Rugged Inductive Sensors Confirm Engagement of Safety Interlocks on Hooklift Trucks


A hooklift truck utilizes a hydraulic system for loading and unloading a demountable container. Once the container is correctly positioned on the vehicle’s load bed, interlocks engage with its base, securing it in position. Rugged sensor systems detect full engagement of the interlocks, ensuring the truck is safely loaded prior to driving away; sensors must be mechanically robust and withstand harsh outdoors conditions.

Customer Values

  • Elimination of human error prevents truck being driven in an unsafe condition
  • Highly reliable sensing performance in highly demanding conditions
  • Corrosion-resistant sensors withstand water, ice, mud and salt
  • Highly cost-effective solution to a potentially serious problem

Specific Product Advantages

  • Single-piece AISI 304 (V2A) stainless-steel construction
  • Rugged, hermetically sealed embeddable sensors
  • High resistance to harsh operating conditions

Customer Application

A specialist vehicle builder manufactures hydraulic hooklift systems for commercial trucks. A hooklift truck employs a series of hydraulic rams to hook, lift and pull a demountable container onto the vehicle’s load bed; a full container may weigh up to 30 tons. Hooklift trucks operate all year round, often in harsh environments such as demolition and construction sites.

Hooklift systems are typically semi-automated. The driver positions the truck, engages the hook with the container and initiates the lifting sequence. Once the container is pulled onto the load bed, two interlock mechanisms, mounted on the truck chassis, actuate and engage with the container body. Once the interlocks are fully engaged, the container is secure and the truck may be driven safely.

A highly reliable sensor system must detect the position of the interlocks, ensuring that they are fully engaged before the truck drives away. Sensors must be robust and must withstand water, mud, dirt and ice. They may also be subject to mechanical shock and vibration.

Customer Solution 

Rugged Full Inox inductive sensors from the Contrinex Extreme range are designed for exceptionally demanding environments, making them ideal for this application. Highly reliable and rated to IP68/IP69K, these embeddable units feature one-piece AISI 304 (V2A) stainless-steel construction with vacuum-encapsulated electronics, making them well suited to the hostile operating conditions.

During operation, a double-ended ram drives a pair of interlock bolts into position. On either side of the truck, two M18-diameter sensors are mounted in the wall of each bolt housing; one detects the bolt as it reaches the fully engaged position; the other detects it in the fully retracted position. A second ram drives a rotating lock mechanism to engage with the vehicle chassis; a further two sensors detect the position of a cam on the locking shaft as it rotates.

Each sensor is configured with an industry-standard PNP normally-open interface, and connection to the truck’s hydraulic control system is via an integral M12 connector and PUR-sheathed cable. This allows easy removal and replacement during routine maintenance.

Rugged sensor construction ensures excellent resistance to shock and vibration and a sensing distance of up to 10mm eliminates any risk of accidental collision with moving parts. These devices are corrosion-resistant and impervious to water ingress, mud, ice and salt. Reliability has been excellent. Contrinex inductive sensors deliver a cost-effective solution that satisfies the customer’s requirements for a simple, fit-and-forget system.