Technical Field

[0001] The present invention relates to a power transmission system. For example, a power transmission system for transmitting mechanical power.

Background

[0002] A power transmission system may generally be used to provide mechanical advantage between an input, e.g. an input gear and an output, e.g. an output gear. The power transmission system may be a mechanical system formed by a plurality of gears, e.g. mounted on a frame. Power transmission system may be a gear down or speed reducer transmission, whereby the output gear turns slower than the input gear, such that the torque is increased between the input and output. Power transmission system may be a gear up or speed increaser transmission, whereby the output gear rotates faster than the input gear such that the torque is decreased between the input and output.

[0003] A power transmission system may be a gear train where a plurality of gears are meshed together to transmit power between an input gear and an output gear. Conventionally, there are various types of toothed gear trains, e.g. spur gear, helical gear, double helical gear, spiral bevel gear, epicyclic gear, worm gear, hypoid gear. Each type of gear train has a typical gear ratio range and are subject to a power loss between the gears. The gear or speed ratio for a pair of meshing gears can be computed from the ratio of the radii of the pitch circles or the ratio of the number of teeth on each gear. A summary of the gear ratio range and the power loss range for each type of gear train is provided below:

Spiral Bevel Gear 1: 1 to 4: 1 1-3%

Epicyclic Gear 3:1 to 12: 1 2-6%

Worm Gear 5:1 to 75: 1 2-50%

Hypoid Gear 10:1 to 200: 1 4-15%

[0004] In addition, there are non-gear train technologies available. Examples of non-gear train technology are cycloidal drive and harmonic drive. The ratio range and power loss range for each of the mentioned type are shown below:

[0005] As shown above, the current available transmissions are limited in the maximum ratio they are able to provide in a single stage. For example, in the current gear train technology, the ratio of the pitch circles of mating gears defines the speed or transmission ratio of the gear set. Therefore, the maximum gear ratio is limited by the maximum and minimum size of the gears. For cycloidal drive and harmonic drive, the maximum ratio is limited by the maximum number of splines or pins that may be built into its outer ring.

[0006] In order to achieve higher gear ratios with current technology, it is required to "stack" multiple stages of gear sets. The primary cause of inefficiency in gear trains is friction between mating gear faces. Therefore, each additional stage of gear set increases power loss. Furthermore, stacking stages of gear sets increases complexity, cost, size, and weight of the gear trains.

[0007] While there are existing forms of gear train that are able to provide a relatively higher gear ratio in a single stage, e.g. worm gear, and hypoid gear, there are disadvantages in such gear trains. Such gear trains have sliding interfaces between the moving parts and the sliding interfaces create friction thereby causing them to be relatively inefficient. In such cases, frequent careful lubrication is required to reduce friction. Furthermore, it is generally not possible or practical to backdrive these types of transmissions at higher ratios. Some forms of gear trains, e.g. double helical gear, may only be stable when rotating in one direction as the gear profile causes parts to thrust apart when rotated in the other direction.

[0008] Existing gear trains are limited in the gain they can achieve in a single stage, and therefore require multiple stages to be connected together for higher ratios. This again increases size, complexity, and power loss.

[0009] It is therefore beneficial to provide a power transmission system that overcomes the abovementioned disadvantages.

Summary

[0010] According to various embodiments, a power transmission system is provided. The power transmission system includes a primary transmission train having a ring gear, a sun gear within the ring gear, and a planet gear rotatably attached to a planet gear carrier, such that the planet gear is coupled to and disposed between the sun gear and ring gear. The power transmission system includes a feedback transmission train having a first wheel and a second wheel having a feedback transmission ratio therebetween. The power transmission system has a first primary wheel being one of the sun gear, the ring gear and the planet gear carrier, such that the first primary wheel is rotationally synchronized with the first wheel, a second primary wheel being one of the sun gear, ring gear and planet gear carrier that is not synchronized with the first wheel, such that the second primary wheel is rotationally synchronized with the second wheel, such that the first primary wheel and the second primary wheel have a primary equivalent transmission ratio therebetween, and a third primary wheel being one of the sun gear, the ring gear and the planet gear carrier that is not synchronized with any one of the first wheel and the second wheel of the feedback transmission train. The power transmission system has a first synchronized wheel set including the first primary wheel and the first wheel, a second synchronized wheel set including the second primary wheel and the second wheel and an unsynchronized wheel comprising the third primary wheel, the first synchronized wheel set and the second synchronized wheel set are adapted to rotate at the feedback transmission ratio between each other, such that the unsynchronized wheel is adapted to rotate with respect to the first synchronized wheel set.

