GB2537620A

GB2537620A - Method of measuring a volume of a lubricant inside a bearing arrangement

Info

Publication number: GB2537620A
Application number: GB1506668.1A
Authority: GB
Inventors: Van Pommeren Jascha
Original assignee: SKF AB
Current assignee: SKF AB
Priority date: 2015-04-20
Filing date: 2015-04-20
Publication date: 2016-10-26
Also published as: GB201506668D0

Abstract

A measurement system 12 for measuring a volume of lubricant in a bearing (10, Fig. 1), the system comprising a pressure regulator 25 connected to an interior of the bearing via a channel 21, a valve 31 in the channel between the pressure regulator and the bearing, and a pressure sensor 27 to measure a pressure in the interior of the bearing. There is also a controller arranged to control the regulator and the valve so as to instantly supply or release a predefined overpressure to/from the interior of the bearing arrangement, and to measure the pressure in the interior during the supplying or releasing of the overpressure. It then determines a volume of gas flown into or out of the interior using the measured pressure, and subsequently determines the volume of the lubricant using the determined volume of gas, the measured pressure or optionally a known empty volume of the interior. There may optionally be a flow sensor in the channel. Leakage from the bearing may be modelled and measured.

Description

METHOD OF MEASURING A VOLUME OF A LUBRICANT INSIDE A BEARING ARRANGEMENT

FIELD OF THE INVENTION

The invention relates to a method and system for measuring a volume of a lubricant inside a bearing arrangement. The invention also relates to a device comprising such a system and to a method of lubricating the bearing arrangement.

BACKGROUND ART

For bearings to properly function they need to be lubricated using lubrication means like grease. Lubrication reduces friction between the components of the bearing so as to improve the performance and to increase the life time.

The optimal grease amount in a bearing is very important. If there is too little grease in a bearing, there will be more wear, and the bearing will have a shorter life time than expected. If there is too much grease inside the bearing, the result will be more friction and thus unwanted heat creation.

To avoid these problems, nowadays methods are used to determine the lubrication condition real-time. Several techniques to determine lubrication film thickness are known, such as Acoustic Emission (AE), Electrical conductivity and Capacitance measuring. These methods can be quite complex and expensive.

Furthermore, knowing the film thickness may not be sufficient for determining how much lubricant actually is in a bearing.

So there is a need for a method of measuring the amount of lubricant inside a bearing.

SUMMARY OF THE INVENTION

One of the objects of the invention is to provide a system and method of measuring a volume of a lubricant inside a bearing arrangement.

A first aspect of the invention provides a measurement system for measuring a volume of a lubricant present in a bearing arrangement. The system comprises a pressure regulator connected to an interior of the bearing arrangement via a channel, and a valve arranged in the channel between the pressure regulator and the bearing arrangement. A pressure sensor is arranged to measure a pressure in the interior of the bearing arrangement. A controller is arranged to control the regulator and the valve so as to instantly supply or release a predefined overpressure to/from the interior of the bearing arrangement. The pressure in the interior is measured during the supplying or releasing of the overpressure, and a volume of gas that has flowed into or out of the interior is determined using the measured pressure. At the end, the controller will determine the volume of the lubricant using the determined volume of gas, the measured pressure, and optionally a known volume of the interior without lubricant. The invention uses the fact that a lubricant is not, or is considerably less compressible than air or another gas used in the bearing. So if the interior of the bearing arrangement is pressurized, the volume of the lubricant will barely or not decrease. So the volume of the space in the interior not occupied by the grease can be determined using pressure models and then used to calculate the amount of grease in an absolute or relative manner depending on the models and measurement means used.

In an embodiment, the controller is arranged to calculate the absolute volume of the lubricant using the determined volume of gas and a predetermined empty volume of the interior (i.e. the volume without lubricant). Alternatively, the controller may be arranged to calculate a relative volume of the lubricant with reference to a free volume of the interior.

In an embodiment, the controller is arranged to calculate a time constant defined as the time required for the measured pressure to reach a certain percentage of the supplied overpressure, and to determine the volume of gas in the interior using the time constant.

