GB2474254A

GB2474254A - Gear drive with loose gears fixed to shafts by fluid couplings

Info

Publication number: GB2474254A
Application number: GB0917568A
Authority: GB
Inventors: Richard Nicholson
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2009-10-08
Filing date: 2009-10-08
Publication date: 2011-04-13
Also published as: GB0917568D0

Abstract

A gear drive, for a gas turbine engine, comprises a first shaft 11, a second shaft 9 and a plurality of gear pairs 2, 4, 6, 8 each having a first gear and a second gear which mesh with each other. Each gear pair 2, 4, 6, 8 has a respective fluid coupling 13, 15, 21, 23 which couples either the first gear 3, 7 of that pair to the first shaft 11 or the second gear 2, 4 of that pair to the second shaft 9. The gear drive further has an activation system for selectably activating or deactivating the fluid couplings 13, 15, 21, 23 by respectively replenishing or removing fluid inside the fluid couplings. When one of the fluid couplings 13, 15, 21, 23 is activated, rotational motion is transmitted from one of the shafts 11, 9 to the other shaft 11, 9 by the respective gear pair 2, 4, 6, 8. The activation system comprises a mechanism which moves together or apart opposing halves of the fluid coupling 13, 15, 21, 23 in order to control circulation of fluid.

Description

GEAR DRIVE

The present invention relates to a gear drive.

In many applications, it is required to select a specific gear ratio between an input shaft and an output shaft. For example, constant mesh gearboxes may use synch ro-mesh devices to select the chosen ratio, or on automatic vehicles a series of electrically operated band brakes can be used to fix either planet, carrier or annulus gears on epicyclic gearboxes. Such gearboxes have many moving parts and several surfaces prone to wear. Due to their inherent complexity, such gearboxes are also often large and have considerable weight.

These considerations can rule out the use of such gearboxes for aerospace applications, particularly as the need for high reliability in such applications can exacerbate problems of weight and size.

It is therefore desirable to provide a gear drive having reduced weight, and which is less susceptible to wear.

Thus, according to a first aspect of the present invention, there is provided a gear drive having: a first shaft, a second shaft, and a plurality of gear pairs, each gear pair comprising a first gear and a second gear which couple with each other, the first gears also coupling to the first shaft, and the second gears also coupling to the second shaft, such that rotational motion can be transmitted from one of the shafts to the other shaft by any one of the gear pairs, wherein each gear pair has a respective fluid coupling which mediates the coupling of either the first gear of that pair to the first shaft or the second gear of that pair to the second shaft, and wherein the gear drive further has an activation system for selectably activating or disactivating the fluid couplings by respectively replenishing or removing fluid inside the fluid couplings, such that when one of the fluid couplings is activated, rotational motion is transmitted from one of the shafts to the other shaft by the respective gear pair.

Different gear ratios can thus be selected by replenishing the fluid in a particular fluid coupling.

Advantageously, each fluid coupling can avoid a need to mechanically mesh together the respective gear and shaft. The gear drive may also have a reduced number of parts compared to conventional gear drives, which can lead to a comparative reduction in weight. By using fewer parts, the complexity of the gear drive can also be reduced. Build and maintenance need not therefore require specially trained gearbox personnel. Failure modes, effects and critical analysis may also be simplified since there are fewer parts to consider.

Advantageously, the gear drive can avoid or reduce the number of highly loaded plain bearings or rolling element bearings, which in aerospace applications can improve operational reliability in high g fields that can disrupt the flow of lubricating oil to such bearings.

The gear drive may have any one or any combination of the following optional features.

Preferably, the fluid inside the couplings is oil.

Preferably, each fluid coupling mediates the coupling of the second gear of the respective gear pair to the second shaft. It is possible to have for one gear pair a fluid coupling mediating the coupling of a first gear to the first shaft and for another gear pair a fluid coupling mediating the coupling of a second gear to the second shaft. However, generally it is mechanically simpler to have all the fluid couplings associated with only one of the shafts. Such an arrangement is also generally preferable if one the shafts habitually serves as the rotational input.

Typically, each gear pair has a different gear ratio. The gear ratios can be selected to match torque characteristics against the speeds of a driver (of one of the first and second shafts) and a load (on the other of the first and second shafts). An appropriate gear ratio can be selected by automatic or manual operation of the activation system. Automatic operation can be implemented by a suitably programmed control processor. The fluid couplings can have different characteristics to match the speed/torque ranges of their respective gear ratios.

Typically, the activation system is configured such that when one of the fluid couplings is activated the other fluid couplings are disactivated.