[0011] According to various embodiments, the first primary wheel and the second primary wheel have a primary equivalent transmission ratio therebetween and the primary equivalent transmission ratio may be different from the feedback transmission ratio.

[0012] According to various embodiments, the primary transmission train may include an epicyclic gear train having a plurality of the planet gear, each rotatably attached to the planet gear carrier.

[0013] According to various embodiments, the first wheel may be engaged to the second wheel directly.

[0014] According to various embodiments, the feedback transmission train may include an epicyclic gear train having a feedback ring gear, a feedback sun gear within the feedback ring gear and a feedback planet gear rotatably attached to a feedback planet gear carrier, such that the feedback planet gear may be coupled to and disposed between the feedback sun gear and the feedback ring gear, and such that the first wheel and the second wheel may include any two of the feedback ring gear, the feedback sun gear and the feedback planet gear carrier.

[0015] According to various embodiments, any one of the feedback ring gear, the feedback sun gear and the feedback planet gear carrier may be rotationally fixed.

[0016] According to various embodiments, the feedback transmission ratio may be adapted to be varied.

[0017] According to various embodiments, the unsynchronized wheel may be adapted to rotate slower than any one of the first synchronized wheel and the second synchronized wheel. [0018] According to various embodiments, one of the first synchronized wheel set and the second synchronized wheel set may include an input wheel, and such that the unsynchronized wheel may be an output wheel.

[0019] According to various embodiments, the unsynchronized wheel may include an input wheel, and such that one of the first synchronized wheel set and the second synchronized wheel set may include an output wheel.

[0020] According to various embodiments, the primary transmission train may include a tooth-typed gear train.

[0021] According to various embodiments, the feedback transmission train may include at least one of spur gear type, helical gear type, herringbone type, sprocket and chain type, pulley and belt type.

[0022] According to various embodiments, the sun gear may be offset from the centre of the ring gear

[0023] According to various embodiments, the feedback sun gear may be offset from the centre of the feedback ring gear.

[0024] According to various embodiments, the sun gear and the planet gear may be rotatably connected to the planet gear carrier and the feedback sun gear and the feedback planet gear may be rotatably connected to the feedback planet gear carrier.

[0025] The present invention relates to a power transmission system, for direct transmission, which is able to provide relatively high fixed ratios, either in a gear up or gear down configuration, in a relatively simple compact package with minimised power loss and backlash.

[0026] The power transmission system of the present invention may be applied anywhere where an efficient and/or low backlash high ratio transmission is desired. For example, turbines and generators, powered vehicles, mechanical energy storage applications, precision measurement, high leverage tools and manufacturing.

Brief Description of Drawings

[0027] Fig. 1 shows an example of a power transmission system. [0028] Fig. 2 shows another example of the power transmission system. [0029] Fig. 3 shows another example of the power transmission system.

[0030] Fig. 4 shows an example of the primary transmission train.

[0031] Fig. 5 shows an exploded view of an example of the power transmission system.

[0032] Fig. 6 shows a front view of the power transmission system in Fig. 5.

[0033] Fig. 7 shows a rear view of the power transmission system in Fig. 5.

[0034] Fig. 8 shows an example of a power transmission system.

[0035] Fig. 9 shows an exploded view of the power transmission system in Fig. 8.

[0036] Fig. 10 shows a front view of the power transmission system as shown in Fig. 8.

[0037] Fig. 11 shows a rear view of the power transmission system as shown in Fig. 8.

Detailed Description

[0038] In the following examples, reference will be made to the figures, in which identical features are designated with like numerals. [0039] Fig. 1 shows an example of a power transmission system 10. Power transmission system 10 has a primary transmission train 100 having a ring gear 102, a sun gear 104 within the ring gear 102, and a planet gear 106 rotatably attached to a planet gear carrier 108, such that the planet gear 106 is coupled to and disposed between the sun gear 104 and ring gear 102. Power transmission system 10 has a feedback transmission train 120 having a first wheel 122 and a second wheel 124 having a feedback transmission ratio therebetween. A first primary wheel 132 being one of the sun gear 104, the ring gear 102 and the planet gear carrier 108 is rotationally synchronized with the first wheel 122. A second primary wheel 134 being one of the sun gear 104, ring gear 102 and planet gear carrier 108 that is not synchronized with the first wheel 122 is rotationally synchronized with the second wheel 124. First primary wheel 132 and the second primary wheel 134 of the primary transmission train 100 have a primary equivalent transmission ratio therebetween. Power transmission system 10 has a third primary wheel being one of the sun gear 104, the ring gear 102 and the planet gear carrier 108 that is not synchronized with any one of the first wheel 122 and the second wheel 124 of the feedback transmission train 120. Power transmission system 10 has a first synchronized wheel set 152 that includes the first primary wheel 132 and the first wheel 122, a second synchronized wheel set 154 that includes the second primary wheel 134 and the second wheel 124, and an unsynchronized wheel that includes the third primary wheel. First synchronized wheel set 152 and the second synchronized wheel set 154 are adapted to rotate at the feedback transmission ratio between each other, such that the unsynchronized wheel is adapted to rotate with respect to the first synchronized wheel set when the feedback ratio differs from the value of the equivalent feedback ratio.

