WO2022063652A1

WO2022063652A1 - Method for determining a viscosity of a lubricant in a bearing

Info

Publication number: WO2022063652A1
Application number: PCT/EP2021/075338
Authority: WO
Inventors: Tobias Schirra; Kerstin HORNEFF
Original assignee: Technische Universität Darmstadt
Priority date: 2020-09-28
Filing date: 2021-09-15
Publication date: 2022-03-31
Also published as: DE102020125236A1

Abstract

Disclosed is a method for determining a viscosity of a lubricant (125) in a bearing (120) with two components (121, 122), which are designed to execute a relative movement, the components (121, 122) being lubricated by means of the lubricant (125) during the movement. The method comprises, as one step, determining (S110) a transition speed of the movement of the components (121, 122) at which there is a release point and thus a transition between a mixed friction and a liquid friction. A further step of the method comprises determining (S120) the viscosity of the lubricant (125), based on the transition speed.

Description

Method for determining a viscosity of a lubricant in a bearing

The present invention relates to a method for determining a viscosity of a lubricant in a bearing, to a device for carrying out such a method, and in particular to a method for measuring a lubricant viscosity when rolling and plain bearings are in operation.

BACKGROUND

Mechanical bearings are characterized by two or more components that move in opposite directions. A lubricant, such as oil or grease, is usually used between the components. In particular, the lubricant prevents friction and wear of the components when the bearing is in operation. For this purpose, an advantageous formation of a lubricating film, and in particular a thickness of the lubricant or the lubricating film, is important. An important property of the lubricant is its viscosity, which is a measure of how viscous the lubricant is. In the course of operation of the bearing, the lubricant is generally subject to a change in its viscosity.

A number of methods are known for determining the thickness of the lubricant, as well as some other properties important to the operation of the bearing. An example from the patent literature is provided by document JP2001311427A, which estimates the quality of the lubricant and, from this, the service life of the bearing by measuring a lubricating film thickness and comparing it with previously recorded data. On the other hand, determining the viscosity of a lubricant that is already in storage proves to be time-consuming in practice. In order to determine the viscosity precisely, it is usually necessary to take the lubricant and analyze it in a laboratory. Alternatively, the viscosity can be estimated from the storage behavior, for example, or can also be precalculated from theoretical considerations. However, these alternatives can only provide an inaccurate result, since there are usually a large number of factors and mechanisms that cause the change in the Lubricant affect or can affect, are unknown.

There is therefore a need for a method for determining the viscosity of a lubricant in a bearing, in particular in a plain or roller bearing, without having to remove the lubricant from the bearing.

BRIEF DESCRIPTION OF THE INVENTION

This object is achieved at least in part by a method according to claim 1, a computer program product according to claim 7 and an apparatus according to claim 8. The dependent claims relate to advantageous developments of the subject matter of the independent claims.

The present invention relates to a method for determining a viscosity of a lubricant containing oil or grease, for example, in a bearing with two components which are designed to move against one another (ie relative to one another), the components performing the movement by be or are lubricated by the lubricant.

The method initially includes determining a transition speed for the movement of the components, at which there is a release point and thus a transition between mixed friction and fluid friction. It is known from the technical literature that the friction between the components lubricated by the lubricant depends on the relative speed between the components. For this purpose, the friction can be described by a friction coefficient, which in the lubricated bearing depends on the speed of the components relative to one another, which is also known as the Stribeck curve. The Stribeck curve shows a course of the coefficient of friction that is initially flat with increasing speed and then decreases monotonously, which finally increases again after a minimum value has been passed. In this subdivision of the Stribeck curve, the flat area corresponds to boundary friction or lubrication, the adjoining sloping area to the already mentioned mixed friction or partial lubrication (mixture of boundary friction and hydrodynamic friction), and the after the minimum, the area of the liquid or hydrodynamic friction or film friction that also increases, as already mentioned. The transition from mixed to liquid friction, marked by the minimum of the friction coefficient, is called the release point. A specific relative speed of the components, referred to here as the transition speed, thus corresponds to the release point. In embodiments of the method, the minimum value of the coefficient of friction, and thus the release point and transition speed, is determined by inspecting a course of friction during a controlled acceleration of the components against one another.

