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Technical Note

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Incredible Shrinking Belt

Due to multiple calls regarding this subject, I decided to add to our Web site some information on the subject of shrinking bare back belt.

In response to two particular applications where we have seen conveyor belt shrinking, both in bare back applications.

1.)  Handling of fertilizer.
Handling fertilizer with a bare back Polyester Warp (length yarn) belt MAY cause shrinking problems.  Whether or not it does create problems is determined by two variables.  The variables are, the content of Urea in the fertilizer, and the amount of humidity/moisture in the working area and/or in/on the product.  Urea has an adverse effect on Polyester; it causes it to shrink.  In granular fertilizers there must be enough moisture present to release the Urea so that it will be absorbed by the Polyester, which in turn will cause the belt to shrink.  Under dry/low humidity conditions the Polyester will not absorb enough of the Urea to cause significant shrinkage.  But, once the moisture level reaches a point that it liquefies enough Urea, which the Polyester will then absorb, the belt will shrink.  Many people have told me "well, we don't see the PVC belts shrinking", and you won't.  Most rubber conveyor belting is made with a Filament Polyester yarn.  This means that the tiny hair sized fibers that are twisted together to make a yarn are as long as the yarn itself, or the belt in this case.  Those continuous fibers, once acted upon by Urea, will shrink causing the entire belt to shrink.  Some plied rubber belts, and almost 100% of PVC belts are made of Spun Polyester.  Instead of the fibers being one continuous length, they are cut into staple fibers, fibers approximately 3" in length.  The staple fibers are then Spun, like cotton or wool, into yarns.  These fibers shrink due to Urea absorption as well, but since they do not run the length of the belt, the belt doesn't shrink, it self destructs.  This is because the only thing that holds a spun yarn together is the entanglement of these fibers.  When the short staple fibers shrink, they pull loose from the other fibers.  This basically causes a PVC belt, or a plied rubber belt which utilizes a spun yarn, to self destruct.  This is why you do not see great belt life from PVC belts in this application.  The belt literally pulls itself apart due to fiber shrinkage.  It takes longer for a PVC belt, utilizing spun Polyester, to self destruct because the carcass, (fabric) is literally soaked in PVC.  This creates a sealing effect which slows down the ability of the Polyester to absorb the Urea.

Legg Company recommends, when handling granular fertilizers with a bare back belt, the use of our Super Slider Back (SSB) belt.  This is a belt that incorporates a highly loaded, low coefficient of friction, high duro rubber back.  It will allow you to utilize a slider bed conveyor and protect the fabric from the adverse effects of Urea.

2.)  Handling of materials which produce (super) fines.
When handling products that contain, or in applications that produce, super fines, bare back Filament Polyester belts May cause shrinkage problems.  Some of these products include potatoes, peat moss, wood bark, nut hulls, lime, rock salt, hay and feed, etc., that either contain or produce super fines when handled.  What happens is the very fine particles of material are forced in between the individual fibers of the warp (length direction) yarns, expanding the yarn which in turn causes the belt to shrink.  We have actually seen tail pulley shafts snapped due to the shrinking forces exerted by this phenomenon.  Applications such as tub grinders, which produce dust clouds large enough to hide the tractor and grinder, are especially bad about causing bare back belts to shrink.  Users have asked "why then, do we see it on some belts, and not others, regardless of the manufacturer?"  The reason some belts do not shrink under the same conditions is due, in our opinion, to the amount of twist in the warp yarns.  A Filament yarn consists of many fibers that are twisted into a yarn.  The amount of twist is specified to yield belt that will fall within the desired parameters in regards to pulley diameters, strength, troughability, load support, initial stretch, etc.  The specification for the minimum and maximum amount of twist, or the window so to speak, that is available in the industry is not controllable enough, at an affordable price anyway, to totally control this phenomenon.  

Legg Company recommends, when handling products that contain or will produce super fines with a bare back belt, the use of our Super Slider Back (SSB) belt.  This is a belt that incorporates a highly loaded, low coefficient of friction, high duro rubber back.  It will allow you to utilize a slider bed conveyor and protect the belt from shrinking due to super fine impregnation of the warp yarns.