Maritime University of Szczecin
Akademia Morska w Szczecinie
2011, 27(99) z. 1 pp. 98–104 2011, 27(99) z. 1 s. 98–104
A comparative analysis of two cases of damage of hookloaders
Analiza porównawcza dwóch przypadków uszkodzeń
dźwigników hakowych kontenerowych
Jerzy Mizgała
Silesian University of Technology, Faculty of Organization and Management Institute of Production Engineering
Politechnika Śląska, Wydział Organizacji i Zarządzania, Instytut Inżynierii Produkcji
41-800 Zabrze, ul. Roosevelta 26, e-mail: Jerzy.Mizgala@polsl.pl; j.mizgala@poczta.onet.pl Key words: hookloader, an avalanche of damage, tacit knowledge, explicit knowledge
Abstract
The article describes two cases of damage to a hookloaders, which had different causes but led to the same result. Factors that may cause such damage and technical solutions in the hydraulic system which increase the extent of damage have been discussed.
Słowa kluczowe: dźwignik hakowy, lawina uszkodzeń, wiedza ukryta, wiedza jawna Abstrakt
W artykule opisano dwa przypadki uszkodzeń dźwignika hakowego kontenerowego, które miały różne przy-czyny, lecz doprowadziły do powstania takiego samego obrazu szkód. Wskazano, jakie czynniki mogą doprowadzić do ich powstania, a także, które rozwiązanie techniczne w układzie hydraulicznym powoduje powiększenie skali uszkodzeń.
Introduction
Hookloaders are widely applied machines for the carriage of containers. They are particularly well-known among companies engaged in scrap or waste paper collection, or municipal service teams dealing with garbage removal, but they are also commonly used by the army and Fire Brigade. The popularity of these machines is constantly growing, as their construction is relatively simple, and opera-tion does not require any special licences [1, 2]. The machine has been shown in figure 1.
The cycle of a hookloader’s work is as follows (Fig. 2): by means of the hydraulically moved hook “4”, mounted on the mobile front half-frame “2”, containers are hoisted and mounted on the chassis (mechanically or hydraulically by container locking mechanism “7”) during transport to the destination. Next they are unloaded in the same way. There is a possibility of work in the dumper truck regime (Fig. 3), owing to which a container to be unloaded
4 2 3 7 6 1 5
Fig. 1. Hookloader [3]; 1 – subframe, 2 – front half-frame, 3 – rear half-frame, 4 – hookarm, 5 – tipping cylinders, 6 – rear pulley, 7 – container locking
Rys. 1. Dźwignik hakowy kontenerowy [3]; 1 – podłużnice podstawy, 2 – półrama przednia, 3 – półrama tylna, 4 – ramię z hakiem, 5 – siłowniki podnoszenia półramy przedniej, 6 – rolki podporowe, 7 – hydrauliczny mechanizm blokady konte-nera
rests on the rigidly connected half-frames “2” and “3”. The tipping cylinders “5” are used to either turn the front half-frame in the process of loading and unloading, or to lift the connected half-frames in the dumper regime. The rear pulleys “6” are used to guide the container runners onto the frame side member during the loading of the device.
Specificity of the hookloader’s hydraulic system
All producers of hookloader apply a parallel connection in the hydraulic system of tipping cylin-ders, used for half-frame lifting (item 5 in Fig. 1). Figure 4 presents a fragment of the hydraulic dia-gram of a hookloader produced by PALFINGER, though the cases described in this article refer to machines produced by other manufacturers.
As shown in the drawing, both hydraulic rams are secured by one hydraulic lock. Therefore, ac-cording to classification presented among others in [4], the system has a series-parallel structure. This is related to the necessity of performing synchronic movements by both hydraulic rams in order to ob-tain appropriate values of forces on both piston rods at the same time.
However, since the working capacities of hy-draulic rams are connected with each other, the hydraulic oil forms one stream before the lock, and contaminations from one ram nearly immediately
get into the other one. Below have been described two cases of damage, which – despite having dif-ferent causes, produced the same result. The first damage occurred in one of the hydraulic rams, and then led to the same damage in the other one.
First case – error in the process of repair
The first case concerns a hookloader produced by MEILLER. The conducted investigations have revealed that after the pump was switched off, the Fig. 2. The cycle of a hookloader’s work [3]: the chassis rideto the container, fastening the hook to the container lift lugs, pulling the container onto the chassis, transport position (travelling with the container blocked on the frame)
Rys. 2. Cykl pracy dźwignika hakowego kontenerowego [3]: podjazd podwozia do kontenera, zaczepienie haka o ucho kontenera, wciągnięcie kontenera na podwozie, jazda transportowa z kontenerem zablokowanym na ramie
Fig. 3. A hookloader’s work in the dumper truck regime [3] Rys. 3. Praca dźwignika hakowego w reżimie wywrotki [3]
Fig. 4. A fragment of the hydraulic diagram of the connection of hydraulic rams for the front half-frame lifting [3]
Rys. 4. Fragment schematu hydraulicznego połączenia siłowni-ków podnoszenia półramy przedniej [3]
loader made some spontaneous working move-ments. The analysis of the machine hydraulic sys-tem (Fig. 5) has not pointed to any specific ele-ments of the hydraulic system. The only obvious thing is that such movements occur due to the pres-ence of residual pressure after the machine has been switched off. Therefore, the test had to be treated as empirical research, additionally containing “…. the researcher’s knowledge of a mathematical or physi-cal model ….” [6].