A further step of the method includes determining a viscosity of the lubricant based on the determined transition speed. From theoretical considerations, a connection between the viscosity and the transition speed can be established at the release point, in which only other material constants of the lubricant and the components, a load of the existing bearing, a temperature and geometric variables of the bearing are included to determine the viscosity. These variables can already be known from the circumstances of a method implementation, in particular as tabulated properties; their determination is not necessarily the subject of the procedure in this sense. The connection between the viscosity and the speed of the components is also based on the theoretical basis that the thickness of the lubricant (lubricating film thickness) between the components at the release point, i.e. at the transition speed, is determined by the roughness coefficients of the components. In the specialist literature, the release point is defined via this relationship between the thickness of the lubricant and a roughness coefficient of each of the two components. Roughness coefficients are usually tabulated material constants of the components. If they are known, the thickness of the lubricant at the release point for the bearing in question is determined.

Optionally, determining the transition speed further includes measuring an electrical resistance through the bearing, wherein the determination of the transition speed or release point is performed based on the electrical resistance. The electrical resistance can in particular be an impedance. Advantageously, the components in relation to Electrically highly conductive in the same way as the lubricant. Various devices are known in the prior art for measuring the electrical resistance through the bearing. In particular, a current can be passed through the bearing for this purpose. The electrical resistance through the bearing can in particular be measured indirectly, for example by a voltage drop across a reference resistor. In embodiments of the method, this can be done, for example, with the aid of a scanning device known in the prior art and an oscilloscope connected thereto.

Advantageously, the electrical resistance is measured while the components are being accelerated relative to each other so that the components successively pass through an interval of speeds that includes the transition speed. The electrical resistance depends on the state of friction of the lubricated components, which is also determined by the speed of the components relative to one another. In particular, the electrical resistance in the area of mixed friction can oscillate between a minimum value occurring when the components touch one another and a maximum value caused by a film of dielectric lubricant between the components, with a change dependent on the speed of the components. With increasing speed, a stable insulation develops between the components from the release point due to the lubricant, and the electrical resistance increases with increasing speed. When accelerating the components, a temperature in the bearing and a load on the components or the bearing must advantageously be kept constant.

Furthermore, as an alternative or in addition to measuring the electrical resistance through the bearing, determining the transition speed may include determining a percentage of contact time (PCT) of the two components. In the area of mixed friction, the components touch each other during movement via roughness peaks, i.e. unevenness on their respective surfaces. A degree of touch is indicated by percent contact time. The percentage of contact time is based on a ratio between a time that the components touch each other at a fixed speed over bumps on their surface and a time that the components run separately. In an ideal model, the percent contact time falls in the Range of mixed friction to release point down to 0%. In order to determine the release point, a threshold value for the percentage of contact time resulting from the measurement, for example 10%, can be defined, below which the release point is reached in a sufficiently good approximation for the embodiment of the method. Depending on the requirement, the threshold value can be adapted to an accuracy of the method result.

In embodiments of the method in which determining the transition speed comprises measuring an electrical resistance through the bearing, for speeds at which mixed friction is present, the electrical resistance may oscillate between a value corresponding to boundary friction and a value corresponding to hydrodynamic friction. The percentage of contact time can then be based on a ratio of two portions of time that each correspond to one of the values.

Optionally, the measurement of the electrical resistance is based on a touching, resistive contact of one of the components. The contacting, in particular of a component that is moved in relation to a measuring device, such as a shaft in a rotary bearing, can take place, for example, via a sliding contact or else a mercury contact. Alternatively or additionally, the measurement of the course of the electrical resistance can also be based on a contactless, capacitive coupling to one of the components. This form of contacting is particularly advantageous for a component moving against the measuring device, since, in contrast to resistive contacting, sources of error such as thermal voltage, contact noise, mechanical friction or temperature-dependent drift of the electrical resistance can be eliminated. Alternatively or additionally, the measurement of the course of the electrical resistance can also be based on a telemetric signal transmission. A telemetric signal transmission can be particularly advantageous for slowly rotating rotary bearings.

Optionally, the method further includes measuring a temperature in or on the bearing and/or measuring a load acting on the bearing. These two variables are usually important for determining viscosity. Methods for determining these variables are known in the prior art. In particular, the load of the bearing can be determined, for example, before or during the determination of the transition speed by moving the components against one another while simultaneously transmitting a current signal and determining an electrical resistance or an impedance. However, a measurement can also be omitted due to external circumstances such as a controlled atmosphere or a bearing load that follows a maintenance standard, for example, as a result of which the variables are already known and can therefore be used to determine the viscosity.