[0040] Fig. 1 shows an example of the power transmission system 10 with the sun gear 104 and the ring gear 102 of the primary transmission train 100 being connected to the first wheel 122 and the second wheel 124 respectively. In this example, the sun gear 104 may be the first primary wheel 132, the ring gear 102 may be the second primary wheel 134 and the planet gear carrier 108 may be the third primary wheel 135. First wheel 122 and the second wheel 124 may be part of the feedback transmission train 120. As shown in Fig. 1, the first primary wheel 132 may be rotationally synchronized with the first wheel 122 such that both of them are adapted to rotate at the same speed. First primary wheel 132 may be connected to the first wheel 122. First primary wheel 132 may be on a common axle as the first wheel 122. For example, the first primary wheel 132 may be on the same shaft as the first wheel 122. Similarly, the second primary wheel 134 may be rotationally synchronized with the second wheel 124 such that both of them rotate at the same speed. Second primary wheel 134 may be connected to the second wheel 124. Second primary wheel 134 may be on a common axle as the second wheel 124. For example, the second primary wheel 134 may be on the same shaft as the second wheel 124. Third primary wheel 135 may be left to rotate freely on an independent axle. As such, the third primary wheel 135 may be an unsynchronized wheel as it is not rotationally synchronized with any wheels in the feedback transmission train. It can be seen that the first primary wheel 132 and the first wheel 122 may form a first synchronized wheel set 152 and the second primary wheel 134 and the second wheel 124 may form a second synchronized wheel set 154. In a synchronized wheel set, the wheels may be rotatably fixed to each other so as to form a unitary piece. For example, the wheels may be attached to a common axle or a shaft. When the first synchronized wheel set and the second synchronized set rotates, the unsynchronized wheel may rotate with respect to the first synchronized wheel set, and vice versa.

[0041] Referring to Fig. 1, either the first primary wheel 132, i.e. the sun gear 104, or the second primary wheel 134, i.e. the ring gear 102, may be an input wheel adapted to drive the primary transmission train 100. Consequently, the third primary wheel 136 may be rotated as an output wheel due to the rotation of the first primary wheel 132 or the second primary wheel 134. As such, the rotating third primary wheel 135 may be the output wheel to drive a component, e.g. a hoist. The transmission ratio between the input wheel, i.e. the first primary wheel 132 or the second primary wheel 134, to the third primary wheel 135 may be denoted as power transmission ratio of the power transmission system 10. It can therefore be seen that the first synchronized wheel set 152 and the second synchronized wheel set 154 may be rotated at the feedback transmission ratio.

[0042] Fig. 2 shows another example of the power transmission system 10. In the power transmission system 10 of Fig. 2, the first primary wheel 132 may be the planet gear carrier 108. Planet gear carrier 108 may be rotatably synchronized to the first wheel 122 to form the first synchronized wheel set 152. Second primary wheel 134 may be the ring gear 102. Ring gear 102 may be rotatably synchronized to the second wheel 124 to form the second synchronized wheel set 154. Sun gear 104 may be the third primary gear 135. Unsynchronized wheel 136 may be the sun gear 104 which may be left to rotate freely on an independent axle. Accordingly, the first synchronized wheel set 152 and the second synchronized wheel set 154 may be rotated at the feedback transmission ratio and the unsynchronized wheel 136 may be the output wheel.

[0043] Fig. 3 shows another example of the power transmission system 10. In the power transmission system 10 of Fig. 3, the first primary wheel 132 may be the sun gear 104. Sun gear 104 may be rotatably synchronized to the first wheel 122 to form the first synchronized wheel set 152. Second primary wheel 134 may be the planet gear carrier 108. Planet gear carrier 108 may be rotatably synchronized to the second wheel 124 to form the second synchronized wheel set 154. Ring gear 102 may be the third primary gear 135. Unsynchronized wheel 136 may be the ring gear 102 which may be left to rotate freely on an independent axle. Accordingly, the first synchronized wheel set 152 and the second synchronized wheel set 154 may be rotated at the feedback transmission ratio and the unsynchronized wheel 136 may be the output wheel.

