Conveyor Belt Maintenance
Special care must be exercised to keep the return rolls and snub pulleys clean. Buildup of material on this equipment has a destructive effect upon training with the result that the belt may run against the structure and damage itself. It is advisable wherever possible that return idlers be suspended far enough below the structure so that any misalignment or dirty idlers can be easily seen. Caution must be used to insure that cleaning devices are used before the materials are allowed to accumulate to the point that the belt is running in it, and it creates more damage than if cleaning it had not been done at all.
Keeping the return rolls and snubs clean requires that the belt be clean when it enters the return run. Scraping is the most common method of doing this.
Rubber scrapers can be made by clamping rubber slabs ½" to 1" thick (not old belting) between two metal or wooden bars. Extend the rubber about twice its thickness beyond the bars and suspend the mechanism with a counter-weight to provide the pressure against the belt. Replace the rubber when it wears down near the bars. Two or three such scrapers can be used in succession.
The most common steel scraper is a series of diagonally set blades mounted on the end of a leaf spring to maintain pressure against the belt. These will scrape sticky materials which rubber scrapers may ride over.
Washing the belt with a water spray before wiping with a rubber scraper will do a good cleaning job on almost any material, including iron ores and mixed concrete.
Dry materials can be cleaned off the belt with rotating bristle or rubber vane brushes, driven at fairly high surface speed, usually three to five times the belt speed. They wear rapidly, require considerable maintenance and are likely to fill up solid if used with wet and sticky materials.
It is preferable to clean just after the head pulley and before the snub. An exception to this is that sticky material often requires scraping on the head pulley. This is because a large part of the fine material sticks to the belt and must be scraped into the chute.
In some cases the best possible cleaning is insufficient and steps must be taken to compensate for the effect of a dirty belt. Snub pulleys can be kept from building up by the use of soft rubber lagging or by scraping directly against the pulley. Diagonal grooving will distort and discharge accumulations on these pulleys. Rubber disc or spiral type return rolls prevent build-up on themselves and thus save a training problem.
The only cleaning required on the pulley side is removal of material, principally lumps, which may fall or bounce onto the return run, and be carried between the belt and tail pulley if not removed. Rubber faced plows immediately in front of the tail pulley are used for this purpose. They are usually held against the belt by gravity and set at an angle to the direction of belt travel.
Another point on a conveyor which is often overlooked is the discharge end. Occasionally at the point of discharge to a pile or bin, material overfills and the belt runs in the material and the belt is worn or torn off and rendered unusable. If there is a possibility of this happening, care must be taken to monitor the operations or possibly install a level control switch to prevent damage.
Generally most wear and tear in a conveyor belt occurs at the loading point because of the material impact on the belt. The loading point of any conveyor or radial stacker is nearly always the critical point, the life determining point of the belt. Here the conveyor receives its major abrasion, and practically all of its impact. The "ideal condition" is to have the material pass from chute to belt at the same speed and direction of travel as the belt with a minimum amount of impact, and strike the belt between and just forward of the idlers.
Receiving material off-center will cause the belt to move sideways after loading as the center of the load seeks the lowest point in the troughing idlers. This can be corrected by proper chute arrangement provided, of course, that the belt is centered as it enters the loading point.
The subject of chute design and arrangement is too broad to be discussed in detail, the following suggestions are offered:
The width of the receiving end of the loading chute should be great enough to accept material lying on the extreme edge of the preceding belt or feeder, and its position determined by the trajectory of the material coming into it. At no place should the chute be less than twice the size of the largest lumps, if fines are present, and 3 1/2 times the size of lumps, if uniform. The discharge width of the chute thus determined should not exceed about 2/3 of the receiving belts' width.
The slope of the chute is determined by the nature of the material, its entering velocity and length of the chute. This value varies with each particular installation, but about 35° has been found satisfactory for most dry industrial materials such as coal and rock.
An attempt to approach the above "ideal condition" should be made continually by adjusting the chute arrangement. Optimum loading and transferring through chutes still requires considerable experimental adjustment in the field.
Skirt boards should be used to further center and settle the load as it leaves the loading point. The steel structure of the chute and skirts never should be placed closer to the surface of the belt than 1", this distance to be made increasing in the direction of belt travel to free any material trapped between the belt surface and the skirt. Skirt boards are usually 4 or 5 times the belt width in length, but may vary considerably due to belt speed, type of material and lump size.
Impact of material being loaded on the belt is often the cause of severe cuts and gouges. The degree of impact can be lessened to some extent by providing a cushion in the form of rubber covered disc type or semi-pneumatic idlers, which also tend to prevent material from crowding under the skirt boards at the instant of impact.
The use of a “Grizzly,” a slightly fanned row of bars, at the bottom of the chute reduces wear on the belt. It distributes the impact of large lumps by allowing the fines to fall onto the belt first to act as a cushion. The fan shape of the “Grizzly” in the direction of travel prevents jamming of the lumps.
A “V-slot” cut in the bottom of the chute is another very satisfactory method of allowing fines to fall on the belt before the lumps and thereby reduce belt wear at this point.
Belts should be stored if at all possible, upright in its factory package until used, in a dry room between 50°F and 70° F, free from sunlight, steam pipes, oil and corrosive fumes. Under no conditions should rolls of belt be laid flat on a concrete floor. Moisture will shrink any exposed fabric which gets damp from such storage and the belt is liable to "bow" on one edge. Upright rolls on a dry wooden floor are recommended. Belts weighing more than 25,000 lbs. should be stored on A-frames and rotated a quarter turn every three months.
Reels or rolls should never be dropped from a freight car, truck, or other means of conveyance since their weight will break the packaging and may damage the belt. Reels or rolls should always be rolled or provisions should be made for hoisting them. For hoisting, a bar is passed through the hole in the center of the roll. Chains or cables looped around the bar ends should be provided with a spreader above the roll to avoid damage to the belt edges.
For additional information regarding these topics and many others, please consult our Radial Stacker / Conveyor manual. The manual is available as a downloadable and printable PDF version.