Belt conveyors are among the equipment where workplace accidents occur most often in crushing and screening plants. The nip points between rotating pulleys and idlers and the belt, and cleaning and interventions carried out on a running belt, can cause serious injuries. Every conveyor should therefore have safety equipment such as emergency stop pull cord switches, belt misalignment switches, speed (slip) sensors, guards and, where necessary, rip detectors, and these should be tested regularly.
In this guide we cover the types of conveyor safety equipment, where and how to install them, test and maintenance frequency, lockout/tagout (LOTO) practice and spare part selection. For general belt conveyor maintenance, see our belt conveyor guide.
Safety Equipment and Its Functions
| Equipment | Function |
|---|---|
| Emergency stop pull cord switch | Stops the conveyor when the cord is pulled at any point along its length |
| Belt misalignment switch | Gives a warning or stops the conveyor when the belt runs too far off |
| Speed / slip sensor | Stops the conveyor if the belt stops or slips on the pulley |
| Rip detector | Detects a longitudinal rip early to limit damage |
| Chute blockage sensor | Detects a blockage at the transfer point |
| Fixed guards | Prevent access to pulley and idler nip points |
| Backstop and brake | Prevent the belt running back on an inclined conveyor |
Installation Rules
- Pull cord switches should run continuously along the entire accessible length of the conveyor on the walkway side; if there are walkways on both sides, they should be on both sides.
- Misalignment switches should be positioned on both edges of the belt, especially near pulleys and at points with a high risk of mistracking.
- The speed sensor is usually located on the tail pulley or the return belt, at a non-driven point.
- Guards should be fixed so that maintenance cannot be done without removing them; mesh or perforated plate can be used for inspection.
Interlocks and Sequential Starting
When one conveyor in a line stops, all the equipment feeding material to it (feeder, upstream conveyors, crusher) must stop automatically. Otherwise transfer points fill up, chutes block and spillage occurs. Sequential starting and interlock logic are the foundation of the automation system; for details, see our article on crushing and screening automation.
Test and Maintenance Schedule
| Check | Interval | Method |
|---|---|---|
| Pull cord switch function test | Monthly (every switch) | Pull the cord to confirm the stop; keep a record |
| Cord tension and integrity | Weekly | Visual |
| Misalignment switch test | Monthly | By moving the arm by hand |
| Speed sensor test | Every 3 months | Through the control system |
| Guards | Daily | In place and sound? |
| Backstop and brake | Every 3–6 months | According to the manufacturer’s instructions |
LOTO: Working Safely on a Conveyor
- The conveyor and connected equipment are stopped.
- The power supply is isolated at the main switch, locked and tagged.
- On inclined conveyors, the belt is secured against belt tension and the risk of running back.
- On a gravity take-up, the counterweight is safely supported.
- Confirm the conveyor will not run by trying the start button.
- When the work is finished, the lock is not removed until the guards are refitted.
Important: A pull cord switch is an emergency stop device; it is not a substitute for LOTO. Maintenance and cleaning must always be done under LOTO.
Common Mistakes
- Not testing or bypassing pull cord switches,
- Removing guards and not refitting them,
- Cleaning on a running belt,
- Disabling a misalignment switch because it keeps alarming (the real problem is the mistracking),
- Neglecting backstop checks on inclined conveyors.
Troubleshooting Table
| Symptom | Possible cause | Solution |
|---|---|---|
| Misalignment switch trips frequently | Real belt mistracking | Eliminate the cause of mistracking |
| Speed sensor false alarm | Sensor distance, contamination | Adjustment and cleaning |
| Pull cord switch does not work | Slack cord, faulty switch | Tension the cord, replace the switch |
| Inclined belt runs back | Backstop failure | Backstop check and replacement |
An Example from the Field
During a safety audit at one plant, it was found that some of the pull cord switches on a long conveyor had not been tested for years and two switches did not work. All the switches were renewed, a monthly test form was introduced and fixed guards were fitted at two pulley areas where guards were missing. The plant had no non-conformities in this area at subsequent audits.
Pre-Purchase Checklist
- Have the conveyor length and number of pull cord switches been determined?
- Have the misalignment switch and speed sensor types been chosen?
- Have the protection class (IP) and environmental conditions been considered?
- Has compatibility with the control system (contact type, signal) been checked?
Frequently Asked Questions
How often should pull cord switches be tested?
Testing every switch at least once a month and keeping test records is good practice.
Does a misalignment switch protect the belt?
In serious mistracking it prevents belt and structure damage by stopping the conveyor; however, the root cause of the mistracking must be eliminated separately.
Is a rip detector necessary?
It is recommended for long, expensive belts, especially on conveyors with a high risk of ripping, such as under the primary crusher.
Typical Causes of Conveyor Accidents
Most conveyor accidents happen in foreseeable situations. The main scenarios are:
- Cleaning on a running belt or near a pulley,
- Intervening at a pulley with the guard removed,
- Trying to adjust tracking by hand while the belt is running,
- Retrieving material that has fallen under the belt while the conveyor is running,
- Someone else starting the conveyor because no lockout was applied,
- An inclined belt running back unexpectedly.
Almost all of these scenarios can be prevented by keeping guards in place, applying LOTO and keeping safety equipment in working order.
Points to Watch in Guard Design
- Guards must comply with the distance and opening dimensions that prevent reaching the nip point.
- Designs that do not require removing the guard for inspection and greasing (extended grease lines, inspection windows) should be preferred.
- Guards should be fixed with fasteners that require a tool and should not be easily removable.
- Guards should not allow material to build up, so as to reduce the need for cleaning.
Selecting Safety Equipment
| Equipment | What to consider |
|---|---|
| Pull cord switch | Protection class, cord length capacity, latching reset |
| Misalignment switch | Two-stage (warning + stop) contacts, roller material |
| Speed sensor | Inductive or mechanical type, mounting distance, resistance to contamination |
| Rip detector | Suitability for belt type and width |
| Backstop | Torque capacity, mounting type |
Test Record Form
| Field | Record |
|---|---|
| Conveyor and switch number | Location of each switch |
| Test date and tester | Signature |
| Result | Stopped / did not stop |
| Correction made | Cord tensioning, switch replacement, etc. |
Safety System and Automation
In modern plants, the status of safety switches is passed to the control system and the operator screen shows which switch has tripped. This lets the fault location be found quickly and allows root cause analysis based on frequently tripping switches (e.g. an area that constantly gives misalignment alarms). Safety circuits should be designed with safety relays or a safety PLC in accordance with standards.
Stock Planning
- Keep at least one spare pull cord switch, misalignment switch and speed sensor of each type.
- Keep spare cord, cord tensioners and fittings in stock.
- For inclined conveyors, add backstop and brake spare parts to the critical spares list.
More Questions
How is cleaning done safely on a conveyor?
No cleaning should be done until the conveyor is stopped and locked out. Measures that reduce spillage (skirt rubber, scrapers, impact beds) reduce the need for cleaning and therefore the risk.
Should there be pull cord switches on both sides?
If there are walkways on both sides of the conveyor, there should be pull cord switches on both sides.
Why does a misalignment switch alarm frequently?
Usually there is a real mistracking problem; rather than disabling the switch, eliminate the cause of the mistracking (loading, pulley build-up, idler alignment).
Spare Parts and Service for Conveyor Safety Equipment
CSP Mühendislik supplies pull cord switches, misalignment switches, speed sensors, guards and backstops as part of its belt conveyor spare parts range, and provides service support for installing and testing belt conveyor safety systems. You can share your requirements on WhatsApp.