Dear George,

I dont know about Garlock Bulldog. I do know there is a ton of cheap often called scab belt from a Jack and Judy shop where QC control and customer satisfaction is low but the purchasing officer says is the best selection based on price not withstanding delaminations, et al.

Large stretch is a product of no pretreatment of the fibers. The higher priced spreads Resorcinol treat the fibers to give a preshrink effect. The individual fibers then can carry greater loads, load share between fiber groups, and stretch less. These belts are in the 1-2% range. Polyester, the normal selection for the tension strand, and nylon for the transverse fiber for impact resistance and compliance for troughing and training.

Cheap belts need to be ironed -- run in for a while to control the excessive stretch. Three days may be a little short. These belts need to be loaded for a while maybe up to 10 days to get to an equivalent level of a pretreated belt tensile fiber.

Lawrence Nordell

www.conveyor-dynamics.com ■

Hello George..

Conveyor belt covers are SBR rubber, and if treated nicely look like black mirrors, especially on the return side.

Regarding stretch, as L K N indicated, there are two main components in the re-inforcing.

The Warp (longways)is often made from EP or polyester

The Weft (crossways) is often made Nylon ans stretches more therefore making it good for troughing.

Sometimes you get a belt made from only nylon, and this can cause a lot of stretch especially over time.

We had a project where each day the counterweights were at the top and next day they were at the bottom. We kept cutting lenghts out until it settled down.

This nasty stretching also caused the belt to form tube shapes in the concave curves.

So don't go for nylon warp.

Otherwise you can buy from where you like really as long as the fabric is crimped in the weft direction and virtually straight in the warp, and there is no ply or cover separation.

Regrds from LSL Tekpro ■

George

I believe what your man is talking about is the permanant stretch as I will attempt to describe. Conveyor belt elongation consists of two type of stretch, permanent & elastic. Permanent is the stretch that occurs during the first few weeks of operation of a new belt and elastic is the normal stretch that a belt undergoes during normal operation of the system. Take-up travel is usually calculated based on a total of these two factors, (which can be determined by your belting supplier) in every case you need to determine the amount of take-up that needs to be designed into a system. If the sytem is not designed to take up the total of the two types of stretch you will need to remove the permanent stretch. In my opinion you will not get all of the permanent stretch out of a belt in only 2 or 3 days. Especially with an all nylon belt such as Graham has mentioned.

It might be better to install a mechanical splice initially until the permanent stretch is out of the belt and then do the vulcanized splice at a later date. Then take-up travel is normally sufficient for the elastic stretch of the belt.

Hope this helps,

Gary ■

Dear George,

First of all, the Garlok brand name represents is a good one. The Garlok belt is definately not a cheap one (produced in Paragould, Arizona, USA).

That being said, high initial belt stretch will depend on factors such as the type of carcass weave, the amount of fabric tensioning during belt manufacture, the type of material used in the warp strands, the tension used during belt rolling, et cetera. Generally, nylon warps will give substantially higher stretch than polyester warps. Loosely rolled belts will have a higher initial stretch than tightly rolled belts. Belts made with highly tensioned carcasses will stretch less than belts of the same carcass material produced under low tension. Etc., etc.

As far as pretensioning is concerned, it only helps to get rid of the minor stretch factors - such as belt roll tensioning, unless you apply a lot of force. Even then, the only part of the belt that gets Pre-Stretched is that section close to the weights - unless you prestretch for a long time (30+ days), which is too long to tie up equipment. The idlers, pulleys, and carcass weft strands all act as tension modifiers as you move away from the pretensioning wieghts (or force) and the initial stretching of the rest of the belt will start once you start to move it around the system. That is, in my opinion, pretensioning is of very limited value.

If you know that you have a belt that is likely to stretch past your take-up allowance, you would be better served to start off with mechanical fasteners and take them out once the rate of stretching has stabilized.

Hope this helps.

Regards, ■

Dear Mr. George Baker,

There are two types of elongation in belts, during course of operation. These are known as elastic elongation and permanent elongation. The belt normally operates at nearly 10% of breaking strength. The belt subjected to the load of the value of 10% of breaking strength, will have following percent of elongation (this is only an approximation).

- Elastic elongation for EP belt : About 1%

- Permanent elongation for EP belt : About 0.8 to 1.5 %

Now, the entire length of belt is not uniformly under 10% of breaking load, so above elongation values are to be adjusted in accordance with tension diagram, and then converted into stroke of the take-up.

Permanent elongation occurs more during initial phase. It is due to straightening of fabric and creep elongation of fiber material. The elastic elongation is in response to tension in the belt. If the belt is not pre-stressed then belt elongation will be more during short time interval and one has to re-vulcanize the belt, as take-up will reach its limit. If the belt is pre-stressed, such need for re-vulcanizing will occur after comparatively more time interval.

In important plants belt is vulcanized on conveyor, operated for certain duration of time as per the contractual condition and then the plant supplier is required to re-vulcanize the belt eliminating all the elongation! So, the belt stretch reality after installation cannot ignored and is to be taken care appropriately.

Regards,

Ishwar G Mulani.

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

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

Everyone above has given valuable information as to belt stretch characteristics. However, little was stated as to the belt manufacturer and their reputations. First, we have found Garlock belting to be of high quality. We have installed and spliced quite a lot of it throughout the US.

In regard to the "Bulldog" brand of belting; this is not Garlock's brand. This is a brand of Scandura belting. The crossing of manufacturers and brands may be an indication of the lack of information or expertise of your belt seller and your apprehension toward his recommendations. ■

The quality of the belt depends on how it suits your needs. Though nylon x nylon has given way to polyster x nylon (warp x weft) as dominant there is no denying the supperior performance of nylon with regard to impact absorption and poking of trapped material. Nylon is more stretchable but if it is superior for your application you accomodate the stretch (longer take-up). The performance is more important.

As mentioned by others stretch is made up of two parts, permanent and elastic. The ISO 9856 modulus test determines the two stretch components by repeated elongation and relaxation between 2% and 10 % of the breaking strength. A stress-strain graph over the 200 loading cycles reveals the permanent stretch component and the permanent elastic modulus (after the manufacturing crimp is largely removed from the belt).

Prestretching prior to splicing does help where the permanent elongation is significant, particularly in nylon x nylon belts. As standard practice we (Dos Santos International) clamp the belt ends with soft timbers and draw them together with come-a-longs. every couple of hours we retension the belts after they have relaxed. We do this typically over a 24 hour period. This helps in removing permanent stretch that woud otherwise occur during the first day or two of continuous running under load. We keep the belt tensioned during the splicing. Only the ends that are being spliced are relaxed. Failure to do this may mean that you must resplice within the first couple weeks of operation.

Mr. Spriggs mentioned the case of a nylon x nylon belt that just kept on stretching. It is not well known in the industry that nylon is chemically a wax. This is meaningless to us when the maximum operating loads remain less then about 1/14 to 1/12 of breaking (as the belting behaves elastically). At higher loads say 10% of breaking, or greater, The nylon molecules will continue to slide past each other manifesting itself as creep elongation and Poisson narrowing of the belt.

I hope this is helpfull in understanding the behavior of fabric belting and why prestraining helps.

Joseph A. Dos Santos ■