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Joanna FABIś-DOMAGAŁA*

ANAlySIS OF DEFECTS AND FAIluRES OF HyDRAulIC GEAR PuMPS wITH THE uSE OF SElECTED QuAlITATIVE METHODS

ANAlIZA wAD I uSZKODZEŃ HyDRAulICZNyCH POMP ZĘBATyCH Z wyKORZySTANIEM wyBRANyCH METOD JAKOśCIOwyCH

Key words: gear pump, failure, FMEA analysis, error diagram.

Abstract: The paper presents an analysis of the defects and failures of a gear pump using the FMEA matrix analysis and error diagram. For each pump component, functions that are performed and potential defects were identified. Failures and defects that are typical for gear wheels were found as the main problem that influences pump malfunction. Therefore, using the error diagram, the main categories of causes were determined. In the next step, the identified causes and subcauses were assigned to defects with the highest possibility of occurrence, which allowed us to specify preventive and corrective actions.

Słowa kluczowe: pompa zębata, wada, analiza FMEA, diagram błędów.

Streszczenie: W pracy przedstawiono analizę wad i uszkodzeń pompy zębatej przy wykorzystaniu macierzowej analizy FMEA i diagramu błędów. Dla każdego analizowanego elementu pompy określono funkcje, jakie pełnią w pompie i potencjalne wady, jakie mogą wystąpić. Dla analizowanej pompy wady i uszkodzenia kół zęba-tych określono jako główną przyczynę wadliwej pracy. Wykorzystując diagram błędów, dokonano określenia głównych kategorii przyczyn, które następnie przypisano do wad o największym prawdopodobieństwie wy-stąpienia. Określono również zakres działań prewencyjnych i korygujących.

INTRODuCTION

Gear pumps are widely used in hydraulic systems of working machineries as a primary or auxiliary power system. Such pumps operate in hard conditions with high temperatures, pressures, and in environment with aggressive agents. Gear pumps have a relatively simple structure and are required to have high performance and reliability, because any failure may lead to stopping machine operation. Detecting pump failures at early stages of its occurrence allows one to undertake appropriate preventive measures to protect other components of the hydrostatic system. On the other hand, a lack of information about possible defects can lead to machine malfunctioning, environment contamination, or, in the worst case, serious hazards for the machine operator. Available research studies [L. 1] say that about 75% of all defects arise during the preparation of manufacturing, but identification at that

stage is negligible. The moment that defects may occur is the manufacturing process. More than 80% of defects are eliminated during the final inspection and operation; therefore, the costs of such defects are much higher than when the defect first appears. Additionally, a late identification of defects is difficult to fix. Therefore, it is important that preventive actions are taken already at the design stage. This approach can reduce costs due to “poor quality” and follows the E. Deming’s “principle of continuous improvement.” Thus, a lot of effort is put on innovative technologies and new solutions for design and manufacturing processes. Methods that allow for detection of defects and failures in pumps during operation are also investigated. One method is the quality improvement method in which a classical and matrix FMEA analysis can be distinguished as well as a classic quality improvement tool – the error diagram. Potential defects can be detected, predicted, and preventive measures planned by using qualitative

* Faculty of Mechanical Engineenring, Cracow University of Technology, Institute of Applied Informatics, al. Jana Pawła II 37, 31-864 Krakow, Poland.

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methods, and, if a defect appeare appropriate corrective actions can be applied. Those methods can be used at any stage of a product’s life cycle, from design to manufacture and operation.

THE PRINCIPlES OF QuAlITy IMPROvEMENT METHODS

FMEA analysis was developed in the United States in the 1940s as the MIL-P1629 “Procedures for Performing a Failure Mode, Effects, and Criticality Analysis” for military missions. Then, in the 1960s, it was used by NASA in the Apollo-Saturn program. The successful flight to the moon has made this method popular in other branches, including aerospace and automotive industries. In the automotive industry, FMEA analysis is mandatory in the QS-9000 standard. According to the IEC 812 standard, this method can also be used in testing of computer software and human activities. FMEA is currently used wherever the “zero defects” approach is required for product/process for the protection of the natural environment and human safety. Therefore,

FMEA analysis is used not only by manufacturers, but also by suppliers for which it is sometimes a condition of receiving contracts. FMEA analysis is consistent with Deming’s “PDCA” (Plan, Do, Check, Action) principle. It requires that each product/process is documented, supervised, analysed, and continuously improved. Therefore, the purpose of the FMEA analysis is continuous and systematic identification of potential product/process defects and then eliminating or minimizing the risks caused by these defects. Two types of FMEA analysis can be distinguished, depending on the way that defects are detected and presented. The first one is a classic method of risk assessment and evaluation of the Risk Priority Number RPN. The second type is the FMEA matrix analysis, which allows one to detect defects based on the product function. The relationships between the investigated element, its functions, and identified defects are determined. Figure 1 shows the algorithm of the FMEA matrix analysis [L. 2, 3].

