Split Washer Placement 9

[wickedmotorhead]

I've seen OE's (and others) put split (lock) washers underneath the head of a bolt or underneath the nut in a mounting configuration. Which one is correct or does it matter? Everywhere I search I get a different answer, I need a educated answer to help make my decision. Thanks.

 

[Tunalover]

Does anyone know of a definitive study where the helical-split lock washer were compared against, say, prevailing-torque locking methods e.g. Nylok nuts, or perhaps, internal- or external-toothed lock washers?


Tunalover

 

[CoryPad]

Those results are in Handbook of Bolts and Bolted Joints. I am not at work, so I can't give you the exact page number.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Tunalover]

CoryPad-
When you get a chance, would you please reply with the ISBN for that book? I'd be obliged. I'm trying to prove to my boss the worthlessness of the helical split washer (among other things).

Tunalover

 

[CoryPad]

tunalover,

The ISBN is 0-8247-9977-1. The book is available at:

http://www.dekker.com

While you are at it, grab NASA RP1228 at:

http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?1990/RP-1228.html

It this NASA Fastener Design Guide, the author states the following regarding helical split washers:

"In summary, a lockwasher of this type is useless for locking."

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[EngJW]

Trying to prove anything to a boss can be difficult and sometimes not worth the trouble. If you are developing a new design you could leave off the lockwashers from the beginning. However, if you have an existing product and want to eliminate them, there are some other considerations.

Besides convincing the boss, you may have to convince his boss, a change review board, and shop personnel.

There might be all kinds of documentation to change. Repair manuals, parts books, etc.

People in the field who know the product might start calling in complaining that the lockwashers were left off.

If anything even unrelated on the part fails and causes an injury, product liability lawyers could point to the elimination of the lockwashers as the cause. Technical explanations could be meaningless to a jury that sees only a greedy management being at fault.

 

[evelrod]

If anything even unrelated on the part fails and causes an injury, product liability lawyers could point to the elimination of the lockwashers as the cause. Technical explanations could be meaningless to a jury that sees only a greedy management being at fault.

That pretty much says it all, I am sad to say!

Rod

 

[Tmoose]

S9086-CJ-STM-010/CH-075R2

NAVAL SHIPS’ TECHNICAL MANUAL
CHAPTER 075
FASTENERS

075-5.6 LOCKWASHER METHOD
075-5.6.1 GENERAL. Many installations aboard ships, especially older ships, still use lockwashers in diesel
engines and similar equipment to prevent threaded fasteners from loosening. These may be replaced with the
same type of fastener if loosening has not been a problem. If loosening has been a problem, however, replace the
lockwashers with self-locking fasteners. Standard lockwashers are detailed in ASME B18.21.1 (inch) and ASME
B18.21.2M (metric). Lockwashers in common use consist of the helical spring, curved or conical spring, internal
tooth, external tooth, and internal-external tooth type.

 

[Tunalover]

Thank you CoryPad! You're a gentleman and a scholar!


Tunalover

 

[MintJulep]

Tuna,

Sorry for the delay in posting this, I have been away from the office, and didn't have access to what I was looking for.

The British Ministry of Defense did a lot of work in this area in the past.

A guy named Light, from British Aerospace published a paper entitled "Vibration loosening of threaded fastenings" that summarized the various MOD studies (along with providing a nice bibliography). The paper was published in the May 1983 issue of some engineering trade magazine. My copy has "CME" in the page footer. I can't recall the actual name of the mag.

 

[RestoGuy]

Hi All:

We're in the business of restoring classic italian sportscars (mostly pre '55) which includes Maserati, Alfa Romeo, Fiat derivatives, etc. Our problems deal not only with good fastening systems but also retaining original appearance.

For example, early Fiat (up to ~ '53) used internal star washers over a beveled flat washer when fastening a nut on a stud to hold an aluminum component. If it was a steel component, then only the star washer was used. Early Alfa is different, they used split lock washers over a similiar beveled flat washer. For rods and mains they used cotter pinned castle nuts. Maserati is different again........and of course the French and Germans had their ways. I point this out because in my business hardware originality (in appearance) is important for the history of the car and judging events,

Having said that, for all critical apps, such as main bearing caps, rods, cylinder heads, flywheels, clutch components, et all, I try to use modern fasterners. For mains or rods, never a split lock or even a wavy washer as I've seen both break, especially, if re-using a split lock. So we always use hardened, surface ground parallel washers. For external non-critical apps and original appearance requirements, we use the orig hardware which usually includes split locks.

Final comment concerns old bolts and studs...........the Italians and other euro countries mostly machined their bolts. Maserati did their own in-house as did Alfa. But Alfa engineering was better and knew to have a radius under the head of the bolt. Maserati machined theirs sharp - often leading to failure. So check your hardware carefully if you're restoring old cars and re-use bolts (which we often do).

Curious if others have made siimiliar discoveries in the resto field?

