Page 15 - MetalForming December 2011

Pulley Diameter Versus recommended Maximum Conveyor Width, Using 8 Class (8N/mm) Belting
24
18
12
6
0
12 15 19 22 25
1 1.25 1.5 1.75 2 Solid Steel Crowned Pulley Diameter, in. (No V– Groove)
component deterioration.
Pay Attention to Tension
Correct tension varies, but typically it’s recommended to be 1 mm of belt stretch per foot of conveyor length. However, with allowable manufactur- ing tolerances on belts, a belt will nor- mally be a little longer or shorter than its stated size.
To ensure correct tensioning, a sim- ple technique has been developed to allow maintenance people and even novice operators to set correct tension in a matter of seconds. The technique uses the tail pulley to set tension. Once all belt slack is taken up, the operator sets tension via a scale on each side of the conveyor, where each increment represents the tension setting per foot of conveyor length. It seems counter- intuitive, but an under-tensioned belt can actually elongate in use, which can lead to a manufacturer discarding belts that actuality are in good condition, due to a conveyor’s limited tensioning capacity.
Again, following the recommenda- tions of the world’s major belt manu- facturers, a crowned pulley with a cor- rectly tensioned belt is the preferred way to achieve automatic belt center- ing. A crowned pulley produces dual, lateral opposing belt forces, which bal- ance each other with the belt centered
over the crown. If the belt moves off center, these lateral opposing forces become unbalanced, resulting in the side with the higher force directing the belt back to its centered position. This action provides virtually wear-free cen- tering. Crowned pulley systems also allow higher belt accelerations and speeds with less belt and component wear.
Properly tensioned belts on crowned pulleys easily withstand moderate short-term lateral forces without major displacement from center. When side loads increase, manufacturers can add a V-retainer on the underside of the belt to limit off-center drift. The V- shaped profile rides in a groove cut into the pulleys (which reduces pulley rigidity), and a groove running the length of the conveyor bed. This approach allows the crowned pulley to quickly center the belt when the external force is removed, minimizing V-retainer wear. Belt manufactures rec- ommend that V-retainers not be the primary belt-tracking system, due to the high wear incurred by the V-profile. If lateral force is sufficient, the V-retain- er will climb out of its groove, possibly damaging the belt.
An alternative is to use the V-retain- er on the top edge surface of the belt, guided and constrained from the top down by a rolling Delrin V-guide locat-
ed opposite the point where the belt experiences side forces. These guides keep the V-profile fully constrained, with reduced wear. A normal V-retain- er alone offers no true noncontact guid- ing, so it tends to drag against the sides of the groove and constantly wear.
Another point to consider on a bot- tom-mounted V-retainer: The weld used to attach the retainer results in a slight high spot in the middle of the belt. This can be troublesome if the belt carries small products prone to tip-over. Because of material incom- patibilities, the V-profile cannot be used with certain belt types, such as sil- icon-based, Teflon and polypropylene belts.
Pulley Diameter—Size Matters
While low-profile conveyor pulleys typically range from 1- to 2-in. dia., this small range can produce a sur- prising difference in conveyor capaci- ty and performance, as well as belt, cleat and bearing life. Smaller-diame- ter pulleys may be needed for applica- tions requiring a minimum height pro- file where speeds and loads are moderate. However, logic dictates that a smaller-diameter drive pulley will have a much greater tendency to deflect as conveyor width increases.
For example, a 1.25-in.dia. pulley without a V-groove will deflect nearly five times more than a 1.86-in.-dia. pulley with the same load. This cre- ates an inherent traction disadvantage for load-carrying purposes, and can negate the crowning effect (belt-cen- tering capability) of the pulley. This is highly problematic in applications that involve reversing, or those with wider conveyors or in accumulating or inclined setups. Flexing a belt over a smaller diameter also accelerates breakdown of the belt structure, lead- ing to erratic operation and shorter life.
When scaled up in width, conveyors with small-diameter pulleys often can- not carry proportionally greater loads. For example, a conveyor with a pulley diameter of approximately 2 in., with a correctly tensioned belt, will carry twice
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Conveyor Width, in.