Según la especificación RCSC 2025 Especificación para uniones estructurales con pernos de alta resistencia, se permiten distintos grados de tuerca para conjuntos sin recubrimiento y con recubrimiento. Por ejemplo, un perno A325 Tipo 1 sin recubrimiento admite más opciones de grado de tuerca ASTM A563 que el conjunto recubierto correspondiente. Un perno A490 Tipo 3 tiene un requisito distinto.
Para compras y control de calidad, esto genera dos preguntas distintas: ¿Está permitido el grado de la tuerca para la configuración de perno especificada? ¿Y la tuerca suministrada cumple con los requisitos aplicables de producto y de montaje? Ambas cuestiones deben responderse antes de la aceptación.
Identifique el perno antes de seleccionar la tuerca
A325 y A490 siguen siendo designaciones conocidas, pero los grados de pernos estructurales de cabeza hexagonal pesada están ahora cubiertos por ASTM F3125/F3125M. ASTM A563/A563M aborda las tuercas, mientras que la especificación RCSC aborda su uso en uniones estructurales atornilladas. Estas normas desempeñan funciones diferentes y no deben tratarse como documentos de aceptación intercambiables.
RCSC 2025 identifica los pernos de cabeza hexagonal pesada ASTM F3125 Grado A325 como Grupo 120 y los pernos de cabeza hexagonal pesada Grado A490 como Grupo 150. Estos grupos distinguen categorías de resistencia para el atornillado estructural; no son designaciones de grado de tuerca ASTM A563.
Antes de consultar una referencia de compatibilidad de tuercas, establezca la configuración real del perno:
- Grado y estilo del perno: F3125 Grado A325 o A490, y si el artículo es un perno convencional de cabeza hexagonal pesada u otro producto de conjunto apareado.
- Tipo de perno: Tipo 1 o Tipo 3. El Tipo 3 está asociado con acero resistente a la intemperie y tiene componentes de acoplamiento permitidos distintos.
- Diámetro nominal y designación de rosca: Estos determinan las dimensiones de acoplamiento requeridas y la compatibilidad de rosca.
- Estado de la superficie: Sin recubrimiento, galvanizado por inmersión en caliente, galvanizado mecánicamente u otro recubrimiento específicamente permitido para el conjunto.
- Especificación y edición aplicables: El proyecto o la orden de compra puede hacer referencia a una edición concreta de ASTM o RCSC en lugar de a la publicación más reciente.
Un perno descrito únicamente como “A325, 3/4 inch” no aporta información suficiente para completar la selección de la tuerca. El tipo y la condición de recubrimiento podrían cambiar el grado permitido, incluso si el diámetro nominal sigue siendo el mismo.
El estilo de la tuerca también es importante. RCSC 2025 especifica generalmente tuercas hexagonales pesadas conformes con ASTM A563 para los conjuntos de pernos estructurales hexagonales pesados correspondientes, sujeto a sus disposiciones de sustitución indicadas. No debe aceptarse una tuerca hexagonal estándar con una designación de grado conocida solo porque rosca sobre el perno. La geometría de tuerca requerida y la norma dimensional también deben coincidir con el conjunto especificado.
Para información general sobre grados de tuercas hexagonales y compatibilidad con pernos, consulte la descripción técnica independiente de SUNHY.
La distinción va más allá de la propia tuerca. RCSC 2025 también exige que el tipo de arandela, cuando corresponda, se corresponda con el tipo de perno. Por lo tanto, en la compra debe identificarse el perno, la tuerca y la arandela como un sistema compatible, y no como tres artículos de catálogo sin relación entre sí.
¿Qué grados de tuerca ASTM A563 están permitidos?
El punto de partida para una conexión estructural según RCSC 2025 es la Tabla 2.2, que establece los grados de tuerca ASTM A563 permitidos según el grupo, el tipo y la condición de recubrimiento del perno. La siguiente explicación resume las consecuencias de la selección y no sustituye a la tabla aplicable.
