Wear, 90 (1983) 89 - 100 89

APPLICATION OF FERROGRAPHY TO CONTAMINATION CONTROL IN FLUID POWER SYSTEMS*

P. J. McCULLAGH and W. E. CAMPBELL

Fluid Power Engineering Business Centre, National Engineering Laboratory, East Kilbride, Glasgow 075 OQU (Ct. Britain)

(Received November 17,1982)

Summary

The use and application of ferrography and associated analytical tech- niques for machinery condition monitoring are outlined.

Specific application to contamination control in hydraulic systems is discussed: contamination in very clean systems is compared with satisfacto- rily operating equipment from the general engineering field. Results from an automatic particle counter are compared with a direct reading Ferrograph and a preliminary relationship between the two entirely different particle measuring and counting ~chniques is discussed.

Ferrography can be used to detect operational differences between machines operating under apparently identical conditions and to identify wear and contaminant particles and is an effective technique for the condi- tion monitoring of hydraulic systems.

1. Introduction

Ferrography, an analytical technique designed to precipitate wear debris and con~min~t particles from a fluid under the influence of a high gradient magnetic field, has been described elsewhere [ 1 - 4 J. Separation of particles, in order of size, is effected either inside a fine bore glass tube (direct reading (DR) Ferrograph) or on a transparent substrate (analytical Ferrograph) for subsequent examination in an optical microscope. The DR Ferrograph is a simple instrument which can be easily operated at depot or plant location where immediate assessment of debris levels enables the operators to decide which sample requires more detailed examination. From the DR Ferrograph sufficient numerical data [ 2,5 - 91 or, by the use of a simple equation [2, 8, 91, a single-figure monitoring parameter can be de- rived and used as the basis of a monitoring code of practice. When more

*Paper presented at the First ~~rnational Conference on Advances in Ferrography, University College, Swansea, Gt. Britain, September 22 - 24,1982.

0043-1648/83/$3.00 0 Elsevier Sequoia/Printed in The Netherlands

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detailed information is required, if for example the DR Ferrograph param- eters are outwith the normal range levels for specific applications, examina- tion by analytical ferrography to investigate particle morphology, size etc. involves the use of optical microscope facilities, such as the bichromatic microscope [ 2,4], possibly sited in a central laboratory remote from operat- ing plant or machinery. More detailed information can be obtained using scanning electron microscopy [ 10, II] or by heat tinting [12] the analytical Ferrogram slides.

Ferrographic analysis of periodic lubricant samples from gas turbines has been used for condition monitoring [2, 8 - 10,131 and preliminary examination of gas stream debris [ 141 shows some promise as a monitoring technique for non-lubricated components of aero gas turbines. The applica- tion of ferrography to the monitoring of earth-moving equipment [ 151, diesel engines [16 - 191 and hydraulic systems [6, 20 - 231 has been success- ful.

Failure of hydraulic components and systems arising out of inadequate design is a rare occurrence and the equipment manufacturers and users are becoming increasingly aware of the role of contamination in hydraulic fluid and its subsequent effect on performance and reliability. Considerable work has been carried out using a variety of particle counters, patch testing, gravi- metric analysis and spectrographic oil analysis for the condition monitoring of hydraulic systems [ 231.

The DR Ferrograph can be considered as a type of particle counter operating on the light interruption technique and the analytical Ferrograph can be related to patch testing or magnetic plug inspection techniques.

The use of a DR Ferrograph, an analytical Ferrograph and a HIAC auto- matic particle counter for condition monitoring the hydraulic power system of a large complex structural testing building operating under ideal condi- tions is described in this paper and the levels of contamination found in some industrial applications in the general engineering field are compared.

2. Hydraulic fluid analysis

2.1. Structural testing building One of the basic concepts of the strong floor building is that of the ring

main of the hydraulic power system: the ring main consists of three main lines with spur lines to adjacent testing areas. The capacity of the system is 3000 gal with a line pressure of 3000 lbf ine2 (210 bar). Fluid from a gravity- feed tank is fed through a series of filters by low pressure pumps which feed a bank of high pressure axial piston pumping units. A high degree of filtra- tion is used and a sample taken from the last low pressure pump is represen- tative of the cleanest fluid in the ring main system.

