smaller gear ratio) or may be positioned as a side gear set, namely not between the two other gear sets. In other words, in a differential unit having a first gear set having the largest ratio (R1), a second gear set with a middle gear ratio (R2) and a third gear set with the smallest gear ratio (R3), i.e., (R1 > R2 > R3), the three gear sets may be ordered R1-R2-R3, or R1-R3-R2, or R2-R1-R3, etc. thus, reference to a gear set having a "middle gear ratio" should be understood as a reference to any of the gear sets, independent of its location (middle gear or a side gear), which has a gear ratio that is not the largest (largest gear ratio) nor the lowest (lowest gear ratio) in a given differential unit. In some embodiments, in a differential unit of the invention, the gear set of the largest gear ratio is positioned between the two gear sets of the smaller ratios (e.g., R2-R1-R3). As a person of skill would understand, in order to achieve a different gear ratio between two similar planetary gear sets, at least one component that is relevant to the ratio determination of a given gear set has to be different in size. In other words, in a given gear set, changing the size of one component of the gear set requires changing the size of another one or more component in the same gear set. Since the planet gears of all gear sets are joined and coaxial when a partial configuration is used, as described herein, the changing of the ratio by changing one of the relevant sizes dictates changing of another relevant component. On the other hand, in a full configuration, it is possible to change the ratio by changing only one relevant component size. Since in a full configuration the planet gears spin freely and independently of the gear ratio, it is possible to change their size to adapt the change of the relevant component in a given gear set. In configurations wherein the ring gears are not connected to each other, one of the three ring gears used acts as an "input port" and the two others as two different "output ports" as further discussed below. The ring gear of a gear set having a middle gear ratio is connected either directly or through the differential case to an input source and each of the other two ring gears are connected each to a separate output shaft that is connected to a respective axle (e.g. left-right wheels or front-rear axles.) In configurations where the sun gears are not connected to each other, the sun gear of a gear set with the middle gear ratio acts as an "input port" and the two others as two different "output ports". As indicated herein, the differential unit of the invention comprises at least three gear sets. In some configurations, a middle gear set may be provided as two sub-sets which are united and operable as a single gear set. In a differential unit comprising three gear sets, said gear sets being a central (or middle) gear set, and two side gear sets adjacent and parallel to the central gear set; each of the gear sets being characterized by a different gear ratio, e.g., at a configuration in which the input is connected to the central gear set, the gear ratio of the central gear set should be greater than that of one of the side gear sets and less than that of the other side gear set; the gear ratios are predetermined. Applying power to the central gear set causes the power to be distributed through the side gear sets so that the differential is in a partially locked state and partially open state. The differential switches between the partially locked state to a partially open state as the difference between the predetermined side gear set ratios and internal friction of the differential. Therefore, the switching is controlled internally by the differential according to conditions. The advantage in the different gear ratios is dramatic. As a person of skill will understand, in a system of two gear sets with the same gear ratio, each comprising a sun gear that is mounted on a common carrier, with a pivot connecting and uniting the two sun gears, a force applied to one ring gear in one direction and an equal force applied on the other ring gear in the opposite direction will lock the system. The two gears will not be able to rotate in opposite directions. Even if the force is applied directly on the carrier, rather than to each of the planetary gears, so the carrier moves relative to the ring gear, there will still not be any movement between the two ring gears, because the gear ratios are same. Where the forces applied are different, the system will remain locked, and will move as one unit in a direction of the greater force. However, in the configuration of the invention any two gear sets are of different gear ratios, so that when the force is applied to a ring gear of the central gear set against a ring gear of one of the side gear sets, the system will be locked. On the other hand, when a force is applied on the ring gears of the side gear sets, in opposite directions, a different rotation will occur and in addition, an accelerated movement of the carrier and the sun gears relative to the ring gears will occur, causing a rotation within the system in the direction of the ring gear of the gear set having a larger gear ratio. This configuration is highly advantageous in cases where one of a vehicle’s wheels loses traction, as described above. The extent of differences between the gear ratios depends on two main parameters: the degree of variation in the relations between any two adjacent gear sets (one side gear set versus the central gear set and the second gear set versus the central gear set) and the inherent friction in the system. Generally speaking, and without wishing to be bound by theory, the greater the coefficient of friction that is built into the system, the greater the degree of variation in the relationship between any two gear sets that will be required for the system to allow a movement between two ring gears when the power is applied to them (namely when moving from a locked state to an open state). Accordingly, the greater the difference in the gear ratio between any two gear sets, the more "open" the system is, so that when an opposing force is applied between the ring gears in the adjacent gear sets, the greater the power that is transferred to drive the carrier and the sun gear. A range of gear ratios allow the differential unit to transition from a partially locked state to a partially open state. This is analogous to a phase change. The differential has a balance point in the range of gear ratios of approximately 5 and 60 %. In other words, the difference in gear ratios, as expressed, e.g., by R1 > R2 > R3, may be reflected in a ratio difference that is between 5 and 60%, or which is between 5 and 30%. A variation in the ratios that is smaller than 5 and 15% or below 10 and 15% should leave the system in a locked state and a ratio of between 10 and 30% results in a limited slip state. Greater ratios up to about 60% may leave the system in an open state. Thus, in some configurations, and depending on a variety of factors known in the art, the ratio difference may be between 5 and 25%, or between 5 and 15%. The terms "locked state", "open state" and "limited slip state" are known in the art. Basically, in a locked differential both wheels on an axle are locked together, thereby forcing both left and right wheels on the same axle to rotate at the same speed under nearly all circumstances, regardless of the tractional differences at either wheel. In the open state, each wheel is capable of rotating at different speeds. An open differential provides the same torque to each of the two wheels on the axle. This means that although the wheels can rotate at different speeds, they apply the same rotational force, even if one wheel is stationary and the other is spinning. Unliked both open and locked differentials, limited-slip differentials (LSD) are a compromise between an open differential and a locked differential. In the LSD case extra torque is directed to the wheel with the most traction. As stated herein, depending on the gear ratio, the transition between the locked and open differentials may be effectively achieved. It is important to note that the transition between the states is not a stable phase state. As the upper ratio limit is approached and even beyond the locking range, resistance to opening the system occurs. As the difference in the ratios between the side gears is greater than 15%, the differential will become more open. In this way, by choosing a difference in the gear ratios between the gear sets, varying differential functions may be obtained for varying implementations. In some configurations of a differential unit, the unit may comprise means for increasing friction resulting from a force transferred from the input port to one of the output ports. Such means may, for example, be bearings and helical gears of different angles, as described herein. Independent of the type of gears used, bearings of different forms may also be used. The bearings may be needle bearings and tapered roller bearings or any other bearing known in the art, including the gear itself that may function also as the bearing. In configurations of differential units or any specific arraignment of the invention as disclosed herein such bearings may be mounted on some or all of the planet gears. Alternatively, some of the planet gear may be structured as bearings. Each gear set may optionally make use of a different type of bearing. For example, sliding bearings may be used in the planet gears of a middle ratio gear set, while roller bearings may be used in the planet gears of the other gear sets, to thereby increase the friction resulting from the force transferred from the input port to one of the output ports. In such a configuration, for example, a degree of system locking may result, even in configurations designed as open differential configurations. Alternatively, as stated herein, helical gears and/or helix angle gears may be used, e.g., in a middle gear ratio, to increase friction resulting from the force transferred from the input port to one of the output ports. As with the use of bearings, this presents another way by which a degree of system locking may be achievable, even in configurations designed as open state configurations. Different helical gears may be used. For example, different helical gears may be used in different gear sets. The difference between the helical gears may be in the helix angle. Spur gears having a zero-helix angle and helical gears having a helix angle of 10 to 30 degrees may be used in different gear sets. For example, a differential unit may comprise a middle gear set or a gear set with a middle gear ratio, as defined, having helical gears of an angle larger than helical gears of the side gear sets.

