WO2000050883A1

WO2000050883A1 - Method and device for determining metallic contaminant particles in lubricating grease

Info

Publication number: WO2000050883A1
Application number: PCT/FI2000/000148
Authority: WO
Inventors: Peter Andersson
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 1999-02-24
Filing date: 2000-02-23
Publication date: 2000-08-31
Also published as: FI990393A0; AU2918300A

Abstract

The invention relates to a method and a device for determining the content of metallic contaminant particles in a lubricating grease. The method comprises sampling of the lubricating grease. In accordance with the invention, the method further comprises exposing the lubricating grease sample (N) to a first magnetic field (H1) alternating at constant frequency and determining the content of contaminant particles in the lubricating grease sample by the influence of a second magnetic field (H2) resisting variations in the first magnetic field (H1), the second magnetic field (H2) being produced by eddy currents induced in the contaminant particles.

Description

Method and device for determining metallic contaminant particles in lubricating grease

The invention relates to a method as defined in the preamble of claim 1 for determining the content of metallic contaminant particles in a lubricating grease.

The invention also relates to a device as defined in the preamble of claim 6 for deteπni-ning the content of metallic contaminant particles in a lubricating grease.

The interface of joints and/or bearings disposed in joints between rotating and axially moving machine parts are usually treated with lubricating grease in order to reduce dynamic -friction and wear. The properties of the lubricating grease deteriorate in use and in the course of time; as the surfaces of the joints and/or bearing parts wear, impurities accumulate from these surfaces and perhaps also from the environment. In order to ensure operation free from malfunctions, the interfaces and/or bearings require cleaning and the lubricating grease must be replaced at suitable service intervals. When a suitable amount of contaminant particles has accumulated in the lubrication grease, the service measures above are called for. The contaminant particles are mostly metallic for the simple reason that machine parts are principally made of steel or any other metal suitable for the purpose.

There are several conventional methods for determining contaminant particles in lubricating greases. In direct microscopy, a sample of lubricating grease is placed in-between two glass sheets and is subjected to a specific microscopic examination (FTM standard 3005.4). In a filtering test (DIN 51813) contaminant particles above 25 μm are collected from the lubricating grease sample into a filter for subsequent determination.

However, conventional determinations involve the problem of being laborious and slow. They are appropriately performed in a laboratory. They are not applicable to practical field measurements.

Published German patent application DE-195 24 353 discloses a method for determining metal particles, which enables the particle content in a flowing lubricating oil to be determined. The method is based on the determination of the change in the permeance of a lubricating oil flowing between the two poles in a magnetic circuit. The metal particles in the lubricating oil constitute part of the magnetic circuit. The permeance of the lubricating oil depends on the particle material and the particle concentration. During the determination, lubricating oil flows through a measurement orifice and varies as a function of time, and thus the variation of the particle concentration can be determined as a relative variable.

The conventional apparatus has the inconvenience of being intended for liquid lubricating oils. It is not suitable for deterrnining the particle concentration of lubricating greases.

The conventional apparatus also involves the drawback of having to be disposed stationary in the lubricating system and of allowing only relative particle contents to be determined.

The object of the invention is to overcome the drawbacks related to the methods and apparatuses mentioned above. Another object of the invention is to provide a new, simple and reliable method and device for deterrnining the content of contaminant particles, especially metal particles, in a lubricating grease.

The method of the invention is characterised by the features defined in claim 1 and the device of the invention is characterised by the features defined in claim 6. The dependent claims set forth preferred embodiments of the invention.

In the method of the invention for determining the content of metallic contaminant particles in a lubricating grease, a sample of the lubricating grease is taken. In accordance with the invention, the method also comprises the steps of - exposing the lubricating grease sample to a first magnetic field which alternates at constant frequency, and

- deteπnining the content of contaminant particles in the lubricating grease sample by the influence of a second magnetic field resisting variations in the first magnetic field, the second magnetic field being produced by eddy currents induced in the contaminant particles.

The invention is based on the phenomenon of eddy currents. When alternating current is conducted to a coil, a magnetic field with alternating directions will be produced, which is perpendicular to the coil wires and parallel with the coil core. When the coil is placed in the vicinity of a metal body, circular eddy currents are induced on the metal surface, and these currents proceed perpendicularly to the direction of the coil core. The eddy currents, in turn, produce a second magnetic field, a "secondary field", which resists the original first, i.e. primary magnetic field. The secondary field partly weakens the intensity of the primary field, and this appears as a measurable variation of the impedance or voltage of the coil. The coil and an electrically conductive body in its vicinity, in this case a plurality of metallic contaminant particles, thus form a system configuration which allows the content of metallic contaminant particles in the vicinity of the coil to be determined.

