Technical Field The present invention relates generally to the field of mechanical engineering and, in particular, to a sprag clutch. The invention provides in particular a design of sprag clutch that addresses lubrication issues and wear associated with conventional sprag clutches.

Background

A clutch is used to control the application and transmission of power. The most common clutches are friction clutches, sprag clutches and roller clutches. The roller clutch is simple and low cost, but sprag clutches can provide higher locking torque than the roller clutch. Sprag clutches are one-way freewheel clutches and they are well known in the art, with many commercially available examples. In basic terms, a sprag clutch resembles a roller bearing, but instead of cylindrical rollers, non-revolving asymmetric figure-of-eight shaped sprags are used. When the unit rotates in one direction the sprags slip or free-wheel, but when a torque is applied in the opposite direction, the sprags tilt slightly, producing a wedging action and frictionally binding to the raceways. The sprags are spring-loaded so that they lock with very little backlash. Because of the design of the sprags and the one-way power application, it is particularly suitable for some designs of automatic gearbox and for helicopters (to allow faster rotation and, hence, autorotation in an emergency). A standard sprag clutch is lubricated by oil circulation. To reduce the size, the weight and the cost of the oil circulation system and to improve the reliability of the lubrication system, grease lubrication is preferred. However, significant wear and seizure may be generated on the engaged surface between sprags and inner ring raceway when lubricated by grease. The extensive wear will reduce the clearance of the contact, leading to loss of locking function.

Accordingly, it is an object of the present invention to provide an improved sprag clutch and/or tackle at least some of the problems associated with the prior art or, at least, to provide a commercially useful alternative thereto.

Summary

In a first aspect, the present invention provides a sprag clutch comprising an inner raceway, an outer raceway and a plurality of sprags between said inner and outer raceways, wherein a contact surface of said inner raceway for contacting a surface of each sprag has a porous coating.

The present disclosure will now be further described. In the following passages different aspects of the disclosure are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. The present inventors have discovered that some of the wear damage which occurs in sprag clutches is due to lubrication starvation on the contact surfaces. In particular, they have discovered that the lubrication starvation may be caused by the following reasons: 1 ) the sliding and the high centrifugal force arising from the rotation of the raceways drives the grease away from the inner ring surface, and 2) there is no re-supplier of the grease to the contact surface as it is a sliding contact.

The present inventors have discovered that it is possible to at least partially address this lubrication starvation by the provision of a porous coating on at least some of the active surfaces within the clutch. In particular, by providing a porous coating on the inner raceway it is possible to (1 ) have the pores acts as a reservoir to supply lubrication to the otherwise lubrication-starved inner raceway contact surface and/or (2) to provide a capillary effect to resist the loss of lubrication from the inner raceway by the centrifugal effects of rotation.

Accordingly, the contact surface of the inner raceway is provided with a porous coating. Preferably, other surfaces may also be provided with a coating as discussed below, including non-contacting portions of the inner raceway, the contact surface of the outer raceway, at least the contact surfaces of the sprags and surfaces of the retainer.

The present inventors have also discovered that it is possible to at least partially address the problem of lubrication starvation by the provision of one or more rollers with the sprags. Preferably the sprag clutch further comprises one or more rollers, each roller being in contact with the inner raceway and the outer raceway and located between a first and a second sprag of the plurality of sprags. By rotation within the clutch between the raceways, it serves to transfer some lubricant from the outer raceway to the inner raceway and, hence, increase the lubrication of the inner raceway and reduce wear.

It is generally the case that a sprag clutch contains as many sprags as possible to give the best torque transfer for a given size and weight of device. That is, the more sprags that are available, the greater the surface area for the transfer of rotational force. Accordingly, the inclusion of one or more rollers in place of sprags is counter-intuitive since this will lessen the torque transfer that is available. However, the inventors have found that there is a balance between the torque transfer and the clutch longevity and wear resistance that can be found which favours the sprag clutch of the present invention. Preferably the sprag clutch comprises two or more rollers, each located between a different pair of sprags. The introduction of additional rollers helps to increase the amount of lubrication which can be returned to the inner raceway. Preferably the sprag clutch comprises three or more rollers, each located between a pair of sprags. The use of three of more rollers helps to distribute the load evenly around the clutch so as to ensure even load balancing and to reduce wear. Preferably the rollers are evenly distributed to further reduce the wear. That is, preferably the rollers are evenly spaced around the clutch.

Preferably the ratio of sprags to roller is from 1 :1 to 10:1 , more preferably from 2:1 to 5:1. The presence of additional rollers reduces the torque that can be effectively applied because the number of sprags is reduced. In one embodiment, when the ratio is 1 :1 , the sprag clutch has alternately sprags and rollers.