Optionally, the viscosity is determined as a function of parameters based on one or more of the following parameters: a roughness value of the components, a relative speed, at least one lubricant property, and/or at least one bearing property. The bearing property can in particular be a geometric property.

Optionally, the bearing is a pivot bearing and one of the components is a shaft. The pivot bearing can be a radial bearing, for example, but also an axial bearing. For a pivot bearing, the speed of movement of the components can be specified as a speed.

Optionally, the bearing is a roller bearing or a plain bearing.

Embodiments of the method presented also relate to a computer program product with software code stored thereon, which is provided when the software code is executed by a data processing machine so that a method according to one of the preceding claims is carried out.

In addition, the present invention relates to a device for determining a viscosity of a lubricant in a bearing with two components which are designed to perform a relative movement, the components being lubricated by the lubricant to perform the movement. The device includes a determination module that is designed to determine a transition speed of the movement of the components at which a release point and thus a transition between mixed friction and liquid friction exercise is available to determine. The device also includes an identification module that is designed to identify a thickness of the lubricant between the components at the transition speed. In addition, the device includes a processing module that is configured to determine the viscosity of the lubricant based on the transition speed and the thickness of the lubricant. In particular, the processing module can contain a computer program product for automatically carrying out the method.

Optionally, the determination module includes a speed measurement module that is designed to measure a speed of movement of the components. In the case of a rotary bearing, this can be a device for measuring the speed; however, the device can also only be designed for manual input of the rotational speed determined in some other way (e.g. read from the bearing). The determination module then also includes a resistance measuring module, which is designed to establish a first electrical connection to a first of the two components and a second electrical connection to a second of the two components, via the first electrical connection and the second electrical connection a current between the to generate the first component and the second component and to measure an electrical resistance. In particular, the resistance measuring module can be designed to measure the electrical resistance via a drop in voltage across the bearing or via a suitably connected reference resistor. In addition, the determination module then includes an evaluation module that is designed to receive the speed and the electrical resistance and to determine the transition speed therefrom.

In summary, the present method is used to measure the lubricant viscosity in the operation of roller and plain bearings, which was previously only possible using oil analysis in the laboratory. The measurement of the electrical resistance of rolling bearings depends on the state of friction. In the case of boundary or mixed friction, roughness peaks touch and the electrical resistance is very small. As soon as there is hydrodynamic friction, the resistance increases very sharply. This change is called the release point. Under constant load and temperature operating conditions, a speed of the load can gers from 0 to an operating speed, the electrical resistance can be measured in parallel. The release point is reached at the speed at which the electrical resistance suddenly increases sharply. The release point is defined as the point at which the lubricating film thickness equals the square root of the sum of the squares of the roughness. Thus, at this point, the lubricating film thickness is known. In addition to the known geometry, speed, temperature and load, the calculation of the lubricating film thickness only depends on the unknown lubricant viscosity. Thus, using this method, the viscosity of the lubricant can be determined as the only unknown in the equation for calculating the lubricant film thickness. The use of the release point as a fixed reference point for the thickness of the lubricating film is particularly new.

Since a conventional analysis in the laboratory is uneconomical, particularly due to the labor costs, regular viscosity measurements have only rarely been carried out on bearings. Previous calculations or estimates are mostly imprecise, since many influencing factors are often unknown. The presented method offers the advantage that the viscosity of the lubricant can be measured during operation of the bearing and no longer has to be carried out in the laboratory, which is time-consuming and costly.

BRIEF DESCRIPTION OF THE FIGURES

The embodiments of the present invention will be better understood from the following detailed description and the accompanying drawings of the various embodiments, which, however, should not be taken to limit the disclosure to the specific embodiments used for explanation and understanding only.

Fig. i illustrates steps of an embodiment for a method according to the present invention.

2 shows a relationship between the thickness of the lubricating film and the friction between two components of a bearing that can move relative to one another. FIG. 3 shows an equivalent circuit for an exemplary embodiment of a device for carrying out the method.

4 shows measured curves of a voltage across a resistor connected in series with the bearing, while the components of the bearing execute an accelerated movement.

