What turns the beacon and shapes its beam?
A motor and gearbox turn the lens carrier, and the lenses gather the lamp light into a narrow white or green beam.
The drive and the lens are the two working parts of a rotating beacon. The drive is a motor, gearbox and shaft: the motor runs fast, the gearbox slows it to the turning speed the beacon needs, and the shaft carries it on bearings to the lens carrier. Because the lamp stays fixed at the centre, only the lenses turn, so the supply does not have to pass through a rotating joint: the lamp cable runs up through the hollow shaft and stays fixed.
The lens turns the lamp's light, which spreads in every direction, into a narrow beam. It is usually a Fresnel lens, which focuses like a thick lens but is made of thin concentric steps, so it is light enough to turn. A clear lens gives the white beam and a tinted lens gives the green one, and the lamp has to sit at the focus or the beam loses strength. At each visit, count the flashes over a minute to check the turning speed, lubricate bearings and gears, clean the lenses and dome, and check the lamp position and the green colour.
Frequently Asked Questions
Q1What is the drive in a rotating beacon?
The motor, gearbox and shaft that turn the lens carrier. The motor runs fast, the gearbox slows it to the turning speed the beacon needs, and the shaft carries that motion up to the lenses on bearings.
Q2What does the lens do?
It gathers the light from the lamp, which would otherwise spread in every direction, into a beam that is narrow in height and sits a few degrees above the horizon, where aircraft see it. Without it the flash would be too weak to see from far away.
Q3Why is the lens a Fresnel lens?
A Fresnel lens has the same focusing effect as a thick lens but is made of thin concentric steps, so it is light and compact. That matters when the lens has to be turned by a small motor.
Q4Where does the green come from?
A green lens or filter in front of the beam. The lens glass or filter is tinted so the light falls inside the green colour limits that apply, and a faded or damaged one can push the colour outside them.
Q5What should be checked on the drive and lens?
Turning speed, by counting flashes over a minute; bearings and gear lubrication; cleanliness and condition of the lenses and dome; lamp position at the lens focus; and the colour of the green lens.
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