GB1577737A

GB1577737A - Hydrodynamic bearings

Info

Publication number: GB1577737A
Application number: GB9461/76A
Authority: GB
Original assignee: Sperry Ltd
Current assignee: Unisys Group Services Ltd
Priority date: 1977-03-07
Filing date: 1977-03-07
Publication date: 1980-10-29

Description

(54) IMPROVEMENTS IN OR RELATING TO HYDRODYNAMIC BEARINGS (71) We, SPERRY LIMITED formerly known as Sperry Rand Limited, a British Company of Sperry House, 78 Portsmouth Road, Cobham, Surrey, KTI I 1JZ, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to bearings and more particularly to hydrodynamic bearings using a fluid as a lubricant.

Hydrodynamic bearings are well known for their low frictional characteristic which make them attractive for sophisticated engineering products such as gyroscopic apparatus, for example. In the main, hydrodynamic bearings to date have employed a gas as a lubricant and these so-called gas bearings have been acceptable in as much as they possess the low frictional characteristic referred to and gasses are generally stable or inert so that the bearing characteristics remain substantially constant over a relatively wide temperature range. That is to say, gasses are predictable in this respect and furthermore, are relatively easy to employ in gas bearings, as regards filling, purging and general handling.

However, gasses have one disadvantageous property so far as gas bearings are concerned, namely a relatively low coefficient of viscosity, which means that for a given bearing stiffness the size of the bearing has to be greater than it would if a lubricant of a relatively high coefficient of viscosity were employed instead of a gas.

With the ever-increasing requirement for smaller envelope sizes for equipments, particularly for the aero-space industry, then the comparatively large gas bearings become unacceptable in some situations.

It is known to employ non-gaseous materials, such as oils and greases as the lubricants in hydrodynamic bearings which means the bearings can be smaller for a given load due to the higher coefficients of viscosity but such bearings (hereinafter referred to as "grease bearings" for convenience) suffer a main disadvantage: they are blind-ended (i.e. the inner member does not extend through the outer member) which results in great difficulty in purging the lubricant of air and makes the bearings inapplicable to situations requiring access at one or both ends of the inner member. Another disadvantage of grease bearings is that the lubricant tends to "bleed" or break down under the high shear forces it is subjected to at the bearing surfaces (known as the bearing film) and this also leads to bearing instability. With regard to purging, it is necessary to minimise inclusion of air bubbles in the lubricant since these render the bearing unstable since they affect the effective coefficient of viscosity of the lubricant. In spite of a high level of purging on assembly of a grease bearing, it is virtually impossible to remove all the air bubbles and so grease bearings to date have suffered from this problem of instability, coupled with that arising from lubricant bleeding, with attendant reduction in performance. It will be appreciated that air bubbles in the lubricant are not particularly troublesome in general and only become a problem when they enter the bearing film which provides the bearing stiffness. Since the lubricant is pumped around a closed lubricant path by grooves disposed between the surfaces between which the bearing film is formed, the grooves often being spiral in form, the likelihood of air bubbles being carried to the bearing film is relatively high.

According to the present invention a hydrodynamic grease bearing (as hereinbefore defined) comprises a casing and a shaft extending through both ends of the casing, with the casing and shaft being capable of relatively rotational movement, the shaft having two spaced portions borne in respective correspondingly-shaped journals disposed within the casing to provide two pairs of opposed bearing surfaces between which the lubricant forms respective bearing films, each of said portions and/or each of the journals being formed with a plurality of grooves in the corresponding bearing surface, the grooves being operable in use of the bearing to pump the lubricant around a closed lubricant path, and barrier means provided around the shaft adjacent the outer end of each journal and in the form of an extension of the associated journal generally parallel to the axis of rotation of the shaft and of a predetermined length relative to thickness of the bearing film, depending on the lubricant employed, the barrier means providing, in conjunction with the shaft, a gas bubble trap, whereby in use any gas bubbles entrained in the lubricant emanating from the bearing films are prevented from being carried around the lubricant path and possibly back to the bearing film.

The barrier means operate principally to prevent the formation of eddy currents of lubricant otherwise created by the flow of lubricant along the bearing film and tending to carry any entrained gas bubbles around the lubricant path, but in so arresting the bubbles adjacent the shaft, the bubbles in fact tend to migrate to, and remain in contact with, the surface of the shaft under centripetal force since they are of smaller mass than the lubricant. It has been found that there is a relationship between the dimension (L) of the barrier means generally parallel to the axis of rotation of the shaft and the gap (D) between the corresponding bearing surfaces, i.e. the thickness of the bearing film. A D:L ratio of 3:1 has been found satisfactory using a mono-ester oil as a/lubricant. The barrier means may be an integral extension of each journal.