 

[stefa78]

Hi all!
From this forum it is clear that split washers are to be avoided as locking systems in most of the engineering applications.
The best device seems to be a self locking nut.
So, in presence of high vibration environment, if the bolt is composed only by the screw (of little dimensions, let say diameter 3 mm), and not the nut, the only way to assure the good locking is the Loctite?
Or exists some other locking system for such an application?
Thanks!

Stefa78

 

[R1100S]

Stefa78,

It is the opinion of many engineers, myself included, that a properly tensioned bolt will not back out, even under harsh vibration. That being said, I still use loctite.

 

[GregLocock]

Yes, I use Loctite on any critical bolt, but in production it is a method of last resort where a joint has not been properly designed. Either the loads were not well understood, or, the hardware was underspecified and somebody or other won't pay for an upgrade.

You could say the same about Nylocs, but they tend to be used where the joint is safety critical but will function correctly with a loose nut and bolt.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Warpspeed]

Perhaps the guiding principle should be, that in a high stress application where the nut can be properly torqued, the threads will self lock. The male threads stretch, and the female nut compresses slightly. This alone will lock a nut and is the proper way to do it.

But not all applications can be tightened with enough torque to create this natural thread locking phenomena. Electrical terminals come to mind, particularly with brass threads. And as already mentioned, mild steel screws through fiberglass panels, and similar very low force applications. There will always be insufficient clamping force to properly lock the nut, and where there is also vibration the result will be obvious.

Either star or split washers can work pretty darned well for low force threaded fasteners, but should be avoided like the plague in rigid high load applications.

 

[Pressed]

For high shock and vibration applications a Nordlock washer works very well. I have not experienced them coming loose, ever. This includes fasteners not being torqued correctly, as in, no torque wrench used.

 

[stefa78]

I agree with you for what concern the good design of the bolts, in terms of proper choose of the torque value and the tightening load, as a way of self-locking.
Nevertheless, i met a situation that seems to put it in discussion.
During a vibration test, in which the equipment was exposed to a random vibration in the range of frequencies of 15-2000 Hz and a level of 50 Grms (!!), the fixing screws (in a tapped hole) having a Metric thread with diameter of 3 mm (M3), torqued at 1.05 Nm, with the placement of a flat washer and a curved washer (DIN 137A) under the head of the screws, unlocked after few minutes... Replacing the flat and curved washers with a split washer, torquing the screws at 1.05 Nm, they remained perfectly locked during all the test.
Saying that i won't to justify the use of the split washers, because i met several applications, with bolts M8 or M10 and torques till 50 Nm, in which they broke after the first assembly.
The fact i don't completely understand is why the screws loosen when torqued at 1.05 Nm, that should be a high value for a diameter of 3mm. So the screw should have been properly tightened, and it shoudn't have loosen even without washers of any kind.
However, in that case i avoided the loose with split washers... that demonstrated a locking function.
I'd like to know your opinion about this.
Regards

Stefa78

 

[thruthefence]

The two main manufacturers of recip aircraft engines, Teledyne Continental and Textron Lycoming, have used for many years, and continue to use 'split' lock washers, mostly to secure accessories such as pumps & govenors. Fasteners subject to rotation are usually secured by castllated nuts & cotter pins. Why haven't they gone to self-locking nuts? I would suspect that it would cost too much to re-certify the engines with the FAA. I have actually seen very few problems with them over the years, but they should never be reused.

 

[EngJW]

During my years with one of the above, lock washers were going away and being replaced by locknuts. In some cases the use of a cotter pin was an FAA requirement. Many components were safety wired. For example, small or light housings with gaskets, oil filters, drain plugs. These do not get tightened with a lot of preload.

If someone forgets to tighten a bolt holding the wing on, it's nice to have safety wire to keep the bolt (and wing) from falling off.

 

[Tmoose]

Stefa78
Could you describe the test fixture and test article a little more? Any inspection of the nut and bolt seats after loosening ?

 

[stefa78]

Hi Tmoose,
the test article is an electronic module having a thin rectangular base shape (the height is very smaller than the base dimensions). Its weight is 0.5 kg,and it is mounted on the fixture by means of 8 screws having a metric thread, diameter 3mm and length 26mm, AISI 304.
The fixture is composed by an alluminium plate fixed on the slip table of the shaker using 12 screws M8, on which is fixed in vertical position another plate, where the test article is mounted (on heli-coil) when vibrated along its vertical axes. The vertical plate is rigidly fixed on the horizontal one using a structural adhesive, screws M8 and two plates mounted in lateral position and perpendicularly to the vertical plate (in a top view the fixture is similar to an H). The fixture's resonance frequency in the area where the test item is mounted is higher than 2000Hz.
An inspection of the screws M3, the ones that loosen after vibrations, was carried out, and no macroscopic damages were evidenced. Nevertheless a little plastic deformation appeared on the thread, and i suspect that the screws were permanently stretched by the tightening and the dinamic loads too (i measured the length of some screws after the unlocking, and they were longer than the new ones, but the measures were not enough and not enough precise to give the best results).
The curved washers became completely flat after the unlocking of the screws, while the split washers left scratches and digs on the surfaces.
I hope that my description is clear enough...(i am sorry for my english...).
Thanks
Regards
Stefa78