GB1577738A

GB1577738A - Hydrodynamic bearings

Info

Publication number: GB1577738A
Application number: GB946276A
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, KT11 lJZ, 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 non-gaseous substance as a lubricant, such as a grease or an oil and for convenience, these bearings will be referred to hereinafter as "grease bearings".

Hydrodynamic bearings are well known for their low friction 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 easv 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 substances such as oils and greases as the lubricants in hydrodynamic bearings which means the bearings can be smaller for a given stiffness due to the higher coefficients of viscosity but such bearings have a serious disadvantage when subjected to a wide operating temperature range. With increase in temperature, the bearing components expand but this in itself is not harmful since the bearing clearance (known as the bearing film) does not change to any significant extent. However, an increase in temperature dramattically reduces the viscosity of the lubricant and thus effects a reduction in bearing stiffness, i.e. a change in bearing film characteristic. Conversely, bearing stiffness increases with a decrease in temperature. This variable bearing stiffness over a wide operating temperature range is unacceptable in many applications such as gyroscopic apparatus.

One of the known ways to attempt to meet this problem is to employ a low stiffness, high compliance spring in the bearing to effect a substantially constant axial preload on the bearing. Thus when the bearing film or clearance increases due to thermal expansion of the bearing components, whilst the lubricant viscosity decreases, the axial load imparted by the spring increases to effect some measure of compensation.

This arrangement, however, is unacceptable for high stiffness bearings since the overall bearing stiffness is approximately that of the lowest spring stiffness in the bearing.

According to the present invention a hydrodynamic bearing employing a nongaseous lubricant comprises a casing and a shaft mounted within and extending through at least one end of the casing, with the casing and shaft being capable of relative rotational movement and with the casing and the shaft constructed at least in part of materials having different coefficients of expansion such that the 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.

Thus the invention provides a bearing in which the expansion or contraction of the components thereof is such as to varv the bearing film, and hence bearing stiffness, to compensate for an increase or decrease in the viscosity of the lubricant.

The invention may be realised in a number of ways: for example, the shaft may be composed of a material having a relatively high coefficient of expansion, or the casing composed of a material of relatively low (even negative) coefficient of expansion.

Alternatively, either the casing or shaft may be composed of two materials of differing coefficients of expansion.

If the shaft is to be composed of materials of differing coefficients of expansion then preferably a slug of material having a greater coefficient of expansion than that of the shaft is interposed between the ends of the shaft. The slug may be of any material meeting the desideratum of coefficient of expansion but synthetic plastics materials have been found particularly useful since generally they have very large coefficients of expansion compared with that of steel, which is the typical material of the bearing shaft, and thus effect the necessary expansion or contraction to compensate for the changes in lubricant viscosity whilst only being of comparatively small dimensions which is necessary in view of the requirement for smaller envelope sizes referred to above. Synthetic plastics materials have coefficients of expansion about ten times that of steel and a particularly useful one is that sold under the trade name DEROTON. Whatever material is chosen for the slug, any thermal expansion of the bearing components, which would normally not affect to any significant extent the bearing film or clearance, results in a greater overall shaft expansion compared to the other components (due to the effect of the slug) which causes an actual reduction in the bearing film to compensate for the decrease in lubricant viscosity. Conversely, the overall shaft contraction due to a decrease in temperature increases the bearing film to compensate for an increase in lubricant viscosity.

If the bearing casing is to be composed of materials of differing coefficients of expansion, then preferably a slug of material having a smaller coefficient of expansion than that of the casing is interposed between the ends of the casing. Again with minimum envelope size in mind, the material of the slug preferably has a much smaller coefficient of expansion than that of the casing, desirably approaching zero and better still, negative. A material having a negative coefficient of expansion is a supercooled ceramic sold under the trade name CERVIT. With increase in temperature, the casing expands less than the shaft thereby effecting an overall reduction in the bearing film to compensate for decreased lubricant viscosity, and vice versa in the case of a decrease in temperature.

