TORQUE TRANSMISSION DEVICE

Abstract

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.

Claims

1. A torque transmission device comprising: an input drive shaft; a housing; an earth ring having a thickness; 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.

2. A torque transmission device according to claim 1, wherein the contact area takes the shape of a curved surface of a cylinder, wherein torque is transmitted from the earth ring to the housing only via the contact area.

3. A torque transmission device according to claim 1, wherein an output is connected to the bearing cage.

4. A torque transmission device according to claim 3, wherein the torque transmission device is arranged such that, in a first operating condition, the output is operable to rotate on rotation of the input drive shaft, wherein the torque transmission device is arranged such that, in a second operating condition, torque is prevented from being transmitted from the input drive shaft to the output.

5. A torque transmission device according to claim 4, wherein the torque transmission device is 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, and optionally the torque limiter may be a roller-jammer or a sprag-type torque limiter.

6. A torque transmission device according to claim 4, wherein the torque transmission device is a one-way freewheel, wherein the first operating is a condition in which the input drive shaft rotates in a first direction, and the second operating condition is a condition in which the input drive shaft rotates in an opposite direction, and optionally the one-way freewheel may be a sprag clutch.

7. A torque transmission device according to claim 4, wherein the torque transmission device is arranged such that, in the second operating condition, the bearings bear against the earth ring to apply torque to the earth ring.

8. A torque transmission device according to claim 7, wherein the earth ring is configured to bear a contact stress applied by the bearings on the earth ring.

9. A torque transmission device according to claim 8, wherein the earth ring is sufficiently thick to bear the contact stress applied by the bearings on the earth ring.

10. A torque transmission device according to claim 1, wherein the earth ring is flexible 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.

11. A torque transmission device according to claim 10, wherein the earth ring is 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.

12. A torque transmission device according to claim 1, wherein the earth ring is 1 to 5 mm thick; and wherein the earth ring is thicker than the depth at which a peak shear stress occurs, and optionally is 4 to 10 times thicker than the depth below the surface at which the peak shear stress occurs

13. A torque transmission device according to claim 1, wherein the earth ring comprises case-hardened metal and wherein: the earth ring is case-hardened to a depth of 2 to 5 times the depth at which the peak contact stresses occur in the earth ring, the earth ring is case-hardened to a depth of about 0.5 mm to 1 mm, or the earth ring is case-hardened to a depth of 40% to 60% of the thickness of the earth ring.

14. A torque transmission device according to claim 1, wherein: the housing comprises a lighter material than the earth ring, and the housing is 6 to 10 times thicker than the thickness of earth ring.

15. A secondary flight system for an aircraft comprising: a centralised power drive unit; a plurality of actuators; a transmission arranged to transmit torque from the centralised power drive unit to the plurality of actuators; and a torque transmission device as claimed in claim 1, wherein the torque transmission device is a torque limiter arranged to limit the torque transmitted from the centralised power drive unit to the plurality of actuators.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0062] Non-limiting examples will now be described, with reference to the accompanying drawings, in which:

[0063] FIG. 1A shows a cross-section of a prior art roller jammer;

[0064] FIG. 1B shows a perspective view of the housing of the prior art roller-jammer of FIG. 1A;

[0065] FIG. 1C shows a perspective view of the earth ring of the prior art roller-jammer of FIG. 1A;

[0066] FIG. 2A shows a cross-section of an exemplary roller-jammer according to the present disclosure;

[0067] FIG. 2B shows a perspective view of the housing of the exemplary roller-jammer of FIG. 2A;

[0068] FIG. 2C shows a perspective view of the earth ring of the exemplary roller-jammer of FIG. 2A;

[0069] FIG. 3 shows a cross-section of an exemplary sprag-type torque limiter or sprag clutch; and

[0070] FIG. 4 shows a schematic of an aircraft flight control surface actuation system, comprising the exemplary roller jammer of FIG. 2.

DETAILED DESCRIPTION

[0071] FIG. 1A shows a cross-sectional view of a prior art roller-jammer 100. The roller-jammer 100 is provided in a mechanical system in which torque is transmitted by an input drive shaft 30. The input drive shaft 30 is supported for rotation within a housing 10 (shown also in FIG. 1B). Mounted adjacent the housing is an earth ring 20 (shown also in FIG. 1C). Torque is transmitted from the earth ring 20 to the housing 10 via dogs 22 on the earth ring that interlock with corresponding dogs 12 on the housing. The housing 10 comprises aluminium and the earth ring 20 comprises steel that has been case-hardened by carburisation.

[0072] The case-hardened steel earth ring 20 is not corrosion resistant, and since it is mounted adjacent to a non-ferrous housing 10 and is not protected from the surrounding environment, expensive corrosion-protection treatments are required. The case-hardened steel earth ring 20 comprises between 5% and 11% of the total weight of the roller-jammer unit. That is, the earth ring 20 is a comparatively heavy component.

