Conventional overload safety links are C-shaped plates that use their deflection under load to operate a switch to stop the crane. In nuclear and offshore applications the deflection on these links is so small that they can stop working in as little as a month. The microswitch and set screws for setting the load limit and switching the signal are very small and soon corrode in a marine environment or in a nuclear containment building. C-shaped links also have to be large to give the necessary 4:1 safety factor and the small deflection, about 0.4mm, means it is difficult to adjust the set screw adjustment to actuate the microswitch, says Igor Victor Kormishkin.
Kormishkin is a project engineer at Allied Systems, a US manufacturer of a range of specialised cranes for marine, research, petroleum, military and power utility applications. Models include fixed knuckle and telescopic boom cranes in capacities up to 100 US ton and boom lengths to 38m (125ft). Allied System has now produced a new type of mechanical overload link.
Kormishkin wanted a link with more deflection so that a larger and more reliable switch could be used. He also wanted a higher safety factor of at least 10:1. A C-shaped link satisfying these requirements would be too large for marine applications, hence the new design. An initial design used an S-shaped plate with with a straight link on the back to take up the load at a certain tension point in the S-link. It worked but was expensive and difficult to make.
The answer was a specially designed and analysed tensor plate with a precisely calculated gap. The gap is large enough to provide a large deflection but keeps stresses in the arches well below the steel’s yield limit to provide good fatigue resistance. As soon as an overload is applied the arches cannot deflect more than the size of the gap. When the gap is closed the neck portions of the plate take up the load thus providing a factor of safety close to 18:1.
The exact deflection of a plate shaped as shown in the pictures can be predicted only by using finite element analysis software but Kormishkin pre-calculated the initial shape on Mathcad using the proven Roark’s stress and strain formulas for arches. To make the plate a drawing is sent by email as a .dxf file to a plate burner where the part can be cut in minutes.
When load is applied the tensor plate stretches, the target lever deflects, and trips the proximity switch as soon as an overload condition is reached. With a larger overload the plate self-locks, thus preventing stretching and providing a high safety factor.
An adjustable length link with coarse and fine threads connects two parts of the tensor. It allows large tolerances between drilled holes because the initial position of the target bar would be off by quite a bit if the position of a hole is off by, say, 0.5mm. For consistency in the target lever deflection a spring is used to eliminate clearances in the pin joints. The spring is stainless steel and has a tension force of about 2.3kg (5lb).
The tensor link shown above was designed for 11.3t to 13.6t (25,000lb to 30,000lb). It is made from 30mm thick (1.25 inch) ASTM 514 steel plated to resist corrosion. All pins and linkages are 316SS (stainless steel). Stainless steel can also be used for the tensor plates but they need to be slightly larger. The plate locks at 22.7t (50,000lb) and stress in the arches is kept below 4,000 bar (60,000 psi). The gap is 2.3mm (0.09 inch) and the lever travel distance at the proximity switch, at 1.1mm (0.045 inch) of plate deflection, is approximately 18mm (0.72 inch).
A 1.25 inch plate can be made up from, for example, five plates of 0.25 inch thickness. So if plates were removed the link would have a different overload value.
