The weir at Grave was in need of accelerated large-scale maintenance. The operation originally scheduled for 2028 was brought forward by the Department of Public Works after cracks appeared in the yokes. These are the supports of the gates that perform the actual weir function. On behalf of contractor combination Mourik-SWARCO (CMD), Mourik Infra B.V. awarded the manufacture of the total of 22 yokes to Dijkstaal B.V. of Maassluis. An order with large dimensions and very small tolerances.
The yokes of the weir at Grave are a fairly faithful copy of the old yokes of this national monument. "A yoke is 16 meters long, 2.8 meters wide, weighs about 12 tons and is made in accordance with the NEN-EN-1090 EXC. 3 and Guidelines for the Design of Art Works (ROK) of the Dutch Department of Public Works," says Mark Pilon, Head of Business Office at Dijkstaal. "When we were approached late last year for this work, the main questions were whether we could meet these requirements and whether we could deliver on time. Capacity was not the problem, it was how to get the right materials on time. The requirements were such that we had to go directly to the rolling mill to get the right HEB sections on time. After a short inventory, we were able to accept the order and received Mourik's confidence. There was great time pressure, so we immediately started working on the design together."
A yoke is a frame assembled from two large HEB 900 profiles and various sizes of tubes. Pilon: "It was designed by the client. We made adjustments with our input, such as welding details for better feasibility. Then the design was converted to production drawings, the welding details were worked out and production was set up. With 22 12-ton yokes and 4.5-ton profiles, it was also a logistical challenge. To minimize handling in production, a routing was made along the different operations: pre-processing, assembling, welding and assembly. First, the HEB 900 profiles and intermediate tubes were loosely welded with plates and parts as much as possible. Then these still manageable components were assembled in a special mold and completely welded and finished. Then they were driven piece by piece, 22 times, by road to GSB (the blasting and conservation company with which Mourik works intensively)."
The most critical part of the yokes is the pivot point. "That's where the whole construction of yoke, sliders and technology hangs," Pilon said. "One millimeter deviation from the axle position means a substantial misalignment at the bottom, which means the yoke no longer enters the lugs. For the axle through the pivots, therefore, an H7 fit applied, i.e., an accuracy of one thousandth. To achieve this, each frame was first fully digitally measured and the exact position of the shaft determined. We then hired a party to boring the holes we had made at 190 mm to the correct diameter of 220 mm and the correct fit at our location."
"You also want to maintain that accuracy when a yoke goes to the coater," Pilon continues. "You have to prepare that well, too. The coating company had indicated, based on our delivery specifications, that handling the yokes would be difficult at its location. So we transported the yokes on the back of a truck, which was able to lower its air springs upon arrival. This immediately put the yoke on supports. We also made a special tool that was inserted through the shaft, where the bushings were also protected by plastic rings." Engineering and production went very quickly and smoothly. "At Christmas we were still calculating, in May the first yoke went on transport and in early October the last one. The result of a fine piece of (collaborative) work."