[0073] A roller cage 50 is coaxial with and encircles a length of the input drive shaft 30 within the housing 10. The roller cage 50 is connected to the input drive shaft 30 by a torsion bar (not shown).

[0074] The roller cage 50 positions a plurality of rollers 40 in pockets 45 defined between the outer surface 30a of the input drive shaft 30 and the inner surface 20a of the earth ring 20. The outer surface 30a of the input drive shaft 30 has a lobed shape, when viewed in cross-section (as seen in FIG. 1A). In the present example, the outer surface 30a of the input drive shaft 30 has an approximately hexagonal shape, with flattened points and inwardly-bowed arcs instead of straight sides. The inner surface 20a of the earth ring 20 is a circle, when viewed in cross-section (as seen in FIG. 1A). A pocket 45 for receiving a roller 40 is defined between each side of the outer surface 30a of the input drive shaft 30 and the inner surface 20a of the earth ring 20. As will be clear from the foregoing and from FIG. 1A, the pocket is thus of non-uniform radial clearance, i.e. the radial distance between the outer surface 30a of the input drive shaft 30 and the inner surface 20a of the earth ring 20 varies circumferentially.

[0075] When the input torque is below a predetermined threshold, the roller cage 50 is held in alignment with the input drive shaft 30 by a torsion bar, and in such a condition the roller cage 50 holds the plurality of rollers 40 in the parts of the pocket 45 of maximum radial clearance (i.e. maximum radial distance between the outer surface 30a of the input drive shaft 30 and the inner surface 20a of the earth ring 20). That is, the rollers 40 are held in the central part of the pocket 45. This allows the rollers 40 to slide, and both the input drive shaft 30 and roller cage 50 rotate together.

[0076] 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 cage 50 relative to the input drive shaft 30 results in the rollers 40 no longer being held in the centre of the pocket 45 (i.e. at the position of maximum clearance between the outer surface 30a of the input drive shaft 30 and the inner surface 20a of the earth ring 20. Instead, the rollers 40 are forced away from the centre of the pocket 45 to bear against both the outer surface 30a of the input drive shaft 30 and the inner surface 20a of the earth ring 20 and become jammed therebetween. As a result, the rotation of the input drive shaft 30 is resisted.

[0077] In such a case, huge contact stresses occur between the rollers 40 and earth ring 20. Additionally, significant hoop stresses are imparted to the roller-jammer. In this prior art example, the earth ring 20 bears the contact pressures and hoop stresses. Moreover, the earth ring dogs 22 must be of a sufficient size and strength to transmit the torque to the housing 10 via the housing dogs 12. As a result, the earth ring 20 must 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 ring 20 is approximately two to three times thicker than the housing 10.

[0078] FIG. 2A shows a cross section of a roller-jammer 200 in accordance with the present disclosure. The roller-jammer 200 comprises: an input drive shaft 30, a housing 10, an earth ring 20, a roller cage 50, and a plurality of rollers 40 held by the roller cage 50 in pockets 45 between an inner surface 24 of the earth ring 20 and an outer surface of the input drive shaft 30. The earth ring 20 is mounted within the housing 10 such that an outer surface 22 of the earth ring 20 contacts an inner surface 12 of the housing 10 along a contact area 15, thereby providing a frictional interface at the contact area 15 for transmitting torque from the earth ring 20 to the housing 10.

[0079] The input drive shaft 30, roller cage 50, rollers 40 and pockets 45 are unchanged compared to FIG. 1A. However, the housing 10 and earth ring 20 have been replaced by a new housing 10 and new earth ring 20 (shown respectively in FIG. 2B and FIG. 2C). The new earth ring 20 is thinner than the prior art earth ring 20, and so the roller jammer unit is lighter (by about 4%). The new earth ring 20 is mounted within the housing 10, rather than adjacent thereto, and as a result, the earth ring 20 need not be treated using corrosion-prevention treatments because the interface between the earth ring 20 and housing 10 at contact area 15 is protected from the environment.

[0080] The thickness of the earth ring 20 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 (Load.sub.roller), the length of the rollers (L.sub.roller), the inner diameter of the earth ring (D.sub.ring), the diameter of the rollers (D.sub.roller), the Poisson's ratio of the material of the earth ring (v.sub.ring), the Poisson's ratio of the material of the rollers (v.sub.roller), the Young's modulus of the material of the earth ring (E.sub.ring), and the Young's modulus of the material of the rollers (E.sub.roller).

[00002]${\mathrm{shear}}_{\mathrm{peak}}:=0.64.Math.\sqrt{\frac{{\mathrm{Load}}_{\mathrm{roller}}}{L_{\mathrm{roller}}}.Math.\frac{D_{\mathrm{ring}}.Math.D_{\mathrm{roller}}}{\left(D_{\mathrm{ring}}-D_{\mathrm{roller}}\right)}.Math.\left(\frac{1-v_{\mathrm{ring}}^2}{E_{\mathrm{ring}}}+\frac{1-v_{\mathrm{roller}}^2}{E_{\mathrm{roller}}}\right)}$

[0081] Two examples are shown in the table below.

