Limit switches have practically limitless applications in regulating crane operation to prevent potential incidents. For instance, when ropes come out of groove or in preventing two trolleys from impacting on the same bridge structure.
In addition, a limit switch keeps a crane from lifting loads above rated capacity. Another kind of limit switch prevents a hoist from reaching the upper block and rope drum. Yet another type protects motors from overheating.
Without overhead crane limit switches, the potential for harm is quite high. For this reason, each limit switch should be inspected and maintained regularly.
As they open or close electrical circuits, limit switches wear out over time. To monitor wear and replace switches before they fail, limit switches should be checked daily before a crane is used.
Knowing how a limit switch works is important for understanding how the crane works and how these established set points keep the crane operating safely and efficiently.
In some cases, there may be multiple limit switches associated with a crane operation such as crane travel or hoist motion. The first switch will signal that the end of the safe travel limit is approaching and slows the crane motion. A subsequent switch will indicate that the end of the safe travel distance has been reached and stops the motion.
Hoisting motions may be served by several limit switches, which provide set points for safety and reliability. An initial limit switch will slow the hoist before it reaches the stop limitation. Slowing the hoist prior to reaching the limit decreases mechanical wear, as the hoist is traveling below maximum speed when the brakes set.
The “up” stop limit switch provides a safe position for the hoist to stop motion for safe travel motions and standby positions.
An additional limit beyond the stop limit—typically called the “ultimate limit”—prevents the hoist block from impacting the drum. The ultimate limit acts as a safeguard in the event the stop limit fails or is set improperly.
This extra limit prevents two-blocking—that is, when the hook block contacts the drum hoisting machinery. Two-blocking is one of the most dangerous incidents involving cranes, as it can snap wire ropes and cause dropped loads and other catastrophic accidents.
After being tripped, the ultimate limit may require a manual reset. This involves resetting a wedge component or lever so that an inspection can be carried out to determine why the crane reached this position and did not stop at the up stop limit position.
Limit switches can also be used for travel motions, providing consistent and predictable motion limitations without impact at end stops. In automated environments, lasers or encoders track positions of crane movements, and limit switches provide consistent calibration points to insure values being used are accurate for safety and reliable production.
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