Loctite 638: High-strength shaft-hub connection in detail
Loctite 638 is a high-strength, anaerobic bonding adhesive for cylindrical joints – shaft-hub, bearing seat, bushing-housing. The green, single-component adhesive cures without air between metal surfaces and achieves shear strengths of up to 31 N/mm². It replaces press fits and thermal joining processes and simplifies assembly thanks to greater tolerances.
Technical specifications
| property | value |
|---|---|
| product type | Anaerobic bonding adhesive (urethane methacrylate) |
| Application | Shaft-hub connections, bearing retention |
| Color | green |
| viscosity | 1,800–3,300 mPa·s (high viscosity) |
| strength class | High |
| Shear strength (steel) | 31 N/mm² (4,500 psi) according to DIN 54452 |
| Shear strength (range) | 20–35 N/mm² |
| gap bridging | 0.15–0.25 mm |
| Fixing time (steel) | 4 minutes (stainless steel: 6 min.) |
| full curing | 24 hours at 23 °C |
| temperature range | -55 °C to +180 °C |
| Chemical resistance | Oil, fuels, weak acids/bases |
Functional principle: Anaerobic hardening
Loctite 638 cures anaerobically, which means that curing begins when the adhesive is sealed off from air (oxygen) and metal ions are present as a catalyst.
Curing conditions:
- Air seal: The adhesive must be enclosed between two tightly fitting metal surfaces.
- Metal contact: Active metals (steel, copper, brass) directly catalyze hardening.
- Passive metals: Stainless steel and aluminum require a primer (Loctite 7471 or 7649) for reliable curing.
- Temperature: Optimal curing at 22–25 °C. Significantly slowed down below 10 °C.
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 wheel, belt pulley, flywheel, clutch hub).
Advantages over press fit:
- Larger manufacturing tolerances possible (intermediate 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 (can be removed with heat)
Bearing Retaining
Securing roller bearings in the housing (bearing seat) or on the shaft. The adhesive prevents migration and micro-movements of the bearing ring under vibration loads.
Typical bearing seat applications:
- Electric motors: Securing the outer and inner bearing rings
- Gearbox: Bearing retention of the drive and output shaft
- Pumps: Fixing bearing housings under vibration loads
- Fans: Protection against axial movement
Bushing and sleeve bonding
- Secure plain bearing bushings in housing bores
- Fixing spacer sleeves 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 holes
Processing: Step by step
1. Surface preparation
- Clean and degrease bonding surfaces with Loctite SF 7063 or isopropanol.
- Surfaces must be bare metal, dry, and free of grease.
- Remove rust and loose particles (abrasive fleece or wire brush)
- For stainless steel or aluminum: Apply Loctite 7471 (spray) or 7649 (liquid) primer and allow to air dry for 1–3 minutes.
2. Apply adhesive
- Apply Loctite 638 evenly to one mating surface (shaft or bore).
- For shafts: Apply in a ring shape around the fitting surface.
- For drilling: Distribute in the hole or coat the shaft
- No excessive amount – the adhesive spreads during joining
3. Insert
- Assemble components within the open time (insert axially or press in).
- A slight twisting motion during joining promotes distribution.
- Wipe off excess adhesive from the joint exit points.
- Align position correctly immediately – once fixed (~4 min. on steel), no further correction possible
4. Curing
- 4 minutes (steel): Fixation (can no longer be moved)
- 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 shaft-hub
Despite their high strength, joints bonded with Loctite 638 can be disassembled:
- Thermal: Heat the joint to 200–250 °C (heat gun, oven, induction). The adhesive softens above its temperature limit and the parts can be pressed apart.
- Mechanical: Loosen using a puller with uniform axial force. Requires high force (corresponding to the shear strength torque).
- Combination: Heating + mechanical loosening is the standard method in maintenance.
Loctite 638 vs. other bonding adhesives
| property | Loctite 638 | Loctite 620 | Loctite 648 | Loctite 680 |
|---|---|---|---|---|
| strength | High | Medium-high | High | means |
| Shear strength (steel) | 31 N/mm² | 17 N/mm² | 31 N/mm² | – |
| viscosity | High (2,500 cP) | Very high (8,000 cP) | Low (500 cP) | means |
| gap bridging | 0.25 mm | 0.25 mm | 0.15 mm | Larger gap |
| Fixing time (steel) | 4 min. | 60 min. | 3 min. | – |
| temperature range | -55 to +150 °C | -55 to +230 °C | -55 to +180 °C | -55 to +150 °C |
| special feature | Strongest universal type | Highest temperature | Fastest curing | For loose fits |
| Color | green | green | green | green |
When which type?
- Loctite 638: Universal high-strength bonding adhesive – strongest option, first choice for most shaft-hub connections
- Loctite 620: When high temperature resistance (up to +446 °F) is required—e.g., exhaust side, engine-related applications
- Loctite 648: Wenn schnellste Fixierung (3 Min.) und enge Passungen (<0,15 mm) gefragt sind
- Loctite 680: For loose fits and easy application
Permabond equivalent
The direct Permabond replacement for Loctite 638 is Permabond HM161 (high-strength bonding adhesive). Both products offer comparable strength and temperature resistance. Please contact SILITECH for advice on switching products.
Design: Calculate shaft-hub connection
The transferable force of a bonded 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 = joint length (mm)
Sample calculation:
- Shaft diameter: 25 mm
- Wing 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 catalyze anaerobic curing sufficiently on its own. Without primer, the setting time increases to 30+ minutes and the final strength can be reduced by 30–50%.
Can I repair a worn fit 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 loosen a connection secured with Loctite 638?
Heat the joint to 200–250 °C and separate the parts mechanically using a puller. The adhesive softens at temperatures above its operating limit. Without heating, very high axial forces are required.
What is the difference between Loctite 638 and Loctite 243?
Loctite 243 is a threadlocking adhesive for threaded joints (medium strength). Loctite 638 is a bonding adhesive for cylindrical joints (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. Loctite 243 (medium strength) or Loctite 272 (high strength, high temperature) are better suited for thread locking, as they are optimized for the gap geometry of threads.
Order Loctite 638 from SILITECH
SILITECH AG stocks the Loctite range of bonding adhesives from its warehouse in Switzerland:
- Loctite 638 (high strength, universal, up to 180 °C)
- Loctite 620 (high strength, high temperature)
- Loctite 648 (highest strength)
- Loctite 680 (fast fixing)
- Primer Loctite 7471 & 7649 (for passive metals)
Contact us for technical advice, samples, or a quote.