In a further embodiment the measurement system comprises a flow sensor arranged in the channel. The flow sensor is arranged to measure a flow of gas through the channel, so that the controller can determine the volume of gas in the interior using the measured pressure and the measured flow of gas.

The controller may be arranged to determine the volume of gas in the interior by integrating the measured flow over a period of time between the moment the valve is opened and a moment in time wherein the pressure in the interior is stabilized. In an embodiment, the controller is arranged to: -control the regulator so as to apply a maximum pressure to the interior in a first step, -record a measured flow into the interior and an interior pressure during the first step until the measured pressure has stabilized, -stepwise lower the pressure in additional steps, -at each step store the measured flow and pressure as soon as the pressure has stabilized, -fit a curve through the stored flow as a function of pressure, to obtain a leakage curve, -estimate the leakage flow as a function of time using the leakage curve, -correct the measured flow into the interior as a function of time with the estimated leakage flow as a function of time, to obtain a net flow as a function of time, -integrate the net flow over time to obtain the absolute volume of the gas in the interior, -dividing the gas volume flowed into the interior by the stabilized bearing pressure after filling to obtain the free volume at atmospheric pressure.

By performing the above steps, the controller is able to accurately calculate the absolute volume of the gas in the interior of the bearing, even in situations where the seal is leaking, which may be the case in many bearing arrangements.

In a preferred embodiment, the curve for fitting is a 4th order polynomial. This has shown to give good results.

According to a further aspect, there is provided a device comprising a bearing arrangement and a measurement system as described above.

According to a further aspect, there is provided a method of measuring a volume of a lubricant in an interior of a bearing arrangement, the method comprising: -supplying or releasing a predefined overpressure via a channel to/from the interior of the bearing arrangement, -measuring the pressure in the interior during the supplying or releasing of the overpressure, -determining a volume of gas in the interior using the measured pressure.

The method of measuring may further comprise calculating a time constant defined as the time required for the measured pressure to reach a certain percentage of the supplied overpressure, and determine the volume of gas in the interior using the time constant.

In an embodiment of the method of measuring, the method comprises -measuring a flow of gas through the channel, -applying a maximum pressure to the interior in a first step, -measuring a flow into the interior and an interior pressure during the first step until the measured pressure has stabilized, -stepwise lowering of the pressure in additional steps, -at each step, storing the measured flow and pressure as soon as the pressure has stabilized, -fitting a curve (e.g. a polynomial) through the stored flow as a function of pressure, to obtain a leakage curve, -estimating the leakage flow as a function of time using the leakage curve, -correcting the measured flow into the interior as a function of time with the estimated leakage flow as a function of time, to obtain a net flow as a function of time, -integrating the net flow over time to obtain the absolute volume of the gas in the interior, -dividing the gas volume flowed into the interior by the stabilized bearing pressure after filling to obtain the empty volume at atmospheric pressure, -determining the volume of the lubricant using the determined absolute volume of gas, the measured pressure, and optionally a known empty volume of the interior.

A further aspect of the invention provides a method of lubricating a bearing arrangement, comprising the method of measuring as described above.

In an embodiment, a rotational speed of a component of the arrangement is measured and used to determine an amount of additional lubricant needed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings Figure 1 schematically shows a bearing arrangement 10, a greasing system 11 and a measurement system; Figure 2 schematically shows an embodiment of the measurement system; Figure 3 shows a model of the measurement system and the volume of the interior of the bearing arrangement; Figure 4 schematically shows a drawing of a valve; Figure 5 show a graph of the time responses of the pressure p in the interior for a measurement with filling the interior; Figure 6 shows a graph of the time responses of the pressure p in the interior for a measurement with releasing the overpressure; Figure 7 schematically shows the measurement system according to another embodiment comprising a flow sensor; Figure 8 shows multiple graphs of the response of the flow sensor as a function of time for three volumes during filling of the bearing; Figure 9 shows multiple graphs of the response of the flow sensor as a function of time for three volumes during releasing of the overpressure in the bearing; Figure 10 shows a graph of a pressure response measurement during an initial filling step and the following pressure reduction steps; Figure 11 shows a graph of the stabilized leakage pressures and flows relating to the measurements of Figure 10; Figure 12A is a graph showing a measured flow into the bearing and the estimated leakage flow out of the bearing; Figure 12B is a graph showing the estimated leakage flow out of the interior; Figure 12C is a graph showing the net flow into the bearing; Figure 13 shows a graph of the integrated net flow into the bearing as a function of time.