The method of the invention is applicable to the deteπr-ination of the total content both of ferromagnetic and non-magnetic metal particles, such as for instance aluminium, titanium and brass particles. Particles determined by the method must be electrically conductive.

In the preferred embodiment of the method of the invention, the absolute value of the contaminant particle content is determined by making a measurement of a first lubricating grease sample whose contaminant particle content is known beforehand, and the measurement value of a second lubricating grease sample to be determined is compared with the reference value in order to determine the absolute impurity content of the latter.

In the apparatus of the invention for determining the content of metallic cont-uninant particles in a lubricating grease, a sample of the lubricating grease is first taken to allow the determination to be made. The apparatus comprises a sampling pole at which the lubricating oil sample is placed; an electric magnet and an alternating current source for this, by means of which a first magnetic field alternating at constant frequency is produced in the lubricating grease sample disposed in connection with the sampling pole; and a measuring unit for determining the influence of a second magnetic field resisting variations in the first magnetic field, the second magnetic field being produced by eddy currents induced in the contaminant particles, and the content of contaminant particles in the lubricating grease sample being determined on the basis of this influence.

In the preferred embodiment of the apparatus of the invention, the measuring unit comprises means for deterrnining the impedance of an electric magnet, which determines the influence of a second magnetic field produced by eddy currents on the electric magnet impedance, the content of contaminant particles in the lubricating grease sample being further determined on the basis of the determined impedance.

The invention has the benefit of rapid and reliable real-time determination of the contaminant particle content, especially in field measurements.

Another advantage of the invention is that its sampling, determinations and analyses utilise the capacity of lubricating grease to retain the metallic contaminant particles that have accumulated in it. The outcome of the determination of contaminant particles yielded by the method and the device of the invention will thus represent the metallic contaminant particle content of the original lubricating grease sample.

A preferred embodiment of the invention has the additional advantage of allowing the value of the metallic contaminant particle content of a lubricating grease sample to be determined as an absolute content value. The invention also yields the advantage of straightforward and easily repeated calibration, so that the absolute values of the determination will consequently be reliable.

The invention also has the advantage of requiring a small sample of lubricating grease and of allowing repeated analysis of the sample in order to ensure the content value, and/or any other kind of analysis.

The invention also has the advantage of automatable determination and of allowing the measurement results to be presented in the desired form. The results can be displayed on a screen and/or printed if necessary.

The invention also has the advantage of the apparatus for deterrnining the content of contaminant particles being easy to use. Another advantage of the invention is that the measuring sensor can be given an ergonomic design. It also has the advantage that the apparatus can be accomplished as a small-sized, portable unit. The apparatus is thus applicable both to field and workshop measurements.

The invention will be explained in detail below with reference to the accompanying drawing, in which

figure 1 is a principal view of the method and the apparatus of the invention;

figure 2 is a schematic view of the electric circuit of the measuring sensor; and

figure 3 is a block diagram of the apparatus in accordance with the invention.

In the device of the invention, the actual measuring sensor comprises a sampling pole 1 and an electric magnet 2, the sensor being schematically illustrated with schematic magnet fields HI, H2 in figure 1. The electric magnet 2 includes a core 2a, around which a coil 2b has been formed with an electric conductor. The electric magnet 2, particularly the coil 2b, has been connected at the poles a, b both to an alternating current source 3 and to the measuring unit 4, as schematically shown in the block diagram of figure 3. The lubricating grease sample N is placed at the sample pole 1 when the determination is carried out. The sample pole 1 of the measuring sensor is a small flat substrate, on whose surface a thin layer of lubricating grease sample is applied. On this surface, the lubricating grease sample is easy to apply, and accordingly, the surface is also easy to clean. The sample pole 1 is preferably disposed at the end of the core 2a of the electric magnet 2, as shown schematically in figure 1. Once the sample N has been spread on the sample pole 1, the determination of contaminant particles can be performed.

In the method of the invention, a sample N of lubricating grease is exposed to a first alternating magnetic field HI, which is produced with an electric magnet 2 in this case. The content of contaminant particles in the lubricating grease sample is determined on the basis of the influence of a second magnetic field H2 resisting alternations in the first magnetic field HI. The second magnetic field has been produced by eddy currents induced in the contaminant particles. The second magnetic field H2, i.e. secondary field, weakens the intensity of the first magnetic field HI, i.e. primary field, and this appears as a measurable variation in the impedance or voltage of the electric magnet 2, especially its coil 2b, this variation being measured.

The method of the invention is based on the utilisation of the eddy current phenomenon known per se. This is illustrated with the aid of figure 2, which shows an electro-magnetic circuit diagram between the coil 2b of an electric magnet 2 and a lubricating grease sample N containing electrically conductive contaminant particles. The impedance Z prevailing at the circuit poles a, b is illustrated by the equation:

in which R_t is the reluctance of the electric magnet 2 and Li its inductance, R₂ is the total reluctance of the contaminant particles 1 and L₂ is their total inductance, and M is their mutual impedance.

