The Hazards of Crane Two-Blocking and How to Prevent Them

Tadano crane with load block to highlight two-blocking area for crane safety.

Crane accidents at the job site need to be avoided at all costs. Operators need to be knowledgeable of the cranes they are operating as well as what can go wrong with them. Two-blocking is one of those situations that is important to avoid to maintain crane safety.

What is Two-Blocking?

The term ‘two-blocking’ is used to describe a situation in which the load block or overhaul ball comes into contact with the boom nose, auxiliary nose or boom extension nose of a crane. This can lead to a dangerous situation that may cause the wire rope to snap and any suspended load to drop. It can also result in other severe crane damage and possible injury to the operator or other personnel. Some past occurrences have caused major injuries and even fatalities. Thankfully, most modern cranes have an anti-two-block warning system, but many older cranes do not.

Crane Damage and Injuries That Can Result

Minor as well as major crane damage can result from a two-block incident. The auxiliary ball can damage the auxiliary nose or the boom extension and the related sheave. The load block can damage the boom nose and its sheaves. The wire rope involved in the two-blocking issue can also become damaged. A stretched and snapped wire rope can itself injure someone due to the sudden release of tension. In addition, along with these potential crane issues, the load could be dropped and damaged. The biggest concern is that the operator of the crane along with personnel at the job site could become injured or killed.


Situations Where Two-Blocking Occurs

During crane operations, two-blocking can happen in several different ways. When the load block or overhaul ball has been hoisted up too high until two-blocking happens with the boom or boom extension tip. Telescoping the boom out without hoisting down as needed can cause two-blocking. Booming down without hoisting down correctly can lead to a two-block occurrence. Also, any combination of these factors can cause two-blocking to occur even faster.


Two-blocking can also happen if the crane’s anti-two-block system is not functioning correctly or because the operator is not properly trained for the crane being used. These situations may even occur if an incorrect crane function is unknowingly activated by the operator.


What is an Anti-Two-Block System?

Modern cranes generally have anti-two-block systems installed (sometimes referred to as an ‘A2B’ or ‘ATB’ system) and are integrated into the crane’s LMI (Load Moment Indicator) system. An A2B/LMI system generally has an operating console with a buzzer and alarm. Typically, there are anti-two-block switches mounted at the head of the main boom, aux nose, and the boom extensions. Generally, there will be some type of weight hanging from the A2B switch on a chain or cable and encircling the main or aux wire rope. As the hoist block or aux ball contacts and lifts the weight, it activates the A2B switch which then signals the A2B/LMI system. The A2B/LMI system then warns the crane operator with a buzzing sound and warning light that a two-block incident is about to occur. Simultaneously, the crane’s hoist-up and boom-down functions are being locked out to prevent the two-block incident.


Even with an A2B system, some situations can still lead to problematic issues such as the A2B system being set up in the wrong operating mode, or the system may be bypassed completely by the operator. Perhaps, the system may have been damaged or have a defect. There can even be a missing or improperly installed A2B weight (or A2B lever such as on a Broderson crane). Some A2B switches can also get pinned (locked out), which will not allow it to signal a two-block condition.


Anti-Two Block systems are helpful, but they are not a substitute for proper operator judgement and a good amount of experience, along with proper operating procedures and safe crane operation. The crane must also be in good working order and the operator must thoroughly understand how to operate it safely.


Two-Block Prevention

As mentioned, it is very important that the crane is properly maintained. This includes keeping up with regularly scheduled maintenance and a thorough daily inspection of the crane before each use. It is highly recommended to install an A2B system if the crane does not have one.


A skilled operator will use safe load control practices such as smooth and controlled hoisting movements. When using the crane, the operator will steadily keep an awareness of where the load block and overhaul ball are at all times. A safe operator will not allow the load block or overhaul ball to get too close to the nose of the boom or the boom extension (jib or swingaway) as insufficient clearance can lead to two-blocking.


The operator will have to make sure to properly set up the A2B/LMI system for crane operations and put it in the correct operating mode. The correct mode may have to be reset in the LMI when the crane setup changes. Once the A2B system is enabled, the function of the switches and entire system can be tested to ensure they are operating properly. If the A2B system is noticed to be damaged or defective, it will have to be repaired before normal crane operations can continue.



Crane safety should be the main focus for everyone at all times. Two-blocking can be a very serious situation, leading to crane damage along with worker injuries and deaths. It is crucial to be aware of how the problem can occur and the steps needed to ensure it does not become an issue during crane operation at the job site.

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