WO2004053346A1

WO2004053346A1 - Liquid pocket bumper for bearings

Info

Publication number: WO2004053346A1
Application number: PCT/IT2003/000799
Authority: WO
Inventors: Pasquale Dell'aversana
Original assignee: Mars-Microgravity Advanced Research And User Support Center S.R.L.
Priority date: 2002-12-06
Filing date: 2003-12-04
Publication date: 2004-06-24
Also published as: ITNA20020070A1; AU2003288537A1

Abstract

The invention relates to a bumper for bearings with a surface which is malleable because of the dynamical action of the lubricant flow under conditions of thermal control and rigidity adjustment, said bumper consisting of at least a compliant membrane (1), a liquid (3), and a rigid construction (2), the temperature of the liquid being adjustable by means of a possible control system (4), and the rigidity being adjustable by a possible integrated mechanism.

Description

Liquid pocket bumper for bearings

The present invention relates to a liquid pocket bumper for bearings.

Bearings are mechanical parts used to support the load of other parts sliding, rotating or pivoting on them or in their inside. There are many kinds of bearings: roller bearings, ball bearings, sliding bearings, carrying bearings, axial bearings etc., the design of which depends on their specific application in view of the load to be supported, lubricant (if used) , operating conditions (vacuum, normal laboratory conditions, industrial environment, presence of dust, debris, oil or other pollution agents), operation temperature, relative speed of the surfaces coming into play etc. Crucial aspects of the technology are: load capacity, friction, wear and tear. Generally, the lubricant is a liquid or air but there are also solid lubricants such as graphite dust . Lubricants are used to help in reducing friction and wear by keeping the running surfaces at a distance from each other (the thickness of the lubricating film interposed therebetween) .

When lubricant fully prevents any contact by forming a continuous film, a perfect fluid-dynamic condition is set up. Under such condition the load ""flies¹¹ above the surface of the other member of the bearing due to the lift produced by the lubricant. However, this is almost always an ideal condition. In most cases, in fact, a contact between the surfaces cannot be avoided and the effects of a ^"limit lubrication condition'¹ or a strong impoverishmen of the lubricant must be faced. Under such conditions the contact between the running surfaces can be sporadic, frequent or continuous and give rise to microlinking between the members of the bearing, the effect of which is the increase in the friction, torque, wear, noise, and the production of debris which can compromise the quality and the efficiency of the lubricant. In many cases the lubricant is pushed between the relative running surfaces by the surfaces themselves or it is forced to enter more or less complicated injection systems. Some bearings do not use external lubricants but only the lubrication capability of the material which they consist of, as it is the case of Teflon or bronze bearings . Sometimes, because of friction, pressure, shearing stress inside the lubricant, the local temperature increases until the lubricant viscosity is reduced considerably (viscosity depends on the temperature) . Generally, the greater the lubricant viscosity, the higher the load capacity of the lubricating film so that an increase in the temperature can cause a significant loss of the load capacity.

Another important aspect influencing the performance of a bearing system is the elasticity of the surfaces involved. Also apparently rigid bodies such as stainless steel ball bearings are subjected to little temporary local deformations in the area of the lubricating film where the involved pressures can be very high. However little, such deformations can be comparable with the thickness of the lubricating film (typically of the order of some microns) and have a considerable effect to the lift of the film. When such elastic deformations cannot be neglected, there is an ""elastic- idrodynamic ' ' lubricating condition.

The natural elastic deformations of a bearing induced by the action of the lubricant cause the load capacity of the bearing to increase. For this reason, even when the fluid-dynamic forces would not be able to produce sufficient deformations of the bearing surface, indentations, grooves, pockets, etc. can be machined intentionally. Furthermore, particularly in bumper bearings, the carrying plate is made with a degree of freedom that allows the same to tilt towards the lubricant flow to take the most suitable coupling angle automatically and naturally, according to the current dynamical conditions . Another kind of bearing that seeks to take advantage of a non-predetermined shape which can be adapted to dynamical operating conditions is the so-called ""foil bearing ' ' . This kind of bearing, used from decades, operates with air or other kind of lubricant and does not need any injection system: the lubricant is forced to enter the room between the relative moving parts by its viscosity.

Typically, they are used in the aeronautical industry, in air recycle installations for the pressurizatio of airliner cabs, because they are light and particularly effective in their operation without oil as they use the environment air as lubricant .

Their load capacity, however, is relatively limited, and even if it is improved in recent designs, it can often be hardly foreseen.

The surface of the bearing consists of compliant foils made of metal alloys (Inconel) or other material, usually secured at only one end and laid out on a set of elastic supports (bumpers) of various design and elasticity to optimise Lhe load capacity and to limit the side leakage of the lubricant .

