Anaerobic retaining compounds

Henkel LOCTITE 638 gives 13.5 N/mm² compressive shear on steel after 15 minutes at 23°C and 29 N/mm² after 72 hours at 23°C, using ISO 10123 pins and collars. At 72 hours, stainless gives 28 N/mm² and aluminum 17 N/mm². Check fit, actual substrate, cure and service loads separately. LOCTITE 638 TDS, pp. 2–3, adhesive properties and product specification

Retaining describes the joint

Retaining compounds cure in cylindrical mating clearances, including shafts, hubs, bearings and sleeves. Specify the fit, adhesive and service load together. Threadlockers serve threaded joints. Henkel, Retaining compounds, applications and “Retaining compound vs. threadlocker”

Product identity matters more than color. Permabond's selector separates threadlocking, thread sealing, retaining and gasketing grades, with different viscosities and strength descriptions. A green liquid is not a complete specification for a bearing fit, and a high-strength grade is not automatically the best choice where maintenance requires removal. Permabond, Anaerobic adhesives and sealants selector guide; Henkel, retaining-compound selection.

Oxygen exclusion is only part of cure

Conventional anaerobic cure requires confinement away from oxygen and a suitably reactive surface. Permabond explains that metal reactivity, gap and temperature change cure speed. Brass, steel and anodized aluminum do not provide the same curing conditions; plating variation can also change behavior. Larger gaps and lower temperatures generally slow the reaction. A cure curve for one substrate at room temperature cannot be transferred unchanged to every plated or passivated component. Permabond, Curing anaerobic adhesive.

Exposed squeeze-out is therefore a poor indicator of the condition inside a confined joint. Conversely, observing that parts resist handling does not establish that the bond has reached its final properties. Henkel defines fixture time as time to initial handling strength, using 0.1 N/mm² in its product-page explanation, while cure time concerns final mechanical properties. That definition must accompany a fixture-time number. Henkel, LOCTITE 638 FAQs, “What is fixture time?” and “What is cure time?”.

Activators change the process

An activator can accelerate cure on less active surfaces or help cure through a larger gap, but its use must be paired with the adhesive manufacturer's instructions. Permabond's ASC10 guidance calls for application to clean, dry surfaces and evaporation of the carrier before bonding. It explicitly prohibits mixing the activator directly into anaerobic adhesive. Permabond, Anaerobic surface conditioners.

Faster cure does not necessarily mean a stronger final bond. Permabond states that ASC10 can improve measured strength where incomplete cure was the limitation, but can reduce strength when complete cure would otherwise occur. That is product-specific guidance, not permission to use any primer to increase a retaining compound's gap capability. Requalify the chosen adhesive, activator, substrate and assembly timing together. Permabond, ASC10 strength and gap-fill FAQs.

Fit, preparation and application

For concentric parts, diametral clearance is D − d and radial clearance is half that. 0.10 mm diametral gives 0.05 mm radial before form error or eccentricity. Check the supplier’s gap convention. Henkel, LOCTITE 638 FAQ, “What is the optimum fit/clearance?”

Oil tolerance is limited. LOCTITE 648's data sheet describes tolerance to minor contamination by specified categories of oils, while its best-results instructions still call for cleaning both surfaces and allowing them to dry. A heavy oil film, an incompatible wash residue or an unidentified coating is not covered merely by the words “oil tolerant.” Henkel, LOCTITE 648 Technical Data Sheet, May 2021, pp. 1 and 3.

The same instructions distinguish assembly methods:

Keep parts undisturbed until adequate handling strength develops. These are instructions for the named product, rather than a universal dispensing recipe. LOCTITE 648 TDS, p. 3, “Directions for use”.

Named example: strength depends on its test conditions

Henkel's LOCTITE 638 TDS, May 2024 identifies a high-viscosity urethane-methacrylate retaining compound and describes cylindrical bond gaps approaching 0.25 mm. Its typical Brookfield viscosity is 2,500 mPa·s at 25°C, spindle 3 at 20 rpm. This describes uncured flow behavior, not cured strength or permission to exceed a joint-clearance limit. LOCTITE 638 TDS, p. 1.

Its compressive-shear results use ISO 10123 pins and collars:

One N/mm² equals one MPa. These typical pin-and-collar results require separate qualification for the actual fit, substrate and load. LOCTITE 638 TDS, pp. 2–3, adhesive properties and product specification

For comparison, LOCTITE 648's May 2021 TDS identifies lower-viscosity material, lists bond gaps approaching 0.15 mm and reports 31 N/mm² on steel pins and collars after 72 hours at 23°C. This difference does not establish that one product is superior for an unspecified shaft. LOCTITE 648 TDS, pp. 1–2.

Calculate engaged area from diameter and length. Validate actual load capacity with the joint geometry, fit, service loads and production variability.

Service exposure and removal

Distinguish strength tested hot from retained strength measured at room temperature after heat aging. Chemical exposure tables likewise state the medium, temperature, duration and test condition. LOCTITE 648's environmental section uses a one-week cure at 23°C and labels these different test arrangements. A temperature range or an oil-resistance description alone does not establish joint life in a particular fluid and load cycle. LOCTITE 648 TDS, pp. 2–3.

Removal belongs in product selection. Henkel notes that serviceable or lower-strength grades may suit maintenance needs better than maximum-strength grades. Its LOCTITE 638 page discusses mechanical separation and, when needed, localized heating followed by separation while hot. The surrounding components' permitted temperatures remain part of the removal method. Henkel, retaining-compound selection; LOCTITE 638, disassembly guidance.

Use the current safety data sheet for the actual product and market for handling information; a technical strength table does not supply that information. Henkel provides its official SDS finder. This reference establishes neither a customer joint's load capacity nor regulatory approval, universal cure time or interchangeability.