It will be seen that the barrier means provide a gas bubble trap which is operative when the bearing is in use. However, when the bearing is inoperative, any gas bubbles will tend to rise and cause the problems discussed above when the bearing is next used. According to a preferred feature of the present invention, a lubricant reservoir is provided for each bearing film, the reservoir being partdefined by the bearing casing and having at least one trap so disposed as to collect gas bubbles. The provision of a lubricant reservoir has the advantage of allowing the lubricant to "rest" following subjection to high shearing forces in the bearing film and before being re-subjected to such forces as it flows around the lubricant path. Preferably a plurality of traps are provided in the lubricant reservoir.

A grooved hydrodynamic grease bearing constructed in accordance with the present invention will now be described in greater detail, by way of example, with reference to the drawings accompanying the Provisional Specification, in which: Figure 1 is a cross-sectional view of the bearing, Figure 2 is an enlarged detail (not to scale) of part of Figure 1, Figure 3 illustrates typical trajectories of gas bubbles in one area of the bearing, and Figure 4 is an explanatory diagram relating to the bubble trajectories of Figure 3.

Referring to Figure 1, the bearing comprises a cylindrical, stainless steel casing 1 having two stainless steel end caps 2 through which extend respective ends 3 of a stainless steel shaft 4 housed in the casing. The ends 3 of the shaft 4 are of reduced diameter compared with the body 5, the latter being located within the casing by a sleeve 6 and more specifically, by inner annular extensions 7 of the sleeve. The casing 1 and sleeve 6 are bonded together at 8 and inspection apertures 9 are provided through both components although for experimental purposes only.

The transition between the enlarged body 5 and the reduced ends 3 of the shaft 4 is formed by respective frusto-conical portions 11 of the shaft on the surfaces of which are etched spiral grooves 12. The shaft 4 is journalled in two stainless steel journals 13 disposed around the respective frusto-conical shaft portions 11, the facing surfaces of the portion and journal pairs Il, 13 providing the bearing surfaces 10, 10' (Figure 2) between which a lubricant 14 forms a bearing film. The journals 13 are located by the corresponding end caps, more specifically by four equiangularly-spaced projections 15 of the caps, these projections serving to space the end caps from the respective outer ends 16 of the journals 13. The peripheral surfaces 17 and inner end surfaces 18 of the journals are also spaced from the inner casing surface 19 and outer end surface 20 of the sleeve 6, respectively. These spaces collectively provide a reservoir 21 for the lubricant 14 at each end of the bearing.

Each reservoir 21 is provided with a plurality of gas bubble traps 22 which appear as recesses off the main lubricant flow path which is from the end of the bearing film adjacent the shaft end 3, radially outwardly along the space between the end cap 2 and the journal 13, axially of the bearing along the space between the casing 1 and the journal 13, and finally radially inwardly along the space between the sleeve 6 and journal 13, back to the bearing film as indicated by the arrows 23.

Each reservoir 21 has a fill hole 24.

Referring now to Figure 2, this shows an enlarged detail of the left-hand shaft end 3 and journal 13 of Figure 1, this detail also applying to the other shaft end and journal since the two are identical. It will be seen that the journal 13 is extended axially of the shaft 2 at 25 from the outer end of the bearing surface 10'. The extension 25 provides barrier means around the shaft end 3 the operation of which will be described hereinafter.

Returning to Figure 1, the body 5 of the shaft 2 has a central portion 26 of a material different from that of the remainder of the shaft for temperature compensation purposes the details of which are described and claimed in copending Patent Application No 9462/76 Serial No 1577738.

On assembly of the bearing, the opposed surfaces in the areas indicated by the arrows 27 are treated with a barrier film which prevents egress of lubricant 14 from the reservoirs 21. On completion of assembly, the reservoirs 21 are filled with the lubricant 14 through the fill holes 24, the lubricant then being purged as far as possible of gas and the holes 24 sealed. A preferred lubricant is a mono-ester oil known under the trade name Anderol L423.

