A new method of preparing ferrograms

Wear, I55 (1992) 277-283

277

A new method of preparing Tan Qing-Chang,

Tang Bo-Jun,

ferrograms

Xin Di and Liu Ming-Jie

Mechanical Engineering Department, Jilin University of Technology, Changehun

130025 (China)

(Received May 24, 1991; accepted October 7, 1991)

Abstract

A new method of preparing ferrograms is put forward to monitor the wear condition of machines whose lubricating oils contain a large number of contaminant particles. In this method of preparing ferrograms, the direction of magnetic force is opposite to the direction of gravitation, and deposition of the contaminant particles onto the substrates is decreased by gravitation. Experimental and practical investigations of the effectiveness of the method applied to mining machine wear were carried out. The results showed that this new method is useful.

1. Introduction

In the lubricated systems of some machines, e.g. mining machines, many contaminant particles are in suspension in the oil. These particles are generally too small to be removed by the filter and remain in suspension in the circulating oil. When the wear condition of the machines is monitored by ferrography, many contaminant particles are deposited onto the substrates and cover the metal wear particles. This is a major problem affecting ferrogram examination and wear monitoring. So far, in order to solve the problem, two methods have been developed: first, the dilution of the examined oil samples is increased; second, ferrograms are prepared using rotary particle depositors. Increasing the dilution reduces the number of wear particles included in the samples. The ferrograms prepared by rotary particle depositors are not convenient for the quantitative analysis of the density and size distribution of wear particles. Here, the effect of magnetic force and gravitation on various particles was studied. A new method of preparing ferrograms was developed. In this method, the direction of magnetic force is opposite to the direction of gravitation, thereby decreasing the number of contaminant particles deposited onto the substrate by gravitation. The method is suitable for wear monitoring of machines and has a high depositing efficiency and good repeatability for the oil samples examined.

2. Principle

of the new method

In general, most of wear particles in oil samples are ferromagnetic, and the magnetic force acting on the particles is great. However, most of the contaminant particles in the samples are diamagnetic and paramagnetic, and the magnetic force acting on these particles is small. The magnetic force, which acts on the particles

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278

when a ferrogram Appendix A):

is prepared,

3/kIz g-% /L+2~lJ n-l

I;,=@&,

can be calculated

using the following

formula

(see

cxp( - KY) ,g14, Pn exp( - &Y)

A comparison between the calculated magnetic force and the gravitation acting on the particles is summarized in Table 1. From the table, it can be seen that the magnetic force mostly causes the ferromagnetic particles to deposit because it is over a thousand times greater than the gravitational force. So, the action of gravitation may be neglected. However, for the paramagnetic particles, gravitation surely causes deposition of the particles since the difference between the magnetic and gravitational forces is not large. Therefore, if the magnetic field of the ferrograph instrument is set above the substrates, and the direction of magnetic force is opposite to the direction of the gravitationat force, as shown in Fig. 1, then when a ferrogram is prepared using this method, the deposition of ferromagnetic particles will not be affected because the magnetic force is over a thousand times greater than the gravitational force. However, for the paramagnetic particles, oxycopper and aluminium, if the magnetic force is greater than the gravitational force, then these partides will be deposited onto the substrate, but they wili be located at relatively distant positions from the entry region of the ferrogram. This is because the magnetic force is smaller, so it takes longer for the paramagnetic particles to be deposited onto the substrates, and the particles arc moved further along the substrates by the action of gravitation. If the magnetic force is smaller than the gravitational force, the particles cannot be deposited onto the substrate. For diamagnetic particles, the direction of magnetic force is the same as

that of gravitation, TABLE

they cannot

be deposited

onto the substrate

either. So, the method

1

Comparison between

magnetic

force and gra~tational

Substances of particles

Magnetic susceptibility

Size of particles

Ferromagnetic Oxycopper Aluminium Carbon

119 2.41 x 1O-4 8.2X 1o-6 -0.46x lo-”

(m)

.5x10-”

force

Magnetic force F, fN)

Gravitational force F,(N)

FJF,

5.97x7o-8 6.04 x10-” 0.21 x lo-l2 -0.012x lo-‘2

4.01 x10-I’ 2.77 x lo-” 1.28X lo--” 1.03x lo-“’

1489 2.18 0.16 -0.01

The magnetic force was calculated according to the parameters Permanent

of the m-1

20

magnet

2e2 x

ontal

~ Substrate Oil fkwng

Fig. 1. Principle of the method.

1 Y

ferrograph

instrument.

279

can decrease the number of contaminant the large wear particles locate.

particles

deposited

at the positions

where

3. Experiments The ferrogram instrument was developed according to the above-mentioned principle, as shown in Fig. 2. When a ferrogram is made using this instrument, the oil sample is absorbed on the substrate. Figure 3 shows a comparison of ferrograms prepared respectively by the developed instrument and standard ferrograph instruments. The oil samples were taken from the lubrication systems of mining machines in which many particles of coal-dust were in suspension. The pictures show the regions of ferrograms where large particles deposit. A lot of coal-dust was deposited onto the ferrograms prepared using the standard ferrograph instrument. This made it difficult to examine and analyse the wear particles on the ferrograms. Also, there were not many wear particles on the ferrograms since the dilution of the oil samples was increased. However, the little coal-dust deposited onto the ferrograms prepared using the developed instrument, did not affect examination of the ferrogram. Many wear particles were also deposited. This illustrates that the numbers of contaminant particles deposited onto the substrates is decreased using the developed instrument. The effective separation of wear particles by the developed instrument can be assessed by the depositing efficiency. First, a ferrogram of an oil sample was prepared by the developed instrument and the used oil sample was collected; then a ferrogram of the used oil sample was again prepared using the standard ferrograph instrument. The densities of large wear particles on the two ferrograms, An and Au, were measured and the depositing efficiency S, was calculated using the following formula:

The calculated results are summarized in Table 2. The depositing efficiency of the developed instrument is 87.2%, showing that most wear particles can be separated by the instrument. The data in the table show that the instrument has good repeatability

Fig. 2. The developed instrument.