In some configurations, a differential unit of the invention is configured and operable to mesh with an auxiliary external gear unit acting as a secondary input port. In such an arrangement, for example, a ring gear of the gear set acting as a primary input port may be provided with internal teeth meshing with teeth of the planetary gears of the same gear set and with external teeth permitting meshing with a ring gear of the external gear set. In such configurations, the auxiliary external gear set may comprise a plurality of planetary gears, a sun gear (acting as the secondary input port or as an input source) and a ring gear that meshes with the external teeth of the ring gear of the gear set acting as a primary input port in the differential unit. The auxiliary external gear set may also be provided with a carrier. Optionally, the carrier of the differential unit and a carrier of the auxiliary external gear set comprise meshing external teeth. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a simplified block diagram of prior art. Figs. 2A-C provide simplified depictions of three different gear arrangements according to some embodiments of the invention. Fig. 3 is a simplified diagram of a partial view of a common carrier. Fig. 4 is a simplified diagram of a partial view of three coaxially united sun gears. Fig. 5 is a simplified diagram of a full configuration differential with a structure of three ring gears that are not connected to each other and used as input and output ports, installed with an input and output ports according to some embodiments of the invention. Fig. 6 is a simplified depiction of a partial configuration of a differential unit absent of sun gears according to some embodiments of the invention. Fig. 7 is a simplified depiction of a partial configuration of a differential unit absent of ring gears according to some embodiments of the invention. DETAIL DESCRIPTION OF INVENTIONAn advantage of the invention is that the differential unit disclosed behaves as a switchable open and locked differential, yet without the friction that many components introduce, among these friction pads and clutches, electrical, mechanical, or pneumatic elements to switch from the locked to the open state. The multiple gear sets provide lower maintenance and more reliable differential.

