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Keeping the Great Clock ticking

Big Ben's famous 'bongs' were silent for seven weeks in 2007, allowing essential maintenance work on the clock mechanism to take place. From 11 August to 1 October, an electric system kept the clock moving, but Big Ben, the name for the Great Bell, and the quarter bells were quiet.

During the first week, a team of specialist technicians abseiled down the south clock dial to clean and repair the glass. Each clock dial contains 312 separate pieces of glass.

The clock mechanism was inspected by a specialist team of engineers. No irreparable damage was found so it was decided to re-install parts of the mechanism after careful restoration rather than replacing altogether.

The Double Three-Legged Gravity Escapement was repaired in the second week. The escapement controls the clock's accuracy by preventing interference from outside pressures on the clock hands, e.g. from the wind.

The escapement was invented especially for the Great Clock by Edmund Beckett Denison, a Member of Parliament and gifted amateur clockmaker. Denison later became Lord Grimthorpe and his invention is officially termed 'The Grimthorpe Escapement'. His revolutionary piece of engineering has been used in large turret clocks ever since.

Over the next three weeks, maintenance work replaced bearings in the clock. These reduce friction in the clock mechanism where moving parts turn against another. They were replaced on the strike train (which operates the hour bell, Big Ben) and the going train (which controls the clock itself). The clock was stopped for short periods over the summer while work was carried out to the drive rod bearings which operate the hands on each clock face.

In week six, following the escapement's refit and refurbishment, the clock was tested for short periods. It is also ran without being connected to the hands or bell, so that engineers could adjust the timing if necessary, as the new bearings altered the amount of weight required to drive the mechanism.

The Great Clock was restarted as the final stages of work were completed. The re-synchronisation of all the bells, including Big Ben, took place on the evening of 29 September 2007 as the bells chimed through their sequences. The clock hands and weights were reconnected and the Great Clock was fully restarted at midday on 1 October. Engineers closely monitored the clock mechanism, to ensure that it continued to keep the correct time.

This was the final phase of a programme of planned works to prepare for the Great Clock's 150th anniversary in 2009.