Q&A Session- Question 1: Why are there 16″ scraper blades on an 18″ belt?

ANSWER:

Telebelts are equipped with “crowned” pulleys. Slightly tapered pulleys allow for easier adjustment when training or tracking of the belt. This is especially true on faster moving belts such as those used to place concrete. This feature is even more important for the purposes of belt training when the conveyor is telescopic, multi-section as is the Telebelt. Trust me, flat pulleys were tried in the very early design and training the belt throughout the telescope range was impossible! Even when only half the pulleys were flat it was still an impossible task to track the belt.

When a tensioned belt wraps around a tapered pulley it leaves that pulley with its edges opposite the pulley taper. That is to say the belt comes off the pulley in a crescent shape, with edges of the belt somewhat lower than the center (see diagram 1, left). For this reason, using a scraper as wide as the belt wears out the edges of the belt pre-maturely.

Getting proper surface tension between the blade and the belt also becomes more difficult because the force being used to pull the scraper against the belt has to deflect the edge of the belt that much more before adding any appreciable force in the middle or center (once again see diagram 1). This problem becomes worse the further behind the pulley the scraper is mounted. This is because you now have to deflect the tensioned belt in both directions, width and length, before significantly effecting the tension of the blade against the surface of the belt (see diagram 2, below, – upper right-hand corner).

You may have read in some of our past literature words like belt “flutter” and scraper “bounce”. These are also symptoms of the scraper being too far behind the pulley. A mistimed or an unlucky scraper bounce can catch a rock and render the scraper useless, or worse wreck a belt! A “fluttering” belt will not run clean. In all cases, an 18” blade on an 18” belt makes every condition or symptom worse!

When you hear customers saying things like; “I just can’t get that thing to run clean no matter how much I tension the scraper!” or “My scraper springs are stretching and I have run out of adjustment on the scraper chains or T-handles!”, the main two causes are scraper blades that are too wide and scrapers that are too far behind the pulley.

Besides these problems, you have to ask why do you want to clean the belt out that wide anyway? The tunnels, even on a machine set up for high volume placing, will only expose a max of 12” or 14” of belt, and most are set up with only about 9” to 11” of belt exposed. The material leaves these transfer skirts on a “V” shaped belt and the material profile on the belt doesn’t reach to those outside edges of the belt. If the concrete is so liquid that the material profile within that the normal “V” shape or in the discharge transition area reaches the edges of the belt, more than likely the belt is running too slow or there is too much belt exposed between the skirts or both. The bottom-line is that on a machine that is properly set up and run, there is nothing out on the last inch of belt on both sides to clean!

The only historical leg to stand on in defense of same width blades and belts is that the earliest Telebelt model I mentioned earlier ran 16” blades with a 16” belt. There are couple reasons for this. Firstly, that machine ran 4” pulleys and they just were not as aggressively tapered, lessening the severity of the systems described above. Second, and the most reality based reason, is that the manufacture just never got around to making a 14” scraper blade (assigning a whole new part number, etc). Every other concrete conveyor built used the ‘2” narrower than belt width scraper blade’, up to and past 48” belts. So, the only machine that ever used same width scrapers did so because the manufacture just never got around to making a 14” blade with some of its components allowing it to get away with that oversight. That doesn’t make it correct!

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