Conveyors can fail for a variety of reasons. Bulk raw materials such as grain or rocks may be damp or wet -- significantly adding to the weight on the conveyor belt and ultimately leading to belt slippage and slowdown. Conveyor belts can break due to overload, wear and tear, or the motor driving the conveyor may fail.

Any one of these scenarios, if left unattended, can lead to lengthy stretches of costly downtime and wasted product. In the worst case scenario, a fault condition in a hazardous environment may lead to a situation that could endanger both people and property. Cleanup along with time taken to carry out repairs can be much more extensive than would have been the case had a speed switch been in place.

How They Work

Speed switches are used to help maintain conveyors' operational integrity and are a critical component of any safe and efficient operation. It is vital in many cases that a relatively constant speed is maintained -- if a conveyor were to experience unwanted slowdown or even complete stoppage, the results could be disastrous and dangerous.

Speed switches come in a variety of forms, each designed to fit a particular application covering virtually every conveying situation. There are 2- and 3-piece systems and versions for explosionproof and corrosive environments, and for dirty, dusty, and greasy environments -- all with the same end result in mind: to protect the operational integrity of the conveyor and larger industrial process. (Shown here, the Model M100 from Electro-Sensors.)

Speed switch systems usually monitor rotation on the tail (or non-driven) pulley via a pulse generator (usually a pulser disc or wrap with alternating polarity magnets set around the perimeter) and sensor, which in turn connects to a switch/relay module. The operator calibrates the setpoint (trip point -- usually set at about 90% of operating speed) at the module using either a rotary dial or potentiometer. When a fault condition occurs, the switch will trip the relay, shutting down the conveyor or activating some type of audible or visual alarm.

Typical Application

For example, suppose a belt conveyor is conveying crushed rocks from one part of a quarrying operation to another. The conveyor runs at approximately 100 rpm and the operator needs to know immediately if the tail pulley speed drops to 90 rpm or below -- any slowdown of 10% or more could indicate conveyor overload, belt breakage, or motor failure, leading to catastrophic results.

This application requires a speed switch system that is simple to install and easy to calibrate to monitor the speed of the conveyor system. Sensor and switch electronics must be unaffected by moisture, dust, grime, and grease, and be in a rugged cast aluminum housing designed for use in a harsh environment where falling rocks are commonplace.

Electro-Sensors provided Model M100, a 2-piece system comprising a UL-listed speed switch with a setpoint range between 10-100 rpm and a 4 in. aluminum pulser disc to generate pulses (M100 with 255 A pulser disc). The unit was installed on the conveyor's tail pulley; the disc was end-mounted on the rotating shaft to be monitored, and the switch was installed adjacent to the disc face.

The speed switch accepts an input voltage of 115 Vac, readily available in most industrial settings, and has a form C relay that can be wired to a PLC, alarm circuit, or motor shutdown in the event of a fault condition.

The M100 speed switch with 255 A pulser disc is a self-contained system; rugged, reliable, simple to install and calibrate, and designed to operate in a harsh environment.

Benefits

An economical solution, the M100 speed switch system ships from stock and combines ease of use with simple, quick installation to avoid lengthy process downtime. The system is delivered onsite complete; installation requires minimal additional components; and minimal maintenance is needed during normal operation. Once installed, the system is designed to alert the operator when a fault condition occurs, enabling a prompt response. In many cases, prevention of just one incident more than covers the cost of purchase and installation.