FMEA analysis is often supported by tools of quality improvement. One of such is a diagram of errors called the Ishikawa diagram (from the name of

its creator) or cause-and-effect diagram. Due to the shape, the diagram is often recognized as the fishbone diagram. The error diagram is used for identifying and graphically presenting problem causes or effects of undertaken actions [L. 4]. It can be used at any branch of industry, from manufacturing to public administrations. The algorithm of the error diagram is shown in Fig. 2.

Fig. 1. Matrix FMEA analysisRys. 1. Macierzowa analiza FMEA

Fig. 2. The error diagramRys. 2. Sposób postępowania w diagramie błędów

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THE OBJECT OF ANAlySIS

The object of analysis is the hydraulic gear pump [L. 5] shown in Fig. 3.

Fig. 3. Pump PZ type [l. 5]Rys. 3. Pompa zębata typu PZ [L. 5]

The analysed pump consists of 1 – body, 2 – drive shaft with key, 3 – drive gear, 4 – driven gear, 5 – cover, 6 – bearing, 7, 10, 11 – sealing rings, 8 – shaft sealing ring, 9 – screw, 12 – spring washer, 13 – spring ring.

Gear pumps belong to displacement pumps, which convert the mechanical energy of the drive motor to the hydraulic energy of the working fluid. Hydraulic PZ gear pumps are constant displacement pumps with axial clearance compensation. In a gear pump, the drive gear mounted on the drive shaft drives a driven gear. Rotating gear wheels interact with each other forming chambers with variable volumes. This makes suction pressure, which takes the working fluid into these chambers. Then the oil is compressed and discharged at the outlet port. The leak tightness of the pump is provided by sealing rings. The body and cover are connected by means of bolts with spring washers. The spring ring secures the pump components mounted on the drive shaft against the axial movement.

MATRIx FMEA ANALYSIS

Matrix analysis of the gear pump was conducted in two stages. The first one was the decomposition of the pump and the selection of nine components that were used in further analysis. These were the body, cover, drive

shaft, spring ring, sealing ring on the shaft and sealing rings considered as one component, bearings, and drive and driven wheel gears. The analysis does not include bolts and washers, which were assumed to be unlikely defective or damaged. Taking into account above assumptions, the FMEA analysis was performed for eight pump components: body (c1), covers (c2), drive shaft (c3), drive gear wheel (c4), driven gear wheel (c5), sealing ring (c6), bearing (c7), and spring ring (c8). In the next step of analysis, functions they perform in the pump were assigned to the elements. Five functions in total were specified:• Secure (e1) for the body, spring ring,• Fix (e2) for the body, cover,• Stabilize (e3) for the body, bearings,• Transmit (e4) to the drive shaft, the drive and driven

wheel,• Prevent (e5) for the sealing ring.

The dependencies between the investigated elements (ci) and the implemented functions (en) are shown in Table 1. For each element of the matrix (EC), a value of 0 or 1 is set. If the function is not performed, value 0 is set, otherwise value 1 is inserted.

In the next step, according to literature review [L. 6, 7] and identified functions for the pump components, 25 potential defects have been identified. Those were then classified and assigned to the 10 categories presented in Table 2.

Table 1. Matrix of dependency function-elementTabela 1. Macierz zależności funkcja–element

Table 2. Similarity diagram of identified defectsTabela 2. Diagram podobieństwa zidentyfikowanych wad

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The relationships between the investigated elements (C) and potential defects (F) are shown in Table 3. The CF matrix was completed with the use of

defects for function e4, while the y-axis is probability of defects (f).

