The answer to this is twofold. For LightRail 3.5 and 4.0 let’s first talk about the strength of the rail then we’ll talk about LightRail Drive Motor that moves your grow light. If only mounted by the mounting holes at the ends of the rail, the rail will begin to deflect when 35 lbs. (16kg) is in the middle of the rail. If all the rails mounting holes are used when mounting the rail first to a board, and then the board to the structure above your garden, it will easily handle 75 lbs. We recommend the board mount method and here is a step by step link to show how that is done: https://www.lightrail3.com/free-shareable-ebooks/how-to-set-up-your-lightrail-3-5-or-lightrail-4-0/
Next, the LightRail 3.5 or 4.0 Drive Motor can easily move 75 lbs (34kg). However, more weight equals accelerated wear and tear mostly on the Trolley Wheels. Our testing has shown that the Trolley Wheels on a LightRail 3.5 or 4.0 Drive Motor will last between 12 and 16 months moving a load of 35 pounds (16kg). Our Extreme Duty Trolley Wheels have a lifetime warranty at 60 pounds per set of four. Plus they have very low drag co-efficiency so they are the best choice for moving heavy or multiple reflectors. And beyond weight, dimensions might matter so specialty grow light systems with very long wingspans should be used with LightRail 5.0 exclusively.
LightRail 5.0 will hold and move 100 lbs (45kgs) per cross arm (limit 3 cross arms) but again, needs to be mounted to a structure properly. Note: The greater the weight, the more frequently trolley wheels will need to be replaced. Trolley wheel kits are available on our website for LightRail 3.5, 4.0. 4.20, the older 3.0, and LightRail 5.0. For any other serviceable work, we make that easy and with a quick turnaround time.