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 bearings, and Figure 2 is an enlarged detail (not to scale) of part of Figure 1.

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.

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 11, 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 equiangularlyspaced 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.

Returning to Figure 1, the body 5 of the shaft 2 has a central portion in the form of a slug 26 of a synthetic plastics material sold under the trade name DEROTON and having a coefficient of expansion of 10 x 10-5 mm/mm/ C compared to that of the stainless steel portion of the shaft of 12 x 10-6 mm/mm/ C, i.e. about eight times greater. The slug 26 is bonded to the respective adjacent shaft body portions 5 preferably using an adhesive having some elasticity to accommodate the relative movement between the shaft body 5 and the slug 26 occurring on changes in temperature. For example, an epoxy resin with various fillers may bel employed or an isocyanate adhesive which is an anaerobicsetting adhesive.

The bearing illustrated has a casing length of 3 8 cms and diameter of 17 cms, the diameter of the shaft ends 3 being 3 mm. The shaft body portions 5 and slug 26 have a diameter of 8 mm and the length of the slug is 1 cm. These dimensions in conjunction with the chosen lubricant provide an axial stiffness for the bearing of 106 x 10 6N.m -I (Newtons per metre) and a radial stiffness of 125 x 10 6N.m -1, which stiflnesses remain substantially constant over the temperature range of the lubricant as a result of the present invention. Furthermore, the quoted stiffnesses are greater than would pertain for a bearing of the same size and using gas as a lubricant. Thus the demand for small envelope size can be met whilst providing a bearing substantially free of instability problems. The dimensions of the slug 26 relative to the shaft 4 depends on thel temperature range to be covered and on the basic desired bearing clearance.

When the illustrated bearing is subjected to an increase in temperature, the resulting expansion of the shaft 4, on the one hand, and the casing 1 and journals 13, on the other hand, is such that there is no significant change in the bearing clearance or film since these components are all of the same material. However, the viscosity of the lubricant decreases with the increase in temperature so that a reduction in bearing stiffness results since there is a less viscous lubricant acting within the same bearing clearance. With the provision of the slug 26 and its greater coefficient of expansion than the remainder of the shaft 4, the latter expands more than the journals 13 and casing 1 giving rise to an overall reduction in bearing clearance, and thus compensating for the reduction in lubricant viscosity. On a reduction in temperature, the lubricant 14 becomes more viscous but the differential contraction between the shaft 4 and journals 13/casing 1 increases the bearing clearance so that again, bearing stiffness remains substantially constant.

The present invention is applicable to any hydrodynamic bearing employing a nongaseous lubricant 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.

Also, the whole of the shaft 4 may be composed of DEROTON or other material having a higher coefficient of expansion than that of the casing 1. This arrangement would be more applicable to a bearing having a relatively large bearing film so that any distortion of the latter due to differential expansion of the shaft therealong arising out of the non-parallel configuration of the shaft at this part can be tolerated. Alternatively, the whole of the casing 1 may be composed of CERVIT or other material having a relatively low coefficient of expansion compared with that of the shaft 4.

The operation of the illustrated bearing is disclosed in co-pending Patent Application No. 9461/76 (Serial No. 1,577,737) and other aspects of the bearing are described and claimed therein.

WHAT WE CLAIM IS: - 1. A hydrodynamic bearing employing a non-gaseous lubricant and comprising a casing and a shaft mounted within and extending through at least one end of the easing, with the casing and shaft being capable of relative rotational movement and with the casing and the shaft constructed at least in part of materials having different coefficients of expansion such that the 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.

2. A bearing according to claim 1 wherein the shaft is composed of a material having a high coefficient of expansion relative to that of the material of the easing.

