684 is an Open Miniature Ball Bearing, 684 Bearing has an inner diameter of 4mm, 684 small Bearing outer diameter is 9mm and 684 Bearing width is 2.5mm, metric 4x9x2.5 mm, 684 Bearing is made of Chrome Steel grade 52100, 52100 is a high carbon, chromium containing low alloy steel, heat treatment and hardening of 52100 alloy steels will get you 60 Rockwell hardness and very common in bearings.

• Item: 684 Bearing
• Design Type: Single row deep groove ball bearings
• Size: 4 x 9 x 2.5 mm
• Size Type: Miniature Bearing
• Closures: Open
• Material: 52100 Chrome Steel
• 684 Dimensions: 4mm x 9mm x 2.5mm
• Standard: Metric
• Bearing Inner Diameter: 4mm
• Bearing Outer Diameter: 9mm
• Bearing Width: 2.5mm
• Load Type: Radial Bearing and can take minimal axial loads
• Quality: Standard Precision
• Quantity: One Bearing

684 is categorized as a miniature deep groove ball bearing, have single row of balls with a deep groove raceway that enables the bearings to take radial loads and axial loads in both directions. 684 is commonly used in high speed applications, and of course they are designed to be mounted on rods (round shafts).