A325 Tipo 1: los conjuntos sin recubrimiento y con recubrimiento difieren
Para un perno ASTM F3125 Grado A325 Tipo 1 sin recubrimiento, RCSC 2025 permite los grados C, C3, D, DH y DH3 de ASTM A563.
Ese abanico de opciones a veces se condensa en la combinación preferida por el fabricante. El grado DH, por ejemplo, es una elección habitual, pero no es el único grado permitido para la configuración sin recubrimiento en esta edición.
La situación cambia cuando el perno A325 Tipo 1 está recubierto conforme a la Sección 2.8 de RCSC. En este caso, la Tabla 2.2 identifica ASTM A563 Grado DH.
Un comprador no debe trasladar una especificación de tuerca Grado C previamente aprobada solo porque el grado y el diámetro del perno permanecen sin cambios. Modificar la especificación del recubrimiento puede cambiar el grado de tuerca permitido y la verificación de montaje requerida.
A490 Tipo 1: una selección más restringida
Para los pernos ASTM F3125 Grado A490 Tipo 1 sin recubrimiento, RCSC 2025 permite los grados DH y DH3. Para los conjuntos Tipo 1 recubiertos conforme a la Sección 2.8, el grado ASTM A563 permitido es DH.
Esta distinción evita dos errores habituales.
Primero, no debe aprobarse una tuerca A563 Grado C para un conjunto A490 argumentando que es aceptable para algunas aplicaciones A325. Las combinaciones permitidas son diferentes.
Los pernos resistentes a la intemperie Tipo 3 requieren su propia decisión de correspondencia
Para las configuraciones Tipo 3 sin recubrimiento, RCSC 2025 permite tuercas C3 o DH3 con pernos del Grupo 120 (A325), y tuercas DH3 con pernos del Grupo 150 (A490).
La designación “3” es significativa porque la selección de material resistente a la intemperie no es únicamente una cuestión de resistencia mecánica. Una conexión diseñada con acero resistente a la intemperie requiere una consideración adecuada de los componentes de unión y de sus condiciones de exposición.
La tabla RCSC no ofrece en estas filas una combinación general Tipo 3 recubierta. Por lo tanto, un comprador no debe extender los permisos del Tipo 3 sin recubrimiento a un conjunto resistente a la intemperie recubierto sin una base de especificación aplicable y la aprobación de ingeniería requerida.
¿Puede una tuerca ASTM A194 Grado 2H sustituir a una tuerca A563?
RCSC 2025 Section 2.3.2 permits ASTM A194 Grade 2H nuts as substitutes for ASTM A563 Grades C, D, and DH.
Se trata de un permiso definido, no de una afirmación general de que cualquier tuerca A194 pueda sustituir a cualquier tuerca A563. En particular, no debe extenderse automáticamente a los requisitos de grado resistente a la intemperie C3 o DH3.
Where Grade 2H is proposed, the buyer still needs to confirm the heavy-hex configuration, correct thread fit, applicable coating requirements, documentation, and complete assembly conditions. A permissible substitute grade does not eliminate the need to verify the delivered component.
ASTM A563/A563M also contains provisions concerning stronger-grade substitutions with notice to the purchaser. Those provisions must be read alongside any more specific structural-joint and purchase-order restrictions. A supplier’s proposed upgrade is not an automatic authorization to change a specified structural assembly.
The practical selection rule is straightforward: identify a permitted grade first, then verify that the proposed nut has the required style, finish, and supporting evidence.
In an illustrative finish revision, an A325 Type 1 order changes from plain to a permitted coated assembly while the original nut line remains unchanged. A nut permitted for the plain configuration may not satisfy the coated row. Recheck the governing pairing, nut treatment and complete assembly evidence. The nut-grade guide supplies background; the cited structural pairing table controls this decision.
Por qué los recubrimientos modifican la compatibilidad de las tuercas
Coating selection can alter the mechanical behavior of a bolting assembly even when the bolt and nut have unchanged nominal sizes.
