The automatic gravity take-up is the most common type used on bulk haulage conveyors. A movable pulley with a weight box maintains slack side tension during starting, stopping and load changes by moving to accommodate elastic stretch in the belt. Underground mine applications commonly use air and hydraulic take-up units to accommodate belt stretch.
Weight take-ups or other automatic types (springs, air or hydraulic) are also used on lightweight belt conveyor applications. Automatic take-ups are generally preferred where space allows them to be used as movable take-ups and they put less stress on the belt. Gravity take-ups are generally low-maintenance and fool proof. When we try to determine where to position the conveyor pulley for its travel during new conveyor belt installation, we incur problems. The problems faced by the installer are:
1. Getting slack pulled out during installation, allowing enough spare belt for re-splicing.
2. Where to set the take-up pulley initially.
3. Factors influencing belt elongation such as load, friction, drive type, carcass type, heat etc.
4. Take-up bottoming out due to combination of slack, and elastic stretch.
5. How much will this belt stretch?
All these problems must be understood and answered in order to string and splice a new belt, as well as, avoid another shut down to remove a section of belt when the belt becomes too long for the take-up system.
There are several ways to pull slack out when stringing a new belt:
1. Use gravity by stringing the belt from the low end around the conveyor and splicing at the lowest elevation.
2. Use a motor to pull the new belt on, then tie and lock one end off while pulling out the slack. It is easier to pull the slack out of the belt down-hill rather than up-hill.
Determine if a mechanical or vulcanized splice will be used and allow enough extra belt for the type of splice chosen. A good rule-of-thumb for vulcanized splices is 2.5 x belt width for belt consumed in a fabric belt splice. Positioning the take-up pulley is important. A few feet should be left at top-of-travel due to possible lift up of the take-up pulley during start-up. At least 75% of take-up travel distance should be available for elongation after the initial vulcanized splice. This available travel can be adjusted to reflect splice length, total elongation, changes in loading, environmental changes, type of textile fiber and the carcass designs.
One factor influencing elongation is percent of load applied verses the amount of working tension available in the belt being utilized, i.e. a belt with a 440 PIW working tension would stretch more at 375 PIW than at 215 PIW. SKE has stress/strain data in Build a Belt and Belt Wizard for each carcass type so that elastic elongation can be predicted.
Heat can also be a problem as hot belts stretch more due to changes in fiber properties.
Higher friction will raise tension resulting in more elongation. Examples of these increases would be frozen idlers, conveyor idlers with higher rolling resistance, heavier conveyor pulleys, skirtboards, plows, wipers, etc. Drive location, belt wrap and lagging types influence tension applied to the belt.
There are various carcass types discussed earlier in this manual that will influence elongation (plain weave, twill, straight warp). The type of fiber used for the strength member in the carcass will also influence elongation, i.e. there are many types of belt carcass materials, including polyesters, nylons, aramids and steel, used in the construction of a belt carcass. The tension member chosen can influence elongation several percent (a variation of 0.5% to 7.0 % is not unusual).
The objective for the user and the installer is to involve the belt manufacturer in selecting the best belt for the application. A part of reaching this objective is to understand the difference between elastic and inelastic elongation. Elastic, by definition, means recoverable elongation. Inelastic, by definition, means non-recoverable elongation. Elastic elongation occurs during acceleration, braking, and load changes during running. This is why automatic take-ups are so common - they compensate for these length changes easily. Typical elastic elongation with polyester fibers is 1% or less, usually more than 1% with nylon fibers and less than .5% with Aramids and steel.
Inelastic elongation is permanent and occurs during the break-in period. The fabric weave will influence the amount of stretch to be expected. The benefit of belts made with polyester fibers in the tensile member is that they break-in quickly, usually within the first 3-4 days of running at full capacity. Nylon warp yarns continue to "creep" and will continue to have permanent elongation (although small in percentage) for the entire life of belt.
To reach our objective of properly setting the take-up, we must calculate a total belt stretch. A 500 ft. center-to-center conveyor with a taped belt length of 1,080 feet runs at 280 PIW using a 3 ply 330 PIW rated belt made with plain weave, polyester warp fabric. The belt runs at an ambient temperature of 72° F and is subjected to fully loaded, "hard" starts at a maximum of 392 PIW.
1. Expected permanent (inelastic) elongation of 1.2 % x 1,080' or 13 feet.
2. Expected elastic stretch of 0.8% (@392 PIW) x 1,080' or 9 feet.
3. Plan to set the take-up at 5 feet from top of travel (Leaving 10ft. of belt in the take-up).
Total take-up travel is then:
1. 5 feet for position.
2. 6.5 feet for permanent movement of 1 ft. = 2 ft. of belt take-up.
3. 4.5 feet for elastic elongation.
4. 16 feet of total travel (32 feet of belt) or approximately 1% of tape length + 5 feet for position (10.8 + 5 = 15.8 feet).
What if a mechanical take-up or screw take-up is used? How do we tension the belt properly to accommodate elastic and inelastic elongation? First, we must calculate operating tension at the worst possible condition and be able to pretension the belt so that the inelastic (permanent) stretch is pulled out. Then we have the belt tightened so that the elastic elongation will be recovered within the length of belt at the slack side. In other words the belt must act like a spring or rubber band. When the belt senses lower tension it must recover and remain tight enough to prevent slippage on the drive.
Usually we cannot achieve the required pretension at initial installation since there is not enough travel and screws cannot exert enough force on the belt. Therefore the belt must be re-tensioned during the break-in period, perhaps several times. Mechanical splices may be necessary, as the belt will require re-splicing to shorten and remain within travel limits of the screw take-up.
Unusual conveyor configurations can also result in abnormal belt stresses. Conveyors such as stacker conveyor that have the drive at or near the tail end will have almost the entire length of belt running at close to full operating tension. This condition can cause abnormal belt stresses. Reversible conveyors with only one motor and one take-up will exhibit greater than normal belt stretch. This is due to the counterweight being heavier than normal. The increased counterweight is required to maintain tension in the system when the motor is reversed. The increased weight results in greater tension applied to the belt.
The amount of take-up tension that will be required is determined by the location of the take-up assembly. The counterweight must apply the maximum static tension to prevent drive slips and belt sag under all operating conditions. For most installations, the counterweight need only equal the combined tension of the carrying and return sides of the belt.
Where take-up travel is restricted, a double-reeved counterweight system can be installed.