Weight-operated limit switches are crucial for controlling the movement of hoisting equipment. When the snatch block or similar component reaches its top position, it pushes a plate connected to a lever on the limit switch. This action causes the lever to rotate, which in turn rotates the cam shaft and cams inside the switch. The cams then open the electrical contacts, disconnecting the motor and halting the hoisting motion. This mechanism effectively prevents over-travel and ensures the safe and reliable operation of the equipment.
The limit switch is designed
to respond to weight or force applied to it. In the context of hoisting
operations, the switch is specifically used to monitor and control the
movement of the hoist or crane.
Activation by Snatch Block:
Snatch Block Positioning: In a crane or similar
hoisting system, a snatch block (a type of pulley) is used to guide the
hoisting cable or rope. As the hoisting motion progresses, the snatch
block moves upward.
Top Position: When the snatch block reaches
the top position, it exerts pressure on a plate that is connected to the
limit switch. This plate is hanging from one end of a lever mechanism
attached to the switch.
Lever and Weight Interaction:
Weight Application: As the snatch block or hoist
reaches its uppermost position, the weight of the block or the force it
exerts pushes the plate downward.
Lever Rotation: The plate is connected to a
lever that pivots around a fixed point. When the plate is pushed by the
weight, it causes the lever to rotate.
Cam Shaft and Cams:
Cam Shaft Rotation: The lever is connected to a
cam shaft within the limit switch. As the lever rotates due to the
applied weight, it also rotates the cam shaft.
Cam Action: The cam shaft has rotating
cams fixed to it. As the cam shaft turns, the cams also rotate.
Contact Operation:
Opening Contacts: The rotation of the cams is
designed to open the electrical contacts within the switch. These
contacts are part of the power and control circuit that operates the
hoist’s motor.
Disconnecting the Motor: When the cams rotate to the
predetermined position, they open the contacts, which interrupts the
electrical circuit. This action disconnects the motor from the power
supply, effectively stopping the hoisting motion.
Preventing Over-Travel: Safe Stop: By disconnecting the motor, the limit switch
prevents the hoist or crane from moving beyond its intended travel limits. This
helps to avoid mechanical damage, ensures safety, and maintains proper
operation of the equipment.
Die-Cast Aluminum Body: Durable and lightweight
construction.
High Performance: Reliable and efficient
operation.
Counterweight: Included to lift the wire rope
and prevent over-hoist positions.
Safety Interlock: Commonly used in industries to
control machinery and ensure safety.
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