Preferably the one or more rollers, where present, are hollow. This serves to reduce the inertia of the rollers and to provide a preload to allow the spinning of the roller. Preferably the width of the roller is slightly larger than the width of the sprags to transport lubricant that has been moved out of the contact by the sprags.

The roller may be provided with a surface pattern to transport lubricant from the edges to the centre of the raceways. In one especially preferred embodiment the surface finish of the roller is double-helix shaped.

In a sprag clutch the sprags are generally held in a retainer or sprag cage between the inner and outer raceways. Retainers are well known in the art and can take various different forms, all having the same effect. The retainer helps to ensure that the sprags are correctly, evenly spaced and suitably aligned to work within the device. The retainer typically incorporates one or more springs or resilient members to ensure that the sprags engage appropriately. The sprag clutch of the present invention preferably comprises a retainer.

In one embodiment, the retainer is secured relative to the inner ring. This means that sliding contact of the sprags occurs at the outer raceway. This is advantageous because the centrifugal effects mean that the outer-raceway is well lubricated.

As discussed above, the porous surface texture provided by the porous coating helps to hold lubrication and this helps to retain base oil from the grease and reduces wear. Preferably a non-contact surface of the inner raceway, i.e. the surfaces immediately adjacent the inner raceway contact surface, is provided with a porous coating; this helps to supply base oil to the contact surface by, for example, oil bleeding. Preferably the sprags are provided with a porous coating; this serves to maintain, transport and supply the base oil to the contact surface with the inner ring.

Preferably the sprags are held in place by a retainer and at least a portion of the retainer is provided with a porous coating to maintain and supply the base oil. Alternatively, at least a portion of the retainer may be provided with an oleophobic coating. This prevents the grease that is driven to the outside by the centrifugal force from sticking to the cage, so it will immediately start dripping back to the inner raceway as soon as the device is stopped. Oleophobic coatings are well known in the art and include, for example, silicon coatings.

Preferably the porous coatings discussed above are independently selected from a phosphate coating or a black oxide coating, or a coating formed by physical vapour deposition (PVD), chemical vapour deposition (CVD) or plasma spraying. The coating preferably comprises a Manganese-phosphate coating or a black oxide coating. Such coatings are well known and a skilled person would have no difficulty applying such a coating to these components. For example, black oxide coatings may be provided to ferrous materials by using a hot bath of sodium hydroxide, nitrates and nitrites. Phosphate coatings may be provided using phosphoric acid and metal phosphate salts.

The porous coating (or coatings) may alternatively be independently selected from a ceramic, oxide, nitride and/or carbide coating. Such coatings are particularly preferred on the contact surfaces of the inner ring and/or the sprags. Such coatings give a high load resistance and scuffing resistance.

The porous coating preferably comprises pores having a longest mean surface diameter of from 0.01 to 10 μηη. Preferably the pores have a longest mean surface diameter of from 0.1 to 1 μηη. By surface diameter it is meant the diameter as measured in the surface plane of the coating. Preferably the pores have a mean depth of from 1 to 100 μηη. Pore measurements are well known in the art and can be investigated by, for example, optical and electron microscopy techniques. The inventors have found in particular that to provide sufficient resistance to the centrifugal forces it is desirable to have a capillary flow speed of 1 mm/s or higher. This can be achieved for deep pores (in the region of 10 to 100 μηη, preferably from 50 to 75 μηη) having a broad range of pore diameters (0.01 to 10 μηη, preferably 0.01 to 1 μηη) or for shallower pores (in the region of 0.1 to 10 μηη, preferably from 1 to 5 μηη) having larger pore diameters (1 to 10 μηι, preferably 2 to 5 μηι). Pores such as these are particularly useful for the inner surface to resist the centrifugal effects on rotation.

The inventors have found that for general resupply of oil by capillary transportation of oil, which is particularly useful for the inner raceway non-contact surface, it is best to use pore having a diameter of from 0.1 to 1 μηη.

Preferably the sprag clutch of the present invention is grease lubricated. Furthermore, empty space within the sprag clutch may be filled with solid oil. Since the unswept volume is filled with solid oil, but the clearance between the sprags and the solid oil is very small, excess lubricant from the outer ring will be transferred to the inner ring.

Figures The disclosure will now be described in relation to the following non-limiting figures, in which:

Figure 1 shows part of a sprag clutch according to an embodiment of the present invention.

Figure 2 shows part of a sprag clutch according to a further embodiment of the present invention.

Figure 3 shows a roller suitable for use in a sprag clutch according to an embodiment of the present invention. Figure 4A shows the effects of capillary pressure and centrifugal force. Figure 4B shows capillary flow speed of an oil inside a pore.

