WO2011057654A1

WO2011057654A1 - Bearing assembly with active oil lubrication

Info

Publication number: WO2011057654A1
Application number: PCT/EP2009/008092
Authority: WO
Inventors: Florin Tatar; Rihad Pasaribu
Original assignee: Ab Skf
Priority date: 2009-11-13
Filing date: 2009-11-13
Publication date: 2011-05-19

Abstract

The present invention relates to a bearing assembly comprising at least one rolling element bearing (102, 105) and at least one lubrication device (100). The rolling element bearing comprises an inner ring (152, 155), an outer ring and at least one set of rolling elements disposed within a cavity of the bearing on opposing inner and outer raceways. The lubrication device (100) comprises an oil reservoir and a pump drive mechanism. The assembly is further provided with an oil supply line, which is in connection with the reservoir and which has a discharge orifice (148, 151 ) arranged inside the bearing cavity in proximity to a rolling contact zone of the bearing. According to the invention, the pump drive mechanism is an electro-osmotic member which pumps a drive fluid, whereby the drive fluid exerts pressure on the oil reservoir in order to effect a supply of oil to the rolling contact zone.

Description

BEARING ASSEMBLY WITH ACTIVE OIL LUBRICATION FIELD OF THE INVENTION

The invention relates to a bearing assembly comprising a rolling element bearing and a pump device for delivering a small amount of oil to a rolling contact zone of the bearing.

BACKGROUND ART

If rolling element bearings are to operate reliably, they must be adequately lubricated to prevent direct contact between the rolling elements, raceways and cage (if present). Loss of lubrication function results in friction and wear, and will quickly lead to bearing failure. The lubricant used and the method of lubrication applied are selected depending on the application. Grease is used in the majority of applications, because it is easy to retain within the bearing cavity and also helps prevent the ingress of contaminants. In high-speed applications, however, the drag and resulting friction losses associated with grease lubrication are too high. Oil lubrication is then employed. Oil not only has the benefit of allowing a higher operating speed, it can also be used a coolant in applications where high temperatures are produced as the result of high speed and/or load. Forced circulation is commonly applied when a cooling function is required. Other types of oil lubrication include oil bath and splash lubrication.

When a bearing must operate at ultrahigh-speeds, such as occur in e.g. machine tool spindles, even a small amount of oil generates an unacceptable degree of oil agitation resistance and friction losses. Oil/air lubrication is generally employed in such applications. Typically, a minimal amount of oil is discharged intermittently by a constant-quantity piston into a pipe carrying a constant flow of compressed air. The oil/air mixture is then spayed into the rolling contact zone. In some cases, this is achieved by drilling a hole through the bearing outer ring and inserting a nozzle of approximately 2 mm in diameter. One drawback of this solution is that compressed air is fed into the bearing, which produces a great deal of noise in combination with the fast-rotating rolling elements. Further, as disclosed for example in EP 1770295, it is known to fit a bearing assembly with an oil supply unit, whereby a tube-like nozzle extends into the bearing cavity from an axial side. The supply unit comprises a piezoelectric diaphragm pump (micropump), which pulsates a spray of oil into the rolling contacts. In one embodiment, the micropump itself is located within the bearing cavity. The assembly as whole can therefore be made compact and the

micropump is capable of delivering a small flow rate of oil that generates very little friction during operation. The piezoelectric micropump in combination with bearing lubrication has a number of drawbacks, however. The maximum length of the tube-like nozzle is limited. If the distance between the discharge orifice and the piezoelectric diaphragm (pump drive) exceeds e.g. 70 mm, there is chance that pulsation will be lost. Consequently, the discharge orifice and the pump drive must be located close together, which places design constraints on the assembly as a whole. Furthermore, a piezoelectric micropump operates at high voltage (e.g. 200 V) and the extremely small flow rates desirable for the active lubrication of high-speed bearings, a pulsated flow is difficult to control with precision. Consequently, there is room for improvement.

SUMMARY OF THE INVENTION

The present invention relates to a bearing assembly comprising at least one rolling element bearing and at least one lubrication device. The rolling element bearing comprises an inner ring, an outer ring and at least one set of rolling elements disposed within a cavity of the bearing on opposing inner and outer raceways. The lubrication device comprises an oil reservoir and a pump drive mechanism. The assembly is further provided with an oil supply line, which is in connection with the reservoir and which has a discharge orifice arranged inside the bearing cavity in proximity to a rolling contact zone of the bearing. According to the invention, the pump drive mechanism is an electro-osmotic member which pumps a drive fluid, whereby the drive fluid exerts pressure on the oil reservoir in order to effect a supply of oil to the rolling contact zone.