In equation 1 the reluctance term of impedance Z is:

R = Ri + (ω²M²R₂)/(R₂ ²+(ωL₂)²) (2)

This shows that the reluctance R increases as the distance of the electric magnet 2 decreases. This is due to the growth of the mutual impedance. In equation 1 the second term of the inductance Z, i.e. the term L of the combined inductance is:

The inductance Li increases as the distance between the electric magnet and the contaminant particles decreases.

The first terms of the equations 2 and 3 are R_t and > and accordingly, the second subsequent terms, i.e. the additional terms of the reluctance and the inductance are determined:

ΔR = ω²M²R₂/(R₂ ² + ( ύ f) (4)

ΔL = ω²M L_>/(R₂ ² + (ω-Lz)²) (5) in which the additional inductance term ΔL represents the reflected inductance produced by the eddy current. The additional term ΔL decreases due to the eddy current as the distance between the electric magnet and the contaminant particles decreases. The joint effect on the impedance Z is that the impedance varies along with a change in the distance.

When the additional terms ΔR, ΔL are divided with each other, the following equation is obtained:

ΔR / ω ΔL = R₂ / ωL₂ (6)

This formula (6) expresses the ratio between the reluctance and the inductance of the object to be examined, i.e. the sample containing contaminant particles, when the angular frequency ω = 2πf is constant (or at least a predetermined variable). The frequency of the magnetic field produced by the alternating current source and at the same time by the electric magnet is f.

Basing the mathematical expression on the additional term ΔR of formula (1) and the additional term ΔL of the combined inductance, and eliminating the reluctance R₂ from these, the following equation is obtained:

(ΔR)² + (ωΔL - (M² ω)/^))² = ((M² ω)/(2La))² (7)

It appears in equation 7 that the impedance graph describes the shape of a hemi- circle if the mutual impedance M and the frequency angular speed ω are kept constant. The centre of the hemi-circle is located on the vertical ωΔL axis and the length of its radius is (M² ω)/(2L₂)- Various hemi-circles can be drawn corresponding to different values of the combined impedance M. The hemi-circles are located such that the horizontal ΔR axis is a tangent of these. The electric conductivity and mutual impedance M of a given metal, i.e. contaminant particle, can be represented by a straight line whose point of intersection with the hemi-circle deteπnines the impedance data of the coil 2b of the electric magnet 2. When the conductivity, the permeance and the frequency F have been selected, the impedance of the coil 2b will depend exclusively on the distance between the coil 2b and the object, i.e. the sample N.

The device of the invention utilises the theoretical data above. A variation in the impedance of the electric magnet 2, in this case of the coil 2b, can be measured by electrical means. This is preferably carried out as follows.

The device, figure 3, preferably comprises, in addition to the first electric magnet 2, a second electric magnet 5, which is identical with the first electric magnet. The actual determination is performed with the first electric magnet. The second electric magnet 5 is adapted to act as a reference electric magnet when the influence of the second magnetic field on the first magnetic field 2 is determined. The second electric magnet 5 is preferably supplied from the same alternating current source 3 as the first electric magnet 2. Protective resistors R are serially connected with the electric magnets 2, 5.

The alternating current source 3 preferably has a frequency in the range from 0.5 to 2 MHz. This ensures that eddy currents with adequate intensity are produced in the contaminant particles of the sample.

The measuring unit 4, figure 2, comprises means 41, 42 for determining the impedance of the electric magnet 1; 5, and these means serve to determine the modified impedance Z of the electric magnet. The means for determining impedance include a comparator 41 and a subsequent amplifier 42. The first input of the comparator is supplied with a test signal from the first electric magnet 2, in connection with which the lubricating grease sample is also disposed, and the second pole is supplied with a reference signal from the second electric magnet 5.

The difference signal from the comparator 41 is amplified normally by means of the amplifier 42 and thus an analogue test signal is obtained. This test signal allows the impedance variation caused by the lubricating grease sample to be determined and, on the basis of this, the content of cont-uninant particles in the sample to be determined as well. The measuring unit 4 may comprise an analytic unit which separates the reluctance component from the inductance component of the combined impedance Z.

In the most preferred embodiment of the invention, a processor 6 has been disposed in connection with the measuring unit 4. This processor 6 converts an analogue test signal obtained from the measuring unit 4 into a digital signal and processes it with a data processing unit 62, stores the results in a database, table 65 or the like, and shows the results on a display screen 66 or any similar output device.