Bearings, driving shafts, and gearwheels are those components that are responsible for the proper operation and durability of the entire gear pump and have an influence on performance and reliability. However, according to literature [L. 8], about 60% of failures are caused by defects and damage of the wheel gears. Therefore, it is necessary to find the cause-and-effect relationships of defects occurring at the wheel gears in the pumps. For this purpose, the FMEA matrix analysis was complemented by a quality improvement tool – an error diagram.

ERROR DIAGRAM OF PuMP GEAR wHEELS

For the investigated gear pump, gear defects (i.e. rupture, fatigue, wear, and corrosion) were identified as the main problems that influence pump operation. Consequently, in the next step, the error diagram was created. Figure 5 presents an error diagram for the investigated problem, which is damage to gears.

Table 4. Matrix of dependency function-potential defects (EF)Tabela 4. Macierz zależności funkcja–potencjalna wada (EF)

Fig. 4. Graphical representation of resultsRys. 4. Graficzna prezentacja wyników

Table 3. Matrix of dependency of element-defectTabela 3. Macierz zależności element–wada

At the second phase of the matrix FMEA analysis, matrix EF was obtained by the multiplication of matrixes EC and CF. It presents the probability of defect (fj) for the analysed elements (ci) in view of realized functions

As indicated by the EF matrix, failures (i.e. fatigue, corrosion, cracking) and wear have the highest probability of occurrence for elements with transmitting function. These elements are the drive shaft, and the driving and driven gear wheel. Figure 4 presents a graphical representation of potential defects (f) for the function of transmit (e4). The x-axis shows potential

a binary system. Value 0 is inserted when the defect does not affect the element being evaluated, while value 1 is inserted when the relationship occurs.

(en) in the pump. The results of the EF matrix are presented in Table 4, where value 3 indicates the highest probability of defect occurrence for a given function.

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Fig. 5. Error diagram for gear wheelsRys. 5. Diagram błędów dla kół zębatych

The 6 main categories of causes which have an influence on this problem have been identified. These are the following: I. Assembly/disassembly, II. Exploitation, III. Cooperating elements, IV. Manufacturing, V. Working liquid, and VI. Design. For each category, the possible causes leading to gear defects are identified. More than 30 causes have been identified that have an influence on investigated problem. Identified errors, causes, and subcauses were assigned to defects with the highest probability of occurrence. Those defects were determined earlier in matrix FMEA analysis (EF diagram): cracking, fatigue, wear, and corrosion. Table 5 presents the defects and potential causes.

As indicated in the above table, the most frequently occurring causes of gear failures are in Group II: Operating, and in Group V: Working fluid. These are improper working conditions, vibration and contact pressure, poor fluid quality, and improper fluid. Therefore, appropriate measures should be taken to ensure that defects related to these causes do not occur or to minimize the risks of those defects. Table 6 presents examples of preventive and corrective measures for the investigated causes.

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SuMMARy

According to FMEA matrix analysis along with the error diagram, it was found that qualitative methods can be useful tools for the identification of potential defects in gear pumps. These methods allow one to identify the

REFERENCES

1. Hamrol A., Mantura W., Zarządzanie jakością. Teoria i praktyka, Wydawnictwo Naukowe PWN, 2006.2. Stone R.B., Tumer Y., VanWie M., The Function-Failure Design Method, Journal of Mechanical Design, 127(3),

2005, 397–407.3. Fabiś-Domagała J., FMEA analysis of hydraulic drives using indexed color mode, Czasopismo techniczne.

Mechanika, R. 108, z. 4-M/1, 2011, 121–128.4. Kiran D.R., Total Quality Management Key Concepts and Case Studies, published by Elsevier Inc., 2017.5. PZL-Wrocław, Technical description and operating conditions hydraulic gear pump, materiały informacyjne.6. Pietkiewicz P., Typical failures of gear pumps. Defects classification, Technical sciences, 2009.7. Parker Hannifin Manufacturing SAS, Hydraulic Pumps & Motors,Vane Troubleshooting Guide, Catalogue

HY29-0022/UK.8. Limmer J.D., Model-based condition index for tracking gear wear and fatigue damage, wear, vol. 241 (1), 2000,

26–32.

Table 6. Preventing and corrective measuresTabela 6. Działania prewencyjne i korygujące

components that have the highest probability of defect occurrence and identify the most important causes that have an influence on those defects. As a consequence, preventive measures can be taken early enough or an appropriate corrective plan can be prepared.

Table 5. Defects and its causesTabela 5. Wady i ich przyczyny