Big Ben falls silent The famous chimes of Big Ben are to fall silent after 157 years of nearly unbroken service. The iconic London landmark needs urgent repairs to prevent its mechanism from failing in a $42m restoration project Elizabeth Tower: Work of Charles Barry and Augustus Pugin. Height: 96m. Renamed in 2012 for Queen’s Diamond Jubilee Work needed in tower to fix cracks in masonry and corrosion to bell frame. Lift will be installed in ventilation shaft to to improve access Mechanism of Great Clock Gravity-driven, consisting of three “trains” – Going, Chime and Strike – each made up of barrel connected to weight suspended below by steel wire. As weights drop, barrels turn, setting in motion hands and bells via series of gears, wires and shafts Built by clockmaker Edward Dent from design by lawyer, MP, and amateur horologist Edmund Beckett Denison with assistance from Astronomer Royal, George Airy GOING TRAIN Moves clock hands via gears driven by single shaft Going train controlled by Double Three-Legged Gravity Escapement – Denison’s ground-breaking invention in which impulse to pendulum is not given by escape wheel, as in deadbeat escapement, but through gravity force of two arms. This frees pendulum from friction, ensuring accuracy Pendulum alternately touches each of two arms every two seconds, unlocking escape weight Locking arms then fall back under own gravity, hitting pendulum to keep swinging Escape wheel Barrel of Strike Train STRIKE TRAIN Rings hour bell via 1 ton strike weight suspended beneath Weight drop rotates barrel, pulling wire connected to bell hammer Pendulum Length: 4.4m Weight: 310kg Ayrton Light, which shines to indicate that Parliament is sitting, needs full restoration Weights Pendulum Fly fans: Air brakes regulate descent of weights Mechanism dimensions Length: 4.7m Width: 1.4m Weight: 5 tons Frame material: Cast Iron girder Barrel of Going Train Barrel of Chime Train Pictures: Getty Images Sources: UK Parliament, Whitechapel Bell Foundry Clock adjustment: Placing old penny coins on ledge alters pendulum’s centre of mass. Adding or removing one penny changes clock speed by 0.4 seconds per day GREAT BELL Nickname “Big Ben” commonly used for clock and tower together. Probably named after Benjamin Hall, First Commissioner of Works QUARTER BELLS On quarter hours four smaller bells play chime based on Handel’s Messiah:First Great Bell cracked while being tested. Recast bell first struck hour on July 11, 1859 “All through this hour Lord be my guide that by Thy power No foot shall slide” Clock faces Four 7m cast iron dials each contain 312 pieces of pot opal glass Hour hands Length: 2.7m Weight: 300kg Material: Gun metal Minute hands Length: 4.2m Weight: 100kg Material: Copper sheet Clock faces will be stripped of black and gold paint applied in 1980s to return to Victorian appearance, thought to feature green and gold Clock stoppages 1962: Clock chimes in New Year ten minutes late due to heavy snow on hands 1976: Clock shut down for 26 days over nine months after chiming mechanism disintegrates through metal fatigue 2005: Clock mechanism stopped for two days to allow inspection of brake shaft 2007: Mechanism replaced by electric motor to allow six-week maintenance work. Clock dials cleaned and repaired Early 2017: Major refurbishment plan due to start – expected to last three years CHIME TRAIN: Rings four quarter bells via steel wires connected to bells’ hammers Every 15 minutes, lifting arm falls off relevant wheel segment. Length of segment determines length of chiming sequence Maintenance Clock wound up by hand three times a week – takes over one hour as not possible to wind while clock is chiming Clock mechanism will be dismantled for repair.Pendulum suspension spring will be replaced