The processing unit 6 includes an analog to digital i.e. AD converter 61, which converts an analogue test signal into a digital signal, which is then fed in the data processing unit 62. The data processing unit 62 is a micro-computer or the like, e.g. a personal computer. This has been provided with a determination unit, which is for instance a unit created by programming. Moreover, the data processing unit 62 comprises a memory unit 63. In this memory unit, data about the absolute reference value of the contaminant particle content are stored, which are acquired by making a measurement on such a first lubricating grease sample for which the contaminant particle content is previously known. In the deteraiining unit 64, the measurement value obtained from the second lubricating grease sample to be actually measured is compared with the reference value of the memory unit 63, and on the basis of this, the absolute impurity content of the lubricating grease sample is obtained. This is stored in the database or table 65 together with the sample identifier data.

It should be noted that lubricating greases used for practical determinations and measurements usually have metal contaminant particle concentrations that are sufficiently high, e.g. 0.05 to 2% by weight, for the sensitivity of the method and the device to be adequate for determining the contaminant particle content.

The invention is not restricted only to the embodiment example given above, many variants being conceivable without departing from the inventive idea defined by the claims.

Claims

1. A method for determining the content of metallic contaminant particles in a lubricating grease, comprising sampling of the lubricating grease, characterised in that the method further comprises: - exposing the lubricating grease sample (N) to a first magnetic field (HI) alternating at a constant frequency (f), and

- deteπnining the content of containinant particles in the lubricating grease sample by the influence of a second magnetic field (H2) resisting changes in the first magnetic field (HI), the second magnetic field (H2) being produced by eddy currents induced in the contaminant particles.

2. A method as defined in claim 1, characterised in that the absolute value of the contaminant particle content is determined by making a measurement of such a first lubricating oil sample whose contaminant particle content is known beforehand, and by comparing the measurement value of a second lubricating grease sample to be determined with the reference value in order to determine the absolute contaminant content of the latter.

3. A method as defined in claim 1 or 2, characterised in that the alternating frequency of the first magnetic field (HI) is arranged as a constant frequency preferably in the range from 0.5 to 2 MHz.

4. A method as defined in any of the preceding claims, characterised in that the first magnetic field (HI) is produced by means of an electric magnet (2), changes in the impedance of this being used to determine the influence of the second magnetic field (H2) and -further the content of contaminant particles.

5. A method as defined in claim 4, characterised in that a sampling pole (1) for the lubricating oil sample (N) is disposed in connection with the electric magnet (2), the lubricating oil sample being applied as a thin layer on the pole and the first magnetic field (HI) being adapted to pass through this pole.

6. A device for deterrnining the content of metallic contaminant particles in a lubricating grease, a sample being taken for the determination, characterised in that the device comprises:

- a sampling pole (1) at which the lubricating oil sample (N) is placed;

- an electric magnet (2) and its alternating current source (3), by means of which a first magnetic field (HI) alternating at constant frequency is produced in the lubricating oil sample (N) which has been disposed at the sampling pole (1); and - a measuring unit (4) for determining the influence of a second magnetic field (H2) resisting changes in the first magnetic field (HI), the second magnetic field (H2) being produced by eddy currents induced in the contaminant particles, this influence enabling the content of contaminant particles in the lubricating grease sample to be determined.

7. A device as defined in claim 6, characterised in that the device also comprises a second electric magnet (5), which is identical with the first electric magnet (2) by means of which the actual measurement is carried out, the second electric magnet (5) having been adapted to act as a reference electric magnet in the determination of the influence of the second magnetic field, and the second electric magnet (5) being supplied from the same or a similar alternating current source (3) as the first electric magnet.

8. A device as defined in claim 6 or 7, characterised in that the constant frequency of the alternating current source (3) is preferably in the range from 0.5 to 2 MHz.

9. A device as defined in any of the preceding claims 6-8, characterised in that the measuring unit comprises means (41, 42) for measuring the impedance of the electric magnet (1; 5), the means being used to determine the influence of the second magnetic field produced by eddy currents on the impedance of the electric magnet, the impedance being further used to determine the content of contaminant particles in the lubricating grease sample.

10. A device as defined in any of the preceding claims 6-9, characterised in that the device comprises a processing unit (6), to which a test signal from the measuring unit (4) is fed and comprising at least - a memory unit (63) in which information about the absolute reference value of the contaminant particle content is intended to be stored, the absolute reference value being obtained by making a measurement of such a first lubricating oil sample whose contaminant particle content is known in advance, and

- a determining unit (64) in which the measurement value of the second lubricating oil sample to be actually measured is compared with the reference value in order to determine the absolute impurity content of the latter lubricating oil sample.

11. A device as defined in any of claims 6 to 10, characterised in that the sampling pole (1) is a small, substantially flat substrate, on whose surface a thin layer of lubricating grease sample is intended to be applied.