Wear, 90 (1983) 101 - 106 101

A F~RROGR~HIC CASE STUDY ~PLIED TO HYDRAULIC SYSTEMS*

G. II. MILLS and F. A. DAVIS

Department of Metallurgy, Brunei University, Uxbridge UB8 3PH (Gt. Britain)

(Received November 17,1982)

The introduction of any new analytical technique to industry must offer at least one of the following advantages: (1) substantial cost saving in terms of unit down-time and/or unnecessary m~nte~ce; (2) increased technical “know-how” leading to improved unit efficiency.

A case study approach to condition monitoring of hydraulic systems by ferrography is reviewed and the benefits are analysed.

1. Introduction

In this modern technolo~c~ age the cost of producing a product has to be kept to a minimum if that product is to be economically competitive in its field. One of the areas in which costs may be cut is in maintenance of the equipment or machinery which is used to manufacture the product. The cost of planned m~nten~ce has increased so ~~atic~ly that in many areas condition monitoring or preventive maintenance would appear to be financially feasible.

Ferrography [l - 31 is one technique which is proving to be viable in many applications, particularly in the monitoring of hydraulic systems r4,51*

2. Results

A Hercules 1140 packaging unit, used for producing filled cartons of soft drinks, was monitored for a period of 18 months. Oil samples were taken from the oil tank which has a capacity of approximately 675 1. The sampling procedure was standardized as far as possible in accordance with

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

@ Elsevier Sequoia/Printed in The Netherlands

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TABLE 1

Direct reading analysis results for the Hercules 1140 packaging unit

Number D, Ds %+& R_ -~ ns DLz --Ds2 of weeks

_

1 7 3 10 4 40 2 6 2 8 4 32 3 7 2 9 5 45 7 5 1 6 4 24

11 6 3 9 3 27 15 6 2 8 4 32 20 4 1 5 3 15 24 5 2 7 3 21 28 7 2 9 5 45 32 7 3 10 4 40 36 6 2 8 4 32 40 22 6 28 16 448 41 5 1 6 4 24 45 6 3 9 3 27 49 7 3 10 4 40 53 5 2 7 3 21 57 5 1 6 4 24

61 6 2 8 4 32 66 5 2 7 3 21 70 7 3 10 4 40 74 6 2 8 4 32

recognized recommended techniques [ 61. The oil samples were taken weekly for the first three weeks to produce a base figure and then every four weeks.

The direct reading (DR) results DL (particles greater than 5 pm in size) and IIs (particles less than 2 pm in size) were measured, and from these results the total wear L+, + I&, the severity of wear DL -- Ds and the severi- ty-of-wear index DL2 - Ds2 were calculated (Table 1). All the results were plotted separately against time with a cumulative plotting method described elsewhere [ 71. The total wear and severity of wear appeared to show the best trends, and these are shown in Fig. 1. Both plots show a steady increase up to point A and then a large increase at point B. A Ferrogram at B (Fig. 2) revealed normal metallic wear particles along with large black carbonaceous debris. On checking, the firm stated that there was doubt about the purity of that sample. This was verified by the Fenogram result. The DR results of the next oil sample (point C) dropped to the previous low value. Continued monitoring of the unit showed the same previous steady slope of the graph, indicative of a satisfactory running condition.

Although no mechanical failure was recorded during the period in which the unit was monitored, the cost of the monitoring programme was justified for the following reasons. A twice-yearly oil change was normally recommended: thus in 18 months 675 1 X 3 = 2025 1 would be used. At a

103

120_ MD SAMPLE

110.

103.

SO-

80

WEEKS

Fig. 1. Trend plots of cumulative total wear and severity of wear for the Hercules 1140 packaging unit: X, DL + Ds;*, D,- Ds.

(a) (b)

Fig. 2. Ferrogram prepared from a sample of 3 ml of oil from the Hercules 1140 packaging unit: (a) debris at the entry area on the Ferrogram; (b) debris adjacent to the entry area on the Ferrogram.

price of 61.5 p 1-l this would cost $1245. The cost of monitoring was $230. Therefore the total saving was $1015 plus labour costs.

During approximately the same period a large multiram closed die forging press was monitored. As the force is developed hydraulically, ferro- graphic oil analysis was used as before. The oil samples were taken at the power pack filter bowl and the majority were taken at weekly intervals.

