Socket Head screws 2

[Peru]

Came across statement recently that read "lock washers are not used with socket head screws." I