Anti-two block switch

A must-have component in lifting machinery, the anti-two block limit switch helps the operator protect equipment, loads, and personnel by preventing the hook block from colliding with the boom. WIKA Mobile Control manufactures the iSENS HES limit switch that is ideal for this task.

Safety is paramount when operating heavy equipment. That’s one reason WIKA acquired WIKA Mobile Control (WMC), formerly known as Hirschmann MCS, to make sure customers have access to the marketplace’s best safety systems for cranes and other mobile machines. One of those safety devices is a switch that prevents two blocking.

What is two blocking?

An essential part of a mobile crane is the hoisting system that is used on the crane boom. This system is what enables the machine to lift and lower loads.  At the end of the boom are sheaves (grooved wheels), also known as the boom nose. The hoisting cable runs through the boom nose down to the hook block at the other end of the double pulley system. A large hook extends from the bottom of this hook block.

A dangerous situation can occur when the hook block comes in contact with the boom nose pulleys; this is referred to as “two blocking.” If this occurs, the impact could weaken or break the hoist rope, resulting in a dropped load and endangering the machine operator and any workers in the area. Unfortunately, two-blocking accidents have caused hundreds of deaths over the years.

How to prevent two blocking

Anti-two blocking switch

WIKA’s iSENS HES anti-two block (A2B) limit switch

Careful operation helps to ensure that the boom nose and hook block sheaves do not come into contact with each other. A more sophisticated anti-two block (A2B) solution is the iSENS HES limit switch.

As an A2B switch is a crucial safety mechanism, it must be extremely robust and reliable – able to withstand heat, cold, moisture, and dust. With ruggedness and flexibility in mind, WMC designed the iSENS HES anti-two block switch.

This switch has a corrosion-resistant housing, and all electronic functional parts are completely sealed. The 3-pole/4-pole electrical connection cable comes in various lengths. The switching unit has an optional divisible operating weight and a ranging resistor for diagnostics to help monitor the integrity of the connection cable.

The A2B switch by WMC operates reliably under a wide temperature range: from −13°F to +167°F (−25°C to +75°C). Built to withstand more than 1 million switching cycles, it has a very long mechanical lifespan.

In addition, installation is easy. Simply attach a metal post to the boom tip, and then mount the switch to the post with a bracket. Chain connectors attach the counterweight to the actuating cable.

The iSENS HES meets several important standards:

  • Compliant with IEC/EN 60947-5-1
  • IP67 ingress protection according to EN 60529
  • 4A gG (IEC 60269-1) short-circuit protection
  • DGUV (Deutsche Gesetzliche Unfallversicherung) certification

WIKA has long been a leader in providing high-quality instruments for mobile working machines, and our solutions help customers work more safely and efficiently. After acquiring WMC, we are now able to broaden our product portfolio to include A2B switches and other devices designed to make lifting loads safer.

Products in this post:

iSENS HES anti-two block switch


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