Talk:Safety wire

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I added a LOT of detail to this article today, but I didn't have too many sources to cite. Please help. Also, please help with formatting/cleanup/quality standards. The info that I added is all good and solid. Thanks. Srajan01 (talk) 11:30, 1 February 2008 (UTC)[reply]

Pics

Oh, and I'll take some pics when I have time - can't find public domain pics online, but if anyone has any of pliers, wires, pliers in action, jigs, tabs, etc, please post them. Srajan01 (talk) 12:18, 1 February 2008 (UTC)[reply]

I shall take some pictures as soon as I can get a good quality camera. I have taken a few with a cell phone, but the quality can greatly improve. VoivodeZmey (talk) 16:02, 2 April 2009 (UTC)[reply]

Fastener retention vs fastener load retention

Something to consider on the topic of fastener joint design.

The primary reason for loosening of fasteners is loss of fastener tension load. Even a small fastener can have several hundred pounds of tension; sub-1 inch fasteners can withstands loads in the 10 ton range. To back out, the tension must be significantly reduced.

Loss of fastener load is usually from fastener embedment - where the material the fastener is bearing on does not withstand the applied loads and dents enough to eliminate the effect of the original stretch in the fastener and the compression of the item being retained. Vibration, blamed for fastener loosening, should be blamed for causing loads in excess of the capacity of the joint design to withstand them. In turn, the joint designer should be blamed for not including such loads in the design calculations.

Occasionally the loss of tension is due to differing material properties - a steel fastener retaining an aluminum part may lose preload at very low temperatures (steel contracting with cold less than the aluminum does) or embed at high ones. Metal fasteners in wood are unaffected by changes in humidity, while wood can change by large amounts.

Of the fastener retention methods, all are designed to make up for an unpredictable or poorly designed fastener installation.

Locking compound (lock patching, lock nuts) prevents the engaged thread from easily turning, but does not retain the unengaged portion of a fastener from departing if the body of the fastener fractures. If preload is lost due to thermal or humidity reasons, the fastener will be in a position to resume the load when those conditions return to the original ones. Locking compound is useless in embedment cases, unless preload is not a requirement for the joint.

Lock washers mostly decrease the spring rate of the fastener by increasing the unengaged faster length, meaning that for a particular preload the stretch of the fastener will increase. To lose preload, the amount of embedment or other tension relieving change is increased. Once preload is lost, a lock washer has no capacity to retain the fastener.

Lock wire is valuable in those cases where loss of the fastener can cause much more serious problems - dropping a loose screw into a gearbox, or turbine, for example. In the case of hydraulic lines, lock wire prevents the loosening of a nut from converting from a minor leak to a major fluid and pressure loss. As mentioned in the main article, it can also be used by attentive workers to demonstrate intended fastener installation has occurred, but it does not show the intended installation was the right one. Lock wire presence cannot show a fastener was installed by an attentive vs inattentive worker.

Of the methods, lock wire can also disguise a problem. If a fastener has failed due to fatigue, it will remain in place, appearing just like all other fasteners. This transfers the load to the remaining fasteners, eventually causing their failure until a catastrophic event occurs.

Locking plates - where a bendable tab is used. If the material is malleable enough to bend, it is soft enough to dent - use with a high strength load spreading washer lower the compression loads.

Other approaches: On city buses I've seen (in the US) there are pre-load indicating washers - lime-green with a triangular tip. These are set with the point tangential to the bolt circle. If the wheel bolt loosens, the pointer is turned by centripetal force to point outward, showing the fastener has loosened in a readily identifiable manner, even if the loss of pretension was a short term event.

There are also tension indicating bolts - precision devices that indicate being under a minimum tension. One-shot load indicating washers (direct tension indicating -DTI) can show the joint was properly tensioned at least one time. In well designed joints, the one-time indication is OK for all the time.

Corrosion:

A means of increasing post-installation preload is allowing corrosion to occur within the fastener load path. On small fasteners with small extensions due to tension, it takes little corrosion to produce very high tensions. Usually this is undesirable and makes removing fasteners difficult.

Dave.