During the period from May 1980 to May 1981 hydraulic fluid samples were analysed with a HIAC automatic particle counter and a DR Ferrograph and the results obtained are shown in Table 1.

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TABLE 1

HIAC and direct reading Ferrograph results obtained on lubricant samples from the hydraulic ring main system sited at the National Engineering Laboratory

Sample date Sample Number of particles per 100 ml DR BS 5540 number with the following sizes Ferrograph code

>5 >15 >25 >50 >lOO results numbera

Pm Pm Pm Pm pm DL DS

May 19,198O LPl 47830 4953 1196 131 5 3.6 0.1 16/13 June 6,198O LP2 27925 1887 440 60 2 0.5 0.3 15/11 June 24,198O LP3 59092 5445 1809 332 32 4.1 0.3 16/13 July 16,198O LP4 2788 298 90 17 0 0.6 0.4 1219 September 24,198O LP5 88143 2312 321 18 1 1.2 0.5 17112 October 9,198O LP6 14049 613 143 18 1 1.8 0.5 14/10 November lo,1980 LP7 42336 1556 314 56 0 2.8 0.9 16/11 December 4,198O LP8 10318 1360 394 57 1 2.0 0.3 14/11 January 12,198l LP9 56188 1495 365 68 5 3.8 1.6 16/11 February 9,198l LPlO 13634 858 280 44 1 1.2 0.2 14/10 March 20,198l LPll 34488 2052 395 68 1 0.8 0.1 16/12 April 23,198l LP12 24654 892 192 24 1 0.6 0.4 15/10 May 12,198l LP13 14380 780 98 10 0 1.0 0.5 14/10

aReference 24.

2.1 .l. Results: National Engineering Laboratory system Although in Table 1 the HIAC particle counter results represent the

number of particles counted per 100 ml of fluid and the DR Ferrograph DL value represents the optical density of particles greater than 5 pm in 1 ml of fluid, a rough correlation between the two sets of results appears to exist, i.e. 15 000 X DL is equivalent to the number of particles greater than 5 pm in size in a 100 ml sample. Although no great confidence can be placed in the accuracy of DR Ferrograph readings where Dr, < 5 the results (Table 1) varying from D, = 1 to D, = 4 do indicate a satisfactorily low debris level. Similarly, when the HIAC results are considered by applying the criterion of the solid contaminant code [24] the hydraulic ring main system has an acceptably low debris level. Generally the HIAC and DR Ferrograph results com- plemented each other.

To elucidate further the nature of debris in the hydraulic fluid samples analytical Ferrograms were prepared from selected samples. A typical Ferrogram prepared from 9 ml of sample LP9 which had high HIAC and DR Ferrograph results is shown in Fig. 1 at low and high magnifications. The Ferrogram contained both non-metallic compound particles and metallic platelets approximately 10 E.trn in major dimensions together with some fibrous material.

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(a) (b) Fig. 1. Ferrogram prepared from 9 ml of lubricant sample LP9 (debris level: DL = 3.8; Ds = 1.6). (Magnifications: (a) 65~; (b) 335x.)

2.2. General engineering systems As part of a general research programme financed by the Mechanical

and Electrical Engineering Requirements Board of the Department of Indus- try a study is being carried out to determine the relative level of contamina- tion or cleanliness of typical industrial or general engineering systems. Hy- draulic fluid samples are being collected from as wide a range of installa- tions as possible and also ranging from very clean systems to highly con- taminated ones.

Typical applications examined to date range from injection moulding machines used for the manufacture of household plastic goods and toys, injection moulding equipment for the formation of hard rubber components such as battery cases and samples from a ring main system of a petroleum products refining plant. In the interests of commercial security the identity of the cooperating firms has been omitted, each sample being given a code number.

2.2.1. Particle counting results: industrial applications The HIAC and DR Ferrograph results obtained on lubricant systems

from general engineering applications are given in Table 2. Although very high counts were obtained in sample NEL 1 and NEL 2 by both HIAC and DR ferrography techniques the equipment operators were entirely satisfied with the machines’ performance and considered the machines to be very reliable when compared with others of similar type and duty.

