Firstly I will start with the purpose of the slew brake, which is to lock the cranes jib so it does not move out of position. This is important when performing lifts when there is wind about.
The slew brake also acts as a safety feature, activating when the crane is turned off, resets or power is lost. This prevents the tower crane and load from being moved about when the operator does not have control of the tower crane, very important because if there is a power failure without the slew brake the crane and load could collide with buildings, people, machinery or even power lines.
At the end of the workday or if work must be stopped in the day due to high winds the tower crane should be free slewed. This simply involves taking off the slew brake so that the tower crane is able to 'paravane' or 'weathervane' that is move freely in the wind on the full 360 degree radius, unrestricted by the slew brake.
The reason for putting the crane into free slew is mostly to prevent damage to the slew brakes, on non luffing tower cranes the wind will not "push the crane over" if it is not free slewed, but brakes will be damaged. On luffing tower cranes there is a danger that if not free slewed and jib at the correct radius (not luffed up too high) the jib can be pushed back and cause major structural damage
In the picture above we see a luffing tower crane at LendLease's New York development One57 the crane was luffed up too high and extreme winds from hurricane Sandy flipped the jib over. It has been suggested that the tower crane was not put into free slew, also contributing to this accident.
The slew brake is a pin that sits on top of the slew motor, when raised the brake is off, when lowered it is locked.Some tower cranes require the operator to manually wind out the slew brake, others automatically release the slew brake when the trolly is closest to the crane tower and others have a button that can be pressed to take off the slew brake.
It is easy to see if a tower crane has been put into free slew, generally a free slewed cranes counter jib points into the wind, the jib the direction the wind is traveling. If there is a group of tower cranes they will all point in the same direction. There are several reasons for a crane not to weathervane; first and foremost is the operator simply not initiating it, next would be electrical or mechanical issues, and most serious – a problem with the slew bearing.
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