It should be noted that items which have the same reference numbers in different Figures, have the same structural features and the same functions, or are the same signals. Where the function and/or structure of such an item has been explained, there is no necessity for repeated explanation thereof in the detailed description.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 schematically shows a bearing arrangement 10, a greasing system 11 and a measurement system 12. The bearing arrangement 10 comprises a housing 14 and a rolling element bearing having an inner ring, an outer ring and a plurality of rolling elements 15 arranged between the bearing rings. In this embodiment, the outer ring is mounted to the housing 14 and the inner ring is mounted to a shaft 13, which is rotational about a rotation axis 16. The bearing arrangement 10 comprises a seal 17 at each axial side of the arrangement, which seals a space between the shaft 13 and the housing 14 also referred to as the interior 18 of the bearing arrangement 10. The greasing system is connected via a tube 19 and a splitter 20 to the interior 18 so as to be able to supply a lubricant, such as grease to the interior 18. The measurement system 12 connected to the interior 18 by way of a further tube 21 and the splitter 20. In this embodiment, the splitter 20 comprises two valves 22, 23 which can be used to connect and/or disconnect the greasing system and the measurement system with the interior 18 of the bearing arrangement 10. The valves 22, 23 could be controlled by grease system 11 or by another system or by hand.

The measurement system 12 is arranged to measure an absolute or relative volume of the lubricant present in the interior 18. This is done by instantly applying an overpressure to the interior via the further tube 21 and the splitter 20 while the valve 23 is closed. Alternatively, the interior 18 may be pressurized after which an overpressure is instantly released from the interior. It is noted that the splitter 20 may be absent and that the measurement system may be directly connected to the interior 18.

Figure 2 schematically shows an embodiment of the measurement system 12 wherein the measurement system 12 comprises a pressure regulator 25, a channel 30 and a valve 31 arranged in the channel 30 between the pressure regulator 25 and an output 32 of the measurement system 12 which in use is connected to the bearing arrangement 10 via e.g. the tube 21, see Figure 1. The measurement system 12 also comprises a controller 26 and a pressure sensor 27 arranged to measure a pressure p in the interior of the bearing arrangement 12.

In an embodiment, the controller 26 is arranged to control the regulator 25 and the valve 31 so as to instantly supply or release a predefined overpressure to/from the interior 18 of the bearing arrangement 10. The controller 26 is arranged to receive input from the pressure sensor 27 to record the measured pressure p in the interior 18 during a the supplying or releasing of the overpressure, and to determine a volume of gas flown into the interior using the measured pressure p. In the embodiments described, the interior 18 is filled with pressurized air, but it should be noted that other gasses may be used to fill the bearing interior 18. The operation of the measurement system 12 will later be explained with reference to Figures 5-13.

Figure 3 shows a physical model of the measurement system and the volume of the interior of the bearing arrangement. The volume of the interior 18 of the bearing arrangement 10 is modeled by a volume V2, see reference 18. As can be seen from Figure 3, the pressure regulator 25 is connected to the interior 18 via the valve 31. The leaking of air out of the bearing is represented by a leakage resistance R2 with reference number 37. An internal resistance of the valve 31 and the tube 21 is also shown in the scheme, see resistance R1 with reference number 38.

It is noted that R1 may represent the internal resistances of valve, tube, transitions, etcetera, but it can also be physically added: the higher R1, the more accurately the bearing pressure can be measured from within the tube. When R1 is low you will first see the rapid pressure rise in the tube (see steep part of figures 5 and 6).