In almost all kind of foil bearings the movement is only possible in one direction; in addition, they are in any case subjected to wear and need to be made by materials and coatings having good self-lubricating features .

The use of Teflon as coating limits the use at temperatures not greater than about 270°C while the use of other compound solid lubricants, such as those of PS300 family, developed for example by Glenn

Research Center of NASA, allows operations at higher temperatures and speeds.

Innovations in this field are so important that it seems to be possible today to develop new engines for aircraft completely without oil due to the last innovations in foil bearings, thus reducing masses, cost and maintenance . It should be appreciated that foil bearings need to be used at not too low speeds in order to operate correctly, otherwise the lubricant is not able to provide enough lift.

Recently, it has been pointed out that lubricating liquids can operate in a not so very different way from the lubricant solids.

In particular, it has been shown that a drop can slide on a smooth solid surface that moves rapidly and tangentially to the drop without the drop wets the solid surface, even if it is pressed against the latter to undergo a considerable deformation.

This is possible because air driven by the running surfaces enters the room between the drop and the solid surface, thus exerting the lubricating action. Interferometric observetion researches have shown that the elastic-hydrodynamic deformations of the surface of a drop at the ""contact'¹ area are very similar to those produced on solid ball bearing.

This could not be easy to guess in view of that a solid bearing operates at thousands of times greater pressures and does not have a moving surface as a liquid does.

Like any bearing, also a drop lubricated by an air film can support a load. As it is easily to understand, however, its load capacity is very small as it is limited by the surface tension of the liquid in the drop. In spite of their typical minimum load capacity, drops lubricated by air have been proposed for use in space applicationπ (in absence of weight a reduced load capacity can not be a too severe limit) as they act as ideal bearings under other aspects.

In fact, the drop bearings always operate under perfect hydrodynamic conditions so that they are not subjected to static friction, do not wear, do not produce debris, and are not noisy because of their capability of being easily deformed under mechanical stress .

They also fit different dynamical conditions assuming naturally the shape as best as possible from the point of view of the energy minimi ation according to the opposed surface, the load applied, and the action of the lubricant, however, with the limit that they break

(i.e. they wet the surface against which they are pressed) as soon as the speed of the running surfaces falls under a critical threshold. Because of the capability of being easily deformed and the very small characteristic loads of the drop bearings, however, this critical speed threshold is much lower than that allowing usually a foil bearing to operate. It should be understood that a bearing device, the features of which are as similar as possible to those of an ""ideal'' bearing such as drops but with the further capability of supporting loads comparable with those of the solid bearings without breakage at the stop of the mobile parts, would be desirable. The present patent for industrial invention aims at a new device for application similar to those of the sliding bearings which associates the. shaping adaptability of the liquid bodies with the strongest construction and the greater load capacity of the solid bearings. Such a device would be :

- less subjected to friction and wear of the bearings currently used; - capable of supporting heavy loads with respect to foil bearings;

- bi-directional, i.e. capable of operating with the same performance whichever the direction of movement of the other member of the bearing may be, without having to be turned;

- able to be positively cooled to solve the problem of the wear and the reduction of viscosity of the lubricant as the speed and the operating temperature increase ; - formed by assembling several parts; the operating wear would be exclusively limited to easily interchangeable parts;

- capable of operating with the same lubricant longer than the bearings of the preceding state of art, changing much less the characteristics of the lubricant because of the release of debris;

- capable of using the environment air as lubricant for applications in which it is important to avoid oils as lubricants and then particularly able to the aeronautical use; - formed also by elastic parts, with the possibility of adjusting the elastic reaction to allow the whole rigidity of the system to be adjusted and then also the performance as damper of mechanical stress, the latter function being reduced or fully absent as and adjustable function in bearings of any kind of the preceding state of art, however excluding magnetic suspensions which need a power supply, and are very difficult to be manufactured. The difficulty to reach the above object is the apparent incompatibility between the necessity of having a surface which is able of being easily deformed locally and the necessity of supporting heavy loads . The problem can be solved with a bumper providing a hydraulic suspension essentially consisting of α given volume of liquid enclosed by rigid walls everywhere except for the vicinity of the lubricated area

(contact area) where the liquid is contained in a pocket consisting of a thin, compliant membrane with limited elasticity.

The load applied to the system is balanced by the force given by the product of the differential pressure (P) between the inside and the outside of the pocket and the contact area.

It is possible to adjust directly the rigidity of the system by modifying the average tension of the membrane by adjusting the liquid pressure. The new solution with respect to the devices of the previous state of art consists in that the reaction to the load in the liquid pocket bumper is perpendicular to the direction of elastic-hydrodynamic deformation of the buinμei ; it is then possible to provide a high load capacity without hardening the system. In fact the pocket is extremely compliant and is easily deformed to a perpendicular direction to its surface, i.e. to the direction to which it should move to avoid the contact with the other member of the bearing, and the supporting function of the load is put into tangential forces to the membrane because the latter, that contains a liquid which is incompressible, tends to widen as the load increases. Conversely, for example in foil bearings, also the lower bump strip must be deformed in order for the compliant foil to be deformed by the lubricant.