For galvanized or certain zinc-flake-coated assemblies, the coating adds material to the mating thread surfaces. To accommodate that thickness, structural nuts are commonly manufactured with intentionally oversized internal threads, known as overtapping.
Overtapping creates clearance for the coated bolt threads, but it also reduces the amount of thread engagement. This matters because the nut must resist thread stripping while the bolt is subjected to the tensile loading associated with installation and service.
A nut that is adequate in a plain-thread configuration cannot automatically be treated as adequate after its internal threads have been enlarged. The relevant nut strength requirements, coating conditions, and assembly tests must be satisfied.
Hot-dip galvanizing and mechanical galvanizing are not interchangeable processes
RCSC 2025 Section 2.8 distinguishes hot-dip galvanizing from mechanical galvanizing. The two processes have separate specification requirements, and the threaded components of a galvanized assembly must use the same applicable process.
For Group 120/A325 heavy-hex bolting, permitted galvanized assemblies require careful control of the corresponding nut treatment and lubrication. Hot-dip galvanized nuts are overtapped according to ASTM A563 requirements, and mechanically galvanized nuts have their own applicable overtapping requirements.
The assembly should not be created simply by purchasing galvanized bolts from one source and nuts from another without establishing responsibility for compatibility testing.
RCSC commentary explains why galvanized structural bolting is treated as a matched assembly. Changes to coating thickness, thread engagement, and lubrication can affect the ability to achieve the required tightening behavior without thread damage or stripping.
For a galvanized assembly governed by these requirements, the supplier must provide the applicable matched and tested components. If separately sourced components are proposed, the purchaser must establish how the necessary assembly verification and certification responsibilities will be fulfilled.
A490 coating restrictions need precise wording
A490/Group 150 heavy-hex bolts are not permitted to be hot-dip galvanized or mechanically galvanized under RCSC 2025.
That restriction should not be simplified into “A490 bolts cannot be coated.”
Section 2.8 separately permits qualifying zinc-flake coating systems complying with the specified ASTM F3393 classification and other applicable conditions. This is a defined coating route, not a general permission to use any commercial zinc-flake, zinc-plated, or galvanized finish.
For a proposed coated A490 assembly, the buyer should verify the coating standard and classification, the supplier’s assembly configuration, the required treatment of the nuts, and the relevant product and assembly evidence.
Older technical literature may refer to different zinc-flake standards used in previous editions. A historic designation or a supplier’s informal coating description is not sufficient evidence that a delivered assembly conforms to the requirements of RCSC 2025.
Hypothetical example: Changing the finish changes the nut decision
Assume an EPC purchaser is preparing a request for 3/4-inch ASTM F3125 Grade A325 Type 1 heavy-hex bolts.
The original specification calls for plain bolts, and the supplier offers plain ASTM A563 Grade C heavy-hex nuts. Grade C is one of the permitted A563 grades for this plain combination under RCSC 2025, provided the other product and project requirements are satisfied.
The design is then revised to require hot-dip galvanized bolting.
The buyer cannot simply keep the original Grade C nut specification and add galvanizing. Under the coated Group 120 Type 1 conditions in RCSC Table 2.2, the specified A563 grade is DH.
The revised procurement request must address the permitted Grade DH nut, correct galvanizing process, overtapped thread configuration, lubrication, matching components, and applicable rotational-capacity evidence.
The bolt diameter has not changed. The required selection and verification work has.
This assumed case illustrates a specification decision; it is not a recorded project, a tested product result, or proof of any supplier’s manufacturing capability.
Verifique el marcado de las tuercas, las roscas y los registros de inspección
A permitted grade on a purchase order establishes what should be supplied. Receiving inspection establishes whether the actual shipment matches the specified requirements.
These are related but different decisions, and neither can be completed by checking the nut grade marking alone.
Confirm the nut’s identity
Inspect the nut for the applicable grade identification and manufacturer’s identification, using the marking requirements of the governing standard and the permitted marking arrangements.
