The conveyor belt is a common piece of mechanical handling equipment that moves or transports materials from one location to another. Conveyors are especially useful in applications involving the transportation of heavy or bulky materials. For moving material, normally two types of equipment are used. Conveyors are used when the path is fixed and industrial trucks are used when the path is free.
Various types of conveyors and mono-rail systems are used in many industries to eliminate manual labor, to expedite the movement of materials, and also to facilitate the processing or assembling. Belt conveyors are widely used and they are of flat or trough type and can be horizontal or inclined. They are used for handling almost all the materials of the modern industry including coal, coke, grain, fertilizers, and building materials such as sand and gravel.
- Conveyors shall be so constructed and installed as to avoid hazardous points between moving and stationary parts or objects. Gears, sheaves, sprockets and all moveable parts shall be securely guarded.
- Where workers have to cross over conveyors, regular crossing facilities affording safe passage and adequate lighting shall be provided. To cross underneath, at least 7 feet headroom is required. Crossovers should be with bridge, stairs and handrails.
- Conveyors shall be provided at loading and unloading stations and at other convenient places with devices (pull wire) for stopping the conveyor in emergency. A pull wire at intervals of 8 m should be provided.
- When two or more conveyors are operated together, the controlling device shall be so designed that no conveyor can feed on to a stopped conveyor. This may be interlocked.
- Where conveyors extend to points not visible from the control station, they should be equipped with gongs, or signal lights to be used by the operators before starting the machinery so as to warn persons who might be in position of danger.
- Conveyors shall be provided with automatic and continuous lubrication system or with lubricating facilities so arranged that oiling and greasing can be performed without the oilier coming into dangerous proximity of the moving parts. Scrappers should be provided to remove sticky material.
- Workers should not ride on conveyors.
- For repair/maintenance work, power must be totally stopped and danger-tag should be displayed.Belt cleaning by flammable solvent should be avoided.
- Tension pulley – nips. and idler rollers should have fixed guard on their complete length of movement.
- Moveable dead/counter weights should also be similarly guarded.
- Static charge collectors should be provided close to the outrunning sections of the drive pulleys and idlers.
- Elevated conveyors should have walkway with toe board (10 cm) and hand rail (I m) all along the length. Flooring should be non-slip type, particularly on sloping walkways. Handrail should also be provided on the belt side. Stop cord must be within easy reach.
- Underpasses should have firm ceiling. Guards should be provided below all conveyors passing. over roads, walkways and work areas.
- Conveyors running in tunnels, pits etc. should be provided with sufficient lighting, ventilation, drainage, guards, escape ways and maintenance clearance.
- If a hopper is used at floor level to feed conveyor, the grill or guard should be provided to prevent falling of a person inside. Scrapper may be provided between the hopper bottom and the moving conveyor. This will restrict the excess flow of material and alert any person under passing it.
- If two or more conveyors operate in series, it should be so interlocked that if one conveyor stops, all conveyors feeding it are also stopped.
- In case of reversing or running away possibility, anti-runaway and backstop devices should be provided so that the load cannot slide or fall in the event of mechanical or electrical failure.
- Overload stop devices like slip or fluid couplings and shear pins are desirable.
- The loading and discharge points of powdered material on conveyor should have exhaust hood for dust removal.
- If the material is combustible, the dust concentration should be below LEL, electric fittings should be flameproof, the conveyor should be grounded and its parts bonded to prevent differences in electric potential. The container into or from which the material is conveyed should also be similarly grounded and its parts bonded.
- Tight fitting clothing, safety shoes, goggles in dusty area and respirators are useful equipment.
- Before starting maintenance or repair work, power should be locked in ‘off position with key in pocket. It should be tagged also with ‘not to start’ notice.
- Fixed scrappers or revolving brushes to remove sticky material from drum or pulley eliminate need of hand operation.
- Static charge collector should be placed nearer to the outrunning sides of the drive pulleys and idlers.