However, the activation system may be configured such that there is an overlap between the replenishment of fluid in one fluid coupling, and the removal of fluid frorri another of the fluid couplings. The overlap can compensate for variations in time taken to drain and replenish respective fluid couplings, as well as differences in the acceleration/deceleration rate of the first shaft and the deceleration/acceleration rate of the second shaft (a driver and a load will have their own inertia and thus in addition to a demanded load, there will be inertial loads to consider).

The activation system may comprise respective fluid supplies to the fluid couplings which can be switched on and off. Additionally or alternatively, the activation system may comprise respective drainage ports which remove fluid from inside the couplings. These ports can remain open during operation. Thus, for example, an activation system comprising switchable fluid supplies and open drainage ports can be configured such that each supply, when switched on, supplies fluid to a particular coupling at a rate greater than the respective drainage port(s) can remove it.

Each fluid coupling typically comprises two opposing halves, torque being transferred from one half of the coupling to the other half via the circulation of fluid inside the coupling. The activation system may comprise a mechanism to separate apart the opposing halves of a disactivated coupling and to move together the opposing halves of an activated coupling. The separation of the opposing halves of a disactivated coupling can help to degrade the ability of the coupling to transmit torque when, for example, residual fluid (e.g. in a fluid mist) remains in the coupling. In this way, a disactivated coupling can be encouraged to "windmill" when torque is being transmitted through a different, activated, coupling.

One or more of the gear pairs may have a set of fluid couplings which mediate the coupling of either the first gear of the pair to the first shaft or the second gear of the pair to the second shaft, each fluid coupling of the set being selectably activatable or disactivatable by the activation system. Using a set of fluid couplings to couple the gear to the shaft can help the gear drive respond more precisely to changes in torque demand than when using a single fluid coupling. In particular, each one of the set of couplings may have different operating characteristics.

The activation system may be configured such that when one of the fluid couplings of a set is activated the other fluid couplings of the set are disactivated. For example, one of the fluid couplings of the set may be activated for part of a duty cycle, and another of the couplings of the set may be activated for a different part of the duty cycle depending on the operating characteristics of the couplings.

However, the activation system may be configured such that there is an overlap between the replenishment of fluid in one fluid coupling of a set, and the removal of fluid from another fluid coupling of the set. Again, the overlap can compensate for variations in time taken to drain and replenish respective fluid couplings, as well as differences in the acceleration/deceleration rate of the first shaft and the acceleration/deceleration rate of the second shaft.

The activation system may include a valve for directing fluid to the respective fluid couplings. The valve may be a spool valve, and may be actuated by an electrically powered servo device. The gears may be contained in a housing, and the valve may be located on the outside of the housing, providing easy access for maintenance.

Typically, the first gears are coaxial with the first shaft. Typically, the second gears are coaxial with the second shaft.

Preferably, each fluid coupling is configured to be at optimum slip of the two opposing halves of the coupling when the coupling is replenished with fluid.

The optimum slip may be determined in order to, e.g. minimise heat generation, maximise power transfer efficiency, maximise apparent stiffness to load variation etc. Typically, such devices are optimised to reduce fluid coupling losses, which are typically dominated by internal oil shear. However, torque is transferred when there is slip between the two opposing halves of a coupling, i.e. there is no torque transfer if the two halves of a coupling rotate at the same speed. So while torque transfer can be maximised by increasing slip, it is at the expense of internal heat generation.

Each coupling may be individually sized to its duty. For example, in relation to the power to be transferred, the speed difference at the moment of selection, the allowable losses and/or the time required for speed synchronisation by the total system of which the drive may be a part.

A second aspect of the invention provides an aero engine (such as a gas turbine engine) having a gear drive according to the first aspect of the invention, the gear drive optionally having any one or combination of the optional features described above in relation to the first aspect. Conveniently, the gear drive can be used to vary power transfer between different turbine shafts.

For example, the gear drive may be used to vary the power transfer from a low pressure turbine shaft to a high pressure turbine shaft on a 2-shaft engine.

On engines having more than two shafts, the gear drive may be used to vary the power transfer between the high pressure shaft and an intermediate pressure shaft, between an intermediate pressure shaft and the low pressure shaft, or between the high pressure shaft and the low pressure shaft. In so doing, the gear drive can be used to enhance the cycle efficiency of the engine.

The gear drive can also be used to take power from one of the turbine shafts, for example the low pressure shaft, and feed it to another device in the engine, such as an electrical power generator. The generator can be used to power accessory systems of an aircraft. There is a trend for higher accessory loads, and it is increasingly challenging to take power for driving accessories from the high pressure shaft of a gas turbine engine without adversely affecting the operability of the engine. The low pressure shaft of a gas turbine engine has more available power, but the large speed range of the shaft is incompatible with many electrical devices. The gear drive, however, can provide gear ratios to match the required speed of the electrical power generator with the large speed range of the low pressure shaft. A conventional "mechanical" variable ratio gear box performing this task would have more wearing elements and be generally heavier. The ability of the gear drive to overlap couplings can also help to ensure there is no undesirable acceleration or deceleration of the shaft or the power generator when different couplings are activated.