--

(cl Fig. 3. Comparison of ferrograms: dilution factor n=lOO; (b) (d) n=lO.

TABLE

(b)

10pm -

Cd)

(a), (c) ferrographs ferrographs produced

--

_. __

.;. ,

. 100 nm

10 pm produced using the standard instrument, using the new method, dilution factor

2

Calculated

Order tests

100 pm

of

results

of the

deposit

AI,

efficiency AIZ

Depositing efficiency

Efficiency

(%) 1 2 3

22.8 24.0 23.3

2.7 3.4 2.9

88.2 85.8 87.6

87.2

of results. The method of use of the developed instrument is the same as that of standard ferrograph instruments; the substrates used are also the same. However, advantages of ferrograms prepared by the new method are that the dilution need be increased, only a small amount of contaminant particles is deposited onto ferrograms, and the ferrograms are convenient for analysis and examination.

the the not the

1

281

4. Application

of the developed instrument

In coal mines, it is very difficult to monitor the wear condition of machines using ferrograph instruments because the lubricating oil contains large amounts of coal-dust. The particles of coal-dust deposited onto the substrates produces a large particle density, A,>40%. The difficulty of examination and measurement of the ferrograms affects the monitoring of wear conditions of these machines. The four gear boxes of the machines of coal mines were chosen to monitor wear conditions using the developed instrument. Much coal-dust is in suspension in the oil of the gear boxes. Before beginning, gear boxes 1 and 2 were cleaned and oiled with the IS0 VGlOO lubricant, but gear boxes 3 and 4 were not cleaned. The oil samples taken from the oil sumps were diluted with a dilution factor n = 10, and ferrograms were prepared using the instrument. The densities of the large particles and small particles on a ferrogram, A, and A,, were measured, and the wear index, Z,, was calculated using the formula from ref. 1:

Figure 4 shows the values of Z, from the gear boxes. The values of Z, for the cleaned boxes were small because a few large particles were in suspension in the oil after cleaning, and the values of Z, of the other boxes were large. However, according to the curves in the figure, cleaning the boxes does not affect the results. After about 2 months of monitoring, the wear index Z, of box 3 reached relatively high values, and the oil filter was changed according to working conditions of the box. The wear

I

2

3

4

5 Running

6 7 time (month)

-

Box 3

v

Box 4

9

10

11

9

10

11

Is

1

Fig.

4. Values

2

of Z, for

3

1,

the

gear

5 Running boxes.

6 1 time (month

8

1

282 index 1, then returned to normal values. The wear index I, of gear box 1 also reached relatively high values after 4 months of monitoring; the oil filter and oil were changed simultaneously, and Z, returned to normal values. Monitoring of the other boxes showed that they worked normally, and they were not overhauled during the period of monitoring.

5. Conclusions

The experiments and application show that the method developed in this paper can decrease effectively the deposition of contaminant particles onto the substrates. The method is especially suitable for monitoring wear conditions of the machines whose lubricating oils contain many contaminant particles. The method has a high depositing efficiency and good repeatability for oil samples.

References

1 D. Scott and V. C. Westcott, Predictive maintenance by ferrography, Wear, 44 (1977) 173-182. 2 R. K. Wangsnees, Electromagnetic Fields, Wiley, New York, 1979. 3 J. D. Jackson, Classical Electrodynamics, Wiley, New York, 1975.

Appendix

A

The magnetic field used in the preparation of the ferrogram is formed by permanent magnets, as shown in Fig. 1. The magnetic field can be regarded as a plane field along the direction of the coordinate Z. In the field, we can write [2] 02u=o

(1)

where u is the magnetic scalar potential. boundary conditions of u are

According

to symmetry

of the field, the

uL= ,=o ul,,o=O

uly=o= 5x

O
fige,

e2
PO

UlY=-0=

PO

where h is the permeability of free space and B, is the magnetic current at the magnetic gap of the ferrograph instrument. The general solution of eqn. (l), obtained by separation of variables, is m

u=

x&

n-1

~0s

&x+L

sinPnx)(K3,chp,y+K~ShP,Y)

(3)

283

By using the boundary derived from eqn. (3):

u=

ZK,sin(&x)

n-1

So, the magnetic

conditions,

eqns. (2), a special

solution

of eqn. (1) can be

exp( - P,y) field intensity

(4) of the field can be written

as [3]

I?= -grad(u)

(5)

Because particles deposited onto ferrograms are very small, their shapes can be regarded as spherical. After a particle is magnetized in the field, its magnetic field intensity is written as [3] H= r

3PaH

(6)

P+ho

where p is the permeability of the particles. Oil is diamagnetic and its permeability is very small. So, the magnetic action of the magnetized oil is neglected. A magnetized particle can be regarded as a magnetic doublet, its magnetic dipole moment is written as [31

nit= vxm12r

(7)

where X,,, is the magnetic susceptibility of the particle and V is the volume of the particle. The magnetic force acting on a magnetic doublet may be shown as [3] F, = /L&i. V)Ei Substitution

of the expressions

(8) from eqns. (4)-(7)

into eqn. (8) yields

m

F,,,=KF&,,

where

(9)