The use of the FMEA method for the evaluation of failures in crawler cranes

The use and operation of crawler cranes in the Czech Republic is dependent on the availability of several major producers (Liebherr, Caterpillar, Volvo, Sennebogen, etc.) and Czech producers (Unex Uničov with its successor ND LOR Uničov, Tatra and others). The authors of this article explain the problems of operation, repairs and risks of failures in specific types of crawler cranes supplied by Liebherr during their operation in the steelmaking company Třinecké železárny, a.s. and Trilet Ground Services, a.s., the company specialized in renting machines that are used for foundations of buildings. The article proceeds from the evaluation of the operational data of both delivered and repaired machines and the data obtained from the repair company needed for the commissioning of the serviced machine, as well as the frequency of repairs during the specified hours of work within the course of operation of the machines. The processed data serves as a basis for the evaluation of risk of possible failures when using machines. Risks are evaluated using the FMEA method.


Introduction
The range of use of crawler cranes in practise is quite extensive. Their performance and scope of work cover many areas of human activities. The machines are often used in various and very poor external conditions; for instance in the Czech Republic they are used in Třinecké železárny under extreme conditions (high temperatures, great dustiness, cyclic operation in foundries, operation in high temperatures). The work of crawler cranes consists in breaking a solidified slag in a large tank in order for the slag to be reused it in the foundry process. In construction industry machines are used for the foundation of buildings (ramming the piles into the ground and thus anchoring the foundations). It means that external climatic conditions (heat, cold, rain, snow) have a great impact on the operation of the machinery. However, the machines are only operated seasonally from spring to autumn. In winter constructions are not initiated due to climatic conditions in Central Europe.
The design of crawler cranes is dealt with in a work [1] while damage to wire ropes, during operation of machines, is solved in the works by authors [2,3]. The operation of wire ropes and their failures are addressed in [4]. In particular, the focus of this article is on the collection and evaluation of data from the operation of machinery using the FMEA risk analysis method for detecting the failures in individual units, and the subsequent evaluation of the highest probability of failure with a proposal for its elimination. *Corresponding author: strohmandl@utb.cz

Operation of machinery
Třinecké železárny is a Czech steel works with the largest domestic capital that produces the major share of steel in the Czech Republic. Two crawler cranes are permanently used there to ensure the continuous running of the iron foundry for the breaking of deposited slag in the steel tank in order for the slag to be reused in the next foundry process. The machines are operated in a very challenging environment; the temperature is very high in the foundry (in excess of 50° C, the humidity of the air is also high due to the cooling of slag with water and there is also a lot of dust, which affects the operation of the machinery). The machines are used in a three-shift operation, however, their operation is irregular based on demand, which is influenced by the tapping of the furnace and the supply of slag, and therefore, the machines are not operated for the whole 8 hour shift. Trilet Ground Services, a.s. was founded 21 years ago in Prague. The company's purpose of business is rental and leasing of construction machinery and equipment, information technology activities, rental and leasing of products for personal use, especially for households, and it is also specialized in six other areas, for example foundation of buildings, bridges and hydraulic structures. The machinery is operated outdoors for about 8 months of the year; it is not operated during wintertime because of frost and snow. The machinery is used in a two-shift operation with approximately 2,000 hours of work, which means that the machines need to be serviced by an external company four times a year.
Within the evaluation of operation, it is possible to monitor the reliability of operation and to evaluate the frequency of failures and dependence of failures on the total number of hours of work. The graph shows that there is no linear dependence against hours of work and defects occurred. After a running-in period between 3 rd and 4 th failures (Series 3) the increase in hours of work is very high (failure-free operation); with increasing age and wear of the machinery the time of occurrence of the next failure shortens even if the technical conditions of operation and regular servicing of the machinery is observed in accordance with the technical regulations. The evaluation of operation of the machinery (Series 2) shows that among the first and the second failure and among the second and third failure the numbers of hours of work are almost identical [6].

Risk assessment using the FMEA method
The Failure and Mode Effect Analysis was used to assess the risks. This method is suitable for assessing the different stages of the life cycle of the system / product and for various cases, each having a different scope of focus [7][8][9][10]. In the said case, the FMEA method was used in the existing process to ensure a reduction of failures in the analysed system, taking into consideration the influence of the environment and location of the existing process.
The following tables ( Table 1, Table 2 and Table 3) define the general assessment criteria for the significance, occurrence and possibility of detecting failures. Simultaneously, the level of risk of analysed failures are assessed in Tab. 4, the FMEA form. Using the FMEA form the failures, which actually emerged during operation, have been analysed, and they will have to be addressed in the future in order to avoid their recurrence. Crawler cranes, which are the subject of this work, operate in extreme heat and dusty conditions. Their operation is not continuous, which is the reason for the occurrence of typical failures resulting from the working environment as well as typical failures related to diesel engines. or a machine or its assembly may endanger the operator with a warning 9 Very high The product / system is inoperative (loss of basic properties).
or 100 % of the product must be scrapped or remade and repaired with a repair time of more than one hour.

8
High The product / system is operating but its performance is reduced, the customer is unsatisfied.
or the product needs to be segregated and a part of it (less than 100 %) is scrapped or remade and repaired with a repair time of half an hour to one hour.

7
Medium The product / system is functional but the operating comfort is reduced. The customer is unsatisfied.
or a part is remade and repaired in less than half an hour without the need to segregate the product (less than 100 % is to be scrapped).

Low
The machine operates at a partially reduced performance and comfort.
or 100 % of the product has to be remade or repaired  Table 6. The probability of occurrence of failure according. Source: own processed data [7,8] PROBABILITY of failure Frequency of failure Grade Very high: Failure is almost inevitable >= 100 out of 1,000 pcs 10 Very high: Failure is almost inevitable 50 out of 1,000 pcs 9 High: Reoccurring failures 20 out of 1,000 pcs 8 High

Suggestions and recommendations
On the basis of the results of the FMEA, the following results can be evaluated: -the highest probability of the failure during operation of the machinery concerns the electrical partsdamage to the wiring, short-circuit and wear of the material; -another area of possible damage is the hydraulic system of the machines; -with increasing environmental protection from the perspective of emissions the risk of frequent failures in the fuel and exhaust system of the engine also increase. In order to eliminate defects in the operation of the machinery and to prevent the occurrence of failures, it is necessary to carry out regular maintenance in specified operating cycles; also, operators of the machines should be trained in the basic knowledge of the machine structure. In addition, checking the condition of the machine upon completion of work should be included in the operators' work duties; any minor defects should be reported and eliminated by means of maintenance to prevent damage in the event of an accident or machine breakdown. This work is a part of the project IGA/FLKŘ/2017/004 and this work is a part of these project "Project Center for the Excellence of Population Protection", 2018/03.