Loctite 638: High-strength shaft-hub connection in detail

Loctite 638 is a high-strength, anaerobic adhesive for cylindrical joints – shaft-hub, bearing seat, bushing-housing. This green, one-component adhesive cures without air between metal surfaces and achieves shear strengths up to 31 N/mm². It replaces press fits and thermal joining processes and simplifies assembly by allowing for larger tolerances.

Technical data

Operating principle: Anaerobic hardening

Loctite 638 cures anaerobically – this means that curing begins when the adhesive is sealed off from air (oxygen) and metal ions are present as a catalyst.

Hardening conditions:

• Airtight seal: The adhesive must be enclosed between two tightly fitting metal surfaces.
• Metal contact: Active metals (steel, copper, brass) directly catalyze the hardening process.
• Passive metals: Stainless steel and aluminum require a primer (Loctite 7471 or 7649) for reliable hardening.
• Temperature: Optimal hardening occurs at 22–25 °C. Below 10 °C, hardening is significantly slowed.

Hardness profile on steel (23 °C):

• Fixing time: 4 minutes (steel), 6 minutes (stainless steel)
• Handling strength (50% final strength): ~1 hour
• Full strength: 24 hours

Applications

Shaft-hub connections

The main application of Loctite 638: Cylindrical connections between shaft and hub (gear, pulley, flywheel, clutch hub).

Advantages over press fit:

• Larger manufacturing tolerances are possible (transition fit instead of press fit)
• No shrink-fit or press-fit devices required
• Uniform stress distribution across the entire joining surface
• No thermal stresses in the component
• Easier assembly and disassembly (removable with heat)

Bearing Retaining

Securing rolling bearings in the housing (bearing seat) or on the shaft. The adhesive prevents migration and micro-movement of the bearing ring under vibration loads.

Typical bearing seat applications:

• Electric motors: Securing the outer and inner bearing rings
• Gearbox: Bearing protection of the input and output shafts
• Pumps: Fixing bearing housings under vibration load
• Fans: Protection against axial movement

Bushing and sleeve bonding

• Securing plain bearing bushings in housing bores
• Fixing spacers and spacer rings
• Repair of worn fits (tolerance compensation up to 0.25 mm)

Other cylindrical connections

• Rotor on shaft (electric motors)
• Encoders/sensors on shaft stubs
• Threaded inserts in light metal housings
• Pins and bolts in bores

Processing: Step by step

1. Surface preparation

• Clean and degrease joining surfaces with Loctite SF 7063 or isopropanol
• Surfaces must be clean, dry, and free of grease
• Remove rust and loose particles (using abrasive fleece or wire brush)
• For stainless steel or aluminum: Apply Loctite 7471 (spray) or 7649 (liquid) primer and allow to flash off for 1–3 minutes.

2. Apply adhesive

• Apply Loctite 638 evenly to a joining surface (shaft or bore)
• For shafts: Apply in a ring-shaped pattern around the entire mating surface
• For drilling: Distribute in the bore or wet the shaft
• Do not use an excessive amount – the adhesive will spread during joining

3. Add

• Assemble components within the open time (insert axially or press in)
• A slight twisting motion during joining promotes distribution
• Wipe away excess adhesive at the joint exit points
• Align the position correctly immediately – no further correction is possible after fixing (~4 min. on steel)

4. Curing

• 4 minutes (steel): Fixation (no longer movable)
• 1 hour: ~50% final strength
• 24 hours: Full strength at 23 °C
• Heat acceleration: 30 min at 120 °C for immediate full strength

Disassembly: Loosen the shaft-hub

Despite its high strength, joints joined with Loctite 638 can be loosened:

• Thermal: Heat the joint to 200–250 °C (hot air gun, oven, induction cooktop). The adhesive softens above its temperature limit and the parts can be pressed apart.
• Mechanically: Remove using a puller under uniform axial force. Requires high forces (corresponding to the torque of the shear strength).
• Combination: Heating + mechanical loosening is the standard method in maintenance.

Loctite 638 vs. other adhesives

Which type, and when?

• Loctite 638: Universal high-strength adhesive – strongest option, first choice for most shaft-hub connections
• Loctite 620: When high temperature resistance (up to +230 °C) is required – e.g., exhaust side, engine-related applications
• Loctite 648: For fastest fixing (3 min.) and tight fits (
• Loctite 680: For loose fits and easy processing

Permabond equivalent

The direct replacement for Loctite 638 is Permabond HM161 (high-strength adhesive). Both products offer comparable strength and temperature resistance. For advice on switching, contact SILITECH.

Design: Calculating the shaft-hub connection

The transmissible force of a glued shaft-hub connection depends on three factors:

Transmissible torque (simplified):

M = τ × π × d × L × (d/2)

Where: τ = shear strength (N/mm²), d = shaft diameter (mm), L = joining length (mm)

Example calculation:

• Shaft diameter: 25 mm
• Joining length: 30 mm
• Shear strength Loctite 638: 31 N/mm²
• → M = 31 × π × 25 × 30 × 12.5 = ~912 Nm

Safety factor: In practice, we recommend a safety factor of 2–3 against the theoretical shear strength. Dynamic loads, temperature, and substrate combination influence the actual strength.

Frequently Asked Questions (FAQ)

Does Loctite 638 work on stainless steel?

Yes, but use Loctite 7471 or 7649 primer. Stainless steel is a passive metal and does not sufficiently catalyze anaerobic curing on its own. Without a primer, the curing time increases to 30+ minutes and the final strength can be reduced by 30–50%.

Can I repair a leaking fitting with Loctite 638?

Yes, Loctite 638 bridges gaps up to 0.25 mm. For larger gaps (up to 0.5 mm), use Loctite 660 (with filler) or combine 638 with iron powder for improved tolerance compensation.

How do I disconnect a connection secured with Loctite 638?

Heat the joint to 200–250 °C and mechanically separate the parts using a puller. The adhesive softens at temperatures above its service limit. Without heating, very high axial forces are required.

What is the difference between Loctite 638 and Loctite 243?

Loctite 243 is a thread-locking adhesive for threaded connections (medium strength). Loctite 638 is a bonding adhesive for cylindrical connections (high strength). The chemistry is similar (anaerobic), but the formulation is optimized for different gap geometries and strength requirements.

Can I use Loctite 638 for threaded connections?

Technically possible, but not recommended. For thread locking, Loctite 243 (medium strength) or Loctite 272 (high strength, high temperature) are better suited, as they are optimized for the gap geometry of threads.