CRANECRITIQUECORNER
Cranes’ A-2-B Systems Explained This month’s topic:
WHEN ARE THEY REQUIRED? by: Dave Barnhouse
A monthly crane and rigging informative column for all personnel directly or indirectly involved with crane safety. Each month we will attempt to explain a different technical issue pertaining to crane operations here on Guam, addressing the sometimes over-looked or misunderstood topics by management and operators alike. First, what is an A-2-B? This is an acronym for anti-two block. Simply put, there must be a means to prevent the lower hook block from accidently contacting the upper boom tip block and possibly damaging sheaves and other components or parting the hoist wire and dropping the load. If you are not familiar with crane operations you might be thinking that would not likely be something a crane operator would do and he would always be careful to maintain sufficient distance between the two considering the possible results of contact. If on the other hand you are familiar with crane operations than you realize it is easy to get caught up with the operations on the ground and forget how little head room you may have between the boom tip and the hoist block. Many headache balls have been two-blocked while the operator was paying attention only to the main block, parting the line and resulting in the hook on the ground. Unfortunately, this has resulted in some fatalities. Lock-outs vs. Warning systems: There are two types of A-2-B systems: the more common type is the hydraulic lock-out, where once the two block switch is activated the hydraulic solenoid on that circuit closes and the function is stopped, not enabling further hoisting for example and causing any damage. The second type is the warning system, where once the two-block switch is activated an alarm goes off in the operators cab and he knows to immediately stop the function. EM385-1-1 and OSHA 1926.1400 differ slightly on the requirements of the A-2-B, but not significantly. The following will explain what cranes require an A-2-B lock-out and which crane types can make do with a warning system only. Let’s establish first that the A-2-B system is not a safety device. It is an operators aid. What this means is that the crane may operate without a functional A-2-B system, but only temporarily. Alternative measures must be taken, however, if the system is not functional. The alternative measures can only be temporary and consist of clearly
28 | MAY2012
marking the cable (so that it can easily be seen by the operator) at a point that will give the operator sufficient time to stop the hoist to prevent two-blocking, and use a spotter when extending the boom. This is identical wording in OSHA and EM385-1-1. EM385-1-1: (A-2-B). Anti-two-blocking devices shall be installed at all points of two-blocking. (a) All cranes and derricks shall be equipped with A-2-B/Hoist limit device that will disengage the function that is causing the two-blocking or an A-2-B damage prevention feature (except as noted). They shall be tested and certified functional by a competent person prior to operating the crane. (b) Lattice boom cranes. Lattice boom cranes shall be equipped with an A-2-B device to stop the load hoisting and boom-down functions before the load block or load contacts the boom tip. Occasionally the above is interpreted word for word before reading the complete section. There are exceptions to the preceding: EXCEPTION 1 – Duty Cycle: Lattice boom cranes that are used exclusively for duty cycle operations are exempt from A-2-B equipment requirements. EXCEPTION 2 – Lattice boom cranes with manually operated friction brakes: Lattice boom crane and hoisting equipment with manually activated friction brakes, A-2-B warning devices may be used in lieu of A-2-B prevention devices. (c) Telescopic boom cranes. (i) Telescopic boom cranes shall be equipped with an A-2-B device to stop the load hoisting function before the load block or load contacts the boom tip and to prevent damage to the hoist rope or other machine components when extending the boom. (ii) Telescopic boom cranes that are used exclusively for duty cycle operations shall be equipped with a two-blocking damage prevention feature or warning device to prevent damage to the hoist rope or other
CONSTRUCTION NEWS BULLETIN
Two blocking as result of non-functioning two-block switch
machine components when extending the boom. (d) Floating cranes. Floating cranes may use an A-2-B alarm system in lieu of a disengaging device unless they are hoisting personnel. (e) Other cranes used in duty cycle operations, to include clamshell (grapple), magnet, drop ball, container handling, concrete bucket, pile driving and extracting operations, drilled shaft operations are exempt from the requirements for A-2-B devices. (f) Temporary alternative measure: clearly mark the cable (so that it can be easily seen by the operator) at a point that will give the operator sufficient time to stop the hoist to prevent two blocking and use a spotter when extending the boom. Interpretation of the above: all hydraulic cranes must have A-2-B lock-outs, (with exception of duty cycle operations) this
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