1 Complete Model & Code Explanation

Model: W 638/2.5-2Z

Code Definition

W: Stainless steel series symbol

638: Miniature deep groove ball bearing series

/2.5: Dimension code for 2.5mm bore specification

2Z: Double metal non-contact shields

Supply form: Single independent bearing, no factory duplex grinding, DB/DF duplex arrangement not supported

Important reminder: W 638/2.5-2Z is a miniature shielded stainless steel bearing with moderate load capacity, mainly designed for radial loads; only slight continuous axial preload is permitted. Double non-contact metal shields block dry airborne dust, without protection against water, moisture vapor or liquid contamination. Grease is pre-filled at factory. No relubrication path, suitable for long-term maintenance-free intermittent or short continuous high-speed operation.

Basic Parameters

Bore d: 2.5 mm

Outer diameter D: 6 mm

Width B: 2.5 mm

Bearing type: Single-row deep groove ball bearing

Number of rows: Single row

Bore form: Straight cylindrical bore

Seal arrangement: Double-sided shields

Seal type: Non-contact metal shields

Oil groove: Without oil groove

Outer ring locating feature: No locating groove or shoulder

Cage: Stamped stainless steel cage

Radial internal clearance: CN standard clearance

Material: Stainless steel

Surface coating: Uncoated

Pre-installed lubricant: Factory pre-filled grease

Relubrication capability: Not available

Duplex arrangement: Not supported, single unit operation only

Basic dynamic load rating: 0.072 kN

Basic static load rating: 0.022 kN

Reference speed: 210 000 r/min

Limiting speed: 105 000 r/min

Net weight: 0.00013 kg

Indicative carbon footprint: 0.00045 kg CO₂e

eClass code: 23-05-08-01

UNSPSC code: 31171504

Series Suffix Comparison

W 638/2.5: Open type without shields, higher speed rating, zero dust protection

W 638/2.5-Z: Single metal shield, single-side dust resistance

W 638/2.5-2Z: Double non-contact metal shields (this model), dual-side dry dust barrier

638/2.5: Bearing steel version with equal dimensions, no anti-corrosion property

2 Structure & Operating Characteristics

Japan NMT W 638/2.5-2Z belongs to W638 miniature stainless steel deep groove ball bearing series, developed for compact rotary mechanisms exposed to minor dry dust. Manufactured entirely from stainless steel, it delivers consistent basic rust resistance compared to bearing steel miniature bearings, ideal for precision instruments which cannot achieve fully dust-free environment and require infrequent disassembly.

Adopting integrated stamped stainless steel cage with low mass and excellent dynamic balance to reduce vibration at high rotational speed. Equipped with double non-contact metal shields; no direct friction between shield and ring, achieving higher permissible speed than contacting rubber seals. The structure prevents dry dust from invading raceways and reduces abrasive wear of rolling elements. Standard CN radial clearance matches common tolerance requirements of miniature shafts.

Limited by miniature overall dimensions, contact area between rolling elements and raceways is restricted. The bearing is optimized for radial loads; only light preload can be applied axially. Continuous heavy axial thrust must be avoided. Shields only defend dry solid particles and cannot isolate moisture, mist or liquid media. For high humidity working conditions, full equipment housing sealing is still required. Micro bearing grease is pre-filled upon delivery for plug-and-play assembly.

3 Core Performance Advantages

1. Compact miniature dimension: 2.5mm bore × 6mm OD × 2.5mm width, fits confined installation space.

2. Stainless steel construction: Provides basic anti-rust performance for slightly humid instrument interiors.

3. Double non-contact metal shields: Blocks dry dust while preserving superior high-speed performance.

4. Stamped stainless steel cage: Lightweight construction ensures stable high-speed rotation.

5. CN standard radial clearance: High universality, compatible with various miniature shaft tolerances.

6. Factory pre-filled grease: Eliminates on-site lubricant filling and simplifies assembly workflow.

7. Low rotational inertia: Fast start-stop response, perfect for precision fine-adjustment assemblies.

4 Typical Application Fields

4.1 Optical Precision Fine-Tuning Devices

Rotary assemblies for microscopy equipment, portable lens adjustment units, micro rotary shafts for optical sensors.

4.2 Portable Precision Medical Equipment

Internal transmission parts of compact diagnostic instruments, micro rotary components for minimally invasive auxiliary devices.

4.3 Miniature Testing & Sensing Equipment

Laboratory compact test fixtures, rotary structures for micro flow and angle sensors.

4.4 Compact Precision Drive Modules

Micro motors, built-in rotary components for smart wearable devices, miniature automated precision actuators.

5 Assembly Specifications & Usage Guidelines

5.1 Assembly Environment & Operation Standards

Assembly of miniature precision bearings should be carried out on clean workbenches to reduce contaminant adhesion. Never strike the bearing directly. Use dedicated micro tooling for press fitting to prevent ring tilt and rolling element damage. Strictly control interference fit of shaft and housing; excessive interference will sharply reduce internal clearance and cause overheating seizure at high speed. Keep good track of bearings during assembly due to tiny size to avoid loss.

5.2 Selection Guidance

Fully enclosed dust-free environment, maximum speed priority → W 638/2.5 open type;

Dust exposure on single side, high speed demand → W 638/2.5-Z single shield variant;

Dry floating dust present, dual-side protection required with balanced speed performance → W 638/2.5-2Z (this model);

Dry environment without anti-rust requirement, higher load capacity expected → 638/2.5 bearing steel version.

5.3 Lubrication Instruction

A small quantity of high-speed grease is pre-filled inside the bearing. Direct assembly is acceptable under standard working conditions. No built-in relubrication channel; periodic re-greasing in service is impossible. Designed for medium-long term maintenance-free machinery. Continuous ultra-high temperature operation is not recommended, which may accelerate grease depletion and premature failure.

5.4 Environmental Application Notes

Double Z-type metal shields can only block dry dust, without water resistance, vapor resistance or chemical liquid resistance. Do not expose directly to humid, salt spray or oily atmosphere. Stainless steel only offers basic corrosion resistance. Complete machine sealing is necessary for long-term high-humidity operation.

6 Daily Maintenance & Fault Troubleshooting

Perform low-speed running-in after assembly. Gradually raise rotational speed to rated level only after verifying no abnormal noise or temperature rise. Fault symptoms of micro bearings are difficult to detect. Stiff rotation, increased torque or rising operating noise generally indicates grease degradation or dust penetration into raceways.

Common failure causes: excessive dust penetrating the shields, grease drying caused by long-term overheating, insufficient clearance due to improper mounting, sustained overload axial force. Repairs are normally not feasible once failure occurs; immediate replacement is recommended.