When R1 is (artificially) made higher, the pressure response will also get closer to first order behavior as described in the equations.

The pressure regulator 25 may receive a constant input pressure pin from a pressure source (not shown). The pressure regulator 25 may use the input pressure pl" to supply different output pressures applied to the interior 18. The valve 31 will be opened by the controller 26 to release the air from the pressure regulator 25 to the interior 18. The valve 31 in the Figure 3 can be of a two-way type such that closing the valve 31 from the pressure regulator 25 will connect the bearing to an outlet through which the pressure can be released. A schematic drawing of such a valve is given in the Figure 4. Alternatively, a second open/close valve could be added to the system to allow the pressurized air to escape.

If it is assumed that the bearing is free of leakage then the leakage resistance R2 will be infinitely large (i.e. °°). The larger the air filled space in the bearing arrangement 10 the longer it will take to reach the regulator pressure. This can be shown by using the ideal gas law: pV2 = nRT in which p is the pressure in the interior, V2 the volume of the interior, n the number of particles (moles), R the ideal gas constant and T the temperature inside the interior.

When the temperature T is assumed to be constant, the gas law can be reduced to: p172 = nC with C = RT which is constant.

A simplified relation between pressure p, resistance R1 and flown to the bearing can be described by: = Ri -(pin -p) or

C

P = RiV2 (Pin p) Let's define a pressure p difference between the supply pressure pin and the bearing pressure p: 13 = P Pin If pin is constant, we can say that P=p" The differential equation that describes the pressure change can now be reduced to: * -c 13 = piv2 P in which p is still the time derivative of 0.

Let's define: P = P(0)eat then 13. = 15(0)A.eAt * c When substituting this into =1172 p we get

-C

p(0)Aem = p(0)eat Ri V2 where A is the decay constant defined as: -C = RiV2 This allows solving the differential equation for p: = 177, 13(0)0(1152L In terms of the original variables we then have: -c P = (P(0)-piri)eRiv2 + pin It can be seen that the time constant (i.e. 1/A) is proportional to V2. So the time to fill the volume V2 to a certain pressure (e.g. 90% of the supply pressure) will be proportional to the air filled volume V2 of the bearing arrangement.

The controller 26 may use the measured pressure level (32) of air in the interior 18 as a function of time during the filling of the interior to determine the time constant 1/A. Once the time constant is known, the volume V2 of the interior 18 is known and thus also the volume of the lubricant.

An alternative for determining the time constant for supplying the overpressure into the interior 18 is to determine the time constant for releasing the overpressure from the interior 18. In an embodiment, the controller 26 is arranged to release air out of the bearing by opening the valve 31, see Figure 4. The controller 6 will use the measured pressure level (p2) of air in the interior 18 as a function of time during the releasing of the air to determine the time constant. This will then be used to calculate the volume of the lubricant. An advantage of looking at emptying the interior of the bearing (i.e. releasing the pressure) is that the measurement will be insensitive to the capacity or bandwidth of the pressure regulator 25.

When the time constant 1/A for some reference (i.e. known) volume with similar filling resistance R1 is known, the measured time constant of the bearing arrangement 10 can be translated to an absolute volume. Another way to use the method can be to measure the bearing interior volume just after installation before the first lubricant is added. In that case the time constant for an empty bearing (so for the total volume) is known and new measurements with lubricants in the bearing can thus be related to this.

Another method to find an absolute free volume V2 is to apply one time constant measurement r1, add a well-known volume of grease AV and then apply a second time constant 12 measurement. Extrapolation can then be used to find the free volume after the second measurement: V2 This last method only helps to find the available volume. To find the already occupied volume prior knowledge or a measurement when empty must be taken.

In Figure 5 and 6 the time responses of the pressure p in the interior 18 are shown for a measurement with filling, see Figure 5, and for a measurement with emptying the bearing interior 18, see Figure 6. These two figures are only given for one volume (=600m1). In both figures the responses were measured a couple of times and put in the same plot.