[0044] As shown in the above figures, the power transmission system 10 may include two portions. The first portion being the primary transmission train 100 and the second portion being the feedback transmission train 120. Primary transmission train 100 may include three primary wheels 132,134,135 of which any two of the three primary wheels 132,134,135 in the primary transmission train 100 may be synchronized to the two wheels 122,124 in the feedback transmission train 120. As described above, the first primary wheel 132 and the first wheel 122 and the second primary wheel 134 and the second wheel 124 may be defined as the first synchronized wheel set and the second synchronized wheel set respectively. Third primary wheel 135 may not be synchronized to the first wheel 122 or the second wheel 124 of the feedback transmission train 120 and therefore is denoted as the unsynchronized wheel 136.

[0045] Power transmission system 10 may therefore include the three primary wheels 132,134,135 that may turn at fixed speed ratios with respect to each other. Further, each of the first synchronized wheel set 152, the second synchronized wheel set 154 and the unsynchronized wheel 136 may be engaged with external inputs and outputs for transferring drive and torque between the inputs and outputs and the speed ratios between the rotating wheels may be utilised to provide mechanical advantage between inputs and outputs. A wheel that is connected to an input may be denoted as an input wheel and a wheel that is connected to an output may be denoted as an output wheel. As such, one of the first synchronized wheel set 152 and the second synchronized wheel set 154 may include the input wheel, and the unsynchronized wheel may be the output wheel. Alternatively, the unsynchronized wheel may include the input wheel, and one of the first synchronized wheel set 152 and the second synchronized wheel set 154 may include the output wheel. For example, one of the first primary wheel 132 and the second primary wheel 134 may be the input wheel and the unsynchronized wheel 136 may be the output wheel and vice versa, i.e. the unsynchronized wheel 136 may be the input wheel, and one of the first primary wheel 132 and the second primary wheel 134 may be an output wheel. As the first primary wheel 132 and the second primary wheel 134 may be on the same axle as the first wheel 122 and the second wheel 124 respectively, the first wheel 122 or the second wheel 124 may be the input wheel or the output wheel. A more detailed explanation on how to obtain the power transmission ratio between the input and the output will be shown below.

[0046] As mentioned, the first primary wheel 132 and the second primary wheel 134 may have a primary equivalent transmission ratio. Depending on which of the sun gear 104, the ring gear 102 and the planet gear carrier 108 are the first primary wheel 132 and the second primary wheel 134, the primary equivalent transmission ratio may be determined from the number of teeth of the sun gear 104 and the ring gear 102. First wheel 122 and the second wheel 124 may have a feedback transmission ratio. When the primary wheels 132,134,135 are disconnected from the first wheel 122 and the second wheel 124 and the unsynchronised wheel 136 is held rotationally fixed, the power transmission system 10 may be denoted as an independent state. When the primary wheels 132,134,135 are synchronized to the first wheel 122 and the second wheel 124 and the unsynchronised wheel 136 is not held rotationally fixed, the power transmission system 10 may be denoted as a connected state.