Thcx^~cfore, it is self-eviden that the greater the load to be supported, the harder the system and the greater the minimum speed to obtain enough lubrication force to deform the compliant foil. As a result, a liquid pocket bumper bearing will operate much better than the foil bearings at low speed, also in the presence of high loads. The innovation brought by liquid pocket bumpers would allow among other things to overcome the hybrid systems which resort to magnetic suspensions at lower speeds .

The solution is shown in the accompanying drawings of an illustrative, not limiting embodiment. Figure 1 of Table I shows the liquid pocket bumper in the form of an axial or thrust bearing.

Figure 2 of Table II shows the liquid pocket, bumper in the form of a coaxial bearing.

Figure 3 of Table III shows the application to a carrying bearing of a cylinder,- and

Figure 4 shows the application to a carrying bearing of a shaft. In this case, the liquid pocket bumper consists of several individually pressurised isolated sectors .

With reference to Figure 1, the bumper consists of a compliant membrane (1) carried by the rigid construction (2), both containing the liquid pocket (3) . Coil (4) is a schematic example of any thermal conditioning system of the liquid in the pocket .

It serves to avoid the overheating of the membrane and the lubricant during the operation.

In part A) of the Figure the bumper is out of operation and then not loaded and separated from the other member (10) of the bearing.

In part B) of the Figure the bumper is operating and applies a load on surface (10) .

The lubricant (not shown) is forced to enter between the membrane and the surface to produce elastic- hydrodynamic deformations in the membrane. In the Figure, the only visible deformation is produced by the flattening of the liquid pocket against the solid

SIIΓ [^~a.ce .

Such deformation would almost be the same even without lubricant .

The elastic -hydrodynamic deformations, instead, are not visible as they are very little with respect to the contact area.

The air space between the liquid pocket and the solid surface at B) is not to scale.

The thickness and the shape of the air space, otherwise called ""lubrication channel'¹ depend on the operating conditions as they are elastic-hydrodynamic .

When the surface (10) stops, the lubricant is drained via the contact area by the flattening of the bumper, and the lubricating film becomes thinner until it disappears .

As it can be seen in the Figure, surface (10) can slide horizontally in any direction. It is also possible for the system to dampen some vertical oscillation or vibration of the other member of the bearing.

Such troubles can be dampened without damages by the total elasticity factor of the bumper consisting essentially of the elasticity of membrane (1) and an integrated system shown schematically in the Figure and consisting of spring (7) .

Upon compressing the spring by piston (5) the pressure of the liquid and accordingly the rigidity of the suspension is changed. The sealing of the piston is ensured by piston rings

(6) .

The compression degree of the spring can be changed by roteting adjustment nut (9) that shifts leadscrew (R) . Instead of the spring system, that is shown to give a schematic example of any way to change the rigidity of the bumper, a pneumatic system or another equivalent system can be provided.

Part C) of Figure 1 shows a bottom view of the bumper: only the compliant membrane of the liquid pocket with its locking ring (11) .

The locking of the membrane to rigid construction (2) by ring (11) is designed and example to make the membrane easily replaceable. With reference to Figure 2 of Table II, the liquid pocket (17) has a cylindric l, shape and moves inside cylinder or coaxial tube (12) . Stationary cylindrical bumpers operating as guide/support for a mobile piston can also be provided. In the schematic Figure liquid (13) is pressurized by only one compression system consisting of the same piston-spring-worm screw system as the preceding

Figure .

Cooling coil (16) has such a shape as to match the cylindrical shape of the pocket.

The sealing ring of the membrane is replaced by flanges (14) ; the rigid construction is formed by part (15) .

With reference to the embodiment of Figure 3 of Table III, a coaxial embodiment with section and front similar to those of Figure 2 is shown with the functional difference that the liquid pocket bumper

(19) and cylinder (18) do not si i t-¹. wi.l.hin each other but rotate to each other about the same axis . The liquid pocket is divided in different sectors (six of them are shown but more or less sectors can be provided if it were necessary) which are individually pressurized. The same pressure can be applied in all of the sectors, however, if the liquid pocket bumper should operate, for example, as a carrying bearing for a cylinder, a greater pressure could be provided in the pockets to support the corresponding greater load. In a more sophisticated system provided with a controlled feedback of the load distributed to independent pockets, the pressure could be adjusted again automatically to compensate load variations due to the accidental breakage of one pocket or changes of the operating conditions. The same considerations for the carrying system of a cylinder are also valid for the system • of Figure 4 of Table III where the six stationary bumpers are carried by a rotating shaft

(20) .

