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More about working load limits (WLL)

More about working load limits (WLL)

Richard Tregoweth - Wednesday, December 09, 2009

Working load limit (WLL) can get confusing. For a start you cannot use a WLL rating in isolation - there are very important relationships between Breaking Strengths (BS), Safety Factors (SF), Working Load Limit (WLL) and Cycles to Failure (CTF). You cannot change one without affecting the other. The WLL figure generally relates to rigging equipment (for arborists) rather than PPE which is expressed in kN.

Adding to the confusion are the different factors applied to each category of rigging equipment. For example, in arboriculture a 10:1 SF applies to ropes, a 7:1 factor applies to slings and strops and a 5:1 factor for hardware. This is based on the 'weakest link' principle. ie the lowering line should always be the weakest link in the rigging system.

The WLL rating is calculated by the above principles. For example if a bullrope has a BS of 45 kN (4500 kgs) by applying the 10:1 factor you have a WLL of 450 kg. This means you should not attach anything  to this rope weighing more than 450 kg. In doing so you account for things like potential shock loading and breaking strength loss of the lowering line over time.

Before you do any rigging make sure you know exactly what you are doing - an excellent start would be talking with Andreas (Rossy) Ross from Pro Climb, Hort Training NZ and Wintec also offer "basic-to-advanced" rigging courses. Rigging is a complicated and potentially dangerous aspect of arboriculture - you can never be too SAFE.

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