Pulley blocks standard
Page 65
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A second standard introduced is one for pulley blocks for use with synthetic fibre ropes (metric units) and is covered in BS 4344, which fills a gap in the series of British Standards for pulley blocks.
In strength the blocks specified are intermediate between those specified in BS 1839, "London pattern pulley blocks for natural fibre rope" and those of BS 4018, "Pulley blocks for use with wire rope to lift up to 25 tons in combination." Very approximately the strengths of natural-fibre, polyester, polythene and wire ropes are in the ratio 1, 2.2, 3 and 10.
Among the ropes that may be used with these blocks are those complying with BS 3758 (polyester). BS 3912 (polythene) and BS 3977 (polyamide or nylon). A block designed for one grade of rope can be used satisfactorily with a rope from a stronger grade which is of smaller diameter yet has the same breaking strength. Such use does not violate the Standard in any way, and the manufacturer can therefore allow for more than one grade of rope to be used. The standard thus provides for existing variations, improvement in the properties of present materials and the development of synthetic materials with new properties.
The Standard applies to pulley blocks designed to lift up to 6 tonnes in combination. To minimize the likelihood of overheating or of electrostatic build-up the maximum rope velocity has been kept to a figure of 0.5 m /s.
To enable full advantage to be taken of the • use of new materials in the design of blocks greater latitude is permitted, for example, in materials and bearing pressures than in earlier standards. Non-ferrous and nonmetallic materials are permitted for parts other than head fittings and beckets. The design does require specific overload performance, however, and provision is made for an optional test.
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