Ball screw assembly

Abstract

A ball screw assembly includes a screw shaft along which is formed a first helical groove; a nut along which is form a second helical groove; the first helical groove and the second helical groove cooperating to define a track, a plurality of balls arranged in the track and configured to move along the track in response to relative motion between the screw shaft and the nut such that rotational motion of the screw is translated to linear motion of the nut via the balls and vice versa. The assembly also includes a bypass shoe arranged between the nut and the track and spaced from the track by a predetermined preload X, wherein when a load applied to the nut exceeds the predetermined preload, the bypass shoe engages with the track such that motion of the screw is transferred to motion of the nut via the shoe and bypasses the balls.

Claims

1. A ball screw assembly comprising: a screw shaft along which is formed a first helical groove; a nut cartridge (2′) in which the nut is housed, and wherein the nut cartridge is provided with means for attachment to a load to be moved by the assembly; a nut along which is form a second helical groove; the first helical groove and the second helical groove cooperating to define a track; a plurality of balls arranged in the track and configured to move along the track from a first end to a second end of the track in response to relative motion between the screw shaft and the nut such that rotational motion of the screw is translated to linear motion of the nut via the balls and vice versa, wherein the balls return from the second end of the track to the first end via a ball return channel in the nut cartridge; and a bypass shoe arranged between the nut cartridge and the nut and spaced from the track by a predetermined preload X selected as being equal to the maximum normal operating dynamic load for the ball screw plus a predetermined error margin, wherein when a load applied to the nut does not exceed the predetermined preload X, torque is transmitted from the screw shaft to the nut via the balls, and wherein when a load is applied to the nut that exceeds the predetermined preload, the bypass shoe engages with the track such that motion of the screw is transferred to motion of the nut via the shoe and bypasses the balls.

2. The assembly of claim 1, wherein the bypass shoe is spaced from the track by a distance in the order of 0.1 to 0.5 mm.

3. The assembly of claim 1, wherein the bypass shoe is responsive to relative motion in a single direction.

4. The assembly of claim 1, wherein the bypass shoe is responsive to relative motion in two directions.

5. The assembly of claim 1, wherein the first and second helical grooves are made of AMS 5659 (15-5 PH).

6. A Thrust Reverser Actuation System, TRAS, comprising: a surface to be actuated; and an actuator comprising: a ball screw assembly as claimed in claim 1, arranged to move the surface responsive to the relative motion between the screw shaft and the nut.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A shows, by way of background and example only, a load plot for the forces acting on a TRAS outer actuator.

(2) FIG. 1B shows, by way of background and example only, a load plot for the forces acting on a TRAS centre actuator.

(3) FIG. 1C shows, by way of background and example only, a load probability plot for the forces acting on a TRAS outer actuator.

(4) FIG. 1D shows, by way of background and example only, a load probability plot for the forces acting on a TRAS centre actuator.

(5) FIG. 2 is a side schematic view of a ball screw assembly according to the disclosure under normal load conditions.

(6) FIG. 3 is a side schematic view of a ball screw assembly according to the disclosure under high load conditions.

DETAILED DESCRIPTION

(7) With reference to FIGS. 1A-1D, and by way of background explanation only, it can be seen that only a very small part of the forces act on the actuator during the majority of the time of operation — i.e. during normal operation (represented as NLD—normal landing deployment). Therefore, for the majority of the time, the ball screw only needs to cope with a relatively small load acting on its components. High forces are rare. In the example shown, the NLD force is in the region of 4000N. The maximum failure load is around 13 times greater but rarely or never occurs.

(8) Nevertheless, because these high forces can occur, the ball screw needs to be designed to cope with them if/when they do occur. For this reason, ball screws are designed with materials hard enough to provide the required static load capacity for such loads. Such materials, e.g. Chronidur 30™, or martensitic alloys with high chromium content are generally very expensive.

(9) The life of the ball screw tracks is also an important design factor. In accordance with ISO 3408-1, the ball screw life with a 95% reliability, L.sub.5, is derived using the following equation:
L.sub.5=(C.sub.dm/P.sub.EQ).sup.3*f.sub.r

(10) Where:

(11) P.sub.EQ is the equivalent load

(12) C.sub.dm=modified dynamic capacity

(13) f.sub.r=corrective factor for reliability (for 95% probability)

(14) And C.sub.dm=f.sub.k*f.sub.a*f.sub.m*C.sub.d

(15) Where:

(16) f.sub.k=(Track Hardness/654 Hv).sup.2

(17) f.sub.a=corrective factor for precision

(18) f.sub.m=corrective factor for material

(19) C.sub.d=dynamic capacity

(20) It can be seen, therefore, that the life of the ball screw is a function of the square of the hardness quotient.

