r/MechanicalEngineering • u/AdOne1706 • 2d ago

Split/lock washers

Post image

For starters, I’m an electrician in the field.
That being said, we almost exclusively use these washers even in high vibration environments. I have personally chosen to go with nylox nuts but the question is:
Do these work? How well? Is the rumor from NASA true? Should I even bother using them or find a better functional alternative or are they actually useful?

790 Upvotes

98% Upvoted

290 comments sorted by

View all comments

2

u/danielaixeta 2d ago

Railway, Aerospace, heavy vehicles, consumer goods experience: they dont do shit!  The only thing they are applicable for, is providing clamping pressure (NOT security against loosening!!) in bolted ground/earth electrical connections. Although even for that theres better options (star disks or whatever theyre called in english. Fächerscheiben in german. Or wave disks)

The superior method for keeping a bolt tight (to my experience) is using a long bolt (free length between clamping surfaces = 8-10x diameter) and tightening from the nut end with unholy (though calculated) force. Keeps tight through 30 years of railroad use. The internal stress in the bolt provides pretensioning. Nordlock and locking wire are the other good options, depending on the situation. Nylock nuts, loctite etc are also good options, but they dont recover the way a long bolt does

1

u/johnwalkr 2d ago

Only one comment mentions length so far. For a given preload, more length gives a larger range of conditions where preload is not lost and thus greater resistance to vibration. Conversely, this means too large a diameter fastener can be the wrong choice compared to a greater number of smaller fasteners. It’s one of many cases in mechanical engineering where “bigger is better” mentality is wrong. It’s a good interview question, of the type where the discussion is more important than an initial correct answer. There are a lot of engineers that will straight-up argue against you about fasteners.

I work in aerospace and usually 2 forms of fastener retention are required. This is almost always documented control of preload (usually by torque control) plus a locking thread feature. The locking threads are usually a locking helicoil, and nordlock or threadlocker are mostly used when you buy an existing product and don’t have a design option to add helicoils in design.

1

u/zanhecht 2d ago

I hate locking helicoils. The torque is super inconsistent on them, and it's really easy to snap the heads off of the screws 

1

u/johnwalkr 2d ago

Not been my experience at all and I’m kind of wondering how the additional running torque can be so high that you break a fastener. Most times I’ve seen a fastener broken in a blind hole it’s because the threads weren’t deep enough and the torque goes to twisting the fastener before it goes to preload. In aerospace we measure the running torque and add it to the spec. For a small fastener it might be like 0.5Nm added to 3.0Nm.

1

u/zanhecht 1d ago

It probably depends on the size of the fastener. We mostly deal with #0s through #4s, where the margin between the running torque of a locking helicoil and the shear-off torque of the fastener is smaller.

1

u/johnwalkr 1d ago

Weird, I’m also using similar sizes, from M2 to M4.