Years ago, I had an old boss that divided parts into two categories Goesintos and Goesontos. Goesinto part went inside of something and Goesonto parts went on or on top of something. I feel that this is an apt description for these types of chair glides. The glide cup on a wrought iron chair takes an insert, it is a a goesinto, if you will, but there is another kind of glide, popular with wrought iron furniture. It is the disk cap.
Our PartsPuppy.com 1 7/16" Disk Cap, fits over 1 7/16" diameter flat metal flange. It has an outside diameter 1 5/8", and is made of clear vinyl. The wrought iron furniture has a flat disk attached to the leg of each chair. Our 1 7/16" Disk Cap, fits over that flat disc. It is a goesonto type of part. Do not assume that because your chairs or table has bare disks that this is how they were intended to be used.
Our 1 7/16" Disk Cap makes these chairs slid so much easer and eliminates scratching of decks and patios.