EP (polyester) belt will not stretch as much as NN. Usually, EP will stretch about 80-85% of a like weave Nylon-Nylon (NN). However, the weave construction and denier, exclusive of material, can also have a profound effect on stretch.

I suggest you obtain certified information about the belt manufacturer's specifications before you purchase. The mfgr. must provide a guaranty on belt performance. No guaranty, no purchase.

Belt's from less known mfgrs. and with a cheap sticker price is a notice of the buyer to beware. ■

Morning Irfan

We touched on the bad points of Nylon Warp and weft belts on one of larry's threads.

An excerpt from my comments is as follows:

"...We had some Korean belts which were delivered incorrectly with Nylon warp (longitudinal) as well as Nylon weft (transverse), and these rolled into almost closed pipes especially in the concaves.

The effect was noticed after a couple of days of varying load.

It looked like the varying load worked on pushing the belts into the troughs, and effectively rolled them into a pipes.

(Incidentally, these belts never stopped stretching, and the supplier had to replace them. I believe that the additional stretch in the bottom of the trough in the concave sections has a direct bearing on the pipe shape formation, as the unstretched edges want to take the shortest route instead of the middle bit)

I have not had anything like this effect on the less stretchy EP warp and Nylon weft...."

I suggest that in future you only buy EP belts, and also specify the fact that they must not elongate over time at constant load for any other reason, such as excessive crimp in the warp strands.

(Crimp refers to the amount of deformation in the warp or longitudinal strength strands, like a sine wave, due to being woven in by the transvers (weft) strands)

The crimp is locked in the rubber. When under load, the crimp wants to straighten out and as a result the rubber slowly deforms to allow this, hence the creep.

I use 1.5% of pulley centres for the take-up using EP, plus enough extra for two splices

Good luck

LSL Tekpro ■

We use NN (nylon x nylon) belts at our sandwich belt high-angle conveyors. This is precisely because of the low elastic modulus which facilitates the tighter convex curves of the troughed belts. The elastic stretch is desirable, the permanent stretch is not but is worth it.

It is important to know that nylon warp requires higher safety factors then polyester, say 12 to 14. At lower safety factors the belt may not stop stretching.

Joe Dos Santos ■

Another important point on NN belt stretch, not previously discussed is the curing of the nylon fabric prior to integration into the belt. Without the cure event, the nylon will not stop cold flowing. When properly treated, the nylon yarn surface acts like a pre-shrink fabric. Its elongaton behavior is far more stable and will elongate in the 1.5 % range rather than the 4% range.

I do not wish to enlighten, producers who do not practice this treatment. If you need further details you can email me with a request. ■

The reason some belt manufacturers do not apply the sizing process is it cost more to produce and takes special skill to apply.

Caveat Emptor. ■

Graham is right using 1.5% for EP

NN requires 3% even pre-stretched.

But the forgiveness of NN is much higher, if you have material trapped between belt and pulley or high impact, then you may stay with your elongation trouble.

With the gentle coal,you should try EP. ■

As I mentioned, we use NN warp/weft belts, at our DSI Snake Sandwich conveyors, for the low elastic modulus and radius of curvature compliance on relatively tight troughed convex transition curves. We typically allow for 5% to 6% combined stretch (never less than 4%) and, at a new belt installation, we specify a rigorous 24 hour pretensioning procedure before splicing. Except typically in the case of Goodyear belting we find that when our pretensioning procedure is ignored, the take-up will soon bottom out and the belt has to be re-spliced.

Joe Dos Santos ■