Conveyor Pull Keys

A pull-key system includes the switches (sometimes called pull-key boxes or trip switches), the cable, supports, tensioners, and often an indicator of which switch tripped. Addressable or coded systems help on long overland belts so the control room can see where the lanyard was pulled. Conventional series-wired switches are still common on plant conveyors. Cable height, support spacing, and whether both sides of the conveyor are covered are installation design, not a single catalogue number.

Reset philosophy is shared with Controls: local reset at the switch, reset in the control room, or both, and whether a visual inspection is required before restart. A pull key that is bypassed because it nuisance-trips from slack cable or poor supports is a failed safety device. Dust, UV on plastics, and corrosion on glands are typical South African gallery conditions.

MHSA-aware underground installations and surface plants may need different enclosures and certification. State underground or hazardous-area needs explicitly. Do not treat a pull-key RFQ as a complete instrumentation upgrade; alignment switches and belt scales will not appear unless you list them.

Common applications

Pull-key work is driven by new conveyors, audit findings, and life-expired cable. Typical scopes:

• New walkway coverage — Full lanyard along accessible lengths. Provide conveyor length, which sides have walkways, and whether the return side is accessible.
• Replacement of tired switches and cable — Like-for-like refresh when glands leak, cables sag, or switches stick. Say if you want the same topology or a redesigned circuit.
• Addressable trip location — Long overland or plant galleries where finding an anonymous series trip wastes hours. This is still pull-key hardware plus a controls interface.
• Reset and indication upgrades — Better flags, beacons or PLC diagnostics. Tick Controls if software and MCC changes are the bulk of the work.

Industries that use this equipment

Mines, coal plants and cement works specify continuous pull-cord coverage on galleries as a basic expectation for people working beside moving belts.

Ports and overland systems need location of the trip across long distances; addressable systems appear for that reason.

Food and warehouse lines use pull cords and E-stops on package conveyors — still this category, lighter hardware, same briefing need for length and reset.

Underground coal and hard rock add enclosure and certification constraints that narrow the product choice.

Specifications buyers should consider

The list below is a briefing checklist, not a datasheet. Exact ratings, dimensions and standards must come from the duty, existing plant records and the supplier quotation — do not treat generic web copy as a specification.

• Conveyor length, walkway sides, and photos of existing switches and cable route.
• Existing system: series, addressable, make if known.
• Reset philosophy required.
• PLC or relay interface already in place.
• Environment: surface gallery, outdoor, underground, wash-down.
• Supply only versus install, tension, and commission with the electrical team.
• Whether other instruments are in or out of scope.

Factors that affect selection

• Hardware without a reset policy is incomplete; involve Controls if the PLC must change.
• Slack cable and wide support spacing cause nuisance trips that train people to bypass.
• Long belts benefit from location of the trip; short plant belts may not.
• Do not bury alignment switches and scales inside a pull-key RFQ.
• Guarding and walkway condition affect whether a lanyard can be reached — that may be a Structure job.

Common problems and failure considerations

• Gaps in coverage where a person cannot reach the cable.
• Sagging cable that trips in the wind or from vibration.
• Switches that cannot be reset locally, so production restarts from a control room with no inspection.
• Bypassed keys after nuisance trips.
• Quotes for a full instrumentation suite when only lanyards were needed — or the reverse, missing the PLC work.

Maintenance considerations

• Walk the lanyard: height, tension, broken supports, cracked enclosures.
• Test trips on a planned, safe procedure; record which switch was tested.
• Replace UV-damaged cable and leaking glands before water fills the switch.
• After structure or walkway work, re-check that the cable is still reachable and not trapped.
• Never leave a jumper on a pull-key circuit without a formal, time-limited waiver.

Questions buyers should ask suppliers

• Switches, cable, supports and tensioners — what is included?
• Series or addressable, and how do we see where it tripped?
• Is installation and tensioning included?
• What PLC or relay interface do you assume?
• Enclosure rating and any underground certification?
• Will you survey walkways, or quote from our length only?

What to include when requesting a quote

Clear briefs produce comparable quotations. Include what you know, and mark estimates separately from measured values. Photos and drawings help more than extra adjectives.

• Length, walkway sides, photos of existing pull keys.
• Reset philosophy in one paragraph.
• Existing PLC/MCC make.
• Supply versus install.
• Underground or hazardous-area notes.
• Explicit exclusion of other instruments unless listed.
• Related Controls tick if logic must change.

Related conveyor information

Related components: instrumentation, controls, structure.

Guides: Industrial Conveyor Maintenance Guide, Types of Conveyor Systems, Choosing a Conveyor Supplier in South Africa.

Resources: Conveyor maintenance guide, How to request a conveyor quote.

Frequently asked questions