TORQUE TRANSMISSION DEVICE

A torque transmission device comprises: an input drive shaft, a housing, an earth ring, a bearing cage, and a plurality of bearings held by the bearing cage between an inner surface of the earth ring and an outer surface of the input drive shaft. The earth ring is mounted within the housing such that an outer surface of the earth ring contacts an inner surface of the housing along a contact area, thereby providing a frictional interface at the contact area for transmitting torque from the earth ring to the housing.

FOREIGN PRIORITY

This application claims priority to European Patent Application No. 17275141.4 filed Sep. 14, 2017, the entire contents of which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a torque transmission device (for example a torque limiter or a freewheel) which comprises an earth ring, and particularly to such a torque transmission device for use in high-torque systems. The torque transmission device may be suitable for use in aerospace applications, for example in the transmission between a centralised Power Drive Unit (PDU) and an actuator which moves flaps, slats or other flight control surfaces on an aircraft wing.

BACKGROUND

Known torque transmission devices (for example torque limiters and freewheels) may comprise an earth ring operable to earth torque. In such devices, the torque is transmitted to the earth ring via bearings (for example, rollers or sprags) which bear against the earth ring and apply a contact stress to the earth ring surface. The earth ring is typically made of case-hardened steel to deal with the high stresses, and is a sizeable and heavy structure that may comprise between 5% and 11%, for example, of the total weight of the torque transmission device. The earth ring may not be corrosion resistant, and may need to be treated with expensive corrosion-protection treatments.

One system where a torque transmission device (for example a torque limiter) may be implemented is in the secondary flight control system of an aircraft. Such systems typically include a series of actuators, each linked to a flap or slat (or other flight control surface) on the leading or trailing edge of the aircraft wings. The actuators may all be connected to a transmission system that delivers torque down each wing from a centralised Power Drive Unit (PDU). The PDU provides sufficient torque to drive all surfaces of the system. In the event of a failure of one of the actuators (for example, if the actuator jams), then all of the torque provided by the PDU may be diverted to that particular actuator, which may cause damage to the actuator.

To protect the actuators, a torque limiter may be provided on the transmission line going from the PDU to each wing. These torque limiters allow only enough torque for each wing to be transmitted. Then, further torque limiters may be incorporated into the actuator inputs in order to prevent the full torque for that wing from being transmitted into any one actuator.

Particularly in aerospace applications, it is desirable to minimise the weight of components, since unnecessary weight is a source of inefficiency in the operation of the aircraft. Moreover, is it desirable to reduce the cost and complexity of manufacture of torque transmission devices. An improved torque transmission device, and particularly an improved torque limiter, is therefore sought.

SUMMARY

The present disclosure can be seen to provide a torque transmission device comprising: an input drive shaft; a housing; an earth ring; a bearing cage; and a plurality of bearings held by the bearing cage between an inner surface of the earth ring and an outer surface of the input drive shaft, wherein the earth ring is mounted within the housing such that an outer surface of the earth ring contacts an inner surface of the housing along a contact area, thereby providing a frictional interface at the contact area for transmitting torque from the earth ring to the housing.

The contact area between the outer surface of the earth ring and the inner surface of the housing may take the shape of the curved surface of a cylinder. Torque may be transmitted from the earth ring to the housing only via the contact area, for example only via the contact area that takes the shape of the curved surface of a cylinder.

Over 50% of the outer surface of the earth ring may be in contact with the inner surface of the housing. Over 60%, 70%, 80% or 90% of the outer surface of the earth ring may be in contact with the inner surface of the housing. The entire outer surface of the earth ring may be in contact with the inner surface of the housing.

An output may be connected to the bearing cage.

The torque transmission device may be arranged such that, in a first operating condition, the output is operable to rotate on rotation of the input drive shaft.

The torque transmission device may be arranged such that, in a second operating condition, torque is prevented from being transmitted from the input drive shaft to the output. That is, a braking load may be applied to the output and/or the input drive shaft. The output and/or the input drive shaft may be prevented from rotating in the second operating condition.

