MATERIAL: Heavy duty case and carrier is cold-rolled steel . Ball bearing wheels in steel or steel with nylon tires. MOUNTING: Mounts in recess in bottom of door rail. Recess dimension and hole spacing conform to general industry practice. Consult drawings for detailed mounting information. Sliding Door Rollers Description of Load-bearing Capacity：You can dependably support nylon wheels rated up to a 150 lb. door with two roller assemblies per door . When a sliding patio door system has an aluminum track, a patio door roller with a nylon wheel must be used.If a steel wheel is used, gauling willoccur, resulting in short track life. Our Promise: If you have any questions about our products, you can contact us through Amazon and House Guard will provide you with the best customer service within 24 hours.