TABLE-US-00001 Example 1 Example 2 Material of earth ring Carburised steel Steel Material of bearing Steel Steel Load.sub.roller 26.7 kN 30.2 kN L.sub.roller 24 mm 12 mm D.sub.ring 70 mm 70 mm D.sub.roller 12 mm 12 mm .sub.ring 0.3 0.3 .sub.roller 0.3 0.3 E.sub.ring 1.76 10.sup.5 MPa 2.1 10.sup.5 MPa E.sub.roller 2.1 10.sup.5 MPa 2.1 10.sup.5 MPa Peak shear stress depth 0.25 mm 0.36 mm Minimum earth ring 1.0 mm 1.44 mm thickness

[0082] In this example, the earth ring 20 is 1.5 mm thick and has an inner diameter of 70 mm.

[0083] The new housing 10 is thicker than the prior art housing 10. The new housing 10 is 9 mm thick in this example.

[0084] As in the prior art roller-jammer, the housing 10 comprises aluminium and the earth ring 20 comprises steel which has been case-hardened by carburisation, in this example. The rollers comprise steel.

[0085] The output may provide drive to an actuator 60. For example, the output provides drive to an actuator 60 (shown in FIG. 4) operable to move a flap, slat or other flight control surface 70 (shown in FIG. 4) on an aircraft wing (not shown). The housing 10 is attached to the aircraft structure such that in an overload condition, torque is transmitted from the earth ring 20, to the housing 10 and to the airframe. This prevents the overload torque from being transmitted downstream of the torque limiter 200 to the actuator 60, and thus protects the actuator 60 from the overload torque.

[0086] FIG. 3 shows a cross-section of a sprag-type torque limiter 300. The structure is similar to that of the roller-jammer 200 shown in FIG. 2, and the earth ring 20 and housing (not shown) are essentially identical. However, in place of rollers 40, the sprag-type torque limiter 300 comprises sprags 140, and in place of a roller cage 50, the sprag-type torque limiter 300 comprises a sprag cage 150 (to which the output is connected) which holds the sprags 140 in pockets 155. Additionally, the input drive shaft 130 of sprag-type torque limiter 300 differs from that of the input drive shaft 30 of the roller-jammer 200, in that the sprag-type torque limiter 300 input drive shaft 130 has a circular cross-section with pockets 145 rather than a lobed cross-section. The radially-inner end of each sprag 140 is contained within the pockets 145 in the outer surface of the input drive shaft 130, and the radially-outer end of each sprag 140 is contained within the pockets 155 in the sprag cage 150.

[0087] The outer surface of the input drive shaft 130 defines an inner race of the sprag-type torque limiter and the inner surface of the earth ring 20 defines an outer race of the sprag-type torque limiter.

[0088] When not in an overload condition, the sprags 140 are free to slide between the inner race and outer race, such that both the input drive shaft 130 and sprag cage 150 rotate together, thereby transmitting drive from the input drive shaft 130 to the output (which is connected to the sprag cage).

[0089] In an overload condition, the pockets 145 in the outer surface of the input drive shaft 130 go out of alignment with the pockets 155 in the sprag cage 150, causing the sprags 140 to tip such that they are no longer free to slide between the inner race and outer race. Instead the sprags 140 transmit torque from the input drive shaft 130 to the earth ring 20 and as a result any further input torque may be transmitted from the input drive shaft 130 to the earth ring 10 and then to the housing.

[0090] A sprag clutch has a similar structure to that of the sprag-type torque limiter described above. However, in the sprag clutch, the sprags 140 are arranged such that when the input drive shaft 130 rotates in a first direction the sprags 140 are free to slide between the inner race and outer race, such that both the input drive shaft 130 and sprag cage 150 rotate together, thereby transmitting drive from the input drive shaft 130 to the output (which is connected to the sprag cage 150). When the input drive shaft 130 rotates in the opposite direction, the sprags 140 tip such that they are no longer free to slide between the inner race and outer race. Instead the sprags 140 transmit torque from the input drive shaft 130 to the earth ring 20 and as a result any further input torque may be transmitted from the input drive shaft 130 to the earth ring 20 and then to the housing.

[0091] FIG. 4 shows a part of a secondary flight control system of an aircraft, including a series of actuators 60 each linked to a flap or slat or other flight control surface 70 on 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 PDU 80 provides sufficient torque to drive all surfaces of the system. To protect the actuators 60, a torque limiter (which in this case is a roller-jammer 200, but could instead be a sprag-type torque limiter) is provided on the transmission line going from the PDU 80 to each wing. These torque limiters 200 only allow enough torque for each wing to be transmitted. Then, further torque limiter devices 200 are incorporated into the actuator 60 inputs in order to prevent full torque from being transmitted into any one actuator 60.