A sprag clutch 1 includes a plurality of sprags 5. Each sprag 5 has a roughly figure of eight cross-section. The sprags 5 are located between an inner ring 7 and an outer ring 9 of the sprag clutch. A radially outer surface of the inner ring 7 serves as an inner raveway 10 and a radially inner surface of the outer ring 9 serves as an outer raceway 15. The inner ring 7 and the outer ring 9 of the clutch are connected to different moving parts. In use, when the inner ring 7 is driven in one rotational direction the sprags 5 will engage with the outer raceway 15 and transmit the torque to the outer ring 9. When the inner ring 7 is driven in the opposite rotational direction the sprags 5 will not engage with the outer raceway 15 and so the inner ring 7 will free-wheel relative to the outer ring 9. The sprag clutch is lubricated with grease.

The sprags 5 are held relative to each other by a retainer 20. The retainer 20 comprises a spring 22 which serves to ensure that the sprags 5 remain evenly spaced and correctly oriented to work. According to the invention, the inner raceway 10 is provided with a porous coating to help retain lubrication at the contact point between the sprags 5 and the inner raceway 10. In one embodiment, the porous coating comprises a manganese-phosphate coating and has a mean pore size of 10 microns deep and 1 micron mean surface diameter.

In figure 2, a further embodiment of a grease-lubricated sprag clutch is depicted. The sprag clutch 2 includes a plurality of sprags 5. Between two of the sprags 5 there is provided a roller 25. The roller 25 is held by a retainer 20 to ensure that it does not interfere with the sprags 5 and can freely rotate. The roller 25 contacts both the inner raceway 10 and the outer raceway 15. This permits it to transfer lubricant from the outer raceway 15 to the inner raceway 10. Although this description includes a roller, it should be appreciated that this is not an essential feature of the present invention. An example of a suitable roller is depicted in figure 3. The roller 25 is preferably hollow. That is, the roller 25 has a cylindrical cavity 30 within the roller 25. Preferably the roller 25 has a surface texture 35. Since the direction of rotation of the cylinder 25 is known due to the locking effect of the sprags 5 in one direction, the surface texture 35 can be selected to encourage lubrication to move from the edges of the roller 25 to the centre of the roller 25.

The inner raceway 10 in the clutch of figure 2 is likewise provided with a porous coating. At portions of the inner raceway which are in contact with the sprags 5, the pores in the coating retain base oil from the grease, to improve lubrication and reduce wear. At non-contacting portions of the inner raceway, adjacent to the contacting portions, the pores can act as a reservoir for supplying base oil to the contacts via, for example, bleeding. In the clutch 2 of figure 2, the outer raceway 15 and an outer surface of the sprags 5 are provided with a porous coating, to further promote lubrication and reduce wear. Furthermore, the retainer 20 is provided with an olephobic coating, meaning that grease and base oil are largely prevented from adhering to the retainer, thus making more lubricant available at locations where it is needed.

Example

The invention will now be described in relation to the following non-limiting example.

A sprag clutch was manufactured as described herein and as follows:

Shaft diameter 25 mm, max. shaft speed 19000 rpm

Base oil property: viscosity 30 est (mPa.s) at 100 ^° C, surface energy 0.032 N/m, density 880 kg/m ³ . The coating property and porous dimension:

Contact angle between oil and coating: 0 deg

· Pores length: 1 μηη, 10 μηη and 100 μηη,

Pore diameter 0.01 to 10 μηη

Capillary pressure, centrifugal pressure and flow rate of lubricant oil through a pore can be calculated as shown in figures 4A and 4B. In figure 4A the axes are pressure in kPa on the y-axis and pores diameter in micrometers on the x-axis. There are four lines shown. The angled line is the capillary pressure. The uppermost line of the horizontal lines is the centrifugal pressure in a pore of length 1 micrometer. The middle line is the centrifugal pressure in a pore of length 10 micrometer. The lowermost line is the centrifugal pressure in a pore of length 100 micrometer.

In figure 4B the axes are capillary flow speed in mm/s on the y-axis and pores diameter in micrometers on the x-axis. There are three lines shown. The uppermost of the lines is for a pore of length 1 micrometer. The middle line is for a pore of length 10 micrometer. The lowermost line is for a pore of length 100 micrometer.

According to figure 4A, the capillary pressure is higher than the centrifugal pressure, e.g. the pore coating can retain the oil and drive the oil to flow against the centrifugal force. The capillary flow speed will be 1 mm/s or higher for certain ranges of pore size. This flow speed is regarded as sufficient to provide re-lubrication, against the centrifugal force.

Although preferred embodiments of the disclosure have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the scope of the disclosure or of the appended claims.