In Figure 5, multiple lines 51 represent measurement of the pressure as a function of time. A dashed line 52 indicates the 90% filling level. In Figure 6, multiple lines 61 (hardly distinguishable since they overlap) represent multiple measurements of the pressure as a function of time during the emptying of the bearing interior 18. A dashed line 62 indicates the 90% emptying level.

= T AV 2 E, When looking at Figure 5 and 6 it becomes clear that the emptying has better reproducibility as was expected. This is explained by the insensitivity to the pressure regulator performance when emptying.

According to another embodiment, the measurement system 12 also comprises a flow sensor 40, see Figure 7, to measure an air flow which goes into the interior 18 during filling or which leaves the interior 18 during releasing. The controller 26 will use the data from the flow sensor 10 to determine an air volume as a function of time. This may be done during the supplying or releasing of the overpressure. The determined air volume as a function of time is then integrated over time to obtain the absolute free volume of the interior 18. The calculated free volume can be divided by the stabilized overpressure so as to obtain the free volume at atmospheric pressure. This free volume can be subtracted from the empty volume of the bearing so as to obtain the amount of lubricant present in the bearing arrangement 10. By adding the flow sensor 10 to the system of Figure 2, direct and absolute measurement of the (available) bearing volume V2 can be carried out.

The valve 31 is used to release pressurized air to the bearing interior 18. When the valve 31 is opened and the bearing arrangement 18 has no leaks, the pressure 132 in the bearing interior 18 will stabilize at the regulated pressure. During the filling process the flow (pressure-independent number of particles or mass flow) Q to the bearing 5 is measured. This flow can be integrated w.r.t. time, what will yield the volume that has flown into the bearing. As the flow will reduce to zero as the pressure stabilizes the measurement will automatically end when the pressure (or the time-integrated flow measurement) has stabilized.

Alternatively, as with the pressure measurement shown in Figure 1, the measurement performed by the system of Figure 2 can be performed when releasing the pressurized air from the bearing 5. Also here this will make the measurement insensitive to the properties of the pressure regulator.

The definition used for time constant in the present example is the time required to reach 90% of the applied pressure in the case of filling. For emptying we take the time required to reach 10% of the overpressure.

Figures 8-13 show some results wherein the measurement system of Figure 7 was used. In this case an absolute volume of the air volume within the interior 18 is determined using the flow sensor 40. It is assumed that the interior 18 is leak-free. Figure 8 shows the response 81, 82, 83 of the flow sensor 10 as a function of time for three volumes during filling of the bearing. Dashed line 84 indicates a threshold level used to define the time constant. Figure 9 shows the responses 91, 92, 93 of the flow sensor 40 as a function of time for three volumes during emptying of the bearing. Dashed line 94 indicates the lower threshold level that can be used to define the time constant.

The methods described above will be useful for bearing arrangements without or with minimal leakage. For arrangements with considerable leakage the filling transient (i.e. time constant 1/A) may change; the bearing pressure end value and also the time constant 1/A will depend on the severity of the leakage. It is noted that the flow into the interior of the bearing will not stabilize at zero when there is a leakage.

Below, embodiments of the measuring method are described in which the system of Figure 2 is used and a more advanced model of the air flow and air pressure in the interior 18 to be able to also model and measure the leakage, see Figure 7. According to an embodiment, the measurement system 12 is arranged to perform additional steps as compared to the method described above. Using these steps a leakage model for R2 can be determined, so that the leakage flow can be estimated. The estimated leakage flow can be used for compensation of the measured flow into the bearing interior 18 to obtain a net flow into the interior 18.