The markings help identify whether the supplied item is represented as Grade C, C3, D, DH, DH3, or another permitted specification grade. A194 Grade 2H has its own designation and should not be silently relabelled as A563 DH.
The inspector must also confirm the required heavy-hex style and nominal size. The grade marking is useful evidence of identification, but it does not independently establish that the nut passed every chemical, hardness, proof-load, or dimensional acceptance requirement.
If the marking is missing, inconsistent with the purchase order, or not traceable to the supplier’s documentation, the discrepancy should be resolved before the component is accepted for the specified structural assembly.
Verify thread fit against the actual finish
Thread compatibility is more than matching the nominal diameter and pitch.
A plain nut and an overtapped coated nut can have different finished internal-thread conditions. That difference may be intentional and necessary for the specified assembly.
Receiving inspection should confirm the relevant thread designation, applicable dimensional requirements, coating condition, and whether overtapping is required. Where gauging or dimensional records are supplied, their acceptance basis should correspond to the specified finished nut condition rather than an assumed uncoated configuration.
An assembler should not attempt to correct a questionable fit by uncontrolled thread chasing, removal of the coating, or applying an unapproved lubricant. Such actions could change the supplied assembly condition and invalidate the basis of its verification.
A bolt and nut turning freely by hand does not demonstrate that the assembled fastener system meets the specified load, coating, or installation requirements.
Distinguish product testing from assembly verification
Three forms of evidence are especially important, but they establish different things.
| Evidencia | What it helps establish | What it does not establish alone |
|---|---|---|
| Nut product conformity and test report | Compliance with applicable ASTM A563 or A194 material, mechanical, and dimensional requirements, within the reported scope | That the nut was tested as part of the actual supplied bolt–nut–washer assembly |
| Rotational-capacity evidence, where required | Compatibility of the applicable coated assembly, including its lubricant and capacity to undergo the specified rotation without unacceptable failure | That the completed structural joint was installed and inspected correctly |
| Pre-installation verification records, where required by the installation method | That the specified assembly and installation procedure demonstrate the required pretensioning behavior before installation | That every nut lot independently satisfies all product specification requirements |
ASTM F606/F606M provides mechanical testing methods for threaded fastener products, including applicable nut proof-load and hardness testing. The product standard and governing purchase requirements determine the actual acceptance criteria.
A proof-load test evaluates the nut’s response to a defined axial test load under specified conditions. It is not the same as the pretension achieved in a completed structural joint. Similarly, a hardness result cannot automatically be substituted for the required proof-load evidence.
The exact numerical test limits, sampling arrangements, and acceptance conditions must be checked against the contractual ASTM A563/A563M edition and the particular product configuration. Published data from an older edition should not be presented as current ASTM A563/A563M-26 acceptance requirements without verification.
RCSC 2025 Section 2.9 addresses the availability of applicable product and assembly test reports to the Engineer of Record and Inspector. Pre-installation verification is separately associated with the relevant installation procedure. These responsibilities should remain distinct in the project’s inspection plan.
Use receiving inspection to close actual discrepancies
Consider a shipment labelled “A563 DH heavy hex nuts” for coated A325 Type 1 bolts.
The receiving decision should not stop at the DH identification. The inspector should determine whether the specified coating was applied, whether the threads were processed for that coating, whether the required lubrication and matched-assembly conditions were maintained, and whether the required records are available.
The documentation should identify the actual supplied product and the applicable lot or other traceability identifiers required by the purchase order. Traceability statements must be consistent with the records; a generic catalogue certificate is not proof of testing for a particular delivered lot.
If a record refers to a different grade, finish, or assembly configuration, the buyer should request clarification or hold the affected material rather than infer compliance from visual similarity.
A hypothetical delivery contains DH-marked nuts with no clear finished-thread or lubricant basis for the ordered coated assembly. The grade mark does not resolve those conditions. Reconcile the product records, coating, overtapping and matched components, retaining the lot-specific evidence relationship. Do not correct an unexplained tight fit by uncontrolled thread chasing.
