Repair of the dish
10 Mar 2011







The Problem

The story of the Chilbolton Observatory is a tale of engineering triumph snatched from the jaws of defeat. A hugely ambitious project in its day, the Chilbolton Observatory was opened in 1967 amid great pomp and ceremony.

It was a celebration of a landmark project, the climax of a two-year effort to design and build a stunning 25 metre radar antenna. Even now it dominates the rolling Hampshire landscape. It was designed to support the research aspirations of the prestigious radio and space research station for decades to come.

As it was put through its paces it initially lived up to everyone's expectations. Their delight at their new experimental toy was to be short-lived - after a matter of weeks a distressing squealing noise could be heard coming from the dish. When the source was found it was devastating news; a bearing had completely failed.

No contingency plan was in place to correct this fatal error - no one had foreseen that such a major flaw could be possible. The failed azimuth bearing was buried in the most inaccessible part of the antenna, between the 400 tonne aluminium dish and the 2000 tonne concrete tower. It was sinking at a rate of 0.4mm per month.

Somehow this vital component had to be extracted and replaced. This difficult task would involve delicately lifting the massive dish away from its supports without causing any distortion to the finely balanced backing structure. It was formidable task for the contractors, but without it the Chilbolton dish would be permanently frozen, its scientific future in serious doubt.

The Solution

Engineers plans to slove this problem 

The repair contractors, GEC/AEI Ltd proposed to raise the 400 tonne superstructure on hydraulic jacks, cut a hole in the side of the rotating cabin and remove the faulty bearing through the gap. A replacement bearing would simply be slid in via the same route and the dish lowered back on to a new support.

This apparently simple strategy in reality required months of careful planning and experiment to work out how to control the enormous forces. Every piece of equipment would have to fit through the 20 foot gash in the side of the cabin, or be winched up inside the narrow confines of the support tower.

The removal of the faulty bearing

The first task was to protect the concrete support tower from collapsing under the strain of the repair. Reinforcing steel 'spiders' were built inside the tower. These spiders supported two massive steel beams that projected out from the side of the tower. The bearing would be slid out along these beams on rollers, before being lifted to ground by crane.

The supports for the jacks used to hold this massive structure                

Before the bearing could be removed, however, the dish superstructure had to be lifted away from it. Four jacks capable of bearing 120 tonnes each were mounted in pedestals, themselves weighing over a tonne. Maneuvering these bulky objects into position inside the tower was a difficult job. The conditions were extremely cramped and it was crucial not to damage the drive mechanism in any way.

The aerial was lifted a cautious 1/8 of an inch at a time, with constant feedback from strain gauges monitoring the forces that threatened to distort it. A screw thread on each jack could take the weight of the aerial in an emergency. Luckily, there was no failure and the aerial was successfully raised 8 inches above the capping ring, just enough to allow the failed bearing to be slid from its moorings. The aerial was then lowered a fraction of an inch at a time back onto a dummy bearing while the old one was removed for inspection.

Installing the new bearing

New bearing is moved into place                

Two weeks later, the whole laborious process was repeated in reverse. The new bearing arrived on-site on a special loader, carried at a jaunty angle so that it could be moved by road without causing serious tailbacks throughout Hampshire. The new bearing was much stiffer than the old one, using two sets of rollers set a right angles to each other instead of the ball bearings that had failed so dramatically.

Once it was in position, the steel supports were removed and the hole in the side of the equipment cabin was repaired to protect both the machinery and the scientists from the elements. Prudently, the lifting equipment was kept onsite, just in case it was needed again. Engineers estimated that the whole process could be repeated in the space of month. Happily, no more major repairs have been needed and today the aerial rotates smoothly on its bearings without a hint of a squeak.

Contact: Jeffery, Judith (STFC,RAL,RALSP)