Whyare V-guided conveyor beltsbetter at keeping conveyor belts centered than crowned rollers?
In the packaging industry, proper belt tracking on conveyors prevents maintenance headaches. Belt tracking keeps belts running straight and true on the end rollers and conveyor frame. Many factors affect tracking, including belt splicing, conveyor rollers, belt tension, side loads from pushing products on or off the belt, and debris build-up on rollers.
But proper conveyor design minimizes problems with belt tracking. Two common design approaches that improve belt tracking are crowned rollers and V-guided belting. Let’s see how they stack up against one another.
Drive rollers on crowned-roller belts consist of a cylindrical center section sandwiched between two tapered sections. The tapered sections are generally half the length of the center section and steer the running belt to the raised center section of the roller. But manufacturing variations in splices on the belt and the belt’s weave, along with variations in the size and spacing of the frame will make the belt drift back and forth across the face of the rollers. On larger rollers, drifts of ±0.5 in. can be expected.
Crown rollers will keep most belts centered. And conveyor-belt manufacturers assume some method of roller crowning will be used, so they make belts that accept a modest level of crown without increasing wear.
Crown rollers can be used with knife-edge idler transfers. These are small-diameter idler rollers that move small or delicate parts on and off conveyors. But idler transfers require belts flexible enough to wrap around rollers as small as 0.25-in. in diameter. Such belts are usually compatible with crowned-roller tracking.
But crowned-roller belt tracking is only done on a conveyor’s end rollers. So there is no control on the belt as it travels the length of the conveyor bed and it is free to move side to side. And if products moving on and off the belt induce side forces on the belt, then designers need to add some sort of belt-steering mechanisms to keep the belt within the frame.
Designers often use lagging or knurling to increase the traction between the belt and roller drives to upgrade a conveyor so that it can handle heavier loads. In lagging, the drive roller is encased in a urethane sleeve, and this creates more friction or traction between roller and belt. Lagging cannot be done to crowned rollers. Knurling, another way to increase traction, machines a pattern of diamonds or lines on the roller to let it “bite” into the belt as it moves the belt along. On crowned rollers, only the center portion of the roller can be knurled. So crowned rollers limit engineers trying to increase traction or loads on a conveyor.
It is also difficult to move small and odd-shaped products over crowned-roller conveyors. The taper on the rollers means the belt is not flat. So if a product is unstable or top heavy, an uneven roller can make it tumble and fall off the belt during transfers.
Another disadvantage is that because belts will drift on crown rollers, both the rollers and frame need to be wider than the belt. This means the conveyor takes up more factory-floor space than it needs to. More importantly from a safety standpoint, crown rollers can create an operator pinch point where the belt enters the roller.
A better method of keeping conveyor belts centered is through V-guided belting. In this approach, the belt has a continuous plastic V or U-shaped guide glued down the center of the underside of the belt. This guide fits into a groove cut into end rollers, holding the belt in place across the width of the conveyor. And the same groove can also run the length of the conveyor bed, letting the belt remain centered along the entire length conveyor.
The V-guide is usually made of urethane or PVC. To get a strong bond between the belt and guide, it’s recommended the guide and the belt’s coating be of the same material. This limits the types of belts than can be V-guided. For example, Teflon, polypropylene, and silicon-based belts cannot use this technique.
The fit between the guide and groove in the roller is relatively tight. Generally there is about a 1/16-in. gap. It takes a relatively severe shock or event to drive the guide out of the groove. The tight controls on the belt means the conveyor frame does not have to be much wider than the belt.
Straight rollers provide consistent transfers and flat surfaces for the vast majority of products. Flat rollers also mean there are no limitations on using lagging or knurling to increasing traction between belt and roller drives. But V-guide belts are not completely flat. The heat used to bond the guide material to the belt can deform the top of the belt. The slight bump is typically less than 0.010 in. However, the bump could mean that light products would not lie flat on the belt.
The V-guide on the belt also makes it impossible to use knife-edge idler transfers because the guide along the bottom of the belt makes it too stiff to run around rollers as small as 0.25 in. in diameter. The smallest roller that accept V-guided belts is about 1 in. in diameter.