Package includes: 4 upper double casters and 4 lower double casters, total 8 piecesComes with screw cover. Spring mechanism that can also be adapted to curved showers. Designed with plastic nut and cover.Easy Side - Mounting Design: These 4 pairs of pulley side - hanging wheels are designed for effortless side - mounting. Their unique structure allows for quick and straightforward installation on various surfaces, saving both time and installation effort.Durable and Wear - Resistant: Made from premium materials, these side - hanging wheels are highly durable and resistant to wear. They can withstand heavy loads and long - term use, providing reliable performance and a long service life.Smooth and Quiet Operation: Equipped with high - quality bearings, the pulley side - hanging wheels ensure smooth movement with minimal friction. They operate quietly, which is ideal for environments where noise reduction is crucial, such as in home furniture or office equipment.