Vertical curves come in two different styles: concave and convex. Both types of curves create conveyor belt operational problems when the curve is designed with a shorter radius than required.
Concave curves, when designed properly, will allow the conveyor belt to lay in the conveyor idlers when the belt is running empty or full without lifting off of the idlers.
When concave curves are designed with a very short radius, belt lift off is a secondary problem. The stresses that develop in the belt are due to the very short radius and belt failure will happen quickly. The only way to combat the situation is by use of a very low modulus fabric. This fabric is extremely elastic or stretchy. These types of conveyor frames are rare and are usually designed around a low modulus fabric.
The simple concave curve is designed around Radii from around 300 feet to 800 feet. This is the type of curve that may create lift off problems. Lift off results from the curve being designed or built with a radius usually being 100 feet or 200 feet short of ideal. Lift off problems from this type of conveyor can be solved fairly simply.
Concave curve belt conveyors that have a hard start usually lift off at start up if the belt is empty. A soft start with an extended time delay to full RPM is the first step to keeping the belt in the idlers. The next step is the installation of a hold down roll in the curve area. The preceding picture utilizes a hold down roll at the beginning of the curve and another roll in the middle of the curve. The hold down roll is set above the belt, so as not to interfere with product movement. The roll should only touch the belt when lift off occurs with an empty belt.
The last step, if needed, is to select a heavier belt. The increased belt weight will also help keep the belt laying in the conveyor rollers. The increase in belt weight will normally not require any change in motor horsepower or take-up weight.
Convex curves create another type of belt problem. A convex curve with a short radius will pull the belt down into the conveyor idler gap as the belt goes over the apex of the curve. This will create idler gap failure in the belt resulting in the belt developing a longitudinal slit in the idler junction area. Due to the lines that become visible on the conveyor pulley cover of the belt, this type of failure can be spotted early. This line later becomes a crease as the carcass starts to deteriorate. Another way to spot this problem early is to look in the idler junction area. The paint on the idlers in the gap area will wear off due to belt tension in the idler junction area.
The apex of the curve, if severe, will cause edge stresses so high that the splice will open up along the belt edge, the belt will suffer from edge delamination and fabric will rupture at the belts edge. A poorly designed curve takes the appearance of a mountaintop with one idler placed at the top of the peak. This problem is a result of a short radius curve causing excessive edge tensions.
Further complicating the problem of idler gap failure is that CEMA, the Association that sets idler standards, currently does not have any set standard on the amount of gap in the idler junction area. This gap between the rolls that the belt must span without being forced into the opening can easily approach 1”.
There are numerous ways to overcome problems associated with idler gap failure due to short radius curves and excessive idler gaps. The following is a series of ways to solve this problem.
1. Change the center distance of the idler sets in the curve area. Place the troughing idlers as close as possible to each other. This will help level out the curve
2. If the conveyor uses 35° idlers, install closely spaced 20° idlers in the curve area, providing spillage does not occur.
3. The most common method is to utilize closely spaced long centered off-set idlers. This type of idler does not have a gap between the rolls.
4. The worst case will require numerous idlers on each side of the apex to be shimmed up to straighten out the curve.
5. The last step is to remove any self-training troughed idlers from the curve area.
Self-trainers are elevated in order to provide tracking and this elevated feature further accentuates the idler gap problem. When dealing with excessive edge stress, the same 5 choices listed above should be used. A low modulus or "stretchy type" fabric may be required, in addition to shims and changing idlers.