Question regarding the optimum use of lock washer

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JJWalecka

Senior Member
Location
New England
I have a question regarding the optimum use of lock washers.

The scenario is such,

A half inch threaded rod is extended from the structure vertically. A half inch nut is threaded onto the rod, followed by, a half inch unistrut washer, inch and five eighths unistrut, another strut washer, the lock washer and a half inch nut on the bottom.

I was always told to install the lock washer on the bottom of the determined load. Granted, by tightening the bottom nut the level of the unistrut could be altered.

Is this a correct assumption regarding the use of the lock washer on the bottom?

Please excuse my ignorance on the matter.

Justin J. Walecka
 

JohnE

Senior Member
Location
Milford, MA
I don't see how it matters whether the locknut is above or below the load, but I'd be interested to hear.

The only thing I was taught is that when using a nut, bolt, washer, lockwasher, then lockwasher is tightened by nut, so bolt head sort of "digs in" to whatever it is fastening. Makes sense intuitively, but possibly the physics will not support the idea.
 

masterinbama

Senior Member
also if you have nuts on the top and bottom of the strut do not over tighten them. This puts a load on the rod just like adding excess weight. This is the biggest problem i see with most of my journeymen. They tell me if tight is good then real tight is real good.Have seen rod failure at hanging transformers and the j-mans answer always is the engineer speced such and such size rods
 

dlhoule

Senior Member
Location
Michigan
I like to use the nylock type nuts for these applications. You eliminate the lock washer and with a 1/2 way intelligent jman they will not go to trouble of overtorquing.
Also; I am not sure, but I'll bet there are specifications as far as torque. :)
 

tonyi

Senior Member
I'm going to recommend THIS book by Carroll Smith on race car design. He goes into why mechanical things fail and the ways to avoid them.

Henry Petroski's To Engineer Is Human is also a good read on mechanical/engineering failures

A strut/rod hanging scenario is basically what failed in that Kansas City Hyatt hotel walkway collapse some years ago.
 

iwire

Moderator
Staff member
Location
Massachusetts
I spent five years as a maintenance mechanic at an amusement park.

In that time I learned that in general lock washers or nylon lock nuts where not acceptable means of securing nuts.

We had quite a bit of written info from hardware makers and the short story is a properly torqued fastener will not loosen itself up.

If it does than the fastener is being used improperly, overloaded, subject to movement of the fastened material etc.

Vibration?

If you have worked on cars you will notice that most fasteners on the engine do not use any lock washers.

Head bolts generally just thread in and are torqued they don't back out even with the thermal cycling.

The only ways you will see critical applications 'locked' is with a definite method such as.

A castle nut with cotter pin (think wheel bearings)

A Steel lock nut (Nuts that have dimples into the threads, they are one time use only and also ruin the bolt as well.)

"Safety Wiring" (Stainless tie wire properly installed through holes in the fasteners.



A washer that keys into the bolt and has tabs that fold up against the nut.

Jam nuts in some applications.

Watch the steel workers put together a building.

They do not use lock washer they use a special tool that ensures the proper torque and therefore stretch have been achieved.

JMO, Bob
 

iwire

Moderator
Staff member
Location
Massachusetts
tonyi said:
A strut/rod hanging scenario is basically what failed in that Kansas City Hyatt hotel walkway collapse some years ago.
It sure was but not do to a fastener loosing.

There was a modification made to the original design.

During January and February, 1979, the design of the hanger rod connections was changed in a series of events and disputed communications between the fabricator (Havens Steel Company) and the engineering design team (G.C.E. International, Inc., a professional engineering firm). The fabricator changed the design from a one-rod to a two-rod system to simplify the assembly task, doubling the load on the connector, which ultimately resulted in the walkways collapse.

http://ethics.tamu.edu/ethics/hyatt/hyatt1.htm[/quote]
 

tonyi

Senior Member
I'm just saying that this is a common scenario ripe for failures. If godzilla honks down and deforms the strut a bit, that's a weak point that can kink and fail under load.

The head bolt thing you mentioned is an interesting one - VW had cylinder head bolt pull out problems for a while back in the 60's with he bug engine and solved the problem by making the bolts thinner along their shank.
 
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