5 illustrates a percentage of contact time for various lubricants as a function of a speed between the components that is described by a rotational speed.

6 illustrates measured viscosities of lubricants compared to tabulated values according to an embodiment of the present method, and a change in viscosity due to aging of the lubricant determined solely by the method.

DETAILED DESCRIPTION

1 illustrates steps of a method according to the present invention. The method serves to determine a viscosity of a lubricant in a bearing. The bearing comprises two components which are designed to move relative to one another, the components being lubricated by the lubricant during movement.

A first step of the method includes determining Sno a transition speed for the movement of the components at which there is a release point and thus a transition between mixed friction and fluid friction. In one embodiment of the method, the components include metallic material, and determining Sno includes measuring an electrical resistance through the bearing. The electrical resistance through the bearing depends in particular on the condition of the lubricant between the components. Different speeds of movement of the components cause different states of the lubricant. Measuring the electrical resistance, for example during a steadily increasing speed, therefore allows conclusions to be drawn about the release point. This occurs at the transition speed and is determinable by an increase in electrical resistance for speeds higher than the transition speed.

The method also includes determining S120 the viscosity of the lubricant from the transition speed and the thickness of the lubricant. The

Determining S120 requires knowledge of a temperature in the bearing and a load on the components, which may also be determined as part of the method. For a rolling bearing in which one of the components is a rolling element and the other of the components is a raceway, the relationship applies at the release point

The constants occurring here can be found in the following table:

For sizes according to Chu and Cameron see K. Witt, Technische Hogeschool Eindhoven, 1974, https://doi.org/1o.61oo/IR161o8; p. 84 f.

2 shows a relationship 10 between a specific lubricating film thickness A and a friction between two components 121, 122 of a bearing that can move relative to one another. Relationship 10 represents a theoretical basis of formula (1) in the description of Figure 1. Lubricant film specific thickness A is a dimensionless measure of the thickness of the lubricant, h, and related to the latter by the relationship

Here, R _ql is an average roughness of a surface of the first member 121, and R _{a 2} is an average roughness of a surface of the second member 122. The relationship 10 between the specific lubricating film thickness A and the friction has three regions, each of which is one of the cases Boundary, mixed and liquid friction correspond. The release point is at the border of the areas between mixed friction and fluid friction. Larger values of the specific lubricating film thickness correspond to higher speeds of movement of the components 121, 122. The three areas and the value of the specific lubricating film thickness A at the release point can be taken from the following table. men will:

A first inserted image A illustrates a situation between the components 121, 122 moving relative to one another in the area of mixed friction. A cross section through the components 121, 122 and the lubricant 125 lying between them can be seen in a lower part of the first inserted image A. A movement of the components 121, 122 is marked by arrows. Unevenness (roughness peaks) of the components 121, 122 touch. For an electrical current that flows through the bearing from one of the components 121, 122 to the other, the bearing can act like an ohmic resistance, illustrated here by a circuit diagram of an ohmic resistance R.

A second inserted image B illustrates a situation between the components 121, 122 moving against one another in the area of fluid friction. A cross section through the components 121, 122 and the lubricant 125 lying between them can be seen in a lower part of the second inserted image 2. A movement of the components 121, 122 is marked by arrows. Here the components 121, 122 are separated by the lubricant 125; Unevenness (roughness peaks) of the components 121, 122 do not touch. For an electrical current that flows through the bearing from one of the components 121, 122 to the other, the bearing can act like a capacitive resistance or impedance, illustrated here by a circuit diagram representation of a capacitor C.

FIG. 3 shows an equivalent circuit 100 for an exemplary embodiment of a device for carrying out the method. The circuit comprises a voltage source 110, the bearing 120, a resistive load resistor 130 connected in series with the bearing 120 and a ground 140. To measure a voltage voltage drop across the charging resistor 130, the equivalent circuit comprises a scanning head 140 and an oscilloscope 150.

The bearing 120 is modeled by an effective capacitance 127 and an effective ohmic resistance 129 connected in parallel. The capacitance 127 varies depending on the operating state of the bearing 120 and can, for example, reach values of up to 50 picofarads. A resistance due to the lubricant 125 can be in the megaohm range with a lubricant film thickness of a few micrometers. In addition to an effect of the lubricant 125, the equivalent circuit diagram of the bearing 120 can record a large number of other effects of the bearing 120, such as, for example, contact resistance at joints.