3. A bearing according to claim l, wherein the casing is composed of a material having a low coefficient of expansion

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. 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. Returning to Figure 1, the body 5 of the shaft 2 has a central portion in the form of a slug 26 of a synthetic plastics material sold under the trade name DEROTON and having a coefficient of expansion of 10 x 10-5 mm/mm/ C compared to that of the stainless steel portion of the shaft of 12 x 10-6 mm/mm/ C, i.e. about eight times greater. The slug 26 is bonded to the respective adjacent shaft body portions 5 preferably using an adhesive having some elasticity to accommodate the relative movement between the shaft body 5 and the slug 26 occurring on changes in temperature. For example, an epoxy resin with various fillers may bel employed or an isocyanate adhesive which is an anaerobicsetting adhesive. The bearing illustrated has a casing length of 3 8 cms and diameter of 17 cms, the diameter of the shaft ends 3 being 3 mm. The shaft body portions 5 and slug 26 have a diameter of 8 mm and the length of the slug is 1 cm. These dimensions in conjunction with the chosen lubricant provide an axial stiffness for the bearing of 106 x 10 6N.m -I (Newtons per metre) and a radial stiffness of 125 x 10 6N.m -1, which stiflnesses remain substantially constant over the temperature range of the lubricant as a result of the present invention. Furthermore, the quoted stiffnesses are greater than would pertain for a bearing of the same size and using gas as a lubricant. Thus the demand for small envelope size can be met whilst providing a bearing substantially free of instability problems. The dimensions of the slug 26 relative to the shaft 4 depends on thel temperature range to be covered and on the basic desired bearing clearance. When the illustrated bearing is subjected to an increase in temperature, the resulting expansion of the shaft 4, on the one hand, and the casing 1 and journals 13, on the other hand, is such that there is no significant change in the bearing clearance or film since these components are all of the same material. However, the viscosity of the lubricant decreases with the increase in temperature so that a reduction in bearing stiffness results since there is a less viscous lubricant acting within the same bearing clearance. With the provision of the slug 26 and its greater coefficient of expansion than the remainder of the shaft 4, the latter expands more than the journals 13 and casing 1 giving rise to an overall reduction in bearing clearance, and thus compensating for the reduction in lubricant viscosity. On a reduction in temperature, the lubricant 14 becomes more viscous but the differential contraction between the shaft 4 and journals 13/casing 1 increases the bearing clearance so that again, bearing stiffness remains substantially constant. The present invention is applicable to any hydrodynamic bearing employing a nongaseous lubricant 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. Also, the whole of the shaft 4 may be composed of DEROTON or other material having a higher coefficient of expansion than that of the casing 1. This arrangement would be more applicable to a bearing having a relatively large bearing film so that any distortion of the latter due to differential expansion of the shaft therealong arising out of the non-parallel configuration of the shaft at this part can be tolerated. Alternatively, the whole of the casing 1 may be composed of CERVIT or other material having a relatively low coefficient of expansion compared with that of the shaft 4. The operation of the illustrated bearing is disclosed in co-pending Patent Application No. 9461/76 (Serial No. 1,577,737) and other aspects of the bearing are described and claimed therein. WHAT WE CLAIM IS: -

1. A hydrodynamic bearing employing a non-gaseous lubricant and comprising a casing and a shaft mounted within and extending through at least one end of the easing, with the casing and shaft being capable of relative rotational movement and with the casing and the shaft constructed at least in part of materials having different coefficients of expansion such that the 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.

relative to that of the material of the shaft.

4. A bearing according to claim 3, wherein the casing is composed of a material having a negative coefficient of expansion.

5. A bearing according to claim 1, 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.

6. A bearing according to claim 5, wherein the slug is composed of a synthetic plastics material.

7. A bearing according to claim 5 and 6, wherein the slug is composed of the material sold under the trade name DERO TON and the shaft is composed of stainless steel.

8. A bearing according to claim 1, 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.

9. A bearing according to claim 8, wherein the slug has a negative coefficient of expansion.

10. A bearing according to claim 9, wherein the slug is composed of a supercooled ceramic.