Others have suggested the crowned roller.
Take a look at how the web is kept centered in the presses. The same methods should work for the belt. It involves adding an extra roller or two to the path so that
You can also use a split roller to do the job. This means that you have to add torque between the sections of the roller to push one part of the belt forwards with respect to the other. This can be made to work on a mechanically "automatic" manner much like the crowned roller.
Think of two independent belts, one each side of the centre line. They would both tend to move away from the centre and run off the edge of the rollers.
Now tie the two belts together. As one tries to run off, it pulls the other towards the centre. Which creates a restorative force, as that belt's tension increases due to the increased running length.
The self-centreing mechanism does rely on the tension in one section of the width of the belt being able to change wrt the other. If the tension across the belt is always uniform (belt is very stiff laterally) then no corrective force is created. Similarly, if the belt is not under tension, no corrective force is created. Also, if the running length does not increase (eg a mid roller, not an end roller) then no corrective force is generated.
Two solutions:
IIRC, it depends on the belt & roller material, also on whether the roller is powered, or an idler.
Again, IIRC, it's a convex profile for elastic belts (eg; leather, neoprene), & concave for other materials (eg; chain, fibre-reinforced).
You could put small rollers on the arms that do this. You really don't want to add a drag to the one side. The added drag will work against the effect of the arm. This method requires that the belt in question be somewhat springy. I don't think the OP stated how springy it is.
OP here...
Crowned looks like the best option. Splitting the roller requires additional support and bearings. We regularly have rollers reground by a specialty service, and I will ask them about doing this to the (rubber) drive and idler rollers. (Would having both rollers crowned be better? Or is one sufficient?)
Second, as someone suggested here, will consider the inclined rollers added in the belt's path at the extreme ends of the belt's width to encourage centering.
FYI, it's a sheet-fed operation, not continuous web printing (and, specifically, this machine is a sheet-fed coater). Light weight product on the belt (less than a few pounds over the entire length of the belt at low speed). Belt is mesh carbon fiber. Seems somewhat springy, overall, but the material, per se, isn't flexible.
Thanks to all for your suggestions.
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