Conveyer belt "wander" auto-correction?

Machine the rollers to put a very small taper into the centre. Slightly increase the tension at the centre line of the belt. The belt will then auto align itself onto the centre line and self-correct any tendency to wander off.

Offhand, you probably have to contract with a competent licensed engineer. My gues is that the rollers and the belt were not designed with the appropriate camber or other features that provide self-centering.

Bill

-- Fermez le Bush--about two years to go.

You could try capacative sensing, since the belts are carbon fibre, and some sort of bridge detector

martin

The usual fix is to "crown" the rollers so that they are bigger diameter in the middle and skinnier at the ends. Then the belt self-centers.

John

Thanks, Sue.

This is a *larger* diameter at the center?

Where might I find more information re. how much taper to apply? This is not a situation for trial-and-error (c;

Very simple! The drive roller, or largest roller, should have a curve in it so that the center is larger than the edges. Or pick two smaller rollers, spaced "far" apart in the loop.

For a quick and dirty test, add some adhesive-backed tape onto a roller, centered and maybe a second, more narrow tape also centered (to get that curve).

When I was in high school, the school had an old-fashioned type printing press with the giant cast-iron flywheel. It had an electric motor to keep it going, but you still controlled the speed pretty much by hand. The shop teacher's favorite way of getting the giant leather belt to track right was to build up the drive pully with masking tape, forming a crown. Seemed to do a good job.

Yes, larger in the center. If the belt tension isn't extreme, try using some temporary bands, hvac type aluminum tape or something, to fake a taper and see if it works. I bet you could google "tapered roller belt" or something for tips, too. In my (limited) experience, the taper is usually pretty significant, and sometimes just near the ends.

I'm surprised that anybody would use a smooth roller for a belt like this. It's guaranteed to be unstable.

Classic leather-belt-driven lathes used rollers that were very rounded, quarter-circle almost. It was uncanny to see a belt start at the very edge and walk right up to the crowned center.

John

You can either put the larger diameter/lower tension at the centre or smaller diameter/higher tension. Both will auto-correct but the latter will tend to make the belt to bunch up in the middle, whereas the former can tend to make things fall off the edge of the belt..

It's just a bit of maths to work out what corrective force is being applied, knowing the difference in tension and the angle of the inclined plane created by the taper(s).

The alternative is to install two extra free-wheeling short rollers, tilted towards the centre and under the edges of the belt. As the belt tries to wander towards one edge, it has to go "uphill", up the free-wheeling roller. The advantage of this method is that you can adjust the position and angle of the rollers to achieve the centreing that you want and it doesn't require any modification to the existing equipment..

One common method is to flare the rollers slightly at the ends. That way if the belt wants to ride up on the roller, there's a centering force due to the stretching of the belt.

Failing that, something like what you propose could probably be made to work--but I'd definitely avoid optical sensors for this if possible. The problem is that they get dirty and stop working, which in an automatic control situation like this would be dangerous.

Another simple method might be to have guide rollers at the edges of the belt. Rather than encoders and motors and microcontrollers, you could have the guide roller turn the adjustment screw via gears or a chain drive. That way, any time the belt got far enough out of line to spin the guide roller, it would be gradually adjusted until it didn't spin it any more. That's sort of an integrating servo--if the gear ratio (i.e. gain) is too high, the control loop will oscillate, and the poor thing will bounce back and forth until it chews itself to bits.

Since the situation doesn't require too many smarts from the controller, this would probably be easier and a good deal more reliable.

Cheers,

Phil Hobbs

You need your rollers crowned. I can't draw one in ASCII, but essentially, the rollers need to be thicker in the middle. Not by much; I don't know if there's a formula, but that's the standard way of keeping a belt centered.

Let me know if you need some custom rollers built - that's one of the things that the company where I sit does.

Good Luck! Rich

Whwn i used to work for a conveyor manufacture we used to crown one quarter of the width to create a 2mm drop from the diameter of the roller

Try this it will help

Gavin

Normally using a crown center roller would fix it how ever, since this is a printing machine, and the belt width isn't very wide (10 mm) I think you said?, If you want automation it can be done via using 2 simple small light weight with following groves with rollers, that have a small spring to keep the two rollers tight on the edges of the belt. These 2 rollers connect to an arm which is connected to a potentiometer that will drive a simple geared head motor on the manual screw via a regen drive.. We had a similar problem also, ours was a much wider belt but that is what we did. In our case we use 2 pots, one on each side of the belt that are join electronically to report a differential signal to an air servo piston that is attached on one side of the roller assembly. The output operates an IP value (Current 4..20 ma's), the electronics is nothing more than a voltage comparator that generates the current signal. normally we would use a gear head motor how ever, money was a problem at the time and this worked out just fine. You may find using an air cylinder and IP valve from a dancer control board may work fine..

And, the important rollers are the turnaround rollers, the end ones that the belt wraps 180 degrees around. The simple idler rollers and the drive roller (if it's a capstan drive and doesn't turn the belt around) can be simple cylinders.

It is generally the rollers where there is a change of direction that are important, because they can most easily translate changes in longitudinal tension forces into lateral movement (inclined planes).

For an outward tapering roller, the belt must be capable of conforming to some extent to the profile of the shaped roller(s) as a result of the belt tension. Any decrease in tension, eg belt stretch, can greatly affect lateral stability.

OTOH, for an inward tapering roller, the belt must be rigid, laterally, or it will bunch into the centre. Belt tension variations are generally not so critical.

The nature of the belt thus determines the roller profile needed to self-centre.

Look for "spreader rollers" or "banana rollers" in your favorite search tool.

How about a very simple electrical "middle of the road" thing? Mechanical switches with rollers that activate a geared-down motor. One switch on either side of the belt, each making the gear motor turn in opposite direction from the other. Rugged, simple, and a lot cheaper than an all-mechanical solution.

robert

Maybe it *has* to be flat? Who knows.

robert

use (slightly) barrel shaped rollers.

Bye. Jasen