What Type of Join is the best – cold or hot vulcanization?
Both methods are equally good and supplement each other. In some cases cold joins will give you what you need. However, sometimes hot vulcanization has to be chosen if, for example, you are dealing with heat resistant belts or cleaner systems. Generally hot is recognised as best.
What are hottest temperatures that cold vulcanization can take?
The recommendations of the manufacturers state a maximum of approximately 70° Celsius. We follow manufacturer’s recommendations as a rule. In practice, the limit is just below 90° Celsius, but to go up to this limit continually is not recommended.
How much do belts stretch in hot conditions?
Actually rubber belts get shorter in hot conditions. This is because the heat causes an expansion of the volume. Since the cord in the fabrics is twined at specially calculated angles, the volume expansion causes a contraction force lengthwise. However, the change in length is so small as to be negligible.
As belts get old they may show signs of crazy cracking more than others. Why?
What breaks down rubber more than anything else is ozone attack. This will occur around electrical motors, and UV-radiation through exposure to sunlight. Belts are fairly well protected against these factors nowadays. For places with a lot of sunlight there are belt types with extra protection, so called ’tropical quality’. Also, heat ages rubber, and 70° Celsius is regarded as an upper limit for common standard quality.
How fast can a conveyor belt travel?
For the belt there is really no limit. At speeds of more than 7 m/s balanced rollers and pulleys should be used, and they must be checked and maintained continuously. This becomes uneconomic in the long run. Therefore, in practice, 6 m/s is the maximum belt speed.
How cold can a belt get before it cracks?
For a standard belt, the limit is about -35° Celsius. However, this point is relatively unimportant. It is more important that the stiffening of the rubber should occur as slowly as possible. Ideally, the belt would keep the same flexibility down to the brittle point, but unfortunately this is not possible In special ’polar qualities’ the brittle point is at about -50° Celsius, which, of course, also positively affects the points at which stiffening occurs.
Why does common wear rubber swell in oil while, for example, nitrile rubber does not?
Wear or abrasion resistant rubber has, in principle, the same molecular structure as oil. This means that it is easy for oil to mix its molecules with rubber molecules with a resulting increase in volume. Nitrile rubber has a different structure and cannot absorb oil. The reason that there is still some swelling is that a rubber blend contains 15-20 different ingredients, of which some can absorb oil.
Splicing problems can be caused by several factors. Contact the manufacturer and solve the problem together. As a rule, splicing should be carried out in accordance with the manufacturer’s recommendations. It is not advisable to splice belts that have already been in service however inserts are often used successfully to replace damaged belts.
You often hear that less counterweight increases the wear life of belt and splice. Is this really true?
No, it is not true. The task of the counterweight is to tension the belt to avoid slippage, and also to prevent excessive sagging between the idlers. This should not exceed 1%, as energy consumption then increases as well as wear, due to larger rolling resistance. Furthermore, the transported material will move up and down every time it passes the rollers. This creates an unnecessarily large amount of belt wear, and at the same time the splice suffers from all the different stretching forces. Also, a belt that is correctly tensioned is more stable in operation.
How long can you store a belt before it is used and how should it be stored?
Using today’s material there are practically no time limits. There are examples of belts that have been stored for more than fifteen years before being put into operation, and their qualities were totally unchanged. The best storage conditions are indoors, in the dark and at normal room temperature. The risks of damage due to ozone or UV-light are then reduced. Often inside storage is impossible so wrapping of the belt is recommended. After many years, it is possible that there may be a slight tendency towards cracking at the belt edges and on the outer wrapping of the belt. The quality of the fabrics, however, still remains the same.
When you are skiving a belt when splicing, does it matter whether you pull the rubber fabric away in narrow strips or whether you pull the whole strip away in one piece?
It depends on what adherence values the belt has. If the adherence values are high and you pull the whole piece there is a risk that the belt ends will be misaligned, which will result in a splice which is not straight. On the other hand, if the adherence values are normal this is not a problem.
How is adherence measured and what values are normal?
Pull a rubber strip or fabric strip straight back along the belt. The pulling force needed is measured in N/mm (kg/cm). The norms state a minimum value of 5N/mm between the layers of fabric and 4.5 between cover plate and fabric. If the cover plate is 1.5 mm or thinner the adherence can be 3.5 N/mm. A good value is around 5-7 N/mm. Skiving gets unnecessarily difficult if this is exceeded, and you do not gain any real advantage. At values of over 9 N/mm you usually have to remove the cover plate in small pieces or grind it off. This is common if you are re-splicing an older belt where the rubber may possibly have deteriorated.