The torque transmission device may be arranged such that, in a second operating condition, the bearings bear against the earth ring to apply torque to the earth ring. The torque transmitted to the earth ring from the bearings may be transmitted from the earth ring to the housing (solely) via the frictional interface at the contact area between the outer surface of the earth ring and the inner surface of the housing.

The earth ring may be configured to bear a contact stress applied by the bearings on the earth ring. In particular, the earth ring may be sufficiently thick to bear the contact stress applied by the bearings on the earth ring.

Any reference herein to the “thickness” (or “thinness”) of the earth ring is a reference to its radial thickness (i.e. its extent in the radial direction). Correspondingly, any reference herein to the “thickness” (or “thinness”) of the housing is a reference to its radial thickness (i.e. its extent in the radial direction).

The earth ring may be flexible (to a necessary degree) such that the torque transmitted from the earth ring to the housing is reacted by the frictional force between the earth ring and the housing such that the earth ring does not slip within the housing. In particular, the earth ring may be sufficiently thin to be flexible enough that the torque transmitted from the earth ring to the housing is reacted by the frictional force between the earth ring and the housing such that the earth ring does not slip within the housing.

The earth ring may be flexible enough that it can be deformed by the applied contact stress from the bearings so that the frictional force between the earth ring and the housing reacts the contact stress and the earth ring does not slip within the housing. That is, friction between the earth ring and the housing may prevent the earth ring from sliding within the housing when the bearings bear against the earth ring.

The contact stress between the earth ring and the housing may be approximately one tenth, or less, of the contact stress between the bearings and the earth ring.

The earth ring may not be bonded to the housing. Alternatively, the earth ring may be bonded to the housing, for example using an organic adhesive.

The earth ring may be less than 5 mm thick, and optionally less than 4 mm thick. These values may be particularly suitable for a torque transmission device designed to receive an input torque of up to 500 Nm. This may allow the earth ring to be sufficiently flexible to be deformed by the applied contact stress from the bearings so that the frictional force between the earth ring and the housing reacts the contact stress and the earth ring does not slip within the housing.

The degree of deformation of the earth ring under loading from the bearings depends on: the inner diameter of the earth ring; the number of bearings; the radius of the bearings; the magnitude of the frictional force applied to the inner and outer surfaces of the earth ring; the elastic modulus of the material of the earth ring; and the thickness of the earth ring.

The earth ring may be thicker than the depth at which the peak shear stress (resulting from the contact stress applied by the bearings on the earth ring) occurs. The earth ring may be about 4 to 10 times thicker than the depth below the surface at which the peak shear stresses occur, and optionally is about 5 to 8 times, or 5 to 6 times, thicker than the depth below the surface at which of the peak shear stresses occur.

The depth below the surface at which of the peak shear stresses occur may depend on the material of the bearing and earth ring, on the force applied on the earth ring by the bearing, and on the geometry of the bearing and the earth ring.

Specifically, the depth of the peak shear stress may be expressed by the following equation:

Loadrolleris the load applied by a roller;

Lrolleris the length of the roller;

Dringis the inner diameter of the earth ring;

Drolleris the diameter of the roller;

vringis the Poisson's ratio of the material of the earth ring;

vrolleris the Poisson's ratio of the material of the roller;

Eringis the Young's modulus of the material of the earth ring; and

Erolleris the Young's modulus of the material of the roller;

The peak shear stresses may occur at a depth of about 0.25 mm to 0.5 mm below the surface.

The earth ring may be about 1 mm to 4 mm thick, for example 1.5 mm to 2.5 mm thick, and optionally is approximately 1.5 mm thick.

The earth ring may have an inner diameter of 50 mm to 100 mm, and optionally has an inner diameter of approximately 70 mm.

The earth ring may have a very hard inner surface, i.e. a surface that can withstand the contact stresses applied by the bearings. The earth ring inner surface may have a hardness of greater than 60 HRC. The earth ring inner surface may have a hardness of less than 70 HRC. The earth ring inner surface may have a hardness of 62 to 64 HRC, for example. The earth ring may comprise case-hardened metal (for example, steel) or a ceramic.

