Torque seal
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Torque sealant, also known as torque seal or inspection lacquer, is a coating used to mark mechanical fasteners for visual inspection. The mark can show that a threaded joint has been assembled or approved and help identify later loosening or tampering. Torque sealant does not replace a technical method of securing the joint against loosening.[1]
Use and operating principle
After tightening, a stripe of torque sealant is applied across the fastener and an adjoining component. Movement between the two parts can break the cured coating, leaving a visible sign that the joint has moved.[1] DIN 32546 describes the seal as an inspection mark and an indicator of loosening. It can also indicate possible tampering where this poses a technical risk.[1]
Documented uses include helicopter components and SMA connectors in radio astronomy and space instruments.[2][3]
Limitations
A seal's ability to reveal movement depends on how the coating behaves and how well it adheres to the parts. An ALMA technical memorandum reports cases in which a connector nut rotated beneath an intact epoxy bead. The adhesive joint under the bead had broken, but the coating showed no obvious sign that the connector had loosened.[3]
The memorandum discusses external adhesive stripes both as movement indicators and as a possible means of resisting loosening. Its treatment of epoxy staking applies to particular connector applications. DIN 32546, by contrast, specifies that torque seal is not a substitute for a technical locking method.[3][1]
Application
Under DIN 32546, the sealant is applied after correct assembly and must bridge the fastener and a mating element. The resulting seal must be detectable by touch and must break when the threaded joint is rotated. A mark placed on a component solely to record inspection is therefore different from a seal spanning the joint.[1]
The application areas may be degreased after assembly where necessary. Full drying can take up to 48 hours, depending on the formulation, unless the manufacturer and user agree otherwise.[1] Preparation requirements and drying times also appear in product data sheets. For example, Lackfabrik Bäder specifies that grease, oil, moisture and rust must be removed before application.[4]
Formulations and properties
Inspection coatings use several types of formulation. Literature on SMA connectors describes epoxy-based torque stripes with titanium dioxide pigment and a silica thickener, as well as polyurethane-based materials.[3] A manufacturer's technical data sheet lists water-based and solvent-based sealants with differing flow properties, drying times and resistance to liquids.[4]
Reduction of hazardous air pollutants
In 2009, the Army Research Laboratory investigated a torque seal inspection lacquer formulated without hazardous air pollutants (HAPs).[5] According to a technical summary, the alternative replaced methanol in the reference product with ethanol. The two formulations performed similarly in laboratory tests of adhesion, fluid resistance, humidity resistance and drying time, and in a demonstration on a helicopter rotor. The HAP-free lacquer was still solvent-based.[2]
DIN 32546
DIN 32546:2026-08 was published in August 2026 with the German title Siegellack and the English title Torque seal. It covers coatings that mark assembly or approval and indicate possible loosening or tampering on threaded fasteners. The user is responsible for assessing the associated technical risks. The standard treats the seal as an inspection aid rather than a technical locking method.[1]
Seal requirements
DIN 32546 requires a seal that can be felt by touch. It recommends that the coating adhere more strongly to the components than it holds together internally, so that relative rotation fractures the coating itself. A fracture test on the intended threaded joint establishes whether the product is suitable for that application.[1]
Classification and designation
Products are classified by application class and flow category. Classes 1 and 2 cover water-based and solvent-based formulations, respectively. Class 3 covers products with increased temperature resistance, and class 4 covers UV-curing products. Neither class 3 nor class 4 is restricted to a single water-based or solvent-based coating system.[1]
| Class | Main distinction | Flow designation | Anti-Flow designation |
|---|---|---|---|
| 1 | Water-based coating | 1.1 | 1.2 |
| 2 | Solvent-based coating | 2.1 | 2.2 |
| 3 | Increased temperature resistance | 3.1 | 3.2 |
| 4 | UV-curing coating | 4.1 | 4.2 |
Table 1 of the standard sets out these class designations.[1] The Flow category denotes moderate to low flowability; Anti-Flow denotes very low or no flowability. A specified flow test determines the category.[1]
The product designation begins with SL and gives the application class and flow category. GHS 02 is added when the flame-pictogram classification applies. For ordering, the designation also includes the container volume and a reference to DIN 32546.[1] Manufacturers must state the storage temperature, expiry date and sealant classification.[1]
Test methods
For the fracture test, the user prepares five examples of the intended threaded joint, assembled to the user's specifications. Sealant is applied across each joint and a mating component, then left to dry for at least 24 hours. Each joint is turned through 90° ± 10° in the loosening direction. All five seals must develop a clearly visible crack for the product to qualify as suitable for that application.[1]
The manufacturer assesses flow behaviour in five tests on cleaned glass plates. In each test, a measured drop of sealant is applied to a plate, which is then placed vertically. The mean distance travelled from the starting mark determines the category: more than 1 cm is classified as Flow, and 1 cm or less as Anti-Flow.[1]
Each test procedure requires a report identifying the product, batch, test conditions and examiner, with photographs taken before and after testing. The fracture-test report also records the joint and drying time. The flow-test report gives the mean flow distance.[1]
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 DIN Deutsches Institut für Normung e. V. (August 2026). DIN 32546:2026-08 — Siegellack (in Deutsch). DIN Media. Search this book on
- ↑ 2.0 2.1 "Validation of Hazardous Air Pollutant (HAP)-Free Torque Seal Inspection Lacquer". Defense Tech Briefs. Vol. 3 no. 6. 2009-12-01. Retrieved 2026-10-01.
- ↑ 3.0 3.1 3.2 3.3 Kerr, A. R. (2006-11-13). Suggestions for SMA Connector Use on ALMA (PDF) (Report). ALMA Memo. National Radio Astronomy Observatory. pp. 2–4. Retrieved 2026-10-01.
- ↑ 4.0 4.1 Lackfabrik Bäder GmbH & Co. KG (July 2025). Technical Data Sheet: Torque Sealant (PDF) (Report). Lackfabrik Bäder GmbH & Co. KG. Standard Range; Zero Range; Fast Dry Range. Retrieved 2026-10-01.
- ↑ Faye R. Toulan; et al. (2009). Demonstration/Validation of Hazardous Air Pollutant-Free Torque Seal Inspection Lacquer (Report). Army Research Laboratory. Retrieved 2026-10-01.
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