Rotary seals

Abstract

A rotary seal comprising: a shaft with a chromium oxide coating provided at a seal contact area; wherein said coating has a hardness of at least 55 Rockwell-C. The coating may have a surface roughness parameter (Ra) of between 0.2 and 0.4 circular. The coating may have a thickness of at least 0.1 mm. The coating layer forms part of a rotary seal with a flexible seal element that is biased into contact with the surface of the coating. The flexible seal element may be biased against the shaft by a spring to increase the pressure of the flexible seal element against the coating. The coating is preferably deposited by a plasma spray process.

Claims

1. A rotary seal comprising: a shaft with a chromium oxide coating provided at a seal contact area; wherein said coating has a hardness of at least 55 Rockwell-C; wherein the coating has a surface roughness parameter R.sub.a at least 0.2; wherein the coating has a thickness of at least 0.1 mm; and wherein the coating has a surface that has been ground using a non-linear grinding technique.

2. A rotary seal as claimed in claim 1, wherein the coating has a surface roughness parameter R.sub.a of no more than 0.4.

3. A rotary seal as claimed in claim 1, wherein the coating has a surface that has been ground using a circular grinding technique.

4. A rotary seal as claimed in claim 1, comprising a flexible seal element that is biased into contact with the surface of the coating.

5. A rotary seal as claimed in claim 4, wherein the seal comprises a spring arranged to increase the pressure of the flexible seal element against the coating.

6. A rotary seal as claimed in claim 1, wherein the coating has been deposited by plasma spray.

7. An actuator assembly comprising: a gear box; an input shaft; and at least one rotary seal as claimed in claim 1 which seals the gear box around the input shaft.

8. An actuator assembly as claimed in claim 7, wherein the gear box comprises a ball screw.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) One or more non-limiting examples will now be described, by way of example only, and with reference to the accompanying figures in which:

(2) FIG. 1 shows an example of part of an aircraft actuator assembly; and

(3) FIG. 2 shows a single rotary shaft seal in more detail.

DETAILED DESCRIPTION OF THE INVENTION

(4) The actuator assembly 1 of FIG. 1 includes a drive shaft 2 which acts as the input to a gear box 3. Inside the gear box 3, drive shaft 2 drives a worm gear 4 which turns a gear 5, causing rotation of ball screw 6, thereby causing linear movement to actuate (extend or retract) a flap or slat.

(5) Shaft 2 extends through gear box 3 and can therefore provide torque to additional units downstream of gear box 3. To protect the interior of gear box 3, two seal assemblies 7, 8 are provided, one at each point of entry/exit of the shaft 2 (in the figure, one to the left and one to the right). The seals 7, 8 must provide sufficient sealing quality to prevent or minimize the ingress of contaminants from the outside of the gear box 3 to the inside thereof. The ingress of water is harmful to the gear box as it dilutes the lubricant leading to an increase in friction and a corresponding increase in wear of the components (e.g. worm gear 4 and gear 5) of the gear box 3 as well as potentially causing corrosion damage. Any particulate matter that enters the gear box past seals 7, 8 will also increase wear within the gear box 3, eroding the components, increasing clearances and friction and decreasing efficiencies. All of these effects reduce the life span of the gear box and eventually result in failure of the gear box and/or the need for replacement and/or serious maintenance and repair. In particular, the ball screw 6 is a high precision component that is particularly susceptible to wear. The seals 7, 8 are required to provide sealing both during intermittent periods of rotation (actuation of flaps/slats) and the intervening stationary intervals. Rotation of the shaft may be in either direction depending on the desired movement direction of the flap/slat.

(6) The arrangement of seal 7 is shown in more detail in FIG. 2. The same principles apply to the seal 8. A layer 10 of chromium oxide (Cr.sub.2O.sub.3) has been formed on the exterior surface of shaft 2 by plasma spraying. The shaft 2 is formed from a steel material such as PH13. The plasma sprayed chromium oxide layer 10 has a Rockwell-C hardness of at least 55. The layer 10 is at least 0.1 mm thick in the radial direction (i.e. with respect to the radius of the shaft) and forms a band around the whole circumference of the shaft 2. The chromium oxide layer 10 has been finished with a circular grinding process that results in a surface roughness (R.sub.a) of between 0.2 and 0.4 (this may be designated R.sub.a=0.2-0.4 circular).

(7) The layer 10 is not formed along the entire surface of the shaft 2, but is only formed in the vicinity of the sealing member 11 that in use presses against the layer 10 of shaft 2. The chromium oxide is relatively expensive and therefore it is desired not to apply the coating in areas other than where it is required for wear resistance, i.e. in the contact area of the seal so as to form a seal land. The plasma spray process to achieve the required properties is also an expensive process and, when it is to be used in aerospace applications, it is treated as a special process with subsequent checks to ensure that the process has performed to requirements.

(8) The seal member 11 is typically made from a flexible material such as a rubber or other elastomeric material. The seal member 11 is biased by spring 12 so as to increase the force between the seal member 11 and the shaft 2 (more specifically the chromium oxide layer 10) so as to improve and maintain sealing quality.

(9) The high hardness of the chromium oxide layer 10 with its particular surface roughness provides excellent sealing qualities as well as being highly resistant to wear even under the increased force applied by spring 12. The coating thickness ensures that these properties can be maintained for a long service life, i.e. with a long MTBF suitable for use in aircraft systems where safety and reliability are critical.

(10) While it will be appreciated that the above description has been provided in relation to an actuator for flaps and slats on an aircraft, being a system in which the shaft typically rotates intermittently at around 1000 rpm with long periods of rest in between and rotation in either direction, the improved seal is also relevant to other applications such as drive shafts that may rotate at significantly higher speeds, for longer continuous operation and where rotation is largely unidirectional.