Patent Application
20180128332
This application claims priority to European Patent Application No. 16306457.9 filed Nov. 7, 2016, the entire contents of which is incorporated herein by reference.
The present invention concerns a friction disk, particularly, but not exclusively, to a friction disk for a no-back system comprising friction disks providing a friction surface for frictional engagement with another friction surface, and preferably operating in an oil-immersed environment. Such no-back systems are used, for example (and not exclusively) in aircraft systems such as trimmable horizontal stabilizer actuators or THSAs i.e. systems for actuating an element for adjusting the horizontal plane or other moveable elements in an aircraft.
Many applications are known where motion is provided or controlled using frictional engagement between two frictional surfaces. One such application is in so-called no-back systems, also known as non-return systems or no-brake systems. Such systems are provided as a safety mechanism in a rotating system where a load is applied to a shaft e.g. in an actuator. The no-back brake helps prevent uncommanded movements or ‘blowback’ if an actuator, for example, malfunctions.
For example, the actuation of a moveable part e.g. on an aircraft, can be effected by means of a worm-screw or a ball-screw actuating system where the screw is attached on the one hand to the aircraft structure and on the other hand to the moveable part to be actuated. The function of the no-back device is to eliminate the effect of any forces which tend to oppose, back drive or favour the normal rotation of the screw (e.g. aerodynamic forces).
If, in such systems, a load were allowed to act against the input torque applied to the actuating screw, it would then be necessary to apply an even larger input torque which would require larger motors. Similarly, an assisting torque acting on the input torque applied to the actuating screw would contribute energy but would also cause difficulties regarding speed regulation and maintaining position of the actuating device.
A no-back device is used to prevent the screw from starting to rotate under the effect of an axial aerodynamic load. Any assisting or opposing force is then eliminated.
No-back systems are disclosed, for example, in EP0960811, FR2989060 and US2008/0000730.
Known systems can be complex and involve many moving parts, each of which may be subject to failure.
In some operating conditions, e.g. in rotating systems such as no-back systems where the axial load is small, combined with a high rotational velocity, a polishing phenomenon can occur. It has been found that in such conditions, particularly where the force of contact between friction surfaces is small, there can be a loss of friction. Loss of friction has been found to occur when hydrodynamic lubrication occurs.
The aim of the present invention is to provide a friction disk as well as a no-back system comprising such a friction disk, which can reduce or avoid this loss of friction.
Accordingly, the disclosure provides a disk having a first major surface and a second major surface, opposite the first, joined by a peripheral edge, at least one of the first and second major surfaces defining a friction surface configured to frictionally contact an opposing friction surface in a rotating system, and wherein a groove formation is provided on the friction surface(s), the groove formation being non-symmetrical with respect to the respective friction surface.
The non-symmetry is with respect to a sliding direction of the disk.
Preferably, both of the first and the second major surfaces define a respective friction surface.
The groove formation has advantages if it is in any non-symmetrical or asymmetrical form, but the most preferred form is a spiral groove extending around and across the friction surface.
Most preferably, the spiral or other non-symmetrical groove extends over the entire friction surface.
To further improve friction, radial grooves may be provided extending radially across the friction surface and across the non-symmetrical groove formation.
The grooves are preferably machined into the disk surface(s).
The disclosure also provides a rotating system comprising a disk as defined above defining a first friction surface and a further component having a surface defining a second friction surface, there being relative rotation and frictional engagement between the first and second friction surfaces.
The rotating system may be a no-back system and then preferably has two such disks which are in frictional engagement with friction surfaces of other components such as ratchet wheels and a screw shaft flange.
Preferred embodiments will now be described by way of example only, with reference to the drawings.
Two ratchet wheels 22 are also mounted rotatably and coaxially about the screw shaft, adjacent the friction disks 21. The ratchet wheels 22 cooperate with pawls 23 arranged to permit rotation of the ratchet wheels in one direction but not in an opposite direction.
In one example, to move a moveable element, a force is applied to the screw shaft 15. The no-back device provides a force resisting rotation of the screw shaft in a direction that would result in movement in one direction (e.g. the direction of aerodynamic force) whilst providing no force resisting rotation in the opposite direction.
When a force is applied in one direction (e.g. ‘nose up’), one surface of the first disk is a ‘first friction surface’ in frictional engagement with the flange 20 providing a ‘second friction surface’ and the other surface of the disk is in frictional engagement with the associated ratchet wheel. The other disk is freely rotatable. In another direction (e.g. ‘nose down’) the other disk is in frictional engagement with the flange and its associated ratchet wheel. In the following discussion and claims, the ‘first friction surface’ is the disk friction surface being discussed and the ‘second friction surface’ is the opposing surface with which it frictionally engages (e.g. ratchet wheel, flange).
The friction disks 21 comprise opposing major surfaces, providing friction surfaces, joined by a peripheral edge (
At the start of relative movement between the first and second friction surfaces, the coefficient of friction is high to compensate for a tangential force depending on the adhesion of the two surfaces, with little lubricant. As movement continues, sliding velocity increases and the coefficient of friction decreases rapidly with a small amount of lubricant.
The decrease in the coefficient of friction then accelerates with the sliding velocity to a lowest critical point where the system is in a mixed or heterogenous lubrication regime. Then, the coefficient of friction increases parabolically with respect to the sliding velocity in an established hydrodynamic regime. The change from the mixed to the hydrodynamic regime gives rise to the problem of loss of friction.
The actual details of the other components of the no-back system will not be described in further detail, as these can be any known system having opposing friction surfaces. One example is described in U.S. Pat. No. 6,109,415. The improvement resides in the form of the friction disks and disks modified according to this disclosure could be used in any known no-back systems of this type.
As indicated above, the friction disks are immersed in and operate in a bath of fluid such as oil or other viscous fluid, to provide lubrication between the first and second friction surfaces.
In earlier systems, a wedge or film of oil was formed between the disks, which was found to have an adverse effect on the coefficient of friction.
This has been resolved by forming, in or on at least one of the major friction surfaces of the disks, drainage grooves. It has been considered preferable for these grooves to be fine and closely spaced and to preferably have their terminal openings outside the friction zone. GB 1,084,958 teaches such drainage grooves.
It has also been discovered that the fluid deposit can be further reduced by providing a second set of grooves transverse to the first grooves (see, again, GB 1,084,958). An alternative pattern of drainage grooves is described in FR 2989060.
The inventors of the present disclosure have determined that the symmetric pattern of grooves in the known systems still result in a loss of surface friction and that surface friction is not, therefore, optimised, particularly during the hydrodynamic lubrication regime.
According to the present disclosure, and with reference to
Most preferably, the non-symmetrical groove pattern is formed as a spiral groove or several spirals around and across the friction surface. Most preferably, the spiral groove(s) covers the whole friction surface—i.e. extends all around and radially right across the disk as shown in
To further optimise surface friction, transverse or radially extending grooves 40 are provided across the friction surface of the disk and across the spiral groove 30 as shown.
Advantages are provided by a symmetrical groove pattern on either friction surface of the disk or on both friction surfaces of the disk.
Such drainage grooves have been found to optimise surface friction in systems where two disks have rotational frictional contact with opposing friction surfaces, such as, but not exclusively, in a no-back system.
| Number | Date | Country | Kind |
|---|---|---|---|
| 16306457.9 | Nov 2016 | EP | regional |
