Cranes on Navy ships and in nuclear power facilities operate under some of the most brutal conditions. Conventional safety links for such equipment, typically C-shaped plates that deflect minutely, often become inoperable within weeks. According to Igor Kormishkin P.E., project engineer at Allied Systems Co., "The microswitch and set screws (used to limit the load and shut the crane) are very tiny and rust easily in a marine environment or inside nuclear containment buildings." In addition, the C links have to be large to provide a factor-of-safety of at least 4:1, but their deflection is small, roughly 1/64 inch, making it difficult to adjust the set screw that trips the microswitch.
As load tension stretches the tensor link, "target" level (A) pivots to a stress-limiting calibrated position (up to 30,000 lbs for the link shown, tripping a proximity switch (B) to shut the crane. For a safety factor of about 18:1 on this 1 1/4 inch thick link, the cap (C) also closes at 50,000 lbs and the neck helps take up the load.
Kormishkin figured that a safety link with a larger deflection could use a more robust proximity switch having greater displacement for increased reliability. But a large deflection C-shape link would be too large to mount on a seagoing crane. "I wanted to combine two contradictions, a large deflection and a higher factor of safety, at least 10:1."
The first design iteration resulted in what he calls a snake-shaped plate—an S-curved link with a straight "safety" link attached to its back. The latter would take up the load at a set tension in the primary S link. "This worked," notes Kormishkin, "but it was complicated to make and expensive."
The resulting configuration (see figure) is a stainless steel plate that uses flexing arches, a lever (for large displacements), and interlocking central necks (to provide safety at high loads). "We eliminated the extra safety link and the mechanism is easy to adjust," he says. An adjustable link, with coarse and fine threads, connects the upper and lower (on the lever) parts of the tensor. This adjustability accommodates large tolerances between drilled pickup holes. A light-tension (5-lb) spring eliminates clearances in the lever-pin joints for consistency in lever deflection. The gap between upper and lower portions of the tensor plate is great enough to provide a large deflection, about 3/4 inch at the proximity switch, but keeps the stresses in the flexing arches below the yield limit for fatigue resistance.
Contact Igor Victor Kormishkin, Allied Systems Co, 2300 Oregon St., Sherwood, OR 97140; Tel: (503) 625-2560; Fax: (503) 625-7980; E-mail: email@example.com ; or Enter 502.
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