At work [7] such a case is described as the use of “tacit knowledge” of the company dealing with repair, as the service documentation does not usu-ally contain a description of particular cases, but merely the simplest and most common types of damage.
As indicated by the repair company’s experi-ence, the cause of the symptoms found in the ma-chine was the presence of solid parts in hydraulic oil, most frequently metal filings, which block the correct functioning of the distributor. As the hoist is equipped with two filters – a high-pressure filter behind the pump, and a return filter in the oil tank, this kind of damage may occur only when one of the filters has let the contamination in, or filings have appeared between the filters, inside the hy-draulic system in operation.
This, in turn, points to mechanical damage of one of the machine components, which is usually related to an event during the operation. The owner
of the machine, however, did not report any events that might have been a potential cause of the dam-age. He only said, that the sealing on tipping cylin-ders had been exchanged not long before. Using the hydraulic diagram, it was then necessary to create a possible “tree of damage” [8] and subject it to analysis, while analysing the results on an ongoing basis.
After dismantling the first servomotor, a Teflon ring was found missing on the piston (Fig. 6), which caused the piston damage (Fig. 7) due to friction against the internal surface of hydraulic rams, damage to the pipes of both hydraulic rams (Fig. 8) and damage to slide valves of both distribu-tor sections (Fig. 9).
Fig. 6. A dismantled piston. Visible lack of Teflon guide ring (author’s photograph)
Rys. 6. Tłok po demontażu. Widoczny brak prowadzącego pierścienia teflonowego (fot. autora)
Fig. 5. Hydraulic diagram of MEILLER’s hookloader [5]
Fig. 7. A MEILLER’s piston after the sealing disassembly. Visible damage caused by metal filings (author’s photograph) Rys. 7. Tłok dźwignika MEILLER po demontażu uszczelnień. Widoczne uszkodzenia spowodowane przez opiłki metalu (fot. autora)
Fig. 8 Damage to the internal surface of the left ram in a MEILLER hookloader (author’s photograph)
Rys. 8. Uszkodzenie gładzi wewnętrznej lewego siłownika dźwignika MEILLER (fot. autora)
Fig. 9. A damaged section of distributor of a MEILLER hook-loader (author’s photograph)
Rys. 9. Uszkodzona sekcja rozdzielacza dźwignika MEILLER (fot. autora)
“An avalanche of damage” (according to the term proposed in [9]) did not finish with that: fol-lowing the interview with the machine owner’s employees, it turned out that when the first
symp-toms of incorrect work were noticed, the inserts were replaced in the return filter on the tank in which metal filings had been detected. However, the system was not rinsed, which caused the filings to get into the oil tank, and next to the pump. As a result, also the pump was damaged (Fig. 10).
Fig. 10. A needle bearing and shaft of the pump in a MEILLER hoist damaged by metal filings from the hydraulic rams (au-thor’s photograph)
Rys. 10. Łożysko igiełkowe i wał pompy dźwignika MEIL-LER, uszkodzone przez opiłki metalu z siłowników (fot. au-tora)
The exact course of “investigation” on the causes of damage and a complete description of damage have been presented in [10].
The following elements had to be replaced as part of the machine’s repair:
both tipping cylinders for the front half-frame lifting,
pump,
both distributor sections, both inserts of oil filters.
Moreover, before the installation of new ele-ments, the whole system had to be rinsed, which additionally increased the repair costs.
Second case – error in operation
In the second case a hookloader produced by the Dutch company TECHNAMICS was reported for repair. The cause of repair was damage to the hy-draulic lock supporting both tipping cylinders of the front half-frame – it was impossible to hold a con-tainer in a not fully lowered position when working in the dumper truck regime, as the container kept falling on its own.
The employees of the repair company, however, noticed damage to the hoist hook (Fig. 11 A) – its part with a weight closing the throat had broken off when lifting the container (Fig. 11 B – the after-repair condition), and a crack of the frame near the fastening of hydraulic rams for the front half-frame lifting (Fig. 12).
A)
B)
Fig. 11. The hook of a TECHNAMICS hoist. A) – before repair, B) after repair (author’s photograph)
Rys. 11. Hak dźwignika TECHNAMICS. A) – stan przed naprawą, B) stan po naprawie (fot. autora)
Fig. 12. A fracture of the frame in a TECHNAMICS hoist (author’s photograph)
Rys. 12. Pęknięcie ramy dźwignika TECHNAMICS (fot. au-tora)
After the lock’s dismantling, fragments of metal filings were found and the situation became clear: during the loading of the container, the chassis must have had an incorrect position in relation to the container, which most probably caused the
con-tainer to fall off the guide rollers, led to the de-tachment of the hook part with an interlock and resulted in frame cracks.
Unfortunately, the force of the resulting impact also caused a temporary change of the piston’s po-sition in relation to the servomotor tube’s surface (in Fig. 12 also the piston rod’s buckling is visible), which led to the “milling” of surfaces by the piston edge when subsequent working movements were being performed. The disassembly confirmed this diagnosis: both hydraulic rams had their internal surface damaged (the result of their parallel connec-tion) and both pistons together with their sealing were seriously damaged as well (Fig. 13 and 14).
Fig. 13. The piston of a hoist produced by TECHNAMICS. Visible damage to the piston and sealing caused by metal filings (author’s photograph)
Rys. 13. Tłok dźwignika TECHNAMICS. Widoczne uszko-dzenia tłoka i uszczelnień spowodowane przez opiłki metalu (fot. autora)
Fig. 14. Damage to the internal surface of a tipping cylinder produced by TECHNAMICS (author’s photograph)
Rys. 14. Uszkodzenia gładzi wewnętrznej siłownika TECH-NAMICS (fot. autora)
In this case, the owner might have talked about “a cloud with a silver lining”, as the hoist reached the repair workshop just after its improper work had been discovered, and the operator had not tried to “cover up” the traces of the accident; there was
no further damage, the divider and other hydraulic system elements were not affected. Admittedly, the filings did damage both hydraulic rams, but they stopped on the lock and did not get to the tank. The repair involved replacement of both servomotor pipes and pistons with sealing, the straightening of one piston rod and thorough rinsing of the hydrau-lic system.
Discussion of both cases
Both cases of damage were undoubtedly caused by errors related to the machine use: in the first case an error was made during the repair, while in the second – during the operation. The result in both cases was identical and was due to the use of an identical constructional solution in the hydraulic system: a parallel connection of both hydraulic rams behind the common hydraulic lock.
Such a construction enables metal filings to penetrate into both hydraulic rams at the same time, which results in their instantaneous damage (it is enough to make a few working movements).
It is obvious that the constructors assumed the machine would be operated in a proper way and the probability of such damage was very low. How-ever, its “picture” related to “an avalanche of dam-age” seems to suggest that changes to the hydraulic system construction should be considered.
A question might be asked whether producers of HOOKLOADER are interested in the introduction of changes which would limit the size of losses in the case of damage to one servomotor.
The introduction of changes is and, at the same time, is not simple from the technical point of view. The author knows constructions other than the ones mentioned in this article, and all of them are based on the same solution. The necessity of the ideal synchronization of both hydraulic rams’ move-ments simply forces their parallel connection, and it is this connection that causes damage to both of them.
Of course, two hydraulic locks could be applied, but this would involve a necessity of their continu-ous monitoring during operation so as to check the synchronization of opening. Additionally, the front half-frame with a hook is characterized by rela-tively low torsional rigidity. For this reason, dam-age to one of the locks, or its non-synchronous (in relation to the other) opening would certainly lead to the frame’s twisting, which would probably en-tail higher costs of the effects’ removal. In this case, the machine would also be threatened with a loss of stability due to possible displacement of the container contents, as it has to be remembered that a container usually holds ca 18 tons of cargo.
It seems that it is a good idea to consider an application of the system which would signal the appearance of steel filings in the hydraulic oil in front of the lock and, at the same time, would allow only one working movement: placing the machine in the transport position with an order of its imme-diate delivery to the repair workshop. Such a solu-tion would be relatively simple: modern hookload-ers are equipped with simple computer drivhookload-ers, which receive a signal on the mutual position of half-frames from inductive proximity sensors and block the possibility of performing movements that are improper from the point of view of the ma-chine’s configuration.
Therefore, connecting a suitable sensor to the “on-board” computer would be enough to solve the problem.
The author, however, has a right to suppose that nobody would be interested in the introduction of such additional protection. For producers it is not an attractive solution, because one servomotor sold as a replacement part costs a few thousand euros (in the case of TECHNAMICS – 8000.00 net), and users would be forced to take care of the hydraulic oil purity, i.e. they would be forced to change it together with filtrating elements on a regular basis (it also costs). Additionally, there might be “false alarms” blocking the machine’s operation, which would probably result in the operators blocking the machines on their own.
Conclusions
The presented study confirms that one type of damage to the same machine may have a few dif-ferent causes. In the process of their occurrence analysis a big role is played by the “tacit know-ledge” of the analyzing person/persons, as the user in many cases does not know or is not able to quote the circumstances leading to damage.
At the same time, the presented material is an example of turning “tacit knowledge” before publi-cation) into “explicit knowledge” (after publica-tion).
The analysis of the reasons for particular kinds of damage allows finding ways to avoid them in the future. However, the practical application of im-provements depends on many factors. The producer could introduce them any moment, but might not be interested in such a solution due to economic con-siderations, such as the wish to sell spare parts. The user, on the other hand, does not necessarily want to buy equipment which is technically more ad-vanced, as this would “force” him to improve “the culture of use”, which is also related to financial expenditure.
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Recenzent: dr hab. inż. Zbigniew Matuszak, prof. AM Akademia Morska w Szczecinie