The probe 150 is represented by a capacitance 152 (approximately 14.8 picofarads in exemplary embodiments) and an ohmic resistor 155 connected downstream of this (for example 9 megaohms). The oscilloscope is modeled by a capacitance 162 of, for example, 17.5 picofarads and a resistance 165 of one megohm in parallel.

To carry out the method, the speed of the movement of the components 121, 122 of the bearing 120 can be increased successively, for example from o. In this case, a resistance in the bearing 120 initially remains low, since in the region of boundary friction and initially also of mixed friction, due to the roughness of the components 121, 122, the components 121, 122 come into contact. A voltage of the voltage source 110 then falls predominantly across the charging resistor 130 . At speeds in a region below the transition speed, the lubricant 125 may phase form a separating layer between the components 121, 122 which electrically isolates the components 121, 122 and therefore increases the electrical resistance to the current through the bearing 120. This is expressed by corresponding fluctuations in the voltage drop across the charging resistor 130, which can be measured and visualized by the probe 150 and the oscilloscope 160. When the release point is reached, fluid friction between the components 121, 122 sets in. The lubricant separates the components 121, 122; the electrical resistance The level of bearing 120 is consistently high and continues to rise as the lubricating film thickness increases. By measuring and visualizing the electrical resistance using the probe 150 and the oscilloscope 160, the release point and the associated transition speed—for example, a speed of the bearing 120—can be determined.

FIG. 4 shows measured curves of a voltage across the resistor 130 (cf. FIG. 3) while the components 121, 122 of the bearing 120 (cf. FIG. 3) execute an accelerated movement. Voltage curves 135 over time for three different speed ranges are arranged one above the other. An unfiltered voltage profile 135 is shown on the right and the same voltage profile 135 after a low-pass filter is shown on the left. The voltage 115 of the voltage source 110 (cf. FIG. 3) is also specified for all curves.

In the speed range shown first, the movement between the components 121, 122 is slow. The bearing 120 works in the area of boundary friction or mixed friction with a high proportion of boundary friction. There is no separating lubricating film between the components 121, 122. The entire voltage drops across the charging resistor 130.

In the speed range shown second, the bearing 120 is in mixed friction with a high proportion of fluid friction. The lubricating film separates the components 121, 122 in phases. Accordingly, the voltage profile 135 across the charging resistor 130 is not constant but oscillates between a value determined by boundary friction and a value determined by fluid friction.

In the speed range shown in the third place, the release point has been reached or already exceeded. The lubricating film forms a separating layer between the components 121, 122. All voltage drops across bearing 120.

The percentage of contact time, or percentage of metallic contact at a total measurement duration PCT, can be determined for each of the three speed ranges

PCT ₌ tu>u _PCT . _10Q% , tot where t _tot stands for the time in which the bearing remains in the respective speed range, and tu>u _PCT is that proportion of time t _tot when the voltage across the charging resistor 130 is above a low threshold voltage U _PCT lies.

Fig. 5 illustrates a percentage contact time for various lubricants 125 as a function of a speed between the components 121, 122 described by a speed. In order to determine the release point by measurement, the percentage contact time or a PCT value (percentage of measured values over a limit voltage between oV and the measurement voltage, for example 3V), which should be below a threshold value 50 (for example 10%), so that a separating lubricating film can be assumed. The figure shows measurement results 22, 24, 26, 28 of the PCT value of four different lubricants 125 as a function of the speed of the components 121, 122 for a pivot bearing, under the same or at least comparable temperature conditions (for example in a range from 22 ⁰ to 25 ⁰ C) and load (for example 1000 Newton). The speed of the components 121, 122 is given here by a speed. The release point is considered to have been reached for each of the four lubricants 125 when the measurement results 22 , 24 , 26 , 28 reach the threshold value 50 .

6 illustrates viscosities 32, 34, 35, 36, 38 of four different lubricants 125 measured according to an exemplary embodiment of the present method in comparison to tabulated values or viscosities of the respective lubricants 125 according to the data sheet. For one of the lubricants 125, the representation also shows a shift in the corresponding viscosity 34 to a shifted viscosity 44. The shifted viscosity 44 is the viscosity of the same lubricant 125 after an aging process, which is an application here. duration of 10 days at an operating temperature of 150°C. The expected decrease in viscosity due to simulated aging can be verified.

