In heavy-duty industrial lifting systems, such as tower cranes, port cranes and overhead cranes, accurate control of the hook travel and boom movements represents a critical engineering challenge.

Unlike standard linear motion applications, where a conventional proximity switch may be sufficient, large lifting systems require the continuous and redundant monitoring of multiple rotations of the drive shaft or the wire rope drum.

In this context, the rotary limit switch is the key component for ensuring functional safety and millimetre-accurate positioning by detecting delayed or critical stopping points before structural failures or hazardous operating conditions can occur.

The selection of a rotary limit switch for cranes and overhead cranes should be based on the analysis of the overall kinematic revolution ratio, the number of microswitches required for the slowdown and stop thresholds, and the possible need for integrated auxiliary devices such as encoders or potentiometers. For heavy-duty applications, the essential requirements include enclosures with a high IP protection degree, high mechanical resistance to severe vibration, and compliance with the applicable safety regulations governing the positive opening operation of contacts.

Rotary limit switch kinematics and operating principle

The operating principle of a rotary limit switch is based on the direct or indirect transmission of the rotary motion from the machine drive shaft, such as the winch drum shaft, to the input shaft of the device.

This connection can be achieved by means of flexible couplings, dedicated pinions or gear wheels.

Inside the device, a gear revolution system, typically consisting of a worm gear and helical gears or planetary gear reducers, reduces the number of input revolutions according to a predetermined revolution ratio. The reduced motion actuates a series of adjustable cams which, as they rotate, operate the actuators of the electrical contact blocks or microswitches.

Unlike general automation solutions not specifically designed for heavy-duty lifting applications, the architecture of TER components is engineered to eliminate internal mechanical backlash. This design ensures consistent repeatability of the operating point even after hundreds of thousands of operating cycles, reducing overall wear and increasing the MTTF (Mean Time To Failure) of the entire control system.

rotary limit switch

Engineering selection criteria: key technical parameters

Selecting the most suitable device requires the combined evaluation of three main groups of parameters: mechanical, electrical and design characteristics.

The first parameter is the overall kinematic revolution ratio. This value shall be calculated so that the entire operating travel of the machine, corresponding to the maximum number of drum revolutions from the lowest hook position to the maximum lifting height, is completed within a cam rotation of less than 360°.

The TER range offers an extremely wide selection of revolution ratios, from standard 1:1 configurations up to highly reduced solutions exceeding 1:8100, covering applications ranging from small jib cranes to large port winches.

The second parameter concerns the type and number of electrical contacts. The number of cams and the associated microswitches defines the safety functions that can be managed by the PLC. Typical configurations include two, four or more contacts to sequentially manage the upward travel pre-alarm or slowdown, the upper limit stop, the downward travel slowdown and the final lower limit stop.

From an electrical standpoint, the choice between snap-action and slow-action contacts allows the switching response to be optimized according to the rotational speed of the input shaft. For emergency circuits, TER uses normally closed contacts with positive opening operation, an essential safety requirement for achieving high-level machinery safety certification.

The third parameter is the integration of auxiliary feedback sensors. Modern automation systems require the rotary limit switch to function not only as a stop device but also as a continuous position sensing device. Many TER series can accommodate linear potentiometers or absolute and incremental encoders within the same enclosure.

This integration provides the PLC with both a continuous position feedback signal and the safety of an electromechanical emergency cut-off.

Manufacturing excellence and reliability: the value of certification

In lifting applications operating in chemically aggressive, marine or extreme temperature environments, construction materials and regulatory compliance make the difference between production continuity and unexpected machine downtime.

TER rotary limit switches are manufactured in Italy under the strict control of a UNI EN ISO 9001:2015 Quality Management System and a UNI EN ISO 14001:2015 Environmental Management System.

The use of impact-resistant technopolymer or die-cast aluminium enclosures provides a high level of protection against the ingress of conductive dust and water jets.

Each series fully complies with the restrictions on hazardous substances established by the RoHS Directive (2011/65/EU and 2015/863) and with the REACH Regulation (EC No. 1907/2006), ensuring a lead concentration of less than 0.1% by weight in electromechanical components and cable glands.

At the end of their service life, environmental sustainability is ensured by the classification of the device as Waste Electrical and Electronic Equipment (WEEE), requiring environmentally responsible disposal separate from household waste streams, supported by fully recyclable packaging.

Technical section for design engineers

Calculation method for the load revolution ratio

To determine the correct revolution ratio to be selected, the design engineer shall follow the calculation procedure described below.

First, multiply the maximum effective lifting height, expressed in metres, by the wire rope reeving ratio.

The value obtained shall then be divided by the product of π (3.1416) and the pitch diameter of the rope drum, also expressed in metres.

As a practical example, assuming a maximum effective lifting height of 40 m, a 2:1 wire rope reeving ratio, and a rope drum pitch diameter of 0.5 m, the total number of drum revolutions is approximately 51. After applying an appropriate structural safety factor, the recommended revolution ratio to be selected in the configurator is 1:60 or higher.

Descriptive Analysis of TER Rotary Limit Switch Series

The product range includes several specialized solutions designed to meet the requirements of industrial applications with different levels of complexity:

  • Base Series: Base rotary limit switches feature compact dimensions, making them particularly suitable for installation where space is limited. They cover revolution ratios from 1:15 to 1:1500 and can be equipped with 2 to 6 switches. They provide IP42, IP65 or IP66 / IP67 / IP69K protection. 
  • Fox Series: This series covers revolution ratios from 1:3 to 1:2870. It provides IP66, IP67 or IP69K protection and allows the integration of encoders, Yankee absolute encoders and potentiometers. It is the ideal solution for jib cranes, small winches and standard overhead cranes. 
  • GF4C Series: Cam rotary limit switch series with revolution ratios from 1:1 to 1:969. It provides IP65 protection and can accommodate 2 cam sets (up to 7 switches), potentiometers and encoders (stand-alone or on cam sets with up to 2 switches) and Yankee absolute encoders (on cam sets with up to 2 switches). It is widely used on tower cranes, winches, hoists, conveyor belts and wind turbines. 
  • Oscar Series: Developed to provide greater flexibility, it offers revolution ratios from 1:1 to 1:1550. It provides IP66, IP67 or IP69K protection and can accommodate potentiometers and encoders (stand-alone or on cam sets with up to 3 switches), Egon 36-AL absolute encoders (stand-alone or on cam sets with up to 2 switches) and Yankee absolute encoders (on cam sets with up to 4 switches). It is the ideal solution for tower cranes and lifting systems for the construction industry. 
  • Top Series: The flagship series for heavy-duty applications, offering revolution ratios from 1:1 to 1:8100. It provides protection up to IP69K and is designed to accommodate 3 cam sets (up to 15 switches), potentiometers and encoders (stand-alone or on cam sets with up to 2 switches), Egon 36-AL absolute encoders (stand-alone or on cam sets with up to 2 switches) or Yankee absolute encoders (on cam sets with up to 4 switches). It is ideally suited for port cranes, wind turbines and stage technology.

Conclusion

The adoption of a TER rotary limit switch represents a strategic choice for operational risk mitigation. Manufacturing excellence and the availability of clear technical data provide design engineers with the confidence to operate in full compliance with applicable international regulations, protecting both the investment and the efficiency of lifting equipment.