13.1 Belt Conveyors: Components, Tracking & Training

Task D-20 (Services conveying systems) carries 5 of the 135 exam questions, and belt conveyors dominate that task. In sawmills, mines, aggregate plants, grain terminals and food plants, the millwright is the trade that installs, aligns, tracks and repairs them.

Conveyor Components

                    TYPICAL TROUGHED BELT CONVEYOR (side elevation)

   Loading skirtboard                                        Primary belt scraper
        |                                                            |
        v      carrying run (troughing idlers, 20/35/45 deg)         v
   [ ]==*==*==*==*==*==*==*==*==*==*==*==*==*==*==*==*==*==*==*==[ HEAD ]---> discharge
    ^                                                            [PULLEY ]
  TAIL PULLEY                                                   (driven, lagged)
    ^                                                                 |
    |    return run (flat return idlers, wider spacing)               v
    +----o---------o---------o---------o---------o------[SNUB]---[BEND]
                                    |                                 |
                       GRAVITY TAKE-UP (vertical counterweight)-------+

Belt Construction and Splicing

A conveyor belt is a carcass (plies of polyester/nylon fabric, steel cord for long high-tension runs) bonded between a top cover (thicker, takes impact and abrasion) and a bottom cover (thin, runs on idlers). Belts are specified by width, ply count or steel cord rating, cover thickness in millimetres, and cover compound (general purpose, oil-resistant, heat-resistant, fire-resistant/FRAS for underground).

A square splice is essential. An out-of-square splice makes the belt run to one side each time the splice passes a pulley, producing a mistrack that appears and disappears once per belt revolution — a distinctive diagnostic signature.

Belt Tracking: The Governing Rule

A belt moves toward the end of the idler or pulley that it contacts first.

Everything about training follows from that one statement. If the leading (first-contacted) end of an idler is skewed forward in the direction of belt travel, the belt migrates toward that end.

        DIRECTION OF BELT TRAVEL  ------------------------>

        Idler skewed like this:            Belt drifts this way:
              /                                    ^
        =====/=====                                |
            /                                   toward the
        (left end advanced forward)            LEFT (advanced) end

Causes of Mistracking, in Order of Frequency

1. Off-centre loading — material dropped to one side of the belt. Fix the chute, not the idlers. This is the single most common cause.
2. Material buildup on pulleys and idler shells — creates a local crown that steers the belt. Fix the cleaners.
3. Frame out of square or out of level — the belt follows the geometry. Check with a string line and level before touching anything.
4. Seized idler rollers — a locked roller drags one side.
5. Out-of-square splice — mistrack recurs once per belt revolution.
6. Belt camber (a bowed belt) — new belt stored improperly, or a belt cut from a bad roll.

Training Procedure

1. Correct the causes first. Clean buildup, free seized rollers, square and level the structure, and fix the loading point. Idler skewing cannot compensate for a crooked frame.
2. Run the empty belt and mark where it runs off. Then run it loaded — a belt that tracks empty but not loaded is almost always a loading-point problem.
3. Work from tail to head, in the direction of belt travel.
4. Adjust the idlers in the region where the belt begins to drift, not where it is already off. Adjust roughly one idler in every ten, and never more than about 3 mm (1/8 in) per adjustment.
5. Wait several complete belt revolutions after each change before judging the result. A long conveyor may need many minutes.
6. Use self-aligning training idlers on both the carrying and return runs for continuing control, but only after the structure and loading are correct.

Never attempt tracking adjustments with bare hands near a moving belt, and never use a pry bar against a running belt. Where a belt must be observed running, use the designated observation points outside the guarding.

Belt Tension, Sag and Take-Ups

Correct tension is the minimum that (a) transmits the drive torque without slip and (b) limits sag between idlers to 2–3% of the idler spacing. Excessive sag lets material tumble and spill and hammers the idlers; insufficient sag overloads bearings, splices and the belt carcass.

Take-up travel must accommodate belt stretch — commonly 1.5–2% of belt length for fabric belts, far less for steel-cord belts. When adjusting a screw take-up, move both sides equally and measure; unequal adjustment is itself a tracking error.

Drive, Lagging and Wrap

Available drive friction depends on the coefficient of friction, the arc of wrap on the drive pulley, and the slack-side tension. A snub pulley raises the wrap from 180 degrees to 210–240 degrees. Lagging — bonded or vulcanized rubber, or ceramic tiles for wet abrasive duty — raises the friction coefficient and improves wet-condition traction. Grooved lagging patterns (herringbone or diamond) must be installed with the apex pointing in the direction of belt travel so water and fines are pushed outward and shed off the pulley ends.

Mandatory Safety Devices

Conveying systems are a high-injury area. Sub-task requirements tie directly to Task A-1:

• Nip-point guards at every pulley, at the drive, and wherever the belt approaches a roller.
• Pull-cord (lanyard) emergency stops running the accessible length of the conveyor, with latching stop switches.
• Zero-speed / belt-slip switch to shut down the drive if the belt stops while the motor runs.
• Belt misalignment switches and plugged-chute detectors on enclosed systems.
• Anti-runback (backstop) devices on inclined conveyors so a loaded belt cannot reverse when power is lost.
• Full lock-out/tag-out including gravity take-up blocking. A counterweight is stored energy: it must be blocked or lowered and secured before any belt work.