On the one hand, the figure illustrates a quality of the method compared to a viscosity measurement in the laboratory. On the other hand, the figure illustrates a possible use of the method by determining the reduced viscosity of the aged lubricant 125 in storage operation.

In exemplary embodiments, in particular for a pivot bearing, the steps of the method can therefore also be described as follows:

- Determine the RPM at the release point based on the following steps:

• Measure at which speed the electrical resistance of the rotary bearing suddenly increases sharply;

• Using the circuit of Figure 3 and measuring a voltage across the charging resistor 130;

• determining the ratio PCT as in the description of FIG. 4;

• Determine the speed at the release point for PCT < 10%;

- Parallel measurement of temperature and bearing load;

- Insertion of all required quantities into equation (1) from the description of FIG. 1 to calculate the viscosity.

The features of the invention disclosed in the description, the claims and the figures can be essential for the implementation of the invention both individually and in any combination. REFERENCE LIST

Silo Determine a transition speed at the release point

S120 Determination of a viscosity of the lubricant io Relationship between specific lubricating film thickness and friction

22, 24, 26, 28 measurement results for PCT value

32, 34, 36, 38 viscosities

44 Shifted viscosity of an aged lubricant

50 threshold

100 equivalent circuit diagram

110 voltage source

115 tension

120 camp

121 first component

122 second component

125 lubricant

127 capacity of the warehouse

129 ohmic resistance of the bearing

130 ohmic load resistance

135 Voltage drop across charging resistor

140 grounding

150 probe

152 Capacitance of the probe

155 ohmic resistance of the probe

160 oscillator

162 capacitance of the oscillator

165 ohmic resistance of the oscillator

A, B inserted images

Claims

CLAIMS A method for determining a viscosity of a lubricant (125) in a bearing (120) with two components (121, 122) which are designed to perform a relative movement, the components (121, 122) in the movement by the Lubricant (125) are lubricated, the procedure includes the following steps:

Determining (S110) a transition speed of the movement of the components (121, 122) at which there is a release point and thus a transition between mixed friction and fluid friction; and

determining (S120), based on the transition speed, the viscosity of the lubricant (125). The method of claim 1, wherein determining (S110) the transition speed further comprises at least one of the following steps:

measuring an electrical resistance through the bearing (120), wherein determining (S110) the transition speed is performed based on the electrical resistance;

determining a percentage contact time of the two components (121, 122). The method of claim 2, wherein measuring the electrical resistance comprises at least one of the following:

Measuring a resistive resistance between the components (121, 122),

measuring a capacitive resistance between the components (121, 122),

Executing a signal transmission.

4. The method according to any one of the preceding claims, further comprising at least one of the following steps:

measuring a temperature (T) in or at the bearing (120), measuring a load acting on the bearing (120).

5. The method according to any one of the preceding claims, wherein the determination (S120) of the viscosity is parameter-dependent based on one or more of the following parameters: a roughness value of the components, a relative speed, at least one lubricant property, a load on the bearing, a temperature of the bearing, at least one bearing property.

6. The method according to any one of the preceding claims, wherein the bearing (120) is one of the following: a roller bearing, a plain bearing, a rotary bearing, wherein one of the components (121, 122) is a shaft.

7. A computer program product with software code stored thereon, which is designed to carry out the method according to one of the preceding claims when the software is run on a data processing unit.

8. A device for determining a viscosity of a lubricant (125) in a bearing (120) with two components (121, 122) which are designed to carry out a relative movement, the components (121, 122) being driven by the lubricant ( 125) are lubricated, the device includes: a determination module that is designed to determine a transition speed of the movement of the components (121, 122) at which there is a release point and thus a transition between mixed friction and fluid friction; and a processing module configured to determine the viscosity of the lubricant (125) based on the transition rate. The apparatus of claim 8, wherein the determination module comprises: a speed measurement module configured to measure a speed of movement of the components (121, 122); a resistance measurement module that is designed to generate a current between the first component (121) and the second component (122) via a first electrical connection to the first component (121) and a second electrical connection to the second component (122). and measure an electrical resistance; and an evaluation module which is designed to receive the speed and the electrical resistance and to determine the transition speed therefrom.