If the earth ring comprises case-hardened metal, the inner surface of the earth ring may be case-hardened. The case-hardened layer may be sufficiently thick that it can withstand the wear and contact stresses applied by the bearings. If the case-hardened layer is too thin, surface failure (brinelling) may occur. The earth ring may comprise a sufficiently thick non-hardened outer layer to impart flexibility to the earth ring. If the earth ring is through-hardened rather than case-hardened (or if the non-hardened layer is too thin), the earth ring may be brittle and may fracture.

The earth ring may be case-hardened to a depth of 2 to 5 times the depth at which the peak shear stresses occur in the earth ring.

The earth ring may be case-hardened to a depth of about 0.5 mm to 1 mm.

The earth ring may be case-hardened to a depth of 20% to 80%, and optionally, 30% to 70%, and optionally 40% to 60% of the thickness of the earth ring.

The earth ring may be case hardened by carburizing. The earth ring may be case hardened by flame or induction hardening, or by nitriding, cyaniding, carbonitriding, or ferritic nitrocarburising. An appropriate process may be chosen depending on, for example, the carbon content and composition of the metal.

The housing may comprise a lighter material than the earth ring, for example, aluminium, titanium, or a composite comprising carbon filament and resin. The housing may have a sufficient thickness to absorb the maximum hoop stresses that could be imparted by the bearings.

The housing may be about 10 mm thick.

The housing may be about 6 to 10 times thicker than the earth ring.

The load applied by the roller (Loadrollerin the equation above) may be 20 kN to 40 kN, and optionally is 25 kN to 30 kN. The length of the roller (Lrollerin the equation above) may be 10 mm to 30 mm, for example 12 mm or 24 mm. The diameter of the roller may be between 10 and 20 mm, and optionally is 12 mm.

The torque transmission device may be used in a high-torque system, where the input torque is, for example, greater than 15 Nm, greater than 50 Nm, or greater than 100 Nm. The input torque is, for example, less than 1000 Nm, or less than 500 Nm. The input torque may be, for example, 15 Nm to 500 Nm. The input torque may be approximately 400 Nm, for example. The torque transmission device may be a torque limiter, wherein the first operating condition is a condition in which the input torque applied by the input drive shaft is below a predetermined threshold, and the second operating condition is an overload condition in which the input torque applied by the input drive shaft is above the predetermined threshold.

The input torque applied by the input drive shaft may exceed the predetermined threshold (in the second operating condition, i.e. the overload condition) as a result of a failure or jam in an associated actuator (or in a control surface associated with an actuator), for example.

The torque limiter may be a roller-jammer. In the roller-jammer, the plurality of bearings may be rollers, and the bearing cage may be a roller cage. The output may be connected to the roller cage.

The input drive shaft may have a lobed shape, when viewed in cross-section. The cross-section of the input drive shaft may have an approximately polygonal shape (for example, having between 3 and 8 sides, for example 6 sides), with flattened points. The sides may be straight or may be defined by inwardly-bowed arcs instead. The cross-section of the earth ring may be annular, such that the inner surface of the earth ring is circular in cross-section, and an inner surface of the earth ring defines a cylinder. A pocket for receiving a roller may be defined between each “side” of the outer surface of the input drive shaft and the inner surface of the earth ring. The pocket may be of non-uniform radial clearance, i.e. the radial distance between the outer surface of the input drive shaft and the inner surface of the earth ring may vary circumferentially. There may be the same number of rollers as there are “sides” of the cross-section of the input drive shaft.

The roller-jammer may comprise a torsion bar arranged to hold the roller cage in alignment with the input drive shaft in the first operating condition.

When the roller cage and input drive shaft are held in alignment (in the first operating condition), the roller cage may hold the plurality of rollers in the parts of the pocket of maximum radial clearance (i.e. maximum radial distance between the outer surface of the input drive shaft and the inner surface of the earth ring). That is, the rollers may be held in the central part of the pocket. This may allow the rollers to slide, such that both the input drive shaft and roller cage rotate together, thereby transmitting drive from the input drive shaft to the output (which is connected to the roller cage).