The conceived liquid pocket bumper for bearings has the following advantages over the currently used devices :

• Any plastic or elastic deformation of a solid causes the change of its atomic lattice structure and then a work. Conversely, the deformations of a liquid enclosed by a membrane not involving variations of P take place almost without work as the liquid dos not have an atomic lattice structure that opposes the deformations, i.e. a bumper consisting of a liquid enclosed in a pockel having compliant walls follows any imperfections or loss of alignment of the other bearing member much more easily than a solid bumper and foil bearings as well.

• Also in case the deformations of the liquid pocket bumper cause P to change, such deformations are never as localized as it is the case for a solid bumper subjected to the same stress so that the wear of the

■ deformed portion is much less; the reason of that is partly the same as mentioned above. In this case, even if the work to produce a given deformation is not zero, it is anyhow much less as it can only be given by the work necessary to change the average tension of the membrane; the transfer of energy that can cause a damage to the lubricated surfaces is lower accordingly. Moreover, because of the malleability of the compliant surface of the bumper, the load can be distributed more evenly all over its surface.

• The malleability of the bumper surface and the reduced transfer of energy capable of damaging such surface along with the formation of more uniform lubricating film cause the friction as well as the operating speed to reduce significantly at macroscopic level with respect to foil bearings.

• It is possible to cool the liquid of the bumper very effectively by an independent cooling circuit, for example, by finching heat from the lubricant through the thin membrane of the liquid pocket. Thus, reductions of the lubricant viscosity connected to increases in the temperature as well as falls of the load c acity are avoided.

• In practice the membrane is the sole portion subjected to wear. It can be placed so as to be easily replaceable .

• As the rigidity of the liquid pocket bumper can be adjusted, it can operate as damper at the same time.

• As the rigidity of the liquid pocket bumper can be easily adjusted by regulating the liquid pressure, such devices can be used as ""intelligent¹¹ components of systems which are responsive to changeable operating conditions.

The proposed embodiments appear for the present the most suitable to carry out the solution of the invention, however, those skilled in the art can modify them without departing from the scope of the present invention as defined by the appended claims.

Claims

1. A liquid pocket bumper for bearings, characterized in that it consists of at least a compliant membrane, a liquid, and a rigid construction, the shape of said membrane being determined by the current fluid-dynamic conditions, said membrane being secured to the rigid construction and being designed to slide on an opposed external surface, said liquid being contained in said construction and said membrane in order to dampen mechanical shocks upon using bearings of different kinds to support loads under operating conditions involving minimum friction and wear.

2. The bumper according to the preceding claim, characterized in that at least one elastic part is integrated in the bumper to improve the capability of damping mechanical shocks.

3. The bumper according to the preceding claims, characterized in that the temperature of the liquid is kept about a predetermined value by means of thermal control system.

4. The bumper according to the preceding claims, characterized in that its rigidity can be adjusted at any moment be means of a device to change the liquid pressure .

5. The bumper according to the preceding claims, characterized in that the compliant membrane has isotropic elastic properties.

6. The bumper according to preceding claims, characterized in that the compliant membrane of claim

1 has not isotropic elastic properties.

7. The bumper according to the preceding claims, characterized in that the elastic properties of the compliant membrane of claim 1 are very low, and the shock damping function is performed almost exclusively by a spring, a compressed gas, or the like, the compression degree of which is adjustable.

8. The bumper according to the preceding claims, characterized in thaL said bumper can be inclined about at least one axis.

9. The bumper according to the preceding claims, characterized in that the compliant membrane can be removed and replaced instead of being secured to the rigid construction of claim 1.

10. The bumper according to the preceding claims, characterized in that the reciprocal distance between the rigid construction of claim 1 and the other member of the bearing is adjustable.

11. A bumper assembly each of them according to the preceding claims, characterized in that said bumpers operate together like a complex structure.

12. The bumper assembly according to the preceding claim 11, wherein the rigidity of each bumper can be automatically controlled individually and positively.

13. A compliant membrane according to claim 1, wherein the surface is reinforced totally or partly by a tissue, a metallic net or the like to limit the elasticity along one or more directions, the compliance of said membrane being not compromised where it is reinforced.

14. The compliant membrane according to claim 1, wherein it is made partly or totally of particularly compliant metals or tissue or other materials with particularly high tensile strength.

15. The compliant membrane according to claim 1, wherein it is corrugated like a bellow or the like outside the lubricated area to locate the greater deformations of its surface.

16. The compliant membrane according to claim 1, wherein it is covered by a solid lubricating layer, said solid lubricating layer being brought to said membrane during the construction or transferred thereto from the surface of the other member of the bearing during the operation.