[0047] To calculate the power transmission ratio between the input wheel, i.e. the first synchronized wheel set 152 or the second synchronized wheel set 154 and the output wheel, i.e. the unsynchronised wheel 136, when the power transmission system 10 is in the connected state, it is necessary to determine the primary equivalent transmission ratio. Primary equivalent transmission ratio may be defined as the ratio of rotational speed between the first primary wheel 132 and the second primary wheel 134 when the third primary wheel 135 is rotationally fixed and the primary transmission train 100 is in the independent state. When the power transmission system 10 is in the independent state, the first primary wheel 122 and the second primary wheel 124, when rotated with the primary equivalent transmission ratio between them, may engage with the unsynchronized wheel at a speed that matches their own rotation around the unsynchronized wheel 136 when the first primary wheel 122 and the second primary wheel 124 rotate the unsynchronized wheel 136 remains rotationally fixed. For example, a single revolution of the first primary wheel 122 causes a certain amount of engagement 'X' with the unsynchronized wheel 136 when the unsynchronised wheel 136 remains rotationally fixed. Based on this understanding, recalling that when the power transmission system 10 is in a connected state, the primary equivalent transmission ratio between the first primary gear 132 and the second primary wheel 134 becomes the feedback transmission ratio, when transmission system 10 is in a connected state and the primary equivalent transmission ratio and the feedback transmission ratio are different, a single rotation of the first synchronized wheel 132 would impart an engagement 'X + δ' to the unsynchronised wheel 136, δ is the change to the engagement caused by the difference between the primary equivalent transmission ratio and the feedback transmission ratio. Unsynchronized wheel 136 is therefore caused to rotate when the first primary wheel 132 is rotated. In other words, the unsynchronized wheel 136 is rotated when the first synchronized wheel set 152 and the second synchronized wheel set 154 are rotated. The primary equivalent transmission ratio may be different from the feedback transmission ratio so as to cause the unsynchronized gear to rotate. The power transmission ratio between the first synchronized wheel set 152 or the second synchronized wheel set 154 and the unsynchronized wheel may be dependent on the difference between the primary equivalent transmission ratio and the feedback transmission ratio, the change in engagement 'δ' with the unsynchronized wheel due to the difference, and the amount of rotation the engagement 'δ' causes on the unsynchronized wheel to rotate for each rotation of the first synchronized wheel set 152 or the second synchronized wheel set 154 to which the ratio with the unsynchronized wheel is being calculated. For the primary transmission train 100 that is an epicyclic gear train, the primary equivalent transmission ratio may be obtained based on the conventional formulae for an epicyclic gear train which calculates the relative speed ratio between the first primary wheel 132 and the second primary wheel 134 when the third primary wheel 135 is held rotationally fixed. [0048] When the primary equivalent transmission ratio is different from the feedback transmission ratio, the power transmission system 10 may be configured to achieve a gear up or gear down configuration. In an example of the unsynchronized wheel 136 being the input wheel and the first synchronized wheel set 152 or second synchronized wheel set 154 being the output wheel, in a gear up configuration, the unsynchronized wheel 136 may be adapted to rotate slower than any one of the first synchronized wheel set 152 and the second synchronized wheel set 154. In a gear down configuration, the unsynchronized wheel 136 may be adapted to rotate faster than any one of the first synchronized wheel set 152 and the second synchronized wheel set 154. If the primary equivalent transmission ratio and the feedback transmission ratio are configured to be the same, it can be anticipated that there will not be any gear up or gear down effect, i.e. the unsynchronized wheel 136 may remain rotationally fixed, which denotes a neutral configuration. It is also possible that the feedback transmission ratio be varied such that the feedback transmission ratio may transit from a gear up or gear down configuration to a neutral configuration and vice versa.

[0049] Using the examples in Fig. 1 to Fig 3, the primary equivalent transmission ratio of the primary transmission train 100 and the power transmission ratio between the first synchronized wheel set 152 (assuming that the input wheel in within the first synchronized wheel set 152) and the unsynchronized wheel 136 of the power transmission system 10 may be calculated as shown in Table 1 below. Row 1 of Table 1 relates to the example as shown in Fig. 1. Row 3 of Table 1 relates to the example as shown in Fig. 2. Row 3 of Table 1 relates to the example as shown in Fig. 3. Row 2, 4 and 6 of Table 1 relates to Fig. 1, Fig. 2 and Fig. 3 respectively as well except that the reference for the first synchronized wheel set 152 and the second synchronized wheel set 154 are interchanged.

[0050] Table 1: Gear transmission ratio

Pr Pc Ps B/(A+B) (A/(R _f *(A+B))-B))

Ps Pc Pr 1/(1+B/A) (B/((R _f *(A+B))-A))

Pc Ps Pr 1+B/A B/(B-(A*(R _f -l))) p _s : the sun gear 104;

p _c : the planet gear carrier 108;

p _r : the ring gear 102;

A: the number of teeth on the sun gear 104, p _s ;

B: the number of teeth on the ring gear 102, p _r ;

Req: the primary equivalent transmission ratio between the first primary wheel 132 and the second primary wheel 134, being the ratio of the rotational speed which the first primary wheel 134 and the second primary wheel 136 of the primary transmission train rotate with respect to each other when not connected to the first wheel 132 and the second wheel 134 and the unsynchronized wheel may be rotationally fixed.

Rt: the power transmission ratio between the unsynchronized wheel and the first synchronized wheel set; UW:SW1 = l:R _t

Rf: the feedback transmission ratio, SW1:SW2 = l:Rf.

[0051] If the values of the feedback and primary equivalent ratio are equal, Rf = Req, then the power transmission ratio Rt is infinite and the unsynchronized wheel 136 remains rotationally fixed when the first synchronized wheel set 152 and the second synchronized wheel set 154 are rotated. When Rf is adapted to be close to Req, the value of Rt is large. For any particular gear configuration, the polarity of Rt inverts when Rf is changed from being greater than Req to less than Req. Therefore, the direction of rotation the output gear may be changed to be either the same as the first synchronized wheel set 152 (positive ratio) or opposite to the first synchronized wheel set 152 (negative ratio) by making Rf either greater than or less than the value of Req.

