HEAVY DUTY WEIGHING
MODULAR BELT SCALES
Design and Installation
|Type of idler||No. of idlers|
|Fig. 6||Flat Idler…………………………………. 20 degree Troughed…………………||2 2|
|Fig. 6A||35 degree Troughed…………………||3 (1-350, 2-200 )|
If the conveyor is longer than 75 feet, a gravity take-up must be provided to maintain a uniform belt tension. The device shall be of the counter-weighted type with either vertical or horizontal travel. The take-up must not introduce excessive belt tension and should be designed to allow the addition or removal of counter weights.
E. RIGID CONSTRUCTION
The conveyor structure (particularly in the area of the scale) must be rigid enough to resist deflection under maximum load. Conveyor stringers at the scale and for not less than 20 feet before and beyond the scale shall be continuous or securely joined and of sufficient size (and so supported) as to eliminate relative deflection between the scale and adjacent idlers when under load. The conveyor stringers should be so designed that the deflection between any two adjacent idlers within the weigh area does not exceed 0.025 inch under load.
F. FRAME WEAKNESS
Weakness in the conveyor frame (see Figure 7) is reduced by reinforcing the frame supporting the scale span or by installing additional support legs (Figure 7b) to equalize conveyor deflection at the weigh idlers. Angled braces (Figure 7c) or supports installed under each idler (Figure 7d) provide the most stable weighing section. Angled braces are usually the simplest solution for tubular frames.
G. IDLER WEAKNESS
Weaknesses in idlers are usually considerable for troughed belt conveyors. Since the load is imposed at the middle of the idlers (seeFigure 8), the idler beams usually deflect 0.04 inches (1 mm) or more. This leads to considerable weighing error. Reinforcement of the idlers adjacent to the scale span (2 approach and 2 retreat) with additional welded beams can minimize weighing errors caused by idler deflection.
Rope conveyors are not acceptable unless a rigid section is provided for the belt scale as shown in Figure 9.
H. RIGID IDLERS
Always use rigid 3-roll in-line type idlers on the belt scale and at least two idlers either side. Do not use offset center roll or garland-type idlers.
I. WIND BREAK
(See Figure 10) If the location of the belt scale is to be subjected to high winds (above 5 mph) the conveyor belt must be shielded by use of a wind break. The wind break must extend to 40 ft. either side of the scale location and 4 ft. above and below the belt line. (The length of the wind break may be reduced on short conveyors less than 80 ft. in length.)
J. FIXED INCLINATION
If the conveyor is inclined, the inclination must be fixed at one specific angle. Portable conveyors are not recommended.
K. STEEP INCLINES
The conveyor inclination must not exceed the material angle of slide to prevent roll back and reweighing of the material.
L. CONCAVE AND CONVEX CURVES (See Figures 11 & 11A)
If the conveyor is designed with a concave curve (Figure 11), the belt scale must be located 40 ft. from the point of tangency as curves induce inherent mis-alignment. Concave curves require special attention, since the belt tends to lift when running empty. As a rule, errors can be avoided if there is a sufficiently long, straight conveyor section between the scale and the curve. The preferred scale location is shown below the curve in ‘SECTION A’.
If the conveyor is designed with a convex curve (Figure 11A), the belt scale should be located in ‘SECTION A’.
Note: If ‘SECTION A’ (Figures 11 & 11A) is not long enough to satisfy a proper installation, alternate ‘SECTION B’ should be considered, providing material tests can be run to establish a necessary correction factor to compensate for the inherent scale location error.
M. TRAINING IDLERS
If training idlers are used, they must not be located within 60 feet from the centerline of the weigh span of the belt scale.
N. STICKY MATERIALS
Belt scrapers or belt cleaners are recommended for materials that tend to stick to the belt and would tend to affect the zero of the belt scale.
O. NON-UNIFORM LOADING
The conveyor loading mechanism shall be designed to provide uniform belt loading over the belt scale through the full range of the scale operation. If the loading is not uniform, a series of diagonally placed plows should be used to eliminate hills and valleys in the material. Sufficient impact idlers shall be provided in the conveyor under each loading point to prevent deflection of the belt during the time material is being loaded.
P. MULTIPLE FEED POINTS
Wherever possible; avoid installing the belt scale on conveyors with multiple infeeds unless all feed points will be used at the same time.
Q. REVERSIBLE CONVEYORS
Avoid trying to weigh accurately in both directions on reversible conveyors. The use of two scales is a better solution.
If the conveyor is provided with a tripper, the belt scale must be located at a point 40 feet from the point of tangency of the belt with the tripper fully retracted (toward the tail pulley). See Figure 12.
SECTION 4 – DETERMINING NUMBER OF WEIGHING IDLERS
The general rule is that control scales require a minimum of one suspended idler whereas an inventory scale (or one that is to be used for a high degree of accuracy) requires at least two or three suspended idlers. There are two overruling factors; (1) belt speed, and (2) weight per foot of material on the belt.
1. BELT SPEED
The material must not travel across the weigh span in less than 0.4 seconds.** If the travel is less than 0.4 seconds, either the number of idlers suspended or the idler spacing will have to be increased to permit a longer time interval across the weigh span. The other alternative would be to slow the belt speed down, which may affect the maximum capacity of the belt conveyor system. (See Figure 13)
Weigh span = idler spacing (feet) X # of weigh units. Speed in feet/second (FPS) = belt speed / 60 seconds Weigh span / FPS = must be greater than 0.4 seconds*
** 1.0 seconds for certified scales
Conveyor belt speed = 500 FPM Idler spacing = 3′-0″ Therefore, for a single weigh unit: 3′-0″ / 8.3 = 0.36 seconds
The example does not satisfy the design requirements; therefore, one of the following three factors will have to be reviewed.
|a.||Increase the idler spacing.|
|b.||Add a second weigh unit.|
|c.||Slow down the belt speed, which may affect the maximum capacity|
of the belt conveyor system.
2. WEIGHT PER FOOT
All scale designs are limited to a minimum weight that may be sensed accurately. The minimum load is calculated as follows:
300 TPH conveyor running at 150 feet per minute. LB/FT belt load on conveyor= 300 * 2000 (short tons) 150 * 60 = 66.66 LB/FT belt load The weight on the load cell = 66.66 * the idler spacing in feet = 66.66 * 3 = 199.98 pounds The minimum load on a model 475 modular belt scale is 10.00 pounds.