The bearing can be used in two ways without any fundamental modification, namely by rotating the shaft 4 relative to the casing 1, or vice versa. The bearing illustrated has been used to carry a rotor (not shown) of a gyroscope mounted on one shaft end 3, the shaft being driven by any desired means such as an electric motor (also not shown). When the bearing is inoperative, any gas (such as air) remaining in the lubricant 14, as it will inevitably in spite of the purging effected during assembly, will form voids or bubbles tending to rise due to their lower mass compared with the lubricant 14. The rising bubbles will collect in the portions of the traps 22 in the reservoirs 21 which are uppermost at the time, it being appreciated that the recesses forming the traps 22 are annular. Thus trapped bubbles will tend to remain in the traps when the bearing is operated although this depends to some extent on whether the shaft 2 is rotated relative to the casing 1 or vice versa. However, if trapped bubbles tend to migrate towards the centre of rotation, they will meet a surface on which they will be "held" and where they will be harmless from the standpoint of having access to the bearing film. The particular surface on which the bubbles will be held depends on the trap 22 in question: it will be the side of the recess in the case of the traps in the end caps 2 and the sleeve 6, and the peripheral surface of the journals 13 in the case of the trap provided in the casing 1.

On rotation of the shaft 4, the grooves 12 in the shaft surfaces 10 act to pump the lubricant 14 slowly around the lubricant path 23. This pumping action causes a pressure rise in the lubricant 14 along the bearing film between the bearing surfaces 10, 10' to provide the necessary bearing stiffness or load capacity. Any gas bubbles appearing in the actual bearing film lowers the viscosity thereof and hence reduces the effective bearing load capacity which is, of course, highly undesirable. Any bubbles appearing in the bearing film will be carried therealong and will eventually appear in the annular area bounded by the journal extension 25 and the shaft end 3.

In issuing from the bearing film, the lubricant 14 normally tends to form into eddy currents which sweep any entrained gas bubbles around the lubricant path 23.

However, the barrier formed by the journal extension 25 prevents or minimises such eddy currents and allows any gas bubbles to be acted upon by the centripetal force created by the rotating shaft 4 and hence migrate to the shaft end 3 where they tend to remain. Thus the action of this gas bubble trap in conjunction with the traps 22 render the bearing self-purging giving rise to a relatively stable bearing performance. Also, the bearing can withstand shock loading and can be operated in any attitude. Furthermore, the shaft is accessible from either end of the bearing, which is not the case in known hydrodynamic bearings, and the provision of a reservoir of lubricant allows the lubricant 14 a finite time to recover from bleeding, as a result of being subjected to high shear forces in the bearing film, before being so subjected again on being returned to the bearing film.

Figure 3 illustrates typical trajectories of gas bubbles of different sizes and it will be seen that the bubbles do not move radially from the journal extension 25 to the surface of the shaft end 3 but have a component of velocity parallel to the axis of rotation of the shaft 2. It has been found that a relationship exists between the dimension L of the barrier formed by the journal extension 25 in the illustrated embodiment and the width D of the bearing film. Using the mono-ester lubricant referred to above, an L:D ratio of 3:1 has proved satisfactory.

The following is an analysis of the bubble trap provided by the barrier around the shaft end 3 and reference is made to Figure 4.

Consider a bubble B on the surface of the stationary journal extension 25: it has two forces acting on it, a centripetal force acting towards the axis of rotation of the shaft 2 due to the velocity gradient, and a retarding viscous force due to its motion. By taking a quasi steady state solution these forces will be in equilibrium and the bubble will move to the centre rotating shaft.

Centripetal force: Fc=m V2Jr (1) where V=angular velocity ;=distance from centre of rotation r=bubble radius U=bubble velocity towards the shaft p=lubricant density (neglecting air density) Viscous force by Stokes Law: FD=6n y U r (2) Expanding equation (1), F=4/3 rr r3 p[(d-xld)R,]Zl(R+x) where w=angular velocity of the shaft Since FcFD. then 4/3 7e r3 p[(d-x/d)R,,12/(R+x)=6 7r ,a U r Hence U=4/3 . 1/6 . r2 p/v(dLx'd)2R2a,2/(R+x) However, the bubble B will not move directly towards the centre since it will be swept along by the axial velocity induced by the pumping action of the bearing.

Whilst the axial velocity and distribution is known in the bearing film, it can only be guessed in the trap formed by the journal extensions 25. Effecting a worst-case analysis, the mean velocity v across the gap between the journal extension 25 and the shaft end 3 is given by: V- l/djVdx.