[0052] The formula may be derived from the following example. Referring to Table 1, the sun gear 104 may be designated as the first primary wheel 132, the ring gear 102 may be designated as the second primary wheel 134 and the planet gear carrier 108 may be designated as the third primary wheel 135. When in an independent state, with the planet gear carrier 108 held stationary, for every revolution of the sun gear 104, the ring gear 102 rotates '-A/B' times. Furthermore, one rotation of the ring gear 102 turns the planet gear carrier 108 'Β/(Α+Β)' times. However, when in a connected state, and the planet gear carrier 107 is free to rotate, for every revolution of the sun gear turns, the ring gear turns Rf times as dictated by the feedback transmission ratio. Therefore 'δ' which is the change in engagement with the planet gear carrier 108 compared to the independent state where it remains stationary is equal to 'B*(Rf-(-A/B) which simplifies to 'Rf*B +A' . Therefore, one rotation of the sun gear 104 rotates the planet gear carrier 108 by '(Rf*B +A)* (B/(A+B))' . Simplifying this equation and inverting is so that it expresses the number of rotations of the sun gear 104 for each rotation of the planet gear carrier 108 results in '(A+B)/(A+(B*Rf))'.

[0053] Fig. 4 shows an example of the primary transmission train 100. Referring to Fig. 4, the primary transmission train 100 may include an epicyclic gear train. Primary transmission train 100 may include a ring gear 102, a sun gear 104 within the ring gear 102, at least one planet gear 106 between the sun gear 104 and the ring gear 102 and a planet gear 106 carrier attached to the at least one planet gear 106. Primary transmission train 100 may include a plurality of planet gears 106, each rotatably attached to the planet gear carrier 108. As shown in Fig. 4, the primary transmission train 100 may include a plurality of gears, e.g. 2 planet gears 106, 3 planet gears 106, 4 planet gears 106, and the planet gear carrier 108 may be adapted to hold the gears accordingly. As shown in Fig. 4, the primary transmission train 100 may be a tooth-typed gear train.

[0054] Fig. 5 shows an exploded view of an example of the power transmission system 20. As shown in Fig. 5, the feedback transmission train 220 may include an epicyclic gear train having a feedback ring gear 242, a feedback sun gear 244 within the feedback ring gear 242 and a feedback planet gear 246 rotatably attached to a feedback planet gear carrier 248. Feedback planet gear 246 may be coupled to and disposed between the feedback sun gear 244 and the feedback ring gear 242. First wheel 222 and the second wheel 224 may include any two of the feedback ring gear 242, the feedback sun gear 244 and the feedback planet gear carrier 248. Feedback transmission train 220 may be shown in Fig. 5 to be a tooth-typed gear train. However, the feedback transmission train 220 may be any type of transmission system. For example, the feedback transmission train 220 may be at least one of spur gear type, helical gear type, herringbone type, sprocket and chain type, pulley and belt type. For example, the first wheel 222 may be a first pulley having a first radius and the second wheel 224 may be a second pulley having a second radius such that the first pulley and the second pulley are connected via a belt. First primary wheel 232 may be connected to the first pulley and the second primary wheel 234 may be connected to the second pulley whereby the feedback transmission ratio between the first pulley and the second pulley may be different from the primary equivalent transmission ratio. First wheel 222 may be engaged to the second wheel 224 directly. For example, spur gears in contact with each other. As shown, while the term "wheel" is used for the power transmission system 20, it can include gears, e.g. spur gears, helical gear, pulley wheel, etc.

[0055] Referring to Fig. 5, the sun gear 204 may be connected to feedback sun gear 244, the ring gear 202 may be connected to the feedback ring gear 242, the planet gear carrier 208 may be connected to the feedback planet gear carrier 248. In the embodiment in Fig. 5, the planet gear 206 and the feedback planet gear 246 may not be connected to each other. Depending on the configuration required, any one of the sun gear 204, ring gear 202, and planet gear carrier 208 of the primary transmission train 200 may be the first primary wheel 232, the second primary wheel 234 and the unsynchronized wheel 236. For example, the sun gear 204 may be the first synchronized wheel set 252 and the ring gear 202 may be the unsynchronized wheel 236 of the power transmission system 20. In this case, the first primary wheel 232 may be the sun gear 204 and is connected to the feedback sun gear 244, hence it is the first wheel 222, and the second primary wheel 234 may be the planet gear carrier 208 and it is connected to the feedback planet gear carrier 248, hence it is the second wheel 224. In this example, the first synchronized wheel set 252 may include the first primary wheel 232 and the first wheel 222 and the second synchronized wheel set 254 may include the second primary wheel 234 and the second wheel 224. Feedback ring gear 242 of the feedback transmission train 220 may be rotationally fixed. As such, in an embodiment where the feedback transmission train 220 may be an epicyclic gear train, there may be a disconnected wheel 226 that is not connected to any one of the wheels in the primary transmission train 200. The disconnected wheel 266 may be rotationally fixed so that the first wheel 222 and the second wheel 224 can be adapted to provide the feedback transmission ratio to the primary transmission train 200. In an example where the feedback planet gear 246 may be rotationally fixed, the feedback planet gear carrier 248 may still be rotatable to allow transmission between the feedback sun gear 244 and the feedback ring gear 242. Feedback epicyclic gear train allows any one of the feedback sun gear 244, the feedback ring gear 242 to be rotated in either direction and torque to transmit in either direction between the feedback sun gear 244, the feedback ring gear 242. Some other existing forms of high ratio gear train may only be rotated in one direction and/or do not allow torque to be transmitted from the load back to the input.