As described, the operation of the novel differential unit will be described based on a structure in which the middle ratio gear set is positioned at the center (central gear set), and the two other gear sets adjacent and parallel to the central gear set from its both sides (side gear set). Also, the structure chosen to describe the operation is based on the "full configuration", where the three ring gears are not connected to each other and used as input and output ports. A central gear set arranged around a central axis and located in the middle slot of a common carrier that can carry three different gear sets; two side gear sets, each gear set coaxially mounted on a face of the common carrier; each of the gear sets having two or more planetary gears independently mounted on the common carrier and mesh both the ring gear and the sun gear of its gear set. The three sun gears are connected coaxially to each other as one unit. The ratio of the central gear set (R2) is greater than the ratio of one of the side gears (R3) and less than the ratio of the other side gear (R1). In this configuration, the central ring gear is connected to the differential case and used as the "input port" of the differential unit that may connect to the power source of the vehicle. The side ring gears are used as the "output ports" of the differential unit that may be connected to the vehicle wheels. The differential can be designed to split the rotation of the output evenly (or symmetrically), namely 50/50 or asymmetrically, e.g., 70/30 etc. In order to achieve a symmetrical rate of rotation, the calculation of the gear ratio rate should be on a basis of an identical reference plane of one component and then adding or reducing the same measure from the other component. For example, in a given gear set according to Fig. 2 , all ring gear sets are 90mm, the sun gear of the middle ratio gear set is 30mm. Since the smaller ratio sun gear is larger by 4mm (34mm) from the middle ratio sun gear, the greater ratio sun gear should be smaller by the same measure (4mm) so its size should be 26mm in order to maintain a symmetrical opposite rotation of the side gear ring gears relative to the middle gear set ring gear. A prior art differential unit is shown in Fig. 1 and generally describes an open differential 100 with a drive train or engine source 102 with a bevel gear 104 meshing with a crown gear 104 that is connected to a rotating case 106. This in turn rotates bevel gears 1and 108. These gears mesh with bevel gears 112 and 114 that are connected to axles 116 and 118. Figs. 2A-C show three different arrangements of sun gears and planet gears, the size of which changes based on the diameter of the sun gear. As an illustration, in each of the three sets depicted in Figs. 2A-C the gear ratios are different. As the diameter of the sun gear increases, maintaining the diameter of the rung gear constant, the diameter of each of the planet gears must be reduced. In the examples three gear sets, a ratio R between a ring gear K (constant) and a sun S is different. For example, in Fig. 2A , the diameter of sun gear S1 = 26, the diameter of the ring gear K1 = 90, and thus the ratio R1= K1/S1 is greater than 3. In Fig. 2B , the diameter of the sun gear S2 = 30, the diameter of the ring gear is K2 = 90, and the ratio R2=K2/S2 is 3. Similarly, in Fig. 2C , the diameter of the sun gear S3 = 34, the diameter of the ring gear K3 = 90, and the ratio R3=K3/S3 is smaller than 3. Fig. 3 schematically depicts a carrier arrangement 500 with three gear sets: a side gear set 510, a central gear set 520 and a second side gear set 530. Each gear set is equipped with planet gears 502A and 502B in gear set 510; gears 504A and 504B in gear set 520; gears 506A and 506B in gear set 530. Each of the gears 502-506 is mounted on a common carrier 508 so that the gears can move freely. This configuration is "full configuration" and thus each of the planet gears is independently mounted on a face of the carrier. Fig. 4 illustrates a sun gear arrangement 600 comprising three sun gears 602, 604 and 606 that are coaxial and joined with a rotatable member. The size of each of the sun gears is different, requiring planet gears of different diameters. Fig. 5 depicts differential 700 in the context of the input port T2 and output ports Tand T3. The drive train or engine source 702 has a bevel gear meshing with a gear on the crown 703 connected to the case 704. Ring gears 706, 708 and 710 mesh with planet gears 712A, 712B, 714A, 714B, 716A and 716B. Sun gears 718, 720 and 722 cooperate with the corresponding planet gears. The diameter of the sun gears is different, while the diameters of the ring gears are the same. This results in different gear ratios, wherein the gear ratio of the central gear set B is greater than the gear ratio of gear set C and smaller than the gear ratio of gear set A. As disclosed herein, sun gears or ring gears may or may not be present. Fig. 6 depicts a differential unit 800 similar to that depicted in Fig. 5 , but without sun gears. Fig. 7 depicts a differential unit 900 that is absent of ring gears. In the configurations depicted in Figs. 6 and 7 , the planet gears of the three gear sets are united and connected coaxially as one unit and mounted on the carrier frame. It is to be noted that in Fig. 7 , T2 defines an input port which is associated with sun gear 710 such that rotation of T2 causes rotation of the sun gear 710 and subsequent rotation of the meshing gear sets.