A third aspect of the invention provides an automotive vehicle having a gear drive according to the first aspect of the invention, the gear drive optionally having any one or combination of the optional features described above in relation to the first aspect. The gear drive may have a manual control, such as a gearstick for controlling the activation system. Alternatively, the activation system may respond to traditional speed and torque demand inputs provided in automatic transmissions.

A fourth aspect of the invention provides a marine vessel having a gear drive according to the first aspect of the invention, the gear drive optionally having any one or combination of the optional features described above in relation to the first aspect. The gear drive can be used to engage or disengage a propeller shaft or to provide a wider engine choice with a multi step gearbox as opposed to traditional fixed ratio speed reducing gearboxes. For example, the gear drive can permit a propeller shaft to vary speed while the engine operates at near constant optimum speed for, e.g. fuel efficiency.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a schematic of a gear drive according to a first embodiment of the present invention; Figure 2 shows a schematic of a gear drive according to a second embodiment of the present invention; and Figure 3 shows a schematic of a gear drive according to a third embodiment of the present invention.

Figure 1 shows a schematic of a gear drive according to a first embodiment of the present invention. The gear drive has two pairs 2, 4 of coupled gears, with different gear ratios.

Gear pair 2 comprises a first gear 1 and a second gear 3 which are rotatably coupled with each other by meshing. The first gear 1 is coaxially connected to a first shaft 9 such that the first gear and the first shaft are rotatably coupled with each other. The second gear 3 is coaxially connected to a second shaft 11 by a fluid coupling 13.

Likewise, gear pair 4 comprises a first gear 5 and a second gear 7 which are rotatably coupled with each other by meshing. The first gear 5 is coaxially connected to the first shaft 9 such that the first gear and the first shaft are rotatably coupled with each other. The second gear 7 is coaxially connected to a second shaft 11 by a fluid coupling 15.

Each fluid coupling comprises two opposing halves 17, 19. Suitable configurations for the fluid couplings are known to the skilled person. One half of each coupling 19 rotates with the second gear in each pair, and the other half 17 rotates with the second shaft. Torque is transferred from one half of the coupling to the other via the circulation of the fluid inside the coupling. Thus, when the fluid coupling contains fluid, the coupling provides a damped connection between the first and second shafts. If the coupling is dry (fluid is removed from inside the coupling), the two halves of the coupling do not operatively engage and the two halves freewheel without any transfer of torque between them. As the fluid is replenished inside the coupling, torque is once more transferred between the two halves, with the additional benefit of a soft start-up due to coupling slippage. The coupling will slip until the power transferred is appropriate to the speed difference across the coupling. The transfer of torque from one half of the fluid coupling to the other then drives the rotation of the second shaft.

The fluid couplings 13, 15 are selectably activated or disactivated by an activation system that respectively replenishes or removes the fluid inside the couplings. When a particular gear pair needs to be selected, the fluid is re-directed by the activation system to the coupling that connects the required gear pair to the second shaft. There may be a deliberate overlap in fluid off/on timing (i.e. the replenishment of fluid in one coupling, and the removal of fluid from another coupling). For example, replenishment of fluid inside fluid coupling 13 may occur at the same time as fluid is being removed from fluid coupling 15.

The size of the overlap will depend on the fluid feed rate into one coupling, the fluid exit rate from the other coupling (affected by speed and detail design features), the acceleration rate from the first shaft, and the deceleration rate (load versus inertia) of the second shaft.

Activating a particular fluid coupling causes the second shaft to rotate at a speed determined by the gear ratio of the second gear connected to the respective coupling, and the first gear with which the second gear is coupled.

In a variant of the gear drive shown in Figure 1, one of the fluid couplings 13, 15 may be moved to the first shaft 9 to mediate the coupling between one of the first gears 1, 5 and the first shaft rather than between the corresponding second gears 3, 7 and the second shaft 11. Such a variant may be desirable if, for example, the first shaft is the rotational input for one of the gear pairs, but the rotational output for the other of the gear pairs.

Figure 2 shows a schematic of a gear drive according to a second embodiment of the present invention. Features which are common to the second embodiment and the first embodiment have the same reference numbers in Figures 1 and 2. In this embodiment, two additional gear pairs 6, 8 are shown, having respective fluid couplings 21, 23. It is possible in this embodiment to select one of four different gear ratios. Each of the fluid couplings is individually sized to its duty, i.e. the power transferred, the speed difference at the moment of selection, the allowable losses and the time required for speed synchronisation by the total system.

