I assume you are speaking of the distance between the wing and center roll supporting the belt.

A 28 mm gap is large. You have not stated:

a) tonnage,

b) material density,

c) belt width,

d) belt speed,

e) max. lump size,

f) idler spacing

g) belt construction ( fabric or steel, cover thickness and total thickness),

h) trough angle and roll diameter

We design for long overland conveyors where the gap distance is controlled, given the above data is known, to keep the belt from failing in the crack. I assume similar to your problem. This type of belt failure was common in the grain industry until they switched to offset rolls. We design with offset rolls for the same reason. Offset rolls have no gap.

Some idlers are designed with a minimal in-line construction gap closer to 6-10 mm gap. The gap must clear the weld spittle on the roll ends or have them ground. ■

Dear tiptop

In case of fixed idlers (excluding garland idlers) we keep maximum 10 mm gap between central and wing roller. we have not faced any problem. We have used garland idlers with about 25 mm gap between rollers and have not faced any problem.

A R SINGH ■

There may be several types of belt damage that may occur in the load zone of a belt conveyor.

1. Entrapment point damage is due to belt sag. The belt sags between the idlers and conveyed material is trapped between the belt and the sealing systems. This would be a grooving of the belt on one or both sides of the conveyor directly under the sealing system. Belt support systems will eliminate this problem.

2. Impact damage from the conveyed material. This will appear randomly on the face of the belt. Impact support systems can reduce this type of damage.

3. Impingement damage caused by dropping material onto the belt. The top cover of the belt is slowly worn away due to the material striking the top cover. Curved chute designs have shown to reduce this problem.

5. Belt cupping may be caused by using trough idlers that exceed the maximum trough angle of the belt being used. this is seen as a permanent curve to the top cover making belt tracking difficult.

6. Idler junction failure is seen as a ripping or tearing of the belt in the junction of the wing roll with the center roll. This is due to the transition distance is too short for the belt being used. Keep in mind the transition distance is a minimum distance (at the head and tail pulley) based on type of belt, trough angle, belt width, and rated operating tension of the belt. ■

The Cema Standard for the pinch point between rolls is 3/8" ■

ISO specifies 10mm. Note however that as trough angle decreases it becomes increasingly more difficult to maintain this dimension whilst keeping the roll standard. ■

Dear Mr. Felipe Velasco C.,

As per European practice, the gap between two consecutive rollers should be limited to 10 mm for belt width below 2000 mm, and 15 mm for belt width 2000 mm and above. This is general practice and can be followed in most of the cases. Now a days belt conveyors generally have higher troughing angle, and achieving such gap should not pose problem for fixed frame idlers.

Regarding, garland idlers, often it is difficult to achieve the above gap if spindle size is very large. In such situation the permissible gap to be decided considering the belt thickness, stiffness and magnitude of load per meter etc. For example, heavy-duty belt conveyors (2400 mm width, 20000 mtph capacity, steel cord belt, garland idlers, etc.) at Neyveli lignite mine, had roller gap more than 15 mm. Therefore such situation to be analysed on case to case basis.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Author of Book : Belt Feeder Design and Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■