Claims (32)

1. -12- 307864/

2. CLAIMS 1. A differential unit comprising three coaxially and parallelly aligned gear sets, each of said gear sets comprising a plurality of meshing primary planet gears, a sun gear member and a ring gear member, wherein a diameter of the ring gear member to the sun gear member defines a gear ratio, such that each gear set of said three gear sets having a gear ratio that is different from that of another of the three gear set in the unit, wherein a middle gear set having a middle gear ratio to gear ratios of side gear sets, the middle gear set being connected directly or through a differential case to an input source, such that applying power to the middle gear set causes the power to be distributed through the meshing of the ring gear, the plant gear and the sun gear of the middle gear set to the sun gear, plant gear and ring gear members of each of side gear sets, and wherein the ring gear members of the said gear sets are connected each to a separate output shaft connected to a respective axle. 2. The unit according to claim 1, wherein the at least three coaxially and parallelly aligned gear sets are mounted on a carrier.

3. The unit according to claim 1, wherein the at least three coaxially and parallelly aligned gear sets comprise three independent and coaxially arranged sun gears.

4. The unit according to claim 1, wherein the at least three coaxially and parallelly aligned gear sets comprise three independent and coaxially arranged ring gears.

5. The unit according to claim 1, wherein the at least three coaxially and parallelly aligned gear sets comprise three independent sets of planet gears.

6. The unit according to claim 1, wherein the sun gear members of the three gear sets are coaxially connected as one unit and the three ring gear members are separated from each other.

