For engineers at CERN (CERN, the European laboratory for particle physics), understanding the co-efficient of friction of a given friction material (including Vesconite Hilube low-friction material, which is being introduced into CERN’s new tooling system) is of utmost importance.
This co-efficient of friction number is an indication of how easily parts will slide, roll or rest on the given material, with a lower co-efficient of friction indicating that there is less friction. Low stick-slip and a more smooth motion is desired.
And this is exactly what CERN project engineer Mike Struik demonstrated this week while testing how the components for the next phase of the Large Hadron Collider (LHC) interface.
He has had considerable experience at the LHC with the various friction materials that have historically been used to assemble the more than 2,000 superconducting magnets at CERN.
Some 1,232 dipole magnets have been assembled in the facility straddling the Swiss-French border in which he works.
Typically weighing 29 tons each, these magnets have to be slid into the 15-meter-long tubes (vacuum vessels) in which they are housed using a system of winches, steel rails and sliding material before being installed in the accelerator.
The sliding material used historically is a 3- to 4-millimetre-thick Teflon-filled bronze wear pad that had to be glued to the base material.
“We don’t like the old material,” says Struik of the old engineering tooling system that has been used since the year 2000.
“If the glue on the sliding material comes off, we have a 29-ton magnet that we can’t mount anymore,” he elaborates.
So, when LHC upgrades were proposed to increase the amount of data that could be collected, using more sophisticated superconducting magnets that cool to 1.9 ºKelvin, this was the ideal opportunity to improve on the tooling system that would also have to be upgraded.
Struik specified that the manufacturer of the tooling use a sliding material that could be fitted into a recess on the housing so as to avoid needing to glue the wear pad to the base. He specified winching speeds of 50 mm/minute and 100 mm/minute. He also specified that the sliding material not grip the high-quality surface of the steel rails on which it was placed and have a low co-efficient of friction with a high yield load strength.
The design Applus+ Laboratories, a worldwide leader in the testing, inspection and certification sector, developed in response included an assembly table; an adjustable table that can be configured to support different-sized vacuum vessels; a synchronised lifting system to lift and hold the magnets in place; and winches to pull the magnets in and out of the vessel.
The design also included temporary extension rails made of steel inside each tube and three sliders, with each slider having a 50-centimetre-long block of Vesconite Hilube low-friction sliding material on each side to safely and efficiently guide the magnets in and out.
Vesconite Hilube was also positioned to guide the magnet laterally and keep the magnet in the middle of the tube. Once completed, Applus performed a functional test with a lighter magnet, simulating what could be expected at CERN, which was still, at that stage, to receive delivery of the 24-ton cryomagnets that are be employed at CERN.
The functional test proved successful and the tooling system was shown to be able to manoeuvre a magnet into a vacuum vessel and keep the magnet in the correct position.
CERN decided that it would perform the real test when the actual magnets that would be used were delivered, and it was able to do so in May 2022.
“We had to try and fit this 24 ton magnet inside another tube and then we had to lift it, we had to align it and we had to drop it,” says Struik.
“Everything went well and we are super happy with it,” he notes of the involved test that included the interface between all the existing and newly-introduced components.
As a conclusion to the test, the Vesconite Hilube pads were removed and tested. No wear was detected.
“The friction co-efficient was also lower than we expected it to be,” Struik notes of the smoothness with which the magnet was manoeuvred into the vessel using Vesconite Hilube wear materials.
With the alignment and equipment tests all completed, the cold tests of the assembled cryomagnet are expected to start in June.
These will demonstrate whether the magnet is able to concentrate a particle beam and that the new upgrade involving an additional 37 more-sophisticated cryomagnets at the LHC will be a welcome addition to the 27-km-long accelerator that is part of the largest particle physics laboratory in the world.