[standfirst] The newly-formed dedicated Reconditioning Service at Sheffield heavy engineering company, DavyMarkham, has overseen the refurbishment and technical upgrading of a Davy 6000 tonne forging press for Valdunes of Dunkerque, one of the world’s leading manufacturers of railway wheels and axles.
Valdunes Forging Press was originally supplied by the Sheffield works in 1956 and has since produced literally millions of forged wheels, at a rate of around one wheel every minute, eventually necessitating the most extensive overhaul in its history. Representing perhaps ‘the ultimate 50-year service’, the DavyMarkham refurbishment package totalled £0.5 million and returned the machine to better than original condition, complete with new columns, bearings and high-tech tensioners, delivering very substantial savings over new capital equipment costs and minimising operational downtime.
Extended service life
DavyMarkham first carried out an extensive engineering study, involving advanced FEA (finite element analysis) tools, to analyse operational problems with the press and recommend design changes for promoting extended service life. It then designed and manufactured 8 new bearings and matched bearing supports, remachined the 70 tonne top and 85 tonne bottom press tables on a fast-track basis, and managed the production of four new 700mm dia x 11m long steel columns, as well as providing technical specifications for the manufacture of 8 custom-made Superbolt tensioners. It also provided technical and inspection support during the 3-week re-build in Dunkerque, with extremely precise dimensional accuracies achieved, and will return during the Christmas plant shutdown, to ensure the press has bedded in without any changes in geometry.
The Valdunes press refurbishment was an exercise in pan-European cooperation with the press column forgings made in Italy, by Lucchini Sidermeccanica, the moving table re-machined locally in Dunkerque, the on-site disassembly and rebuild carried out by French heavy mechanical installation experts, Ponticelli Frères, and the Superbolt tensioners produced by Nixon Industrial of Chesterfield, with DavyMarkham inspecting all the key processes and organising the logistics. All components were returned to France on schedule and Valdunes has now asked DavyMarkham to carry out design work on the hydraulic system, to further refine operation of the press.
Part of a global manufacturing group, Valdunes specialises in the design and manufacture of railway running gear and its products are in service in over 60 countries throughout the world. Its wheel and axle technology is focused on improved performance and operational profitability for customers and the company is the sole supplier of wheelsets for TGV high speed trains, holders of the world rail speed record at 574.8 km/h.
The Davy forging press, itself weighing 600t and with a force rating of 6000t, has been in service since 1956 and consists of an upper and lower table, each weighing 70t, which are held together and tensioned by four forged steel columns, with extremely large nuts securing them at each end. The lower table carries the bottom workpiece die, holding red hot metal blanks, and a hydraulic ram fixed to the upper table drives down the traversing crosshead carrying the upper die and presses the blank; this produces an almost finished locomotive wheel, which then moves to another machine that punches out the central axle hole. Recently, fatigue failures of the forged steel columns had resulted in considerable downtime and temporary replacement columns were also starting to develop significant fatigue cracks, so Valdunes turned to DavyMarkham for an analysis of the situation and help in returning the press to full working condition.
The principal load on the lower table arises from the 6000t thrust of the press, with vertical restraint provided by axial tensions in the columns. If correctly assembled, the tables and columns act as one, contributing to inherent stiffness, but it was suspected that a partial preload, or insufficient tightening, on the column nuts may have subjected the table to greater bending moment than intended.
Traditionally, tensioning was achieved employing thermal tightening, whereby stud heaters induce thermal growth in the columns, then the nuts are tightened and preload is achieved as the metal cools. However, it is difficult to achieve accurate results this way, due to the large thermal masses involved and problems of achieving consistently high temperatures. It is likely, therefore, that the columns were not preloaded to a sufficient level, either at the initial install or during a substantial rebuild in the early 1990s, although given the age of the machine, thermal tightening must have been largely satisfactory.
Nevertheless, DavyMarkham recommended that the most effective means of achieving and retaining accurate preloads on joints of this scale was using modern Superbolt technology, where no heat or hydraulic pressure is required. Superbolt tensioners utilise a series of small jackbolts threaded through the main body of the nut, to create large clamping forces, and can be torqued up using hand tools. Very high and accurate preloads can be exerted and, as there is no relaxation over time, the bolted connection will not loosen even under the most arduous conditions. DavyMarkham thus calculated the preload required for the Valdunes press and subcontracted Nixon Industrial to tailor-make eight Superbolts, each weighing 1¼ tonnes and with a thread diameter of 635mm.
Further FEA studies were carried out by DavyMarkham, using the latest Lusas 3D engineering analysis software, to identify which areas of the press tables and other forgings were exposed to stress and should be subjected to non-destructive testing. The design of the press columns was subsequently revised, with a new bolted thread pattern, to prevent future fatigue failure and new polymer bearings and cast iron housing supports were produced to accommodate the re-designed columns. The top and bottom tables were also remachined to suit the new components and eliminate existing fatigue cracks, a time-critical task as the bottom table was the first item due to be reinstalled on site. Ponticelli managed the subsequent re-assembly in Dunkerque, just five weeks after it had first dismantled the press, and the entire reinstall took just three weeks, with Nixon fitting the Superbolts in a matter of hours.
Hydraulics and Pneumatics Magazine Jan/Feb 2008