Thus, in an assembly according to the invention, the pump is configured for indirect pumping of oil. The pump is an electro-osmotic pump which causes a drive fluid to be transported from a first side of the electro-osmotic member to a second side of the electro-osmotic member. The oil reservoir is arranged at the second side and suitably, the drive fluid and the oil in the reservoir are separated by a deformable membrane. As the drive fluid is pumped from the first side to the second side of the electro-osmotic member, the fluid exerts pressure on the deformable membrane, which in turn causes oil to be pumped from a pump nozzle. An amount of drive fluid displaced equals an amount of oil displaced, and thus oil flow rate is controlled by controlling the flow rate of the drive fluid. Controlling the flow rate of the drive fluid has several advantages. The principle of electro-osmosis allows extremely small, continuous flow rates to be achieved and controlled with great precision. Furthermore, there is an excellent linear relationship between a voltage applied across the osmotic member and the resulting flow rate, even at very small rates of flow. The precise, direct control of the drive fluid enables precise, indirect control of the oil, meaning that extremely small, continuous flow rates of oil can be supplied and controlled with great precision. Therefore, an optimal amount of oil can be supplied that results in minimized friction losses. In an advantageous embodiment of the invention, the bearing assembly is adapted for rotational speeds of between 10,000 and 20,000 rpm. The bearing assembly may, for example, support a machine tool spindle or form part of a turbocharger. As explained above, when bearings operate at such high rotational speeds, it is important to minimise friction losses from oil agitation resistance.

Suitably, the lubrication device used in the invention may be configured to pump oil at a flow rate of between 1 and 10 micro-litres per minute. In a further example, the lubrication device is configured to pump oil at a flow rate of between 1 and 5 micro-litres per minute.

In one embodiment of the invention, the bearing assembly comprises a first and a second bearing which are supplied with oil from a single electro-osmotic pump. Suitably, the oil supply line is split into two parts and comprises a first discharge orifice that opens in the cavity of the first bearing and comprises a second discharge orifice that opens in the cavity of the second bearing. Alternatively, the bearing assembly may comprise a first electro-osmotic pump for lubricating the first bearing and a second electro-osmotic pump for lubricating the second bearing

In further development of the invention, the at least one electro-osmotic pump is arranged at a distance from the at least one bearing, such that a length of the supply line between the oil reservoir and a discharge opening is greater than 10 cm or greater than 20 cm or greater than 30 cm. The advantage of this development is that a bearing assembly according the invention is suitable for applications where the one or more bearings are arranged at an inaccessible location or at a location where there is no room to mount a lubrication device. The electro-osmotic pump of the invention delivers oil in a continuous flow and thus enables placement at such distances from a bearing.

Consequently, a bearing assembly according to the invention has several advantages. The at least one rolling element bearing can be supplied with a continuous flow of oil that may be controlled with extreme precision. Flow control is a straightforward process and is achieved simply by varying a supply voltage. Only low voltages and currents are needed, making the lubrication device economical to run and because the device comprises no moving parts, it is highly reliable. Furthermore, the at least one lubrication device need not be located in close proximity to the one or more bearings.

Other advantages of the invention will become apparent from the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail, by way of example, and with reference to the accompanying drawings, wherein:

Figure 1 : is a partial cross-sectional view of an example of a bearing assembly according to the invention;

Figure 2: is a cross-sectional view of a pump device suitable for use in the invention.

DETAILED DESCRIPTION

Figure 1 shows an example of part of a bearing assembly according to the invention. In this example, the bearing assembly comprises first and second angular contact bearings 102, 105 which support a spindle 107 relative to a housing 110. The spindle 107 may be a machine tool spindle which, in use, is adapted to operate at speeds of between 10,000 and 20,000 rpm. Oil lubrication is needed, so as to form a film that prevents metal-to-metal contact between rolling contact surfaces of the first and second bearings 102, 105. At such high operating speeds, however, too much oil will cause significant oil agitation resistance and unacceptable friction losses. The assembly is therefore provided with active lubrication, whereby a minimal supply of oil is delivered directly into a cavity of each bearing, to lubricate the rolling contacts. According to the invention, oil delivery is effected using an electro-osmotic pump 100.

