GB2223819A - Shroud means for a gear assembly - Google Patents

Abstract

First and second inter-meshing bevel gears (10, 16) each attached to a respective shaft (14) are provided with shrouds (26, 28) to reduce windage losses and disturbance to oil drainage. Each shroud comprises a first portion (flange 30) which covers the front face (13) of the gear at least to the depth of the gear teeth and a second portion (flange 32) covering the back face (22). The second flange (32) extends radially inward to the respective shaft (14). Each shroud has a cutout adjacent the meshing point of the gears and the two shrouds are butted together adjacent the meshing point. <IMAGE>

Description

SHROUD MEANS FOR A GEAR ASSEMBLY The invention relates to shroud means for a gear assembly.

Inter-meshing gears rotating at high speed in a confined space in an oil rich environment create severe windage which disturbs the oil. In a pumped lubrication system this severely impairs drainage, and oil in the chamber is continually re-circulated so that constant heat input to the oil raises its temperature to problem levels. By improving circulation oil temperatures can be substantially reduced.

British Patent No. 292,540 shows a gear casing having a special guide wall placed close to the periphery of a gear wheel to act as a return guide for lubricant thrown off the gear wheel to drain back to collection region in the bottom of the casing. The guidewall extends up to and slightly beyond the meshing point of the gears, and for bi-directional working two such guide walls may be provided on diametrically opposite sides of the gear.

However, without continuous peripheral shrouding of the gear wheels the powerful vortices set up by high rotational velocities are not constrained and prevented from whipping-up the reservoir of oil, consequently lubrication is severely impaired.

British Patent No. 1,322,585 discloses a gear shroud formed by a foraminated or porous screen which surrounds the circumference of the gears. A collection chamber is formed radially outboard of the foraminated screen. The screen is substantially cylindrical and the air movement induced by the bevel gears is radially outwards along the inclined faces of the gears. Although some air-oil mixture flows through the screen the majority of the flow is circumferential along the surface of the screen to be eventually re-entrained by the gears. Meanwhile, the radially outward component of the flow creates a powerful vortex system in the gearbox which reduces the effectiveness of the collection chamber (and drainage from the collection chamber) but also seriously inhibits the drainage from the whole gearbox.

The present invention has for an object the elimination of the effects of windage on the behaviour of the oil by total containment of vortices produced by the gears.

According to the present invention shroud means for reducing windage effects in a gear assembly working in a confined space comprises an impervious shroud extending substantially fully over the exposed toothed portions of the gears.

By shrouding the front and back faces of a gear around a substantial part of its periphery windage can be substantially reduced, allowing oil from outside the shroud bearing to drain naturally and without disturbance.

The wind age created by the gears is contained within the shroud. The flanges extend radially inward to define a generally circular opening through which the respective attached shaft protrudes, the shaft and circular opening are dimensioned relative to each other substantially to prevent oil or air passing therebetween.

In a bevel gear arrangement a second shroud is positioned around a substantial part of the periphery of the second gear in a like manner and the first and second shroud are joined or butted together adjacent the meshing point of the first and second gears so that the constrained vortex within the shroud does not escape and disturb oil drainage.

According to another aspect of the invention a tangential outlet pipe is provided on the first shroud, said pipe adapted to withdraw lubricating oil from the shroud which, in use, is forced radially outward to form an annulus of oil within the first shroud. In another embodiment radial slots or holes are provided between front and back faces of the shroud to allow oil to drain from the shroud. Preferably the slots or holes are sufficiently small to avoid emitting air jets which could set up secondary vortices.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows in section a set of shrouded bevel gears; Figure 2 shows in perspective part of Figure 1 illustrating the abutting shrouds of the bevel gears; Figure 3 is a detail view of a section through a bevel gear and shroud; Figure 4 shows a shroud with a tangential oil drain.

Referring to Figure 1, a first bevel gear 10 comprises an annular array of gear teeth 12 formed on the front face 13 of the bevel gear 10. The gear is carried on a shaft 14 and is driven thereby. A second bevel gear 16 is likewise provided with an annular array of gear teeth 18 on a front face 19 thereof and is attached to a second co-axial shaft 20 arranged at 900 to the first shaft 14. Both gears also have back faces 22 and 24 respectively. The first and second bevel gears 10, 16 inter-mesh whereby the shaft 14 drives the shaft 20.

Figure 2 shows that each gear 10,16 is similarly provided respectively with first and second shrouds 26,28 which eliminate or greatly reduce windage effects created by high speed rotation of the gears. The first shroud 26 extends generally circumferentially around the periphery of the first bevel gear 10 atd has a front portion or flange 30 which substantially covers the array of gear teeth 12 at least to the depth of the gear teeth on at least the front face 13. The front flange 30 preferably fits relatively closely to the shaft 14, it need not form a complete seal. The material of the shrouds is impervious to air and is preferably formed of a light metal such as aluminium.