Especifique las tuercas y los conjuntos correctos en una solicitud de cotización
A useful RFQ does more than state “ASTM A563 nuts for A325 bolts.” It defines enough information for a supplier to propose the correct items and for the purchaser to verify the delivered assembly without making assumptions.
The following checklist separates selection information from verification evidence.
| Ítem de RFQ o de recepción | What to specify or confirm |
|---|---|
| Governing documents | Applicable ASTM F3125/F3125M, ASTM A563/A563M, RCSC, and project editions |
| Bolt configuration | Grade A325 or A490, Type 1 or Type 3, style, nominal diameter, length, and thread designation |
| Nut requirement | Permitted A563 grade or approved substitute, heavy-hex style, size, and marking requirements |
| Washer configuration | Required washer specification, Type, finish, and special configuration where applicable |
| Revestimiento | Exact coating method, governing coating specification, classification, and finish condition |
| Tratamiento de rosca | Whether overtapping is required and how finished-thread conformity will be verified |
| Assembly condition | Matched component requirements, specified lubrication, packaging, and handling controls |
| Acceptance evidence | Product conformity reports, applicable lot identification, and assembly test records required for the configuration |
| Changes and substitutions | Written identification and approval of any departure from the specified nut grade, finish, or assembly |
Not every project requires an identical document package. The evidence requirement should follow the applicable standards, connection and installation conditions, and contract documents. A buyer can request additional traceability or inspection records, but those requests should not be represented as universal ASTM requirements.
For coated assemblies, it is useful to ask the supplier to identify who is responsible for the complete assembly verification, rather than simply providing separate bolt, nut, and washer certificates.
For a plain A490 Type 3 assembly, the focus changes to confirming the permitted weathering-grade nut and mating component configuration. Asking for an unrelated galvanizing record would not solve the relevant acceptance question.
The final procurement decision should follow a clear sequence: establish the bolt identity, confirm the permitted nut configuration, verify the coating and thread conditions, and review the actual product and assembly evidence. If a proposed substitution changes that basis, obtain the necessary specification and project approval before accepting it.
SUNHY’s structural bolt product information may serve as a starting point for a technical inquiry. Buyers should provide the bolt grade and Type, nut grade, dimensions, finish, quantity, and required documentation for a configuration-specific review. Availability, manufacturing capability, and conformity must be confirmed for the particular order rather than inferred from this guide.
Technical reference basis
- Research Council on Structural Connections (RCSC). Especificación para uniones estructurales con pernos de alta resistencia, June 5, 2025. Particularly the bolt Group definitions, permitted nut-grade requirements, coated assembly provisions, and test-report requirements.
- ASTM International. ASTM A563/A563M-26 — Standard Specification for Carbon and Alloy Steel Nuts (Inch and Metric).
- ASTM International. ASTM F3125/F3125M-26 — Standard Specification for High Strength Structural Bolts and Assemblies.
- ASTM International. ASTM F606/F606M-26a — Standard Test Methods for Determining the Mechanical Properties of Externally and Internally Threaded Fasteners, Washers, Direct Tension Indicators, and Rivets.
The 2025 RCSC document is the edition used for the pairing explanations in this guide. Actual acceptance requirements must follow the standards and editions governing the project; this article does not reproduce the complete ASTM product specifications.
Preguntas frecuentes
¿Qué tuerca A563 debe utilizarse con A325?
Identifique primero el tipo de perno y el acabado, y después utilice el grado permitido en la tabla aplicable. Las combinaciones sin recubrimiento y con recubrimiento no tienen opciones idénticas.
¿Puede la tuerca A194 2H sustituir a cualquier tuerca A563?
No. El permiso de sustitución de la RCSC citado se limita a los grados A563 especificados. No lo extienda automáticamente a requisitos de grado resistente a la intemperie.
¿Una tuerca más resistente aprueba automáticamente una sustitución?
N.º Compruebe el grado de permiso aplicable, el estilo, el estado de la rosca, el recubrimiento, la documentación y cualquier aprobación requerida para el montaje real.
