Locking of Threaded Fasteners (Article-19)
The purpose of a locking device is to prevent the loosening of mating components which may be operating in conditions
of varying stress, temperature or vibration. Information about commonly used locking devices like Nylock Nuts, Castle Nuts, Heli Coil Inserts, Spring Washer, Tab Washers, etc. is given in this article. Special care shall be taken during maintenance to verify component healthiness and correct reinstallation when the effectiveness of the locking device is critical for the safety of persons / system.
There are two main principles used for locking devices - friction and positive locking. It should be noted that locking
systems using friction should be selected with care because under high vibration friction between adjacent surfaces can be significantly reduced. With positive locking device e.g. use of tab washer very high reliability results provided the
locking device is used correctly.
Lock Nut /Jam Nut
The term is used for thin nuts used to lock a thicker nut. Usually a thin nut is fixed above a normal nut. It is recommended by many persons that the thin nut should be adjacent to the joint surface and tightened against the thick nut as shown above. As per them, if thin (lock) nut is placed on top of the thick nut the thin nut would sustain loads it was not designed
to sustain. The lock nut principle can also be used to fix a nut in any position on the male screw thread and therefore create a shoulder.
Usually nuts are free spinning, but Nylock nuts have a plastic or fiber collar set into the nut which is an interference
fit on the male thread that causes resistance to nut turning. This resistance is called "prevailing torque". Prevailing torque is the torque required to turn the nut. The prevailing torque does not go toward tightening the bolt. On assembly
the male thread forces its way through the collar and the resultant friction restricts the tendency to unscrew.
The rule of thumb is to add the prevailing torque to the torque value when applying torque to a Nylock nut. This is because
the prevailing torque doesn't contribute to bolt tightening. You can use torque wrench to measure Nylock nut torque and then add this value to the bolt's required torque. Usually you wouldn't add prevailing torque to the torque value published
by the equipment manufacturer. The equipment manufacturer has already done this for you.
The prevailing torque locknut retains its locking ability even when the preload or tightening torque has been lost. The nylock nuts are more friendly to the threads, locks out moisture and prevents corrosion.
For more information on nylock nuts please refer IS 7002 (Identical to ISO 7040), Prevailing Torque Type Hexagon Nuts
(With Non-Metallic Insert), Style 1 - Property Class 5, 8 and 10. The DIN standard number is 982.
All-metal Lock Nuts
As nylock nuts are not suitable in areas exceeding 120 degrees C, for high temperature application all-metal lock nuts
are used. Various types of all-metal lock nuts are as under.
Distorted Thread Nut (CLEVELOC Nut)
In this type of nut, the collar of the nut has been slightly crushed at the top to make it oval. When the round bolt reaches
the oval portion of the nut it springs the nut back round. This spring action grips the bolt and adds friction that prevents loosening. Such type of nut is also known as Cleveloc nut. Cleveloc is a registered trade name of Forest Fasteners.
Spring Beam Nut (FLEXLOC Nut)
This type of nut has thin slots cut down through the top few threads with the resulting fingers bent slightly inward. At installation, the bolt springs the fingers out and the fingers grip the bolt with a prevailing torque. This type of
lock nut is also known as FLEXLOC nut. FLEXLOC is a trademark of SPS Technologies. For more information please visit internet
site of SPS Technologies - http://www.spstech.com
In slotted nut, a slot is machined into the nut and the nut is deformed to compress the slot as shown above. When the nut is tightened onto the male thread it forces the nut back to its original geometry. The thread system is locked by
the built in friction.
Slotted / Castle Nuts
These nuts have slots in the top face. The nuts are fully tightened and a hole is drilled through the male thread to align with one of the slots. Split cotter pin is then inserted through the nut and the male thread and bent to hold it in position.
This is a very effective and positive locking device but is expensive to install.
For more information on slotted / castle nuts please refer IS 2232, Specification for Slotted and Castle Nuts. This standard prescribes the requirements of precision and black grades of slotted and castle nuts in the following diameter ranges.
Precision grade slotted nuts: 4 to 39 mm
Black grade slotted nuts: 12 to 33 mm and
Precision and black grades castle nuts: 12 to 100 mm.
Durlok? self-locking system