Checking bolts8 March 2007
In January, the UK HSE warned crane users to check the high-tensile bolts that hold together tower cranes. This is a summary of that guidance.
High tensile bolt connections are used on tower crane joints, including those on mast, jib and slew ring bearings. They are subject to repeated cyclic loading as the crane lifts and lowers, loads and slews.
When correctly installed and pre-loaded high tensile bolt connections can transmit very large loads. If they are not correctly installed and pre-tensioned, they are more likely to suffer fatigue during crane loading cycles.
They consist of a bolt, nut, and hardened washer, and on occasions a spacing sleeve. All of the components should be manufactured from high strength materials. Bolts and nuts are typically marked with a manufacturer’s logo or name, strength grade and a batch number. Only components of the same strength grade should be used together.
Replacement parts should be the same strength grade as the originals and their thread specification should be rolled rather than cut.
Users should replace these connections before the manufacturer's maximum recommended in-service lifetime runs out.
Those bolted connections with additional lifetime should not be re-used unless the manufacturer says so.
Wear, damage and corrosion create surface defects on connection components that significantly reduce their fatigue life. Users should clean and examine components before use. Components should be treated to guard against corrosion, and lubricated before installation as the manufacturer recommends.
Users need to follow manufacturers' instructions about how to load a crane when a joint is tightened. An operator may need to place the crane in a certain position, or hang a load on the hook. If users do not follow these instructions, the bolt can be pre-loaded incorrectly, with very short service life before failure.
When tightening bolted connections, tighten the nut, not the bolt head. The bolt head can catch on the bearing surface, or the bolt shank can touch the side of the hole, creating friction that prevents proper pre-loading.
Torque wrenches or hydraulic tensioners should be maintained and calibrated properly, and there should be records on file to prove it. Those that work with springs should be dialled down to the lowest setting before storage to safeguard the springs.
All high-tensile bolt connections need to be re-tensioned typically three to six weeks after installation to make sure that the pre-loading does not decline as the connection beds in.
The connections should be regularly inspected by a competent person to check that joints do not seem to move as the crane raises and sets down a load; that there are no rust stains (which indicate movement at the joint); that nuts have not worked loose.
If a joint has come loose, or if bolts are stretched or broken, a competent person should examine the joint faces to check that they are flat and undamaged; that components have been properly assembled in the correct sequence to the correct pre-load. Samples of components that have been submerged in water should be examined using non-destructive testing (NDT). If components have worked loose, an examiner might decide to do thorough examinations at shorter intervals of time.
If an inspector suspects that a bolted connection has been overloaded or has failed, all components making up the joint should be replaced according to the crane manufacturer’s procedures.
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HSE crane bolt alert