When any crane is overloaded, disaster can strike. Installing an overload guard on your crane can increase safety and help cut maintenance costs. Overloading any equipment dramatically increases the wear of wire rope, which is the most vulnerable part of the crane. And 100% overload increases fatigue on crane components eight times.
Besides repairs, consider that lifting equipment is often an integrated part of a costly manufacturing process, where damage to people or equipment can seriously affect and impair competitiveness.
Overload guards can be especially beneficial on overhead travelling cranes and container handling equipment, where operators may not know they have an overload condition. When considering an overload guard, it should be reliable and it should be the right type for the application at hand.
There are two basic categories of overload guards: mechanical and electronic.
Mechanical overload guards are often more resistant to damage from mechanical impacts, severe shock loads, sudden unloading, overloading and tough environments, such as heat, cold, dust and humidity. Many types are easy to install and adjust.
In many mechanical overload guards, there are power-absorbing elements in a package of specially made Belleville washers designed to resist fatigue. The movement of the washers operates a device, which controls one or more microswitches.
Overload guards are mostly built into the dead end of wire rope or clamped on the rope near the rope end. Clamp-on types are the most common. They operate according to the deflection principle. The rope deflected through a slight angle between two wheels and clamping jaws, which hold the device on the rope. When loaded, the rope tends to straighten. When the preset force on the load cell is exceeded, a switch opens the electrical circuit.
With the deflection method, there is no need to adjust the set value when wire ropes must be changed. It is important that the deflection angle on the wire rope be small, or it will increase the wear on the wire rope. Also, if the deflection angle is big, wear on the rope or device will dramatically deteriorate operating accuracy for the overload guard. This type of overload guard does not affect the construction of the crane and is usually chosen when a crane is going to be retrofitted with overload protection.
Normally, an electronic load guard consists of a power transducer (load cell) with an amplifier and signal processing unit. The transducer senses either tension, compression, bending, shear and torsion or a combination of these.
Foil gauges are fed with a constant voltage from the transducer amplifier. A signal is received in return, which is proportional to the force in the load cell. The signal can indicate switching, overload control or load.
An advantage of electronic overload guards is the ability to update technical solutions. One load cell can control slack ropes, warnings and overload, while providing voltage or current signal for readout-instruments and/or recording remaining lifetime of the crane. It can also provide load summation of two or more hoists.
The most frequently used type of electronic overload guard works according to the rope deflection principle. Instead of a package of Belleville washers, transducers with built in foil gauges are used.
Normally, the signal is amplified and converted into a 4mA to 20mA signal before being processed in the electronic field. This type of electronic overload guard has all the features of the mechanical overload guard, but can also show the load or pulling force.
Another type of electronic overload guard is a load pin. It can be dimensioned to replace an existing shaft at an equalising pulley or in another part of the crane.
The load pin should be installed as close as possible to the force and/or load to achieve best possible accuracy. A load pin does not reduce the lifting height of the hoist. Installation and calibration are more extensive in comparison with a load cell working according to the deflection principle.
Since January 1995, regulations require all European Union countries to install overload guards in hoists with capacities over 1000kg.