Sample NEL 3 was the cleanest industrial sample analysed and, although it originated from a ring main system which had centrifugal filtration, it was not as clean as samples obtained from the National Engineering Laboratory hydraulic ring main.

The hydraulic fluid samples designated NEL 4 and NEL 5 are from a similar application to samples NEL 1 and NEL 2 and the difference between

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TABLE 2

HIAC and direct reading Ferrograph results obtained on lubricant samples from general engineering systems

Sample number Number of particles per 100 ml with DR Ferrograph BS 5540 the following sizes results code numbera

>5 >15 >25 >50 >lOO D, Ds

Pm Pm Pm Pm Pm

NEL 1 1326014 15059 1642 172 0 120 50 21114 NEL 2 1073001 7004 847 97 0 45 31 21/13 NEL 3 58766 256 86 23 0 6 3 1619 NEL 4 802970 7310 400 33 3 20 9 20113 NEL 5 620302 23832 6542 1159 156 35 8 20115

aReference 24.

(b)

(d)

Fig. 2. Ferrogram prepared from 1 ml of lubricant sample NEL 1 (debris level: DL = 120; Ds = 50): (a) entry deposit; (b) - (d) details of entry deposit. (Magnifications: (a) 65~; (b) 335x; (c), (d) 650X.)

(b)

Cd)

Fig. 3. Ferrogram prepared from 1 ml of lubricant sample NEL 2 (debris level: DL = 45; Ds = 50): (a) entry deposit; (b) -(d) details of entry deposit. (Magnifications: (a) 65~; (b) 335x; (c), (d) 650x.)

the particle counts and the DR Ferrograph results for the samples is related to a more complex maintenance schedule.

2.2.2. Analytical ferrography results: industrial applications Analytical Ferrograms were prepared from each of the samples to

examine the particle content and morphology in more detail. Typical photo- micrographs are shown in Figs. 2 - 6.

Comparison of Ferrograms prepared from 1 ml of each of samples NEL 1 and NEL 2 show that although they have very similar particle levels, as indicated by the HIAC results (Table 2), sample NEL 1 contains more metal- lic debris. The presence of translucent polymeric material evident in Figs. 3(a) and 3(b) probably accounts for the difference in the HIAC particle counts. Qu~i~tive identification, by heat tinting, of the composition of metallic debris indicates most of the debris is low alloy steel. It is also apparent from the change of the straw colour of the large (10 pm in diam- eter) platelets in Fig. 3(d) to blue, brought about by heat tinting, that the machine represented by sample NEL 2 was operating at higher temperatures

(a) (b)

(cl Cd)

Fig. 4. Ferrogram prepared from 3 ml of lubricant sample NEL 3 (debris level: DL = 6; Ds = 3): (a) entry deposit; (b) -(d) details of entry deposit. (Magnifications: (a) 65~; (b) 335x; (c), (d) 650x.)

than NEL 1. It is also possible that the extensive deposits of translucent polymeric material are also associated with operating temperature and condi- tions.

Sample NEL 3, from a hydraulic ring main system employing centrifu- gal filtration of the hydraulic fluid, is apparently the cleanest of the five sys- tems under discussion (Table 2). The Ferrogram prepared from 3 ml of the sample contained finely divided platelets 5 - 10 km in size, fibrous con- taminant and dark compound material (Fig. 4). Figure 4(d) shows a large platelet 30 pm long and 10 pm wide. After heat tinting, most of the particles are tempered a light blue colour and the remainder are unaffected, thus sug- gesting a high alloy or stainless composition.