[0056] Fig. 6 shows a front view of the power transmission system 20 in Fig. 5. As shown in Fig. 6, the primary transmission train 200 may be connected to the feedback transmission train 220. For example, the sun gear 204, e.g. the first primary wheel 232, and the ring gear 202, e.g. the second primary wheel 234, of the primary transmission train 200 may be rotatably synchronised with the feedback sun gear 244 (not shown in Fig. 6), e.g. the first wheel 222, and the feedback ring gear 242, e.g. the second wheel 224, respectively. Planet gear carrier 208, e.g. the unsynchronized wheel 236, may not be rotatably connected to the feedback planet gear carrier 248 and is left to rotate freely.

[0057] For the example in Fig. 6 where the primary transmission train 200 and the feedback transmission train 220 are epicyclic gear trains, the power transmission ratio of the power transmission system 20 may be calculated as shown in Table 1 above. However, the power transmission ratio of the power transmission system 20 may alternatively be calculated as shown in Table 2 below, where values of the number of teeth on the feedback sun gear and on the feedback ring gear 242 are substituted directly into the formula in place of the feedback transmission ratio.

[0058] Table 3: Power transmission ratio with epicyclic feedback gear train

p _s : the sun gear 204;

p _c : the planet gear carrier 208;

p _r : the ring gear 202; A: the number of teeth on the sun gear 204, p _s ;

B: the number of teeth on the ring gear 202, p _r ;

X: the number of teeth on the feedback sun gear 244;

Y: the number of teeth on the feedback ring gear 242;

Rt: the power transmission ratio between the unsynchronized wheel to the first synchronized wheel set, UW:SW1 = l:R _t

[0059] Fig. 7 shows a rear view of the power transmission system 20 in Fig. 5. As shown in Fig. 7, the feedback transmission train 220 may be attached to the primary transmission train 200.

[0060] Fig. 8 shows an example of a power transmission system 30. Power transmission system 30 may include offset epicyclic gear trains. As shown in Fig. 8, the primary transmission train 300 may be an offset gear train such that the sun gear 304 may be offset from the centre of the ring gear 302. Planet gear 306 may be disposed between and connected to the sun gear 304 and the ring gear 302. Sun gear 304 and the planet gear 306 may be rotatably connected to the planet gear carrier 308. Similarly, the feedback transmission train 320 may be an offset gear train such that the feedback sun gear 344 may be offset from the centre of the feedback ring gear 342. Feedback transmission train 320 may include a feedback planet gear 346 disposed between and connected to the feedback sun gear 344 and the feedback ring gear 342. Feedback sun gear 344 and the feedback planet gear 346 may be rotatably connected to the feedback planet gear carrier (not shown in Fig. 8). Sun gear 304 and the planet gear carrier 308 of the primary transmission train 300 may be rotationally connected to feedback sun gear 344 and the feedback planet gear carrier respectively. In this example, the feedback ring gear 342 may be rotationally fixed. In this example, the sun gear 304 and the feedback sun gear 334 may be the first synchronized wheel set 352 and the planet gear carrier 308 and the feedback planet gear carrier may be the second synchronized wheel set 354. Offset primary transmission train 300 may be connected to a non-offset feedback transmission train. Feedback transmission train 300 may include a non-gear train with a first wheel and a second wheel where the sun gear may be connected to the first wheel and the second wheel of the feedback transmission train. [0061] Fig. 9 shows an exploded view of the power transmission system 30 in Fig. 8. As shown in Fig. 9, the planet gear carrier 308 may be rotated along a rotational axis 312 through the centre of the ring gear 302. Planet gear carrier 308 may include a main shaft 3082 extending along the rotational axis 312 such that the planet gear carrier 308 is rotated at about the main shaft 3082. Main shaft 3082 may be mounted to bearings (not shown in Fig. 9). Planet gear carrier 308 may include a crank 3084 adapted to crank the planet gear carrier 308 to rotate about the rotational axis 312. It can be appreciated that the crank 3084 is optional as the main shaft 3082 may be connected to an actuating component, e.g. a motor, to drive the primary transmission train 300. Planet gear carrier 308 may include a first arm 3086 and a second arm 3088, each extending radially from main shaft 3082. Planet gear carrier 308 may include a first gear shaft 3086G extending from the first arm 3086 and parallel to the rotational axis 312 and connected to the sun gear 304. Planet gear carrier 308 may include a second gear shaft 3088G extending from the second arm 3088 and parallel to the rotational axis 312 and connected to the planet gear 306. When the crank 3084 of the planet gear carrier 308 is being rotated, the first arm 3086 and the second arm 3088 may rotate the sun gear 304 and the planet gear 306 within the ring gear 302.