Figure 3 shows a schematic of a gear drive according to a third embodiment of the present invention. Features which are common to the first, second and third embodiments have the same reference numbers in Figures 1, 2 and 3. In this embodiment, the gear drive has three pairs of gears 2, 4, 6, and four fluid couplings 25, 27, 29, 31. The second gear 7 in the second gear pair 4 is connected to the second shaft by a set of two of the fluid couplings 27, 29.

The second gear 7 is coupled to respective halves 33, 35 of the two couplings 27, 29. The fluid couplings 27, 29 are of different sizes and have different operating characteristics.

Activating either one of the two fluid couplings 27, 29 connected to the second gear 7 causes the second gear 7 and second shaft 11 to rotatably couple with one another. When both of the fluid couplings 27, 29 connected to the second gear are disactivated, the second gear 7 and second shaft 11 no longer couple, and the second gear 7 rotates freely around the second shaft.

The activation system can be configured to alternate the supply of fluid to the two couplings 27, 29 connecting the second gear 7 to the second shaft. For example, one of the fluid couplings 27 may be activated for part of a duty cycle, and the other fluid coupling 29 may be activated for a different part of the duty cycle. This can help to provide a more precise response to the required torque demand than is possible by using a single coupling. Both couplings 27 and 29 would not normally be simultaneously activated, although such operation is not excluded.

As in previous embodiments, a different one of the second gears may be selected to rotatably couple with the second shaft by replenishing the fluid inside a corresponding fluid coupling, and emptying the fluid from the coupling (or couplings) connecting the other second gears to the second shaft. In each case, the activation system may replenish fluid in one of the fluid couplings at the same time as fluid is removed from the other fluid coupling.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

Claims

CLAIMS1. A gear drive having: a first shaft, a second shaft, and a plurality of gear pairs, each gear pair comprising a first gear and a second gear which couple with each other, the first gears also coupling to the first shaft, and the second gears also coupling to the second shaft, such that rotational motion can be transmitted from one of the shafts to the other shaft by any one of the gear pairs, wherein each gear pair has a respective fluid coupling which mediates the coupling of either the first gear of that pair to the first shaft or the second gear of that pair to the second shaft, and wherein the gear drive further has an activation system for selectably activating or disactivating the fluid couplings by respectively replenishing or removing fluid inside the fluid couplings, such that when one of the fluid couplings is activated, rotational motion is transmitted from one of the shafts to the other shaft by the respective gear pair.
2. The gear drive according to claim 1, wherein each fluid coupling mediates the coupling of the second gear of the respective gear pair to the second shaft.
3. The gear drive according to claim 1 or 2, wherein the activation system is configured such that when one of the fluid couplings is activated the other fluid couplings are disactivated.
4. The gear drive according to any one of claim 1 to 3, wherein the activation system is configured such that there is an overlap between the replenishment of fluid in one fluid coupling, and the removal of fluid from another of the fluid couplings.
5. The gear drive according to any one of the previous claims, wherein each fluid coupling comprises two opposing halves, torque being transferred from one half of the coupling to the other half via the circulation of fluid inside the coupling, the activation system comprising a mechanism to separate apart the opposing halves of a disactivated coupling and to move together the opposing halves of an activated coupling.
6. The gear drive according to any one of the previous claims, wherein one or more of the gear pairs has a set of fluid couplings which mediate the coupling of either the first gear of the pair to the first shaft or the second gear of the pair to the second shaft, each fluid coupling of the set being selectably activatable or disactivatable by the activation system.
7. The gear drive according to claim 6, wherein each fluid coupling of the set has different operating characteristics.
8. The gear drive according to claim 6 or 7 wherein the activation system is configured such that when one of the fluid couplings of the set is activated the other fluid couplings of the set are disactivated.
9. The gear drive according to any one of claims 6 to 8 wherein the activation system is configured such that there is overlap between the replenishment of fluid in one fluid coupling of the set, and the removal of fluid from another fluid coupling of the set.
10. The gear drive according to any one of the preceding claims, wherein the first gears are coaxial with the first shaft.
11. The gear drive according to any one of the preceding claims, wherein the second gears are coaxial with the second shaft.
12. A gas turbine engine having a gear drive according to any one of the preceding claims.
13. The gas turbine engine according to claim 12, wherein the first and second shafts are, or are operatively connected to, the high and low pressure turbine shafts.
14. The gas turbine engine according to claim 12, wherein one of the first and second shafts is, or is operatively connected to, a turbine shaft of the engine, and the other of the first and second shafts is, or is operatively connected to, a device, such as an electrical power generator, to provide power to the device.
15. A gas turbine engine according to any one of claims 12 to 14 which is an aero engine.
16. A gear drive as any one herein described with reference to and/or as shown in the accompanying drawings.