In an overload condition (in the second operating condition), the plurality of rollers may bear against the inner surface of the earth ring and an outer surface of the input drive shaft, and the rotation of the input drive shaft may thereby be resisted. In more detail, in an overload condition, an angular movement of the roller cage relative to the input drive shaft may result in the rollers no longer being held in the centre of the pocket (i.e. at the position of maximum clearance between the outer surface of the input drive shaft and the inner surface of the earth ring). Instead, the rollers may be forced away from the centre of the pocket to bear against both the outer surface of the input drive shaft and the inner surface of the earth ring and become jammed therebetween. As a result, the rotation of the input drive shaft may be resisted.

For a given twist of the torsion bar (i.e. for a given differential angular displacement of the torsion bar), the rollers may be more readily forced away from the centre of the pocket if the sides of the cross-section of the input drive shaft are defined by inwardly-bowed arcs, compared to the case that the sides of the cross-section of the input drive shaft are straight lines.

The torque limiter may be a sprag-type torque limiter. In that case, the plurality of bearings may be sprags (for example, non-revolving asymmetric figure-of-eight shaped bearings), and the bearing cage may be a sprag cage. The cross-section of the input drive shaft may be circular in shape, and the outer surface of the input drive shaft may define an inner race of the sprag-type torque limiter. The cross-section of the earth ring may be annular, such that the inner surface of the earth ring is circular in cross-section, and an inner surface of the earth ring may define an outer race of the sprag-type torque limiter. The output may be connected to the sprag cage. The radially-inner end of each sprag may be contained within a pocket in the outer surface of the input drive shaft, and the radially-outer end of each sprag may be contained within a pocket in the sprag cage.

When not in an overload condition, the sprags may be free to slide between the inner race and outer race, such that both the input drive shaft and sprag cage rotate together, thereby transmitting drive from the input drive shaft to the output (which is connected to the sprag cage).

In an overload condition, the pockets in the outer surface of the input drive shaft may go out of alignment with the pockets in the sprag cage, causing the sprags to tip such that they are no longer free to slide between the inner race and outer race. Instead the sprags may transmit torque from the input drive shaft to the earth ring and as a result any further input torque may be transmitted from the input drive shaft to the earth ring and then to the housing.

The torque transmission device may be a one-way freewheel. In this case, the first operating condition (in which the output rotates on rotation of the input drive shaft) is a condition in which the input drive shaft rotates in a first direction, and the second operating condition (in which torque is prevented from being transmitted from the input drive shaft to the output) is a condition in which the input drive shaft rotates in the opposite direction.

The freewheel may be a sprag clutch. Such a sprag clutch may have a structure similar to that described above in relation to the sprag-type torque limiter, except that the sprags are arranged to tip when the input drive shaft rotates in the opposite direction (rather than when the input torque exceeds a predetermined threshold, as in the case of a sprag-type torque limiter).

The sprags may be arranged such that when the input drive shaft rotates in a first direction the sprags are free to slide between the inner race and outer race, such that both the input drive shaft and sprag cage rotate together, thereby transmitting drive from the input drive shaft to the output (which is connected to the sprag cage).

The sprags may be arranged such that when the input drive shaft rotates in the opposite direction, the sprags tip such that they are no longer free to slide between the inner race and outer race. Instead the sprags may transmit torque from the input drive shaft to the earth ring and as a result any further input torque may be transmitted from the input drive shaft to the earth ring and then to the housing.

DETAILED DESCRIPTION

FIG. 1Ashows a cross-sectional view of a prior art roller-jammer100. The roller-jammer100is provided in a mechanical system in which torque is transmitted by an input drive shaft30. The input drive shaft30is supported for rotation within a housing10(shown also inFIG. 1B). Mounted adjacent the housing is an earth ring20(shown also inFIG. 1C). Torque is transmitted from the earth ring20to the housing10via dogs22on the earth ring that interlock with corresponding dogs12on the housing. The housing10comprises aluminium and the earth ring20comprises steel that has been case-hardened by carburisation.

The case-hardened steel earth ring20is not corrosion resistant, and since it is mounted adjacent to a non-ferrous housing10and is not protected from the surrounding environment, expensive corrosion-protection treatments are required. The case-hardened steel earth ring20comprises between 5% and 11% of the total weight of the roller-jammer unit. That is, the earth ring20is a comparatively heavy component.