To determine the leakage resistance R2 the pressure regulator 25 in this embodiment is arranged to stepwise vary the pressure applied to the bearing interior 18. In a first step a maximum pressure is applied. The flow Q and pressure p2 (see Figure 7) are traced during the filling step, until the pressure p2 has stabilized. In additional steps the pressure is brought down in steps. In Figure 10 the pressure response during the initial filling step and the following pressure reduction steps are shown. In this example the reduction steps took relatively long (i.e. between 30-100 seconds) due to the used pressure regulator which had limited control bandwidth. At each step the flow Q and pressure p2 are stored by the controller 6 as soon as the pressure p2 has stabilized. When flow and pressure have stabilized it can be assumed that all flow is flow through the leak in the bearing to the atmosphere. This implies one can determine the leakage flow rate for the stabilized bearing pressure. In this way a pressure-leakage curve can be created. The more steps are used to characterize the leak, the more accurate the model will be. The created leakage model can be used to estimate the leakage flow during the initial filling step of the bearing. For each sample the flow measurement is corrected using the measured pressure and corresponding leakage flow from the leakage model. The net flow into the bearing interior 18 can now be integrated over time to obtain the absolute free volume of the interior 18 as in the describe method without leakage. The obtained absolute free volume can be divided by the stabilized overpressure so as to obtain the free volume at atmospheric pressure.

This free volume can be subtracted from the empty volume of the bearing so as to obtain the amount of lubricant present in the bearing arrangement 10.

It should be noted using this leakage model allows volume estimation of systems with leakage that varies non-linearly with pressure. A variation to the method to obtain the leakage model can be to first release the pressure and obtain the pressure-flow relation while building up the pressure. This can be used to avoid inaccuracy due to hysteresis effects in the pressure-flow curve.

In the schematic drawing of Figure 2 a resistance R1 is shown. As mentioned above, such a resistor can be included physically to slow down the filling process of the bearing. This can be used to avoid saturation of the flow meter 10 and to allow the use of sensors with limited bandwidth. A possible drawback of an additional flow resistor can be that for larger leaks at the bearing (or small values of R2) the resistor R1 can become dominant. A drawback then is that it may become hard to reach the desired pressure in the bearing.

After having performed the steps of applying decreasing pressures, the controller 26 may determine the stabilized leakage pressures and flows.

In Figure 11 the stabilized leakage pressures and flows are plotted (see dots) relating to the measurements of Figure 10. A curve 115 through the measurements is fitted. In this case, a 4th order polynomial provides a good fit. This polynomial is referred to as the leakage curve.

Figure 12A is a graph showing a measured flow into the bearing 15 and the estimated leakage flow out of the bearing 15. Figure 12B is a graph showing the estimated leakage, and Figure 12C is a graph showing the net flow into the bearing 15.

When looking at Figures 12A-12C it can be concluded that the leakage model works well as the net bearing flow nicely goes to zero each time the total and leakage flow stabilize. The negative net flow at each pressure step down is also as expected: air must leave the bearing to lower the pressure.

When the net bearing flow of the initial filling step (i.e. first 53 seconds of Figure 12C) is integrated over time, the curve of Figure 13 will be obtained.