The DR results are shown in Table 2. As before, the results are expressed as cumulative plots of the total wear Dr, + Ds and the severity of wear DL - Ds (Fig. 3). At first the results showed a steep slope and then a more steady and apparently normal running value for the unit. The initial

104

TABLE 2

Direct reading analysis for the multiram die forging press

Number D, DS DL+ D, D,-DS Number DL D, D,+ DS DL-D, of weeks of weeks

1 70 13 83 57 23 6 2 8 4 2 43 6 49 37 24 7 2 9 5 4 13 3 16 10 25 27 6 33 21 5 11 2 13 9 27 7 2 9 5 6 10 3 13 7 29 7 2 9 5 7 11 4 15 7 30 10 2 12 8 8 9 3 12 6 31 4 1 5 3 9 8 2 10 6 32 5 1 6 4

10 43 11 54 32 33 3 1 4 2 11 4 1 5 3 34 4 2 6 2 12 2 1 3 1 35 4 1 5 3 13 2 1 3 1 37 6 1 7 5 14 2 1 3 1 40 6 1 7 5 15 3 1 4 2 42 7 2 9 5 18 3 1 4 2 43 6 1 7 5 19 6 2 8 4 44 5 2 7 3 20 5 2 7 3 46 4 1 5 3 21 8 2 10 6 48 6 2 8 4 22 7 2 9 5 50 6 1 7 5

Fig. 3. Trend plots of cumulative total wear and severity of wear for the multiram die forging press: X, DL + Ds; 0, D, -D,.

high values were due to a sampling deficiency caused by the inexperience of the personnel. The very high value of the oil sample at point A on the graph was also due to a sampling error.

105

In spite of the apparent improvement in sampling technique, apart from point A, during the first ten weeks the DR values were too high for what was expected from a normal hydraulic system. Examination of various Ferro- grams from this period revealed the presence of contamination debris from an outside source (Fig. 4). This was traced to a poor seal in the storage tank. At this point the system was drained, flushed and refilled with a new filter installed. The next sample showed a significant lowering of the DR values (point B) and over the next eight weeks the unit maintained a steady level of debris, as shown by the slope of the graph.

(4 (b) Fig. 4. Ferrogram prepared from a sample of 2 ml of oil from the forging press: (a) debris at the entry area on the Ferrogram; (b) debris adjacent to the entry area on the Ferrogram.

Between weeks 20 and 24 there was a steady increase in the DR values and then a relatively large jump during week 25. Examination of prepared Ferrograms at those points showed only normal metallic wear particles suggesting possible filter malfunction. This proved to be the case and when the filter was renewed the unit functioned perfectly over the remaining period of 24 weeks.

The condition monitoring programme was initially started because of pump losses incurred during the previous six months. No exact cost savings due to the monitoring programme were calculated. However, one of the results has been the better housekeeping of the unit, resulting in less inspec- tion and down-time and thus less expenditure. No pump failures have occurred in the unit since the introduction of the condition monitoring programme.

Oil sampling points are being installed to enable consistent and accurate sampling to be employed. Since this work was undertaken more refined techniques involving the ferrographic examination of hydraulic oil samples have been devised. This involves the use of magnetizing solutions enabling the recovery of any plastic or polymeric wear debris. The successful identification of such seal material has added a new powerful dimension to condition monitoring of hydraulic systems.

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3. Conclusion

The successful condition monitoring of these hydraulic systems by ferrographic analysis resulted in substantial financial savings to the firms involved.

In one case this success has led to further units being placed on a new condition monitoring programme.

References

1 W. W. Siefert and V. C. Westcott, A method for the study of wear particles in lubri- cating oil, Wear, 21 (1) (1972) 27 - 42.

2 D. Scott and G. H. Mills, Spherical debris - its occurrence, formation and significance in rolling contact fatigue, Wear, 24 (2) (1973) 235 - 242.

3 A. W. Ruff, The characteristics of wear particles recovered from wearing systems, Wear, 42 (1977) 49 - 62.

4 M. V. Hofman and J. H. Johnson, The development of ferrography as a laboratory wear measurement method for the study of engine operating conditions on diesel engine wear, Wear, 44 (1) (1977) 183 - 199.

5 W. Hoffman, Some experiences with ferrography in monitoring the condition of air- craft engines, Wear, 65 (1981) 307 - 315.

6 Oil sampling technique, Ferrogruph Tech. Bull. 1, 1972, revised April 1979 (Fox- boro-Trans-Sonics Inc., Burlington, MA).

7 D. Scott, P. J. McCullagh and G. W. Campbell, Condition monitoring of gas tur- bines - an exploratory investigation of ferrographic trend analysis, Wear, 49 (2) (1978) 373 - 389.