The present invention relates to telescopic booms. Telescopic booms with three or more sections are often used with mobile cranes or with access or work platforms and such booms use a chain or other suitable extension means to extend and contract the outer telescopic boom section. The extension and retraction of intermediate telescopic section(s) is usually by means of a hydraulic cylinder, which in turn causes the extension or retraction of the outer telescopic section by means of the chain or other suitable extension means. Such a system is well known in the industry, and has been used successfully for many years.
The major disadvantage of the use of a chain or other suitable extension means is that mechanical failure of the extension chain or extension means itself or its end attachments may allow sudden and catastrophic retraction of the section being supported, with the possibility of causing death or serious injury to the occupants of an access platform, or others in the vicinity.
Regulatory Authorities in some countries have recognised this danger, and have dictated in particular that access platform booms which incorporate chain systems must be provided with a safety device, which in the event of extension chain failure arrests the linear motion of the boom within 100 mm and also causes the power source--i.e. the engine--to be shut down.
An object of the present invention is to mitigate or overcome one or more of the above problems.