A rear portion or flange 32 of the shroud covers the back face 22 of gear 10 and extends radially inwards almost as far as the shaft 14. The inner rime 34 of the so-formed annular flange 32 forms a circular aperture through which the shaft 14 extends. The diameter of aperture is designed to be only slightly greater than the diameter of the shaft, ie so that it has a running clearance typically 0.5 mm. This close fit between the shaft and shroud effectively contains the vortex within the shroud preventing air inside the gear casing generally being stirred-up by the revolving gear teeth.

The shroud 26 extends round over at least 80% of the circumference of the gear 10 and has a cutout 33 adjacent the meshing point of the gears 10,16. Small drain holes 15, see Figure 3, which in the example are radial, are provided at intervals around the outermost periphery of the shroud between the first and second shroud portions 30,32 to allow oil inside the shroud to escape. The outermost margins of the shroud portions 30,32 may be formed with peripheral flanges which abut each other except at intervals where one or both may have a radial indentation to form a drain hole or passage 15. The front and rear portions of the shroud may be joined together along these flanges.These holes, of which no more than about six are needed, ensure a continuous circulation of fresh oil which has a beneficial effect on reducing oil temperatllre by avoiding continued recirculation of entrapped oil.

A second shroud 28, essentially similar to the first shroud 26, extends around the second bevel gear 16 in like manner.

As previously mentioned to be completely effective the shrouding of the gear wheels must be continuous around the periphery of the gears. Therefore, in the present example of meshing bevel gears the two shrouds 26,28, which are disposed at right angles to each other, are designed to continue around the periphery gears until the shrouds abut. At their intersection the shrouds are preferably sealingly joined to prevent a jet of high velocity air or oil mist being ejected by the vortex inside. Alternatively, the front sides of both shrouds may be formed as a single component, for example as an integral pressing.

The portions of the shrouds as will be evident from Figure 1 may be mounted on nearby parts of the fixed structure defining the walls of the gear casing, for example the part 34. Where permissible the said part 34 of the fixed structure may be designed to approach one of the faces of the gear wheel so as to form part of the shroud 32 itself. The remainder of the shroud then comprises a plate-like member 26 attached directly to the wall of the fixed structure.

One of the shrouds may incorporate a tangential offtake 38 see Figure 2 to siphon away oil that collects under the shroud. This oil may be driven from the shroud by the vortex, irrespective of the attitude of the gear assembly. This has particular utility in aircraft applications to accommodate negative "g" manoeuvres or other unusual attitudes of the aircraft during flight.

Generally, the impervious shroud members fully cover all toothed portions of the gears, this means that the shrouds cover the peripheral portions of the gears and extend radially inwards to cover at least the toothed portions. The radially inwardly extending parts of the shrouds reach as far as necessary to effectively contain the vortex. In the case of a shaft driven gear the side portions extend almost up to the surface of the shaft.

Where the shrouded gear is carried on one end of shaft the shroud should extend to within the predetermined running clearance of the shaft but on the opposite side the face of the gear wheel may be completely enclosed.

The inner surface of the shroud is made to fit closely to the gear teeth so that as little volume of air as is practicable is entrained by the gears.

Claims (12)

1. Shroud means for reducing windage effects in a gear assembly working in a confined space comprises an impervious shroud extending substantially fully over the exposed toothed portions of the gears.

2. Shroud means as claimed in claim 1 wherein the impervious shroud substantially fully encloses the exposed portions of the gears.

3. Shroud means as claimed in claim 2 wherein the walls of the or each impervious shroud fit closely over the teeth of the said gear.

4. Shroud means as claimed in any preceding claim adapted for reducing windage effects in a bevel gear assembly comprises around each of the gears impervious shrouds which extend as far as they abut one another in the region of engagement of the gears.

5. Shroud means as claimed in claim 4 for bevel gears wherein abutting portions of adjacent impervious shrouds are sealingly joined.

6. Shroud means as claimed in claim 5 wherein the said abutting portions of adjacent shrouds are formed integrally as a single component.

7. Shroud means as claimed in any preceding claim wherein at least one shroud face extends radially inwardly to cover at least a substantial portion of the exposed face of the gear.

8. Shroud means as claimed in claim 7 for a gear mounted on a shaft wherein the radially inwardly extending shroud face terminates in close proximity to the shaft.

9. Shroud means as claimed in any preceding claim wherein a shroud comprises in combination on one side of a gear a plate-like member attached to a wall of a structure defining the confined space within which the gear assembly is located, and a surface of a portion of the said wall which is made to closely approach opposing side of the gear.

10. Shroud means as claimed in any preceding claim wherein a plurality of relatively small drain radial holes or passages are formed leading from the periphery of the shroud.

11. Shroud means as claimed in claim 11 wherein the said radial drain passages are formed by indentations formed in flanges at the periphery of the shroud.

12. Shroud means for reducing windage effects in a gear assembly substantially as described with reference to the accompanying drawings.