Figure 2 shows an example of an electro-osmotic pump that may be used in the assembly depicted in Figure 1. The pump 200 comprises a pump housing 202 that defines a first chamber 205 and a second chamber 207. The first chamber 205 contains a drive fluid 210, such as de-ionized water, which exhibits electro- osmosis. The second chamber 207 contains a lubricant 212, such as high-speed spindle oil. The first and second chambers 205, 207 are separated by a deformable membrane 215, which prevents contact between the drive fluid and the lubricant. The pump 200 further comprises a drive mechanism 217, consisting of an electro-osmotic member 220 and first and second electrodes 222, 225 for generating an electric field across the electro-osmotic member 220. The drive mechanism 217 separates the first chamber 305 into an inlet side 227 and an outlet side 230, whereby the first electrode 222 is located at the inlet side of the electro-osmotic member 220 and the second electrode 225 is located at the outlet side. In the depicted example, the drive mechanism 217 is housed in a narrow portion of the first chamber, formed by a rigid wall 231 of the pump housing 202 that extends inwardly into the first chamber 205. In other examples, the drive mechanism spans a full diameter of the pump housing when higher flow rates are required

When a DC voltage is placed on the electrodes 222, 225, the drive fluid (water) is transported from the inlet side 227 to the outlet side 230 of the first chamber 205. The drive fluid 210 therefore exerts pressure on the deformable membrane 215, which in turn causes lubricant 212 to be pumped out of a nozzle 232. Pumping occurs on the basis of electro-osmotic flow, which will now be briefly explained.

The electro-osmotic member comprises a porous silica, such as glass fibre, and is preferably also hydrophilic, so that the drive liquid (water) is absorbed into the porous silica. In effect, the porous silica acts a set of capillary channels, whereby silanol groups form on an inner surface of each capillary channel. These silanol groups are ionized above pH3, meaning that the inner surface of the channel is negatively charged. In solutions containing ions, the cations will migrate to the negatively charged surfaces, thereby forming an electrical double layer. When an electrical potential is then applied to the channels, with an anode at one end of a channel and a cathode at another, the cations will migrate towards the cathode. Since these cations are solvated and clustered at the walls of the channel, they drag the rest of the solution with them, even the anions. In the depicted example, the first electrode 222 is the anode and the second electrode 225 is the cathode. Therefore, when a DC voltage is applied across the electrodes, electro-osmotic flow occurs in the direction indicated by the arrow on Figure 2. The flow is continuous and may be precisely controlled by varying the voltage. A voltage of approximately 2 V is sufficient to induce flow, and flow rate increases with voltage in a linear manner. Precision control is possible, even at flow rates of less than 5 microlitres per minute, meaning that a high-speed bearing can be lubricated with an optimal amount of oil that generates minimal friction.

A further advantage of an electro-osmotic pump is that when most or all of the oil in the second chamber 207 has been pumped out, the chamber can be refilled using the principle of electro-osmotic flow. By reversing the polarity of the first and second electrodes 225, 227, the flow direction of the drive fluid 210 is reversed. Thus, drive fluid that is present at the outlet side 230 of the electro-osmotic member 220 is transported to the inlet side 227. This creates an under pressure at the inlet side, and if the pump nozzle 232 is placed in e.g. a bath of oil, oil will be sucked up to refill the second chamber 207.

In some embodiments, a lubricant supply tube 235 is attached to the pump nozzle 232. For lubricating a bearing, this tube may be inserted into e.g. a hole drilled in a bearing outer ring and a corresponding hole drilled in the bearing housing, to spray oil onto the rolling elements. Alternatively, with reference to Figure 1 , the pump housing may be clicked into a specially adapted holder device 130 that is fitted to the bearing housing 110, whereby the holder device comprises a supply line that links up with suitable channels provided in the bearing housing.

In the example of Figure 1 , the supply line 135 comprises a T-junction and splits into two parts. A first part links up with a first channel 112 in the bearing housing 110 and a second part links up with a second channel 115 in the housing. The first and second bearings 102, 105 are axially spaced apart by first and second annular spacers 142, 145. Suitably, the first spacer 142 comprises a channel 147 that links up with the first channel 112 in the housing and the second spacer 145 comprises a channel 150 that links up with the second channel 115 in the housing. The channel in each the first and second annular spacers has a corresponding orifice 148, 151 that opens just above an inner ring 152 of the first bearing 102 and inner ring 155 of the second bearing 105.

As will be understood, the assembly of Figure 1 may be provided with a second electro-osmotic pump, whereby the discharge orifice 148 of the first spacer 142 is in connection with the oil reservoir of the first pump and the discharge orifice 151 of the second spacer 145 is in connection with the oil reservoir of the second pump. The aforementioned channels 112, 115, 147, 150 in the annular spacers and in the housing may be used as oil supply lines. Suitably, these channels may be provided with a low-friction coating or an oleophobic coating, to ensure that the lubrication oil experiences minimal flow resistance. Alternatively, a flexible supply tube may be inserted through these channels, which supply tube may be connected directly to the pump 100.