It is sometimes said that a belt with high elasticity has a longer wear life because it is more flexible than a belt with low elasticity. Is that right?
Yes and no! A loaded belt with high elasticity sinks deeper between the carrying idlers. Since the upper cover plate then stretches at every carrying idler and since the belt is stretched from the beginning, heavily increased wear results. All stretched rubber has less resistance against wear. Being more flexible also means larger steering problems, even when only small defects appear on the conveyor. There are conveyors designed in such a way that a belt with larger elasticity is required for it to function on the conveyor. So, sometimes it can be an advantage to have a belt with high elasticity.
How much does a belt weigh?
The density is approximately 1.1 g/cm3. So, a belt which is one meter wide and ten millimeters thick weighs approximately 11 kg/m. There are also cheap fillers, for example, clay, without wear properties, which can be added to the rubber. This means that a belt constructed with ’cheaper’ rubber quality becomes heavier.
Today one belt is as good as another. I mean, a belt is a belt. So I can buy the cheapest alternative, can’t I?
If you had said the same about car tires a number of opinions and experiences would be proposed straight away. And naturally this is the same for belts too. In conveyor belts we generally see different belts in the market as specifically affective in either Abrasion, Impact, chemical or Heat applications. What is a good abrasion resistant belt will not be the best impact belt and what is a good impact belt is not the best abrasion resistant belt. The cheapest will give you the poorest result unless you have no abrasion, impact, heat or chemical attacking issues to deal with. Chemical resistant belts have very little abrasion or impact qualities as another example.
In the old days there were rubber edges on the belts. Today most belts are delivered with a cut edge. Why is that?
Cotton belts were sensitive to moisture and capillary reaction would see moisture working its way into the middle of the belt. This will cause delamination and premature failure to the belt and or join. Modern belts are not sensitive to moisture these days and can be manufactured to the most suitable and economic width and then cut to the desired width. This method may make the belts cheaper and at the same time the properties have been improved. So, the reason for the rubber edge was not just to protect against damage to the edge but also to stop moisture contamination.
Which is the best troughing angle?
The best troughing angle is 45° on a three-roller idler. It gives the highest and most economic conveying capacity and, at the same time, the best belt steering if the conveyor is designed for this. Compared with older types of belts, it is even possible to use this troughing angle for very narrow belts today. However consideration has to be given to the thickness of the belt. For example, if a belt is 600mm wide and has a 4 ply with 6mm x 2mm covers, then it may struggle to trough at this angle. If this happens then steering this belt will be very hard if not impossible.
Are there rules for minimum cross strength in a belt?
Yes and no. For all belts up to EP 400 the cross strength must be at least 40% of the length strength. For belts stronger than EP 400 there are no minimum values
How long can a conveyor be if you use a screw take-up?
The tasks of a take-up device include creating enough pre-tensioning to avoid slippage. With horizontal conveying there is little difference in load between an idler belt and a fully loaded belt. The usual opinion is that the maximum length is approximately 50 mt. However, there are examples of Conveyors as long as 150 mt. For inclined conveyors, where there is a high difference in tension, the limit is approximately 30 mt. The lengths stated can be more than doubled if you use belts with low stretch. Although these are manufacturers and design engineer recommendations, there are many applications where these numbers are ignored successfully.
Conveyor belts sometimes burn. How much energy is there in a belt?
The energy value for a normal belt is about the same as for oil, 40 MJ/kg. As a rough comparison you can say that the energy in a normal belt roll (300m, 1200 mm width) corresponds to what you would need to heat up a normal house for a year. But, in fact, a standard belt does not burn quite like that. To start with it has to be a heated to 350° Celsius before the inflammable gases are liberated, and then approximately another ten degrees before it ignites. For natural rubber the equivalent temperature requirement is 230° Celsius.
When do you have to use terpene-resistant or oil resistant belts?
That is a tough question. The problem with terpene-resistant belts is that they are already extremely slippery at a temperature of 5° Celsius. The incline should therefore, not be greater than approximately 6°. By mixing SBR-rubber into the cover plates of terpene-resistant belts, you can make them somewhat less slippery hence MOR belts.
How much can rubber be compressed?
Rubber is easy to compress provided that it can expand in one direction or another at the same time. If there is no room for expansion it is not possible to compress it. This means that rubber, in fact, can be looked upon as uncompressible