A roller cage50is coaxial with and encircles a length of the input drive shaft30within the housing10. The roller cage50is connected to the input drive shaft30by a torsion bar (not shown).

The roller cage50positions a plurality of rollers40in pockets45defined between the outer surface30aof the input drive shaft30and the inner surface20aof the earth ring20. The outer surface30aof the input drive shaft30has a lobed shape, when viewed in cross-section (as seen inFIG. 1A). In the present example, the outer surface30aof the input drive shaft30has an approximately hexagonal shape, with flattened points and inwardly-bowed arcs instead of straight sides. The inner surface20aof the earth ring20is a circle, when viewed in cross-section (as seen inFIG. 1A). A pocket45for receiving a roller40is defined between each “side” of the outer surface30aof the input drive shaft30and the inner surface20aof the earth ring20. As will be clear from the foregoing and fromFIG. 1A, the pocket is thus of non-uniform radial clearance, i.e. the radial distance between the outer surface30aof the input drive shaft30and the inner surface20aof the earth ring20varies circumferentially.

When the input torque is below a predetermined threshold, the roller cage50is held in alignment with the input drive shaft30by a torsion bar, and in such a condition the roller cage50holds the plurality of rollers40in the parts of the pocket45of maximum radial clearance (i.e. maximum radial distance between the outer surface30aof the input drive shaft30and the inner surface20aof the earth ring20). That is, the rollers40are held in the central part of the pocket45. This allows the rollers40to slide, and both the input drive shaft30and roller cage50rotate together.

In the event that the applied torque exceeds the predetermined threshold, for example as a result of a failure or jam in an associated actuator or control surface, the angular movement of the roller cage50relative to the input drive shaft30results in the rollers40no longer being held in the centre of the pocket45(i.e. at the position of maximum clearance between the outer surface30aof the input drive shaft30and the inner surface20aof the earth ring20. Instead, the rollers40are forced away from the centre of the pocket45to bear against both the outer surface30aof the input drive shaft30and the inner surface20aof the earth ring20and become jammed therebetween. As a result, the rotation of the input drive shaft30is resisted.

In such a case, huge contact stresses occur between the rollers40and earth ring20. Additionally, significant hoop stresses are imparted to the roller-jammer. In this prior art example, the earth ring20bears the contact pressures and hoop stresses. Moreover, the earth ring dogs22must be of a sufficient size and strength to transmit the torque to the housing10via the housing dogs12. As a result, the earth ring20must be thick (typically around 10 mm thick for a torque limiter designed to receive an input torque of approximately 400 Nm). Since the earth ring is so thick, the housing need only be relatively thin (typically around 4 mm thick for a torque limiter designed to receive an input torque of approximately 400 Nm). In the present example, the earth ring20is approximately two to three times thicker than the housing10.

FIG. 2Ashows a cross section of a roller-jammer200in accordance with the present disclosure. The roller-jammer200comprises: an input drive shaft30, a housing10, an earth ring20′, a roller cage50, and a plurality of rollers40held by the roller cage50in pockets45between an inner surface24of the earth ring20′ and an outer surface of the input drive shaft30. The earth ring20′ is mounted within the housing10′ such that an outer surface22of the earth ring20′ contacts an inner surface12of the housing10along a contact area15, thereby providing a frictional interface at the contact area15for transmitting torque from the earth ring20′ to the housing10′.

The input drive shaft30, roller cage50, rollers40and pockets45are unchanged compared toFIG. 1A. However, the housing10and earth ring20have been replaced by a new housing10′ and new earth ring20′ (shown respectively inFIG. 2BandFIG. 2C). The new earth ring20′ is thinner than the prior art earth ring20, and so the roller jammer unit is lighter (by about 4%). The new earth ring20′ is mounted within the housing10′, rather than adjacent thereto, and as a result, the earth ring20′ need not be treated using corrosion-prevention treatments because the interface between the earth ring20′ and housing10′ at contact area15is protected from the environment.

