A belt conveyor has the advantages of simplicity of construction, high efficiency and low frictional loss resulting in low power requirements and economy in upkeep. It may be horizontal or inclined or a combination of both and can be arranged to convey material up or down an incline.
There are two main types of belt conveyors – flat and troughed. A flat belt conveyor is generally used for bags, bottles, boxes and other packaged materials, although under certain conditions, loose material in bulk, such as soap, may also be satisfactorily handled. A troughed belt conveyor enables the loading capacity to be increased by over 60% with out danger of spillage and is suitable for almost any bulk material in a dry damp or semi – wet condition. It is due to this flexibility, that belt conveyors are used in almost all industries handling bulk material viz, cement, steel, fertilizers, food, etc., to name a few.
1. CONVEYOR BELTING
The belt consist of layers or piles of fabric duck, impregnated with rubber and protected by a rubber cover or both sides and edges. The fabric duck supplies the strength to withstand the tension created in carrying the load while the cover protects the fabric carcass.
2. CONVEYOR BELT DRIVING GEAR
The belt is usually driven by means of an electric motor through
There are several methods of obtaining maximum driving efficiency.
2.1 Short and medium length conveyor generally allows a plain open drive
from which traction is available.
2.2 The tension requirements of longer conveyor may be obtained by one ore more of the following:
• Increasing the initial or slack side tension
• Using a snub pulley to increase the arc of driving contact (Fig.4)
• Using a tandem drive to further increase the arc of contact
• Lagging the driving pulley with a covering of rubber as the co-efficient of friction between rubber and rubber is more than that between rubber and steel. Further, pulley wear can be avoided by this arrangement and only the lagging need be changed from time to time.
2.2.1 Conveyor Belt Pulley Sizes
For good service, terminal pulleys should be as large as possible. The pulley diameters are selected based on the recommendations from belt manufacturers depending on belt tensions. Small pulleys are cheaper in initial cost, but pulleys below the minimum recommended diameter may cause ply separation or even break the outer plies of the belt prematurely, thus increasing replacement cost.
2.2.2 Belt Edge Clearance & Pulley Widths
The life of conveyor belt would be curtailed if the edges were allowed to come in contact with structure guards or chutes. The minimum pulley widths are specified in IS 1891 (Part 1.)
3. BELT CONVEYOR IDLERS
The belt runs on rollers, known as idler rollers or idlers, because running freely, power is neither received nor transmitted to, the belt. Most conveyor installations are now equipped with 3 roll troughing idlers for the carrying side and single roll idler for the return side.
The rollers are usually made of ERW pipes. The components within the ERW shell are a central shaft , bearing housing, antifriction bearings, multi – labyrinth seals and circlips. The idler rollers are supported on conveyor frame by means of brackets often called as transom.
Carrying idlers support the belt as well as the load and they should be so placed that they limit the amount of belt sag under load between adjacent idlers to maximum of 3% of idler spacing. Carrying idlers are commonly 100 to 150mm in diameter and the troughed shape increases the carrying capacity of the belt by over 60%. The angle of troughing idlers is normally 200 to 300 . With availability of synthetic duck belting in the country, the use of idlers with deeper troughing angles, such as 350 and 450 is on increase.
Return idlers are usually 75 to 150 mm in diameters and spaced at approximately 3 meter intervals.
3.1 Self Training Conveyor Idler Rollers
Intermediate sets of troughing and return idlers are mounted in a manner which permits the idler axis to be adjusted a few degrees on either side of the correct centerline in order to train the belt centrally on the idlers. On long conveyors, additional guide roller are sometimes installed to overcome any tendency of belt to depart from its true path.
4. CONVEYOR BELT TAKE-UPS
A take-up has three functions.
• It applies the initial tension necessary to provide tension between the driving pulley and belt.
• It removes accumulation of slack in the belt due to belt stretch.
• It provides sufficient reserve belt length to enable a new joint to be made if necessary.
There are two types of take-up devices in general use, namely :
4.1 Conveyor Positive or Screw Take-up
In this arrangement the tail pulley is installed over slides so that it can be moved in the line of the belt with screws. It is commonly used on conveyors upto 60 meter length. The arrangement is simple as it requires no extra bends. The disadvantage is that it is not self – adjusting and the exact tension applied by screws cannot be ascertained.
