Hex Coupler Nut

What a coupling nut does

A coupling nut is a long hex nut with one continuous internal thread. Hobson describes its hex couplers as a connection of two male threads, most commonly threaded rod: screw a rod into each end and the two become one longer length. They are also called long nuts, extension nuts and threaded rod couplers.

Common uses are extending threaded rod for hanging cable tray, HVAC ducting, pipes and other services, and joining studs or rod in fabrication. Hobson publishes proof loads for some ranges (see the table) but no working load for any coupler, so a hanging, suspension or bracing system that carries a load needs the rod, coupler and anchor specified by its designer or an engineer.

How to join allthread with a coupler

Hobson's coupler data sheets set the rule: to reach the coupler's stated mechanical properties, each rod must be engaged by at least 1 x its diameter (D). The standard couplers are 3 x D long, which leaves room to adjust at either end.

1. Match the coupler to the rod: same diameter and same thread (metric coarse for M sizes, BSW for the imperial couplers).
2. Mark each rod at half the coupler length, for example 18mm on each rod for an M12 x 36mm coupler. That is more than the 12mm minimum and leaves the joint centred.
3. Screw the coupler onto the first rod up to its mark.
4. Screw the second rod into the other end up to its mark, holding the coupler with a spanner of the size in the table.
5. Check both marks sit at the ends of the coupler, so each rod has at least 1 x D of thread inside it.

Coupling nut sizes, spanner sizes and proof loads

Every coupler we carry, grouped by finish and class, smallest to largest. The spanner size and proof load come from Hobson's data sheets for the zinc plated and galvanised class 5, galvanised class 8 and 316 stainless couplers; Hobson publishes no sheet for the Hobkote, zinc yellow, Neptune and BSW couplers, so those cells are blank.

Class 5, class 8 and 316: what the proof loads mean

• •Class 5 couplers are made to the class 5 nut requirements of ISO 898-2 / AS/NZS 4291.2, and class 8 couplers to the class 8 requirements. Their proof loads match the class 5 and class 8 hex nut values on Hobson's hex nut properties sheet.
• •The 316 stainless couplers are made to ISO 3506 property class 50 (A4-50), with a stress under proof load of 500 MPa. That is lower than class 5 steel, so a 316 coupler has a lower proof load than a steel one of the same size: 42.1kN against 51.4kN at M12. On A4-70 or A4-80 stainless rod, the coupler is the lower strength part. Hobson's 316 figures are from testing at an ambient 10 to 35°C; properties can vary outside that range.
• •The Neptune M12 x 50mm coupler is listed as A4-80. Hobson publishes no data sheet for it.
• •The M36 x 70mm zinc yellow coupler is shorter than the 3 x D pattern: at 70mm it cannot give 36mm (1 x D) of engagement on both rods, and Hobson publishes no sheet for it. To join M36 rod, use the 108mm M36 couplers.
• •Proof loads are test values for the coupler alone. What a joint can carry depends on the rod, the anchors and how it is installed.

Which coupler finish: indoors, outdoors or near the sea

Hobson's corrosion guide for metal connectors sets the finish by exposure: zinc plated only for enclosed work in very low and low corrosion areas, and anything exposed to the weather in hot dip galvanised or 304 or 316 stainless, with stainless in the sea spray zone and 316 within 2m of a pool edge. Use the same finish as the rod and nuts on that job.

• •Zinc plated: a 5 micron zinc coating, for indoor and non-corrosive conditions.
• •Zinc yellow: also 5 micron, with a yellow passivate that resists corrosion better than clear zinc; for low corrosion areas or temporary fixing.
• •Hobkote (M10, M16 and M20): a 5 to 8 micron zinc coating with a specially formulated passivation that Hobson says gives 33% more corrosion resistance than zinc plated.
• •Hot dip galvanised: a 50 micron coating for long term outdoor work. Hobson has found no zinc whiskers on its hot dip galvanised products, which suits sensitive sites such as data centres, where whiskers from electroplated zinc can cause electrical faults.
• •316 stainless: for coastal, pool and wet areas. Stainless threads can gall (cold weld) when tightened; Hobson suggests a slower installation speed and a moly based thread lubricant. The ASSDA article Hobson publishes says mixing grades (a 316 nut on a 304 bolt) is not a reliable fix: galling is worse with fine threads, tight clearances, poor thread profiles, no lubrication, dirt and over-tightening, so keep threads clean and lubricated and do not overtighten.

Coupling nuts in Adelaide

Allmat supplies coupling nuts, threaded rod, nuts and washers to electricians, plumbers, HVAC installers and builders across Adelaide and South Australia. Add the couplers to a quote with your rod and anchors, and we will price the lot.

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