What is Rated Capacity and Working Load Limit?
Rated Capacity is, (as defined in BS EN 13000:2004, Cranes – Mobile Cranes)
“load that the crane is designed to lift for a given operating condition (e.g. configuration, position of the load). For mobile cranes the mass (weight) of the hook block is part of the load”
Working Load and Working Load Limit
Standards bodies in the developed world now encourage the use of Working Load Limit (WLL) for lifting accessories like hooks, slings and shackles.
A Working Load is the “load on the hook plus mass (weight) of the hook and block”
(as defined in BS EN 13000:2004, Cranes – Mobile Cranes).
This is a concise definition for a Working Load on a crane line, however it does give us an understanding of what the Working Load for any lifting gear, accessory or component should be. That is, the Working Load for any component is the load on the component plus the mass (weight) of the component itself.
Working Load Limit (WLL)
A Working Load Limit is the maximum Working Load for an accessory under a set configuration for expected or normal working conditions.
It is calculated using the Minimum Breaking Load (MBL) required to fail or permanently deform and weaken that piece of equipment, divided by a Factor of Safety (FoS). An equipment’s Factor of Safety depends its construction, risk to life and expected working conditions (wear, corrosion, dynamic loads, inspection periods).
The lowest WLL for any component or set of components working together within a lift configuration determines the overall Gross Capacity for that configuration.
The Working Load Limit for an individual piece of equipment is set by the equipment’s manufacturer. Any requirement to “derate” (change the WLL) of a piece of equipment should only be done by a competent person (qualified engineer) and after consultation with its manufacturer.