All-Metal Prevailing Torque Nut

Prevailing torque nuts: how they lock

Hobson defines a prevailing torque nut as a lock nut with a built-in element that creates friction on the mating thread and stops the nut turning freely. Nyloc, cone lock, met lock, stover and deformed thread nuts are all prevailing torque nuts. The ones on this page are all metal, with no nylon ring; for nylon insert lock nuts see the nyloc nut page.

• •Cone lock nuts (DIN 980 class 10 and grade C): in Hobson's words, cone lock protects against loosening by adding prevailing torque to the connection.
• •ESL dual slot self lock nuts (NFE 25 411): Hobson lists consistent prevailing torque, no galling, and locking slots that work independently of bolt tension, for mining and processing, railway and transport, energy, military and industrial work.
• •K-lock nuts (DIN 934K): a nut and a free-spinning external tooth lock washer with 12 teeth, assembled as one piece. The washer resists loosening. Hobson lists electrical and electronic equipment, white goods, IT and communications equipment and robotics.
• •BUMAX 88 collared lock nuts: Bumax's Lock data sheet says the root of the thread is cut at a larger angle to form a wedge-shaped surface, so once tightened the bolt thread bears metal to metal with no play. The nut locks in one direction, with the collar against the joint, and does not lock until it is tightened hard enough to meet resistance.

When does a joint need a prevailing torque nut?

Hobson's anti-vibration paper finds that a preload above about 65% of the fastener's yield load is enough to stop loosening at the vibration levels commonly seen in the field. Where a joint is subject to vibration and the bolt cannot be pretensioned to at least 65% of its proof load, the paper says a locking device must be used, and under severe vibration one may be needed anyway.

The same paper warns that two identical nuts locked against each other will hardly ever work, because both nuts rotate together under vibration.

Every all-metal lock nut we carry

Select a row to open that range in the selector, then pick the size.

Cone lock nut tightening torque (DIN 980 class 10)

Hobson says standard torque values should not be used with prevailing torque nuts: the extra torque needed to turn the nut along the thread leaves a lower clamp load than expected. Add the prevailing torque to the target torque, T = F x k x d + Tpr, where F is the clamp load (kN), k the nut factor, d the thread diameter (mm) and Tpr the prevailing torque (Nm). The installation torques below already include it: for M12, 45.48 x 0.14 x 12 + 21 is about 97Nm. They use the maximum prevailing torque from ISO 2320, and Hobson notes this will likely give a higher clamp load than expected. Measuring the actual prevailing torque with a torque wrench as the nut runs down the thread, and adding that, gives a closer result.

Prevailing torque nuts must have full threads right through the locking element, and at least one full thread should show past the nut.

ESL dual slot self lock nut sizes

Reusing prevailing torque nuts, and temperature

Hobson says prevailing torque nuts can be reused but every use reduces their effectiveness; from the second installation on, the install and removal torques will be similar. When reusing one, check with a torque wrench that its prevailing torque still suits the job.

For the BUMAX collared lock nut, Bumax says the nut can be locked and unlocked up to 10 times without losing its function, but recommends changing the screw after each unlocking. Being all metal, in 316L high molybdenum steel, Bumax says it may be used at relatively high surface temperatures, up to about 300°C. Hobson publishes no temperature limit for the cone lock, ESL or K-lock nuts in the documents we hold. Hobson's anti-vibration paper notes that nylon insert nuts are not suitable for joints exposed to high temperatures.

Finishes: zinc, stainless and BUMAX

Hobson rates zinc plated (5 micron) for indoor and non-corrosive conditions only. For nuts exposed to the weather, Hobson's corrosion guide for metal connectors (written for timber connectors) gives stainless (304 or 316) or hot dip galvanising as the minimum, stainless only in seaspray zones and 316 within 2m of a pool; on this page that means the A2-70 or A4-80 ESL nuts or the BUMAX nuts.

The class 8 ESL nuts have a zinc finish: use them indoors and in non-corrosive conditions.

Stainless threads can gall (cold weld) as they tighten. Hobson's galling sheet suggests slowing the installation speed, lubricating the threads, and using different stainless grades for the bolt and the nut; the ESL nuts are listed as no galling. BUMAX 88 is a 316L high molybdenum A4 steel, and Bumax says BUMAX 88 fasteners have strength corresponding to 8.8 carbon steel fasteners.

Cone lock nuts and prevailing torque nuts in Adelaide

Allmat supplies all-metal lock nuts, nyloc nuts, bolts and washers to fabricators, mechanics, maintenance crews and trade across Adelaide and South Australia. Add them to a quote with the rest of your list, and we will price the lot.

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