Mining operations can be dangerous places. Giant earth-moving machinery, drills, blasting tools, crushing equipment, feeders, and free-falling materials are just some of the hazards one can encounter. A conveying system can also present a potentially dangerous situation.
The job of the conveyor is to transport bulk materials over long distances or to feed materials to crushers, mills, screens and other processes. The belt is constantly moving, usually loud, and contains bouncing rocks, minerals, and other materials. Inevitably a hazard will arise out of those conditions.
Conveyor protection switches for bulk monitoring systems are important tools in helping to prevent accidents, protect equipment, and reduce unscheduled shutdowns (which helps to keep your profits and production at their highest levels). These switches are used for position information, control signals, and to identify potentially hazardous situations with your process equipment. When hazardous situations occur, conveyor protection switches should activate alarms to stop your equipment and protect your personnel.
There are three common types of conveyor protection switches to help ensure the safety of your operations: belt misalignment switch, safety cable pull switch, and a tripper position switch.
Belt Misalignment Switch
The belt misalignment switch, also known as a run off switch, is used to monitor the position and tracking of conveyor belts. It is usually mounted on the conveyor structure and adjusted so the roller arm is the proper distance and angle from the outside edge of the belt. When a belt drifts out of alignment it contacts the roller actuator arm which rotates the actuation shaft, causing the belt misalignment switch to send an alarm signal. The actuation shaft has two cams inside the housing. Each cam actuates an independent SPDT micro-switch for alarm signals. The first micro-switch actuates with a 10° rotation of the actuation shaft for a warning alarm signal, allowing operators to address the situation before it becomes a problem. The second micro-switch actuates with a 20° rotation of the actuation shaft for a shut down alarm signal in order to prevent or minimize equipment damage. The roller arm should have internal stainless steel bearings to allow the switch to be used on conveyors with belt speeds up to 1250 feet per minute with outstanding service life.
Safety Cable Pull Switch
The safety cable pull switch is used as an emergency shutdown device for conveyors or other equipment. They should meet OSHA and MSHA requirements for safety shutdown devices. Safety pull cables should be attached to the actuating arm of the switch, and when force is applied to the pull cable it rotates the actuating arm and actuating shaft. At a 20° rotation the switch should enter a tripped and locked position.
The actuating shaft usually has two cams inside the housing. Each cam simultaneously actuates an independent SPDT micro-switch for shutdown and alarm signals. Usually, the safety cable pull switch has one housing layout that may be used interchangeably in mid-run or end-run positions of the safety pull cables. Standard safety cable pull switches include a manual reset lever that keeps the switch locked in an alarm condition until it is manually reset by an operator.
Tripper Position Switch
The tripper position switch is a heavy duty limit switch, often used to indicate the position of a tripper on a conveyor with multiple discharge points. However, it can be used anywhere you need an extremely heavy duty limit switch with physical activation. The actuating arm of the tripper position switch usually utilizes a large Acetal contact roller, 3 in diameter by 1.75 in wide. When the roller makes contact it rotates the actuation shaft which has two cams inside the housing. Each cam actuates an independent SPDT micro-switch for position or alarm signals. Some switches offer an option where one of the SPDT switches is actuated with clockwise rotation and the other SPDT switch is actuated with counter-clockwise rotation, which allows you to identify which direction the switch is activated from.
All your switches should be NEMA 4 (or NEMA 4X) weatherproof, corrosion-resistant, rugged, and designed for heavy duty applications. Optional nickel plated finish provides extra corrosion protection. Switches for use in hazardous area applications should have CSA approved for CL II, Div 1 & 2, Group E, F & G applications ratings. The recent addition of incorporating uniquely identified network addressable modules in the switches which are the wired to a customer PLC give the user immediate information on where the alarmed switch is located.
When a potentially dangerous situation arises, your main concerns should be keeping your personnel safe and minimizing damage to your equipment. Make sure you have safety programs in place, including conveyor protection switches on your equipment, so you have the equipment in place to protect your people, process and profits.