Safety wire? 1

[Roktgr]

Hi guys,

Recently someone mentioned the need of using safety wire on bolts someone had on a application that keep coming loose and shearing.

I said safety wire doesn't keep the bolts from loosening just from completely falling out once they loosen. To this I got several people give me blank stares and question my statement. Well I remember seeing it somewhere and started to looks for some more research on the matter. All I could find off-hand was Carroll Smith's reference to safety wire in his book Nuts and Bolts, that it doesn't work, and in a book called how to build a Hot Rod.

In my mind the only thing keeping a bolt tight is the clamping force between the threads of the bolt and hole and I don't see how a piece of wire that is wrapped around the head and produce enough force to prevent the bolt from loosening? I did some searching and see it has somewhat been covered before but the threads were locked so wonder if I can get some more insight on this subject. Thanks.

 

[MintJulep]

Fastener Design Manual; NASA Reference Publication 1228 Although lockwiring is a laborious method of preventing bolt or nut rotation, it is still used in critical applications, particularly in the aerospace field.

Acceptable Methods, Techniques and Practices – Aircraft Inspection and Repair; FAA Advisory Circular AC43.13-1B These practices
are not a means of obtaining or maintaining
torque, rather a safety device to prevent the
disengagement of screws, nuts, bolts, snap
rings, oil caps, drain cocks, valves, and parts.


Bickford, John H., 1981 An Introduction to the design and behavior of bolted joints Can effectively prevent total loss of the nut – but are not very effective in preventing substantial loss of preload

The U.S. Navy’s Naval Ships’ Technical Manual; S9086-CJ-STM-010/CH-075R2 The primary purpose of safety wiring is to prevent complete loss of a fastener. It’s not particularly effective in preventing minor relative nut and bolt rotation and subsequent loss of preload.

It’s effective in preventing complete loss of the fastener. It is, however, an ineffective
method for preventing loss of preload; some rotation can still occur, even with the best of tying techniques.

Safety wire may be used to replace previous existing safety wire. If loose or broken wires or
loose fasteners are found at disassembly, however, self-locking fasteners may be installed at reassembly

Verein Deutscher Ingenieure, Systematic Calculation of High Duty Bolted Joints. Joints with One Cylindrical Bolt VDI 2230, Part 1 Listed 3rd of 4 “Positive locking” methods in table A14.


SAE AS567J Safety cable or wire is not a means of obtaining or maintaining torque, but a safety device used to prevent disengagement of the part.

NASM1515 Fastener Systems for Aerospace Applications
(Formerly MIL-STD-1515) Self-locking fasteners are preferred over auxiliary devices (such as cotter pins, safety wire, tab washers, etc.)

 

[JJPellin]

I agree completely. If I look at typical turbo-machinery in our plant, I find that the only bolts that are safety-wired are the ones that are internal and could be drawn into moving parts if the bolt were to come completely out. It is not typically used on critical fasteners that are external (bearing housing mounting bolts, bearing housing cover bolts, split-line bolting). Lastly, the bolts that I am most concerned about coming loose (coupling bolts) use self-locking nuts. The engineers that designed these machines were apparently familiar with the limitations and value of safety wire when they created these designs.

Johnny Pellin

 

[btrueblood]

Gee, Mint, maybe you should copy that post to a FAQ!

 

[MintJulep]

Good idea. A FAQ it is.

faq725-1549