Figure 13 shows the integrated net flow into the bearing interior 18 as a function of time. To obtain the bearing volume V2, the final volume value in Figure 13 needs to be scaled to compensate for the used filling pressure p2 (the higher the used pressure the more air needs to be pumped into the bearing). In the example of Figure 13 (see also Figure 10 at t=40s) the pressure p2 stabilizes at 1.78 bar overpressure and the volume signal stabilizes at 2.83 liters. From this the bearing volume V2 can be calculated: V2= 2.83/1.78 = 1.59 liters. This comes very close to the estimated 1.55 liters that was extracted from a CAD model of the bearing arrangement 10.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS1. Measurement system (12) for measuring a volume of a lubricant present in a bearing arrangement (10), the system comprising: -a pressure regulator (25) connected to an interior (18) of the bearing arrangement via a channel (30), a valve (31) arranged in the channel between the pressure regulator and the bearing arrangement, a pressure sensor (27) arranged to measure a pressure in the interior of the bearing arrangement, a controller (26) arranged to: * control the regulator (25) and the valve (31) so as to instantly supply or release a predefined overpressure to/from the interior of the bearing arrangement, * record the measured pressure in the interior (18) during supplying or releasing of the overpressure, * determine a volume of gas flown into or out of the interior using the measured pressure, and * determine the volume of the lubricant using the determined volume of gas, the measured pressure, and optionally a known empty volume of the interior.
2. Measurement system according to claim 1, wherein the controller is arranged to calculate the absolute volume of the lubricant using the determined volume of gas and the known empty volume of the interior.
3. Measurement system according to claim 1, wherein the controller is arranged to calculate a relative volume of the lubricant with reference to the empty volume of the interior.
4. Measurement system according to any of the preceding claims, wherein the controller is arranged to: -calculate a time constant defined as the time required for the measured pressure to reach a certain percentage of the supplied overpressure or underpressure, -determine the volume of gas in the interior using the time constant.
5. Measurement system according to any of the preceding claims, wherein the system further comprises a flow sensor (40) arranged in the channel and to measure a flow of gas through the channel, the controller being arranged to determine the volume of gas flown into the interior using the measured pressure and the measured flow of gas.
6. Measurement system according to claim 5, wherein the controller is arranged to determine the volume of gas flown into the interior by integrating the measured flow over a period of time between the moment the valve is opened and a moment in time wherein the pressure in the interior is stabilized.
7. Measurement system according to claim 5, wherein the controller is arranged to: - control the regulator (2) so as to apply a maximum pressure to the interior in a first step, - measure a flow into the interior and an interior pressure during the first step until the measured pressure has stabilized, -stepwise lower the pressure in additional steps, -at each step store the measured flow and pressure as soon as the pressure has stabilized, -fit a curve through the stored flow as a function of pressure, to obtain a leakage curve, -estimate the leakage flow as a function of time using the leakage curve, -correct the measured flow into the interior as a function of time with the estimated leakage flow as a function of time, to obtain a net flow as a function of time, -integrate the net flow over time to obtain the absolute volume of the gas flown into the interior, - divide the gas volume flown into the interior by the stabilized bearing pressure after filling to obtain the empty volume at atmospheric pressure.
8. Measurement system according to claim 7, wherein the curve is a 4th order polynomial.
9. Device comprising a bearing arrangement (10) and a measurement system (12) according to any of the preceding claims.
10. Method of measuring a volume of a lubricant in an interior of a bearing arrangement, the method comprising: -supplying or releasing a predefined overpressure via a channel to/from the interior of the bearing arrangement, -measuring the pressure in the interior during the supplying or releasing of the overpressure, and -determining a volume of gas in the interior using the measured pressure.
11. Method according to claim 10, the method further comprising: -calculating a time constant defined as the time required for the measured pressure to reach a certain percentage of the supplied overpressure, -determining the volume of gas in the interior (18) using the time constant.
12. Method according to claim 10, the method further comprising: - measuring a flow of gas through the channel (40), -applying a maximum pressure to the interior in a first step, -measuring a flow into the interior and an interior pressure during the first step until the measured pressure has stabilized, -stepwise lowering of the pressure in additional steps, -at each step storing the measured flow and pressure as soon as the pressure has stabilized, -fitting a curve (115) through the stored flow as a function of pressure, to obtain a leakage curve, -estimating the leakage flow as a function of time using the leakage curve, -correcting the measured flow into the interior as a function of time with the estimated leakage flow as a function of time, to obtain a net flow as a function of time, - integrating the net flow over time to obtain the absolute volume of the gas in the interior, -dividing the gas volume flown into the interior by the stabilized bearing pressure after filling to obtain the empty volume at atmospheric pressure, - determining the volume of the lubricant using the determined absolute volume of gas, the measured pressure, and optionally a known empty volume of the interior.
13. Method of lubricating a bearing arrangement comprising the method of measuring according to any of the claims 10-12 and supplying additional lubricant to the interior depending on the amount of lubricant measured.
14. Method of lubricating a bearing arrangement according to claim 13, wherein a rotational speed of a component of the arrangement is measured and used to determine an amount of additional lubricant needed.