Some photos might help us to understand the application conditions.
For instance:
Is this the chain that drives the bogie wheels that move the tip of the skybridge?
Or is it the chain that swivels the bogie?
Or is the chain that extends/retracts the bridge extension?
I think I have seen roller chains in all those applications on various skybridges.
Skybridges are an odd market; there was a huge buildout, complete saturation, then all the suppliers, AFAIK, disappeared because the market was gone. Or maybe there are a few skinny survivors. If so, they might be able to help.
As far as protecting chain drives from dust, there are a few solutions from the motorcycle industry.
I think you can find spray-on lube that wicks into the spaces where it needs to be, then runs off or dries in place.
For chains that run between two fixed pulleys, it's possible to fit tubes or gaiters over the chain runs and housings around the sprockets. This has been used on a few high-end motorcycles, and provides improved durability, especially with an oil bath, but adds weight, which you probably don't care about.
It may be possible to fit an open-bottom shield, which will protect the chain from falling dust, but only partially from wind-driven dust.
If you have cheapish labor, you might consider hosing down the chains regularly with some sort of lubricant and wiping off the excess. You have to do it fairly often, and it's likely to make a mess on the apron, even with groundcloths spread before every application, and you generate a waste stream of oily wipes and spotted groundcloths.
If your skybridge once had guards around the chains that were removed because of damage or wear, replace them. If you need to fabricate replacements, you have an opportunity to make them better, or less awful.
Mike Halloran
Pembroke Pines, FL, USA