Angular contact ball bearings are particularly suited for high speed applications because during operation, the balls spin around two axes of rotation. The lubricating film therefore gets well distributed over inner and outer raceways of the bearing, in comparison with e.g. a deep-groove ball bearing or a tapered roller bearing. These latter-mentioned types of bearing are more susceptible to wear at high operating speeds.

As can be seen from the first and second bearings shown in Figure 1 , the inner ring of each bearing, at opposing axial sides, has an inclined surface 157. In other words, at a peripheral edge of each inner ring 152, 155, the ring has a minimum radial thickness that increases towards the centre of the bearing. As mentioned, the channels 147, 150 in the first and second annular spacers 142, 145 have a corresponding orifice 148, 151 that opens just above the inclined surface 157 of each inner ring 152, 155. Each orifice may have a diameter of approximately 0.1 to 1 mm. A continuous flow of oil, of approximately 5 L/min, is delivered onto the inclined surface 157. The bearing assembly is adapted for inner ring rotation, meaning that upon contact with the inclined surface 157, the high centrifugal forces will fling out part of the oil in a radial direction. Some oil will remain on the surface 157, however, and because of the surface inclination, the centrifugal force has an axial component that urges the oil towards the centre of the bearing and into the rolling contact zone.

Due to the nature of an electro-osmotic pump, the continuous flow of oil can be precisely controlled, allowing the flow rate to be optimised according to factors such as spindle operating speed and load. Furthermore, the pump 100 need not be located "close" to the orifices from which the oil is delivered. It has been found that an electro-osmotic pump can indirectly pump oil through a supply line of at least 50 cm in length. Consequently, a bearing assembly according to the invention allows greater freedom of design with regard to placement of a lubrication device.

A number of aspects/embodiments of the invention have been described. It is to be understood that each aspect/embodiment may be combined with any other aspect/embodiment. The invention may thus be varied within the scope of the accompanying patent claims.

Reference numerals

Figure 1

100 Electro-osmotic pump

102 First angular contact bearing

105 Second angular contact bearing

107 Spindle

110 Bearing housing

112 First channel in bearing housing

115 Second channel in bearing housing

130 Holder for pump

135 Oil supply tube

142 Fist annular spacer

145 Second annular spacer

147 Channel in first annular spacer

148 Discharge orifice of channel in first spacer

150 Channel in second annular spacer

151 Discharge orifice of channel in second spacer

152 Inner ring of first bearing

155 Inner ring of second bearing

157 Increase surface of bearing inner ring

Figure 2

200 Electro-osmotic pump

202 Pump housing

205 First chamber of pump housing

207 Second chamber of pump housing (oil reservoir)

210 Drive fluid

212 Oil

215 Deformable membrane

217 Drive mechanism

220 Electro-osmotic member First electrode

Second electrode

Inlet side of first chamber Outlet side of first chamber Wall section of pump housing Nozzle

Oil supply tube

Claims

1. Bearing assembly comprising at least one rolling element bearing (102, 105) and at least one lubrication device (100, 200), the lubrication device comprising an oil reservoir (207) and a pump drive mechanism (117), the assembly further comprising an oil supply line connecting the oil reservoir with a cavity of the bearing, the oil supply line having a discharge orifice (148, 151 ) arranged in proximity to a rolling contact zone of the bearing,

characterized in that

the pump drive mechanism (217) is an electro-osmotic member (220) which pumps a drive fluid (210), whereby the drive fluid exerts pressure on the grease reservoir in order to effect a supply of oil to the rolling contact zone.

2. Bearing assembly according to claim 1 , wherein the bearing assembly is adapted for operational speeds of between 10,000 and 20,000 revolutions per minute.

3. Bearing assembly according to claim 1 or 2, wherein the at least one lubrication device (100, 200) is configured to pump oil from the oil reservoir (207) into the bearing cavity at a rate of between 1 and 10 pL per minute.

4. Bearing assembly according to claim 3, wherein the at least one lubrication device (100, 200) is configured to pump oil from the oil reservoir (207) into the bearing cavity at a rate of between 1 and 5 μΙ_ per minute.

5. Bearing according to any preceding claim, wherein the at least one lubrication device (100, 200) is arranged at a distance from the at least one bearing (102, 105), such that a length of the oil supply line between the oil reservoir (207) and the discharge orifice (148, 151) is greater than 10 cm or greater than 20 cm or greater then 30 cm. Bearing assembly according to any preceding claim, wherein the assembly comprises a first rolling element bearing (102) and a second rolling element bearing (105) and the oil supply line comprises a first discharge orifice (148) which opens in a cavity of the first bearing and further comprises a second discharge orifice (151 ) that opens in a cavity of the second bearing.