The thickness of the earth ring20′ is at least 4 times the depth at which the peak shear stresses occur. The depth at which the peak shear stresses occur is determined according to the equation below and is based on the expected load applied by a roller (Loadroller), the length of the rollers (Lroller), the inner diameter of the earth ring (Dring), the diameter of the rollers (Droller), the Poisson's ratio of the material of the earth ring (vring), the Poisson's ratio of the material of the rollers (vroller), the Young's modulus of the material of the earth ring (Ering), and the Young's modulus of the material of the rollers (Eroller).

Two examples are shown in the table below.

In this example, the earth ring20′ is 1.5 mm thick and has an inner diameter of 70 mm.

The new housing10′ is thicker than the prior art housing10. The new housing10′ is 9 mm thick in this example.

As in the prior art roller-jammer, the housing10′ comprises aluminium and the earth ring20′ comprises steel which has been case-hardened by carburisation, in this example. The rollers comprise steel.

The output may provide drive to an actuator60. For example, the output provides drive to an actuator60(shown inFIG. 4) operable to move a flap, slat or other flight control surface70(shown inFIG. 4) on an aircraft wing (not shown). The housing10′ is attached to the aircraft structure such that in an overload condition, torque is transmitted from the earth ring20′, to the housing10′ and to the airframe. This prevents the overload torque from being transmitted downstream of the torque limiter200to the actuator60, and thus protects the actuator60from the overload torque.

FIG. 3shows a cross-section of a sprag-type torque limiter300. The structure is similar to that of the roller-jammer200shown inFIG. 2, and the earth ring20′ and housing (not shown) are essentially identical. However, in place of rollers40, the sprag-type torque limiter300comprises sprags140, and in place of a roller cage50, the sprag-type torque limiter300comprises a sprag cage150(to which the output is connected) which holds the sprags140in pockets155. Additionally, the input drive shaft130of sprag-type torque limiter300differs from that of the input drive shaft30of the roller-jammer200, in that the sprag-type torque limiter300input drive shaft130has a circular cross-section with pockets145rather than a lobed cross-section. The radially-inner end of each sprag140is contained within the pockets145in the outer surface of the input drive shaft130, and the radially-outer end of each sprag140is contained within the pockets155in the sprag cage150.

The outer surface of the input drive shaft130defines an inner race of the sprag-type torque limiter and the inner surface of the earth ring20′ defines an outer race of the sprag-type torque limiter.

When not in an overload condition, the sprags140are free to slide between the inner race and outer race, such that both the input drive shaft130and sprag cage150rotate together, thereby transmitting drive from the input drive shaft130to the output (which is connected to the sprag cage).

In an overload condition, the pockets145in the outer surface of the input drive shaft130go out of alignment with the pockets155in the sprag cage150, causing the sprags140to tip such that they are no longer free to slide between the inner race and outer race. Instead the sprags140transmit torque from the input drive shaft130to the earth ring20′ and as a result any further input torque may be transmitted from the input drive shaft130to the earth ring10′ and then to the housing.

A sprag clutch has a similar structure to that of the sprag-type torque limiter described above. However, in the sprag clutch, the sprags140are arranged such that when the input drive shaft130rotates in a first direction the sprags140are free to slide between the inner race and outer race, such that both the input drive shaft130and sprag cage150rotate together, thereby transmitting drive from the input drive shaft130to the output (which is connected to the sprag cage150). When the input drive shaft130rotates in the opposite direction, the sprags140tip such that they are no longer free to slide between the inner race and outer race. Instead the sprags140transmit torque from the input drive shaft130to the earth ring20′ and as a result any further input torque may be transmitted from the input drive shaft130to the earth ring20′ and then to the housing.

FIG. 4shows a part of a secondary flight control system of an aircraft, including a series of actuators60each linked to a flap or slat or other flight control surface70on the leading or trailing edge of the aircraft wings (not shown). The actuators are connected to a transmission system that delivers torque down each wing from a centralised Power Drive Unit (PDU)80. The PDU80provides sufficient torque to drive all surfaces of the system. To protect the actuators60, a torque limiter (which in this case is a roller-jammer200, but could instead be a sprag-type torque limiter) is provided on the transmission line going from the PDU80to each wing. These torque limiters200only allow enough torque for each wing to be transmitted. Then, further torque limiter devices200are incorporated into the actuator60inputs in order to prevent full torque from being transmitted into any one actuator60.