Now axial velocity will have its greatest effect on a bubble on the surface of the journal extension 25, hence if we take a mean velocity it will sweep the bubble further downstream than would the actual velocity. Thus the bubble trap will tend to be overdesigned.

The bearing illustrated in Figure 1 has a casing length of 3.8 cms and diameter of 1.7 cms. The diameter of the shaft ends 3 is 3 mm. This bearing provides an axial stiffness of 10.6x106 N. m-' (Newtons per metre) and a radial stiffness of 12.5x10e N . m-'. These stiffnesses are greater than would pertain for a bearing of the same size using a gas for the lubricant and thus the demand for small envelope size can be met whilst providing a bearing substantially free of instability problems due to the self-purging characteristics thereof resulting from the features of the present invention.

The present invention is applicable to any hydrodynamic bearing and the construction thereof is not limited to that shown in Figure 1. For example, the transition between the body and end portions of the shaft 4 does not have to be frusto-conical: it may be hemi-spherical or have a journal and thrust plate (H-form) configuration. Furthermore, the transitions at either end of the shaft 4 may be opposed in different configurations from that shown, i.e. they may flare, as opposed to taper, from the body 5 to the shaft end 3.

Claims

WHAT WE CLAIM IS:

1. A hydrodynamic grease bearing (as hereinbefore defined) comprising a casing and a shaft extending through both ends of the casing, with the casing and shaft being capable of relative rotational movement, the shaft having two spaced portions borne in respective correspondingly-shaped journals disposed within the casing to provide two pairs of opposed bearing surfaces between which the lubricant forms respective bearing films, each of said portions and/or each of the journals being formed with a plurality of grooves in the corresponding bearing surface, the grooves being operable in use of the bearing to pump the lubricant around a closed lubricant path, and barrier means provided around the shaft adjacent the outer end of each journal and in the form of an extension of the associated journal generally parallel to the axis of rotation of the shaft and of a predetermined length relative to thickness of the bearing film, depending on the lubricant employed, the barrier means providing, in conjunction with the shaft, a gas bubble trap, whereby in use any gas bubbles entrained in the lubricant emanating from the bearing films are prevented from being carried ar6und the lubricant path and possibly back to the bearing film.

2. A bearing according to claim 1, wherein the barrier is in the form of an integral extension of the associated journal.

3. A bearing according to claim 1 or 2, wherein the lubricant is a mono-ester oil and the ratio of thickness of each bearing film to the dimension of the barrier means generally parallel to the axis of rotation of the shaft is 3:1.

4. A bearing according to any of the preceding claims, wherein a lubricant reservoir is provided for each bearing film, each reservoir being part-defined by the casing and having at least one trap so disposed as to collect any gas bubbles contained in the lubricant.

5. A bearing according to claim 4, wherein each trap is in the form of a recess off the main lubricant flow path.

6. A bearing according to any of the preceding claims, wherein the casing and the shaft are constructed at least in part of materials having different coefficients of expansion such that each bearing film characteristic (as herein defined) is maintained substantially constant irrespective of variations in temperature of the bearing within the temperature range of the lubricant.

7. A bearing according to claim 6, wherein the shaft is composed of a material having a high coefficient of expansion relative to that of the material of the casing.

8. A bearing according to claim 6, wherein the casing is composed of a material having a low coefficient of expansion relative to that of the material of the shaft.

9. A bearing according to claim 8, wherein the casing is composed of a material having a negative coefficient of expansion.

10. A bearing according to claim 6, wherein the shaft is composed of two materials of differing coefficients of expansion and comprises a slug of material having a greater coefficient of expansion than that of the shaft and interposed between the ends of the shaft.

11. A bearing according to claim 10, wherein the slug is composed of a synthetic plastics material.

12. A bearing according to claim 10 or 11, wherein the slug is composed of the material sold under the trade name Deroton and the shaft is composed of stainless steel.

13. A bearing according to claim 6, wherein the casing is composed of two materials of differing coefficients of expansion and comprising a slug of material having a smaller coefficient of expansion than that of the casing interposed between the ends of the casing.

14. A bearing according to claim 13, wherein the slug has a negative coefficient of expansion.

15. A bearing according to claim 14, wherein the slug is composed of a supercooled ceramic.

16. A hydrodynamic grease bearing substantially as herein particularly described with reference to the drawings accompanying the Provisional Specification.