7. The unit according to claim 2, wherein the sun gear is mounted for rotation about a rotational axis of the carrier and meshes with the plurality of primary planet gears arrayed around its periphery.

8. The unit according to claim 1, wherein the ring gear has internal teeth meshing with all primary planet gears in a given gear set.

9. The unit according to claim 1, wherein the ring gear is positioned coaxially to the sun gear and around the primary planet gears in a given gear set, and is operable to drive the primary planet gears or is operable to be driven by the primary planet gears. -13- 307864/

10. The unit according to any one of the preceding claims, wherein the each of the different gear ratios is selected to allow the unit to transition from a partially locked state to a partially open state.

11. The unit according to any one of the preceding claims, wherein the different gear ratios reflect a ratio difference of between 5 and 60 %, or between 5 and 30%, or between 5 and 15%, or between 5 and 10%.

12. The unit according to any one of claims 1 to 11, being a locked differential unit, an open differential unit or a limited slip differential unit.

13. The differential unit according to claim 1, comprising at least three coaxially and parallelly aligned gear sets, each of said gear sets comprising a plurality of meshing primary planet gears, a sun gear member and a ring gear member, wherein a diameter of the ring gear member to the sun gear member defines a gear ratio, such that where a difference in gear ratios between the at least three gear sets is between 5 and 15 %, the unit is locked; or where a difference in gear ratios between the at least three gear sets is between 10 and 30 %, the unit is limited slip; or a difference in gear ratios between the at least three gear sets is between and 60%, the unit is open.

14. The unit according to any one of the preceding claims, the unit comprising means for increasing friction resulting from a force transferred from an input port to an output port.

15. The unit according to any one of the preceding claims, wherein the planet gear members comprise a plurality of different types of bearings.

16. The unit according to claim 14, wherein the bearing is selected from needle bearings and tapered roller bearings.

17. The unit according to claim 15, wherein the planet gears of the middle ratio gear comprise or consist sliding bearings.

18. The unit according to claim 15, wherein planet gears of the other gear sets comprise or consist roller bearings.

19. The unit according to claim 15, wherein planet gears of the middle ratio gear comprise or consist sliding bearings, and wherein planet gears of the other gear sets comprise or consist roller bearings.

20. The unit according to claim 1, wherein the unit comprises helical gears.

21. The unit according to claim 1, wherein the unit comprises helix angle gears.

22. The unit according to claim 1, wherein the middle gear set having a middle gear ratio comprises planet gears comprising or consisting sliding bearings, and planet gears of the other gear sets comprising or consisting roller bearings. -14- 307864/

23. The unit according to claim 1, wherein the middle gear set comprises helical gears.

24. The unit according to claim 1, wherein the middle gear set has helical gears of an angle larger than helical gears of the side gear sets.

25. The unit according to claim 1, wherein the middle gear set having a middle gear ratio comprises planet gears comprising or consisting sliding bearings, and planet gears of the other gear sets comprising or consisting roller bearings.

26. The unit according to claim 1, wherein the middle gear set comprises helical gears.

27. The unit according to claim 1, wherein the middle gear set has helical gears of an angle larger than helical gears of the side gear sets.

28. The unit according to any one of the preceding claims, configured and operable to mesh with an auxiliary external gear unit acting as a secondary input port.

29. The unit according to claim 28, wherein a ring gear of the unit gear set acting as a primary input port is provided with internal teeth meshing with teeth of the planetary gears of the same gear set and with external teeth permitting meshing with a ring gear of the external gear set.

30. The unit according to claim 28, wherein the auxiliary external gear set comprises a plurality of planetary gears, a sun gear member acting as the secondary input port and a ring gear member that meshes with the external teeth of the ring gear of the gear set acting as a primary input port in the differential unit.

31. The unit according to claim 2, wherein carrier of the differential unit and a carrier of an auxiliary external gear set comprise meshing external teeth.

32. The unit according to claim 30, wherein carrier of the differential unit and a carrier of the auxiliary external gear set comprise meshing external teeth.