- Operators should be trained in safe work procedures near belt conveyors. They should be trained to avoid pinch points to save their hands and fingers.
Injury due to entanglement of clothing, body parts, etc with running parts of conveyor system, falling of material from height, Falling from height, snapping of belt joint. Before starting any activity in a conveyor belt, the worker must be aware of ‘six direction hazards’ in the working area.
Before changing/replacing any component of the drive (coupling, gearbox, or motor), do the positive isolation of the potential energy source (take-up unit) of the belt by holding it with a pair of suitable chain blocks.
A belt is considered to be aligned properly, when under full load or in no-load the edges of the running belt consistently remain within the width of the pulley faces and within the confines of other rolling components, such as idlers. If the running belt deviates from the above definition of aligned tracking, it is called ‘belt sway’.
At site lagging of gravity, take-up pulley must not be done. If required, the pulley should be taken out from its position and then lagged. For pulleys other than gravity take-up pulley, If lagging has to be carried out at the site, the pulley must be freed from the belt, the belt must be held away from the pulley by at least 1 m on all sides. If it is not possible to clamp the belt at least 1 m away in all directions of the pulley. The site lagging should not be done. Standard scaffolding to be made for lagging purposes. The pulley should be checked for its lagging condition (if lagging is present), axial shift and its bearing condition at regular intervals. Weld-joint condition of the disc with hub and shell of the pulley should be checked every six months.
The jammed or damaged idlers should be replaced immediately, as they have a high potential of damaging the belt (especially the belt joint). The space between two idlers should be maintained as per standards, based on belt width and bulk density of the material. For example, for a belt of width, 1200 mm and carrying material of bulk density 1.2 t/m3 the distance between two troughing idlers should be 1.0 m and that for return flat idlers should be 3.0 m. The minimum distance between two troughing-trainer idlers should be 15 m and that for return trainer should be 30 m.
For better control on belt sway, the ‘true-track idlers can be used, such as, In return side, fix the idler (i) at 4.0 m away from the tail pulley. (ii) At about 2.0 m towards the head end from the bend pulley of GTU and (iii) In return side, at 4.0 m away from the head pulley. For heavy-duty belts, it is advisable to lift the counterweight for idler changing. Belt lifting devices should be used for idler changing.
Deck plate :
Deck plate must be given below the troughing belt. It protects the entrapment of spillage material between the rotating pulleys and the belt. The locations where the conveyor gallery is passing over the road or working area, the deck plate below the return belt must be provided.
Belt safety switches –
- The switch allows smooth running of the conveyor and protects it from damages by over swaying which occur due to uneven loading of material, womout idler roller bearings etc.
- It is also known as run off switch.
- The length of lever of ‘belt sway switch’ should be in the range of belt during belt sway. The ‘pull cord switch’ with LED indicator should be used.
Pull cord switch-
- Pull cord switch also know as rope operated emergency switch is used as a safety switch to stop the conveyor belt in case of an emergency.
- It is also known as pull switch cord.
Zero speed switch-
- Zero speed switches also known as speed activity sensing switches are used to detect whether a rotating shaft is turning in various machines conveyors, powerplant, sugar, textile, paper, etc.
- Control actions are:-
- Zero speed protection
- Under speed protection
- Over-speed protection can be achieved.
Associated Hazards of conveyor belt
- Belt sway (Belt Tracking)
- Do not wear loose clothing or jewelry near the conveyor belt.
- Do not put your hands on a moving conveyor belt.
- Provide proper illumination at the working zone.
- Ensure the ‘OK’ condition of the pull cord and emergency stop switch of the conveyor belt.
- Keep one skilled person with a ‘walky-talky’ near the pull cord.
- Communicate with the control room before aligning/tracking the running belt.
- Move the trainer idler by pushing/pulling its frame from the ends only.
- Do not try to move any damaged/jammed trainer idler.
- Do not touch trainers directly by hand, use hammers for training idler.