The Ferrograms prepared from 3 ml of each of the samples NEL 4 and NEL 5 are similar to those prepared from samples NEL 1 and NEL 2: the Ferrogram prepared from NEL 4, when viewed at low magnification (Fig. 5(a)), contains less polymeric material than NEL 2. More detailed examina- tion shows dense deposits of platelets 5 pm in size (Fig. 5(c)) and platelets in the range 5 - 15 pm (Figs. 5(b) and 5(c)), some of which appear to have

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(a)

(d)

Fig. 5. Ferrogram prepared from 3 ml of lubricant sample NEL 4 (debris level: DL = 20; D, = 9): (a) entry deposit; (b) - (d) details of entry deposit. (Magnifications: (a) 65~; (b) 335x; (c), (d) 650x.)

an oxide coating. Heat tinting of this Ferrogram indicates that the majority of wear debris particles are of low alloy steel composition and only a few particles are of higher alloy steel. The Ferrogram prepared from sample NEL 5 is extremely dense (Fig. 6) suggesting that both HIAC and DR Ferrograph results are misleadingly low, a considerable quantity of translucent and non- metallic contaminant being visible at low magnification (Fig. 6(a)). Because of the pile-up of large metallic debris the Fenogram shoufd have been pre- pared from a smaller sample vofume, possibly 1 ml. Heat tinting reveals the presence of high alloy stainless steel material and it is intended to carry out a more detailed examination of the sample at a future date.

3. Discussion

The HIAC particle counts during the period May - June 1980 shown in Table 1 seem to indicate some degree of contamination of the fluid in the hydraulic ring main system operating in the strong floor building but since

(a) (b)

(cl (d) Fig. 6. Ferrogram prepared from 3 ml of lubricant sample NEL 5 (debris level: DL = 35; Ds = 8): (a) entry deposit; (b) -(d) details of entry deposit. (Magnifications: (a), (b) 65x;(c) 325x; (d) 650x.)

the DR Ferrograph results exhibited DL < 5 it was considered that the HIAC results were indicative of non-metallic contaminant in the fluid. Analytical ferrography confirmed the generally low metal content of the fluid and no further action was considered necessary. As the monitoring programme con- tinued throughout the year it became increasingly apparent that the low DR Ferrograph results with Dr, < 5 signify a satisfactorily clean fluid and in most cases the monthly HIAC samples correlated with DL values for concurrent samples.

Not surprisingly, the empirical relationship between the HIAC particle counts and D, values derived during the monitoring of the National Engi- neering Laboratory hydraulic ring main system does not seem too appropri- ate for the industrial hydraulic samples. However, the relationship does give the count derived from DL values about the same order of magnitude as sam- ples measured by the HIAC and perhaps, when more data are obtained from industrial systems, a more positive correlation can be established.

HIAC results from injection moulding machine samples NEL 1 and NEL 2 are similar but the DR Ferrograph results are significantly different

98

(Table 2). The other two injection moulding machines (samples NEL 4 and NEL 5) have less debris than NEL 1 and NEL 2. This may be attributed to differences in the maintenance system employed by the two companies. Although the DR Ferrograph results suggest a higher debris concentration in sample NEL 5 than in sample NEL 4 this may be explained by the higher concentration of debris greater than 15 pm in size as shown in Table 2. Further work is obviously required to study the relationship between the Ferrograph and HIAC results, Analytical ferrography indicates higher operat- ing temperatures in NEL 2 and NEL 5 contains significant quantities of non- metallic contaminant together with very high levels of metallic debris.

The ring main system, sample NEL 3, appears to be satisfactorily clean according to both the HIAC and the DR Ferrograph results but analytical ferrography detects large platelet particles of high alloy material. Further samples from this system may yield more useful information: although the equipment is considered reliable it is interesting to note that such large par- ticles are present in hydraulic fluid cleaned using a centrifugal filtration system.

4. Conclusion

Ferro~aphy is an effective technique for the monitoring of hydraulic fluids. It may well be more acceptable to monitor industrial systems from the general engineering field using a DR Ferrograph since the particulate level is very high: sufficiently high Dr, values which would trigger the making of an analytical Ferrogram are obtained to enable the detection of non- metallic material.

Most of the contamin~t in hydraulic fluid samples from general engi- neering systems consists of self-generated metallic platelets. Debris from injection moulding equipment operated by two users at different locations was similar containing both metallic wear particles and translucent polymeric material.

Acknowledgments

This paper is pubIished by permission of the Director of the National Engineering Laboratory, Copyright Controller, H.M. Stationery Office. This work was supported by the Mechanical and Electrical Engineering Require- ments Board of the Department of Industry.

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