(21) Further, in accordance with ISO 3408-1, the static load capacity C.sub.oam of the ball screw is derived using the equation:
C.sub.oam=C.sub.oa*f.sub.ho*f.sub.a

(22) Where f.sub.ho=(Track Hardness/654 Hv).sup.3

(23) Here, again, it can be seen that static load capacity is a function of the cube of the hardness quotient.

(24) Using the current design strategy, it can be seen that track hardness is the dominant factor in the determination of both life and static load capacity and that ball screws are having to be made of very hard, expensive materials for the rare event that a high failure force occurs and that less hard and, thus, less expensive materials would provide sufficient static load capacity and adequate life for the majority of operation.

(25) The solution provided by this disclosure allows ball screws to be used in these same applications but to be made using less hard, less expensive materials and lower production costs.

(26) The ball screw according to this disclosure is designed such that a static load that exceeds a predetermined threshold, based on the normal operating dynamic loads with an error margin factored in, effectively bypasses the ball track so that the ball screw tracks and balls only need to be designed to handle static loads up to the predetermined threshold.

(27) In an example, the predetermined threshold might be, say, 1.2 times the maximum normal operating dynamic load, rather than the conventional 13 or so factor mentioned above. This design would enable a significant reduction in track hardness requirements.

(28) Furthermore, depending on the application, the target life margin of safety can be less than is currently designed for, since not all ball screw applications will require the margins that are currently built into the design of all ball screws. In aerospace applications, for example, TRAS actuators do not require the high life margins that are currently designed into ball screws for e.g. primary control actuator ball screws.

(29) From an endurance life perspective, according to this disclosure, it is possible to specify an appropriate life margin of safety for the particular application and, from that, derive the minimum hardness requirement to achieve that margin, from the equations above.

(30) Because the assembly of this disclosure incorporates a bypass function for higher loads, the wear on the components will be less and this bears on the required life margins and calculations.

(31) The bypass function according to this disclosure will now be described in more detail with reference to FIGS. 2 and 3.

(32) The bypass function is provided, according to the disclosure, by introducing a pre-loading system between the ball screw nut cartridge and where the cartridge attaches to an actuator.

(33) With reference to FIG. 2, a ball screw arrangement is shown under normal load operating conditions.

(34) As in conventional systems, the ball screw comprises a screw or shaft 1 and a nut 2. The nut 2 and the screw shaft 1 are provided with opposing helical grooves or turns 3, 3′ acting as a ball track for balls 4 arranged therein. Rotation of the screw 1 relative to the nut 2 causes the balls 4 to move along the helical grooves which drives the nut axially or linearly along the screw. The nut includes a cartridge through which the balls 4 return to the start of the grooves. The nut cartridge 2′ is provided with an attachment housing 5 that attaches to the load to be moved e.g. to an actuator (not shown). A load is applied to the ball screw by the actuator.

(35) To provide the load bypass function when the load exceeds a predetermined threshold X, a bypass shoe 6 is provided between the nut housing 5 and the nut 2. The bypass shoe 6 sits in the groove 3 of the screw 1 with a small clearance 7 of e.g. approximately 0.10 mm. The nut 2 is preloaded to the housing 5 at the load X. X is selected to include the maximum normal operating load plus a predetermined margin. X may be e.g. 1.2 times the maximum normal operating load.

(36) Thus, during normal operation, so long as the load does not exceed X—i.e. for all normally occurring dynamic loads—the bypass shoe 7 is spaced from the screw groove and torque is transmitted from the screw 1 to the nut 2 via the balls 4 in the usual way.

(37) If, however, the load exceeds value X, as shown in FIG. 3, the nut overcomes the preload and moves such that the shoe 7 engages with the groove 3 in the screw. The application of the load exceeding value X is then transferred to the nut housing 5 via the bypass show 7, thus bypassing the balls 4.

(38) The bypass function can be designed to be either uni-directional or bi-directional.

(39) As well as meaning that the ball screw components do not need to be designed with capacity for excessive failure loads, so that less hard materials e.g. AMS 5659 (15-5 PH) can be used, the bypass feature has an additional benefit. The bypass function effectively converts the ball screw into a lead screw when the threshold load is exceeded. At this stage, the drive efficiency of the actuator is reduced which means that conventional mechanical load limiters are not required. This also contributes to reducing cost, size and weight of the assembly.

(40) As well as being less expensive, the less hard materials mean that the ball screw tracks can be formed by simple turning operations rather than requiring special grinding processes.

(41) As an alternative to taking the opportunity to use less hard materials, the by-pass function could also be used to reduce the number of balls in the ball screw, which has the effect of increasing efficiency as well as reducing costs.

(42) The bypass function has particular benefits in relation to TRAS applications, but the design of this disclosure is not limited to such applications and can provide advantages in many ball screw applications.