[0062] In the above example in Fig. 9, the first primary wheel 332 may be the sun gear 304 and the second primary wheel 334 may be the planet gear carrier 308. However, it is possible that the first primary wheel 332 may be the planet gear carrier 308 and the second primary wheel 334 may be the sun gear 304 as both gears are actuated by the planet gear carrier 308. Similarly, the first wheel 322 may be the feedback sun gear 344 and the second wheel 324 may be the feedback planet gear carrier and vice versa. Ring gear 302 may be the unsynchronized wheel 336. The operation of the transmission is otherwise similar to the earlier examples. The transmission ratio between the wheels may be calculated as shown in Table 1 and Table 2.

[0063] Fig. 10 shows a front view of the power transmission system 30 as shown in Fig. 8. Fig. 11 shows a rear view of the power transmission system 30 as shown in Fig. 8. By allowing the centre of the sun gear 304 to be offset from the centre of the ring gear 302, a greater variety of gear combinations may be used. Furthermore, the gears may be arranged in a more compact layout. [0064] Power transmission system 10,20,30 may be adapted to allow a wide range of transmission ratios including ratios significantly above that provided by any single conventional gear train mentioned above in a single stage of gear set, i.e. the epicyclic gear train and the feedback transmission train 120,220,320. Therefore, the power transmission system 10,20,30 is not restricted to the standard ratio of conventional gear sets and non-gear sets. Even if it is possible to stack conventional gear sets to achieve a desired gear ratio of the power transmission system 10,20,30, the number of the gear sets utilised and therefore the size of the gear sets would be substantially larger than the power transmission system 10,20,30. As shown in the example, the power transmission system 10,20,30 of the present invention allows this to be achieved with a relatively small number of moving parts and possibly using standard toothed gear interface which are relatively efficient, and easy to manufacture. The relatively simple compact package reduces space constraints, the use of materials and manufacturing/maintenance costs. As it minimises the number of meshing gears, power losses and backlash may be reduced.

[0065] Feedback transmission ratio may be fixed or adapted to be varied. The embodiments described earlier provide a feedback transmission train 120,220,320 with a fixed feedback transmission ratio which provides a power transmission with a fixed power transmission ratio. However, if the feedback transmission train 320 alternatively has a variable feedback transmission ratio, it may be useful for varying the power transmission ratio, which may be suitable for certain mechanical applications, e.g. a clutch, a brake, a speed governor, or similar applications. Feedback transmission train 120,220,320 that is an epicyclic gear train as shown in Fig. 5 is an example of a fixed feedback transmission ratio where one of the gears of the feedback transmission train 120,220,320, e.g. the feedback ring gear 142,242,342, is held fixed whilst the other two gears are rotatable so as to provide a fixed feedback transmission ratio to the primary transmission train 100,200,300. Referring to the same example in Fig. 5, to achieve a variable feedback transmission ratio, the feedback ring gear 142,242,342 may be rotated. Consequently, the feedback transmission ratio may then be varied. The rotation of the feedback ring gear 142,242,342 could be achieved by, for example, attaching to an external drive, attaching to a clutch that rotates in only one direction or slips at a certain torque, attaching to a housing of a system by a spring that allows some movement of the ring gear 102,202,302 under force. Power transmission system 10,20,30 may include a mechanism to allow slip/disengagement between connecting gears/wheels and/or rotationally fix gears/wheels in order to vary the ratio or disengage the transmission between rotating wheels/gears or halt the rotation of a wheel.

[0066] A skilled person would appreciate that the features described in one example may not be restricted to that example and may be combined with any one of the other examples.

[0067] The present invention relates to a power transmission system generally as herein described, with reference to and/or illustrated in the accompanying drawings.