4.2 Conveyor Floating or Gravity Take-up
In this arrangement, there are two types – vertical and horizontal. The vertical type requires more headroom for installation and consists of weighted pulley on return run of belt as shown in Fig.9.
Two other pulleys are required to complete the arrangement and hence it gives the belt three extra bends. The horizontal type shown in Fig.10. consist of carriage on which tail pulley is mounted. The carriage is forced to retract under the influence of weights attached to the carriage.
The advantage of gravity take-up is that it is self – adjusting and the tension in the belt can be accurately controlled by the weights. Also, much greater movement of take-up is possible for which reason gravity takeups are strongly recommended on long conveyors.
5. LOADING AND UNLOADING OF RUBBER BELT
The rules for correct loading of conveyor belt are :
• The material to be placed on the belt should be fed uniformly in the direction of belt travel at the speed as close to the belt speed. The belt has then only to carry the load and not bring it upto its own speed.
• The material should be fed along the middle of the belt and the height of fall kept to a minimum
• The belt should never have an idler under it at the point where the material lands.
• Spillage, especially on to the return belt, should be prevented by means of skirt board.
5.1 Belt conveyor Loading Chutes
The feed to belt conveyor is usually achieved directly by means of a chute or indirectly by means of a chute or indirectly through vibratory feeder or transfer feeders. Except where the material is transferred from one belt to another, indirect loading is usually more satisfactory.
Chutes are generally made of wood or steel. The wood or steel may wear out rapidly due to abrasion. Chutes are therefore fitted with renewable liners. Chutes may be open or closed depending on material being carried. A closed chute will decrease dust hazard but it prone to get jammed . A jammed chute will rapidly ruin the belt. Hence, any chute should be wide enough to deliver the two largest lumps that it would ever handle side by side and be 70% to 80% of the belt width. The general arrangement is shown in Fig.11.
5.2 Skirt Boards Rubber
These are used in conjunction with chutes. They are essential to guide the material centrally on the belt while loading until it has settled down to prevent spillage. They consist of strips of steel not less than 1.5 meter long and placed in direction of belt travel. At their commencement, they may be placed two-thirds the belt width apart and they should widen out in direction of belt travel.
The gap between the skirt board and the belt should be sealed by a wear strip of soft rubber.The skirts should not touch the belt and old conveyor belting should never be used for wear strips. The skirts should flare slightly upward from the chute to prevent the material from wedging.
5.3 Unloading of Belt
Usually the belt is discharged over the head pulley. The path of trajectory of the material leaving the belt is determined by belt speed and gravity and this has to be determined by belt speed and gravity and this has to be determined while deciding the location and form of the discharge chute or hopper. The chute should be so placed that sufficient clearance is available between the belt and chute so that even the biggest lump handled by the belt shall not jam the two.
6. CLEANING & PROTECTION OF BELTS
Most materials handled in bulk by belt tend to cling to the belt after passing the discharge point and then fall off on the return run on meeting the return idlers. Dirt on the belt while passing over snub or tandem pulleys can seriously affect the belt.
Thorough cleaning of belt immediately after the discharge point and prior to return run should receive careful consideration.
Four methods of cleaning the belt are in common use, but for particularly sticky material a combination of two or more methods may be required.
• If the material carried is lumpy, then all that may be required for effective cleaning is a rapping roller. This usually takes the form of an eccentric roller which, as it turns, exerts a rapping effect on the belt, shaking the adhering particles free.
• A rubber scrapper can be very effective for materials such as gravel, coal, etc. It is usually mounted on the counter weight pilot arrangement to bear on the belt immediately under the discharge pulley.
• For really sticky material such as clay, a steel scrapper may be essential but great care must be taken when adapting this methods as a badly fitted or misaligned metal scraper can cause extensive damage to belt.
• Rotary brushes are becoming more popular for dealing with all materials. The brushes should rotate in the direction opposite to belt travel and at a sufficient speed to keep the dristles clear of material.
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