Edited by Mary C. Gannon
senior associate editor
Avoiding and eliminating leaks in the car and car supplier industries
Finding the origins of leaks is the just the first step in permanently eliminating them. In many cases, cutting ring connections are not properly assembled, which eventually leads to slow leaks and safety issues. A tube connection system, such as the ES-4 or the new VOSSFormSQR, can achieve or exceed the mechanical characteristics of cutting ring connections and help the assembly meet required specifications. Tube forming systems simplifies installation, so even unpracticed fitters can easily use it in the field, and ensure their safety as well as that of the system.
Maintenance in the auto industry
If one looks at current mechanical processing in the car industry and their suppliers, then it is hard to believe the old picture of dirty green installations in dark work halls, oil impregnated air or the light oil coated floors and machine parts. Today we see white painted machines on white floors in brightly lit, almost sterile looking production areas by which everywhere keywords such as "TPM","5S" or "orderliness and cleanliness" appear with striking relevance.
Here the question can be posed, how much is real maintenance service and how much is pure cosmetic?
Looking behind the scenes, this means oil tanks and oil disposal costs, the need for fresh oil as well as the applications for the cleaning of installations and work hall floors, presents us with a different picture. Machines and installations are currently designed for an application period of 10 to 15 years. The guarantee of the machine manufacturer and, therefore, the minimum calculated life period of machine components, is for a period of between 2 and 5 years. Existing systems are frequently converted or extended, whereby today the technical changes are mostly carried out by external services and hardly by in house staff. The mechanical and hydraulic stress of individual components and, therefore, the complete system can hardly be compared to that of former installations. This can soon be recognized when the size and performance capability of hydraulic components of old installations are compared with new, modern installations. Operating pressures increase, smaller and more compact valves with higher performance, increases the total hydraulic load of the installation. The work load of installations differ a great deal and can at peak times be operated in a 7 day mode, whereby, preventative maintenance is hardly possible with this work load. In general, the maintenance resources were reduced in the past years or with increasing machines parks, held constant so that maintenance is carried out more and more as coordinated actions incorporating outside service providers for the actual work. Within the scope of TPM, simple maintenance work is carried out by machine operators — in practice, this does not always lead to the required success and the required sustainability. In addition, to this there is a considerable cost pressure with the procurement of new installations, whereby a deviation from the regulations for operating machines as a cost increase is accepted. With the acceptance of new installations, the machine regulations are hardly checked because during the guarantee period the machine manufacturer is obliged to take over a large portion of the maintenance work.
Summing up, this means that installations with a high application level pose special challenges on maintenance. The required long life duration of the whole unit, with low wear reserves of the installation components, bring problems to the forefront. Furthermore, the know-how of utilized external personnel reduces which is also coupled with the ever-present massive production pressure.
Leaks in hydraulic installations
Leaks in hydraulic systems of production machines represent only a relatively small portion, however, these are also connected with consequential costs. In addition, to the visual impression, whereby, less oil can be seen on the floor but, if anything, the oil collection tanks are full as well as oil contamination on the actual hydraulic unit, there is here an increased danger of an accident. Waste and costs are produced due to oil disposal and filling of new oil.
Here maintenance capacity is used which could be applied more profitably elsewhere. If the replenished new oil is contaminated with solid particles then this can cause pump failure or valve blockage or complete failure of the installation. The costs for floor cleaning considerably increase when the cleaning water, now contaminated with oil, is to be disposed as special waste or must even be processed before disposal via an emission separation installation.
One can ask where the causes of these leaks originate and how these leaks are to be permanently avoided.
In the past, the cutting ring was implemented as the standard system precisely in tool machines. With the cutting ring connection a metal ring cuts into the tube and owing to this has a holding and sealing function. For a few years now, this cutting ring is fitted with an additional elastomer sealing as a standard which reduces the risk of leaks even further. The advantages with this cutting ring system is that it is economically priced, easy to apply and available worldwide. With respect to the mechanical characteristics, such as pull-out strength, bending strength and pressure impulse stability, modern cutting ring connections used today achieve a level which is absolutely sufficient for most applications.
However, there is a challenge posed by the cutting ring connection with initial assembly and especially with maintenance and repair work. In the field of assembly of an average machine engineering company many thousands of cutting ring connections can be produced per year. If one looks here at the possible failure potential, then the tube tolerances as well as the layer thickness of the tube layers must firstly be regarded. When the tube tolerances (tube ovality) or the largest maximum layer thicknesses are exceeded then assembly failures are already just a matter of time. If machine-controlled pre-assembly is carried out, the tolerances of the pre-assembly tools (wear) and the adjustment values of the pre-assembly machine are to be continually checked. That is how things stand with respect to theory.
However, in practice, it is frequently the case that the specifications according to DIN 3859T 2 and the assembly specifications of the manufacturer cannot be met. This then inevitably leads to a percentage of cutting ring connections not being correctly assembled. With incorrect assembly the first hundred to thousand operating hours seem as though everything is sealed, but then at some time later the leaks slowly start. In turn, incorrect assembly can cause a fast burst influenced by mechanical stresses in subsequent operation. The tolerance fields of further connection components must be especially taken into consideration here. These can, with a marginal construction in connection with seating effects, lead to the fact that long-term stability and therefore permanent leak safety is no longer guaranteed. The frequently used soft sealing systems do offer evident improvement, however elastomer seals, especially under dynamic stress, are not designed for machine life and therefore count as wear parts. This in turn means that leakage is retarded in time and is not avoidable. In a worse case scenario, incorrect assembly is covered-up by soft sealing and causes leakage a few years later. It is not without reason that some maintenance sectors in the car industry use metallic sealed cutting ring connections so that incorrect assembly can be directly detected immediately after the machine is started.
If one makes a deduction, then the cutting ring connection represents the correct alternative from the constructive side and on the basis of the normal mechanical and hydraulic stresses which exist. The cutting ring connection is to be put in question owing to long machine operation times, the marginal maintenance resources, the high technical demands on assembly made according to standards, which above all overburdens the practiced maintenance technician.
A tube connection system was sought out for achieving or exceeding the mechanical characteristics of a cutting ring connection and also supporting the fitter optimally with respect to an assembly which meets specifications. In addition to this, it should be designed so that the unpractised fitter is able to correctly use it in the field of servicing. As a solution, tube forming systems have emerged which imitate the so-called "conical connection" in the contour of the formed tube. The advantage of this "conical connection" compared to the cutting ring connection is that with final assembly it is only the friction forces of the tube connection which must be overcome as well as a corresponding pre-tension in the system after the end stop in the taper seating. A component "seat" can then be excluded. Additional safety can be ensured here by using an extra o-ring as elastomer sealing. The complete pre-assembly is then made mechanically, controlled by a forming process of the tube by which the tube is calibrated thus compensating the tolerances. In this way, all possible sources of failure for cutting ring assembly are therefore excluded. The break resistance, bending resistance and the pressure impulse stability are, with this system, on a comparable level with the unweakened tube because the tube is not damaged compared to the cutting ring connection. Structure changes occurring in the cold-worked hardening phase of the tube, and therefore the weaknesses of the tube, can be compensated by a clamping ring inserted into the nut in which the bending stresses which occur are removed from this point into the tube. From a purely constructive point of view this system is over-dimensioned for normal applications in tool machines but it is exactly these over-dimensions which guarantee leak safety in a normal operation period of 10 to 15 years. The disadvantages of this system are not concealed. A hydraulic press with a press force of about 100 t is necessary for tube forming, whereby the whole range of normal tube diameters and wall thicknesses in the hydraulics can be processed. This hydraulic press (forming equipment) is only economically practical at a corresponding capacity so that it is mainly feasible for the manufacturer when it can be utilized with large scale inspection work and modification work. For daily maintenance requirements these machines are not practical. It is for this reason the so-called flared coupling BV-10 has emerged in the field of maintenance, especially, also as a repair solution for defective cutting ring connections. With these flared couplings a hardened flared cone is driven into the tube with a small hand pump. In doing this the tube is expanded by 10°. A saw-tooth profile holds the tube and seals it. The conical seal mentioned above with integrated o-ring is on the connection side of the tube thus guaranteeing very simple and reliable assembly. Even here the tube is not weakened because the expansion of the tube is within the yield point. Structure changes are therefore effectively excluded. The cutting ring connection is therefore replaced by a connection with much higher stability characteristics whereby correct assembly is ensured by the conical seal principle. The extension of the tube using the flared cone enables the repair of defective cutting ring connections. The defective tube area can be sawn off whereby the previous total length of the tube can be obtained again via the long construction of the flared cone.
Therefore, at least in the area of leak prevention on tube connections, a long-lasting anti-leak capability is guaranteed during the life duration of the machine when using the tube forming system in the original equipment in combination with the flared coupling as a repair solution.
Special paragraph – Audi AG
Already in 2002, Audi AG decided to use cutting ring connections in metal machining only with specific approval. At the beginning, soft sealing flared adapters were used as an alternative to cutting ring connections. However, due to the fact that these present a very costly solution and are only supplied by few connection manufacturers, trials were made with form connections from various manufacturers from 2002. In 2004 these form connections were finally prescribed at Audi and exclusively used in the new part/extension of the car plant at Gyr. Decisive for this were the high demands on cleanliness on the one side and on the other side the excellent anti-leak capability and a very high degree of reliability against breakages. In the four years of operation no known negative occurrences have happened so that tube forming systems are now specified as standard at Audi.
From 2007 this regulation was extended to all Power Train Plants at VW.
Special paragraph – Grob
Due to the trend and regulations in the market, especially in the car and car supplier industries with respect to tube forming systems and the resulting very good experiences in their own production, the Grob company decided in 2007 to use this new type of system as an option in the processing centres of the company. For Grob this was linked to an individual investment in the corresponding forming equipment. Today Grob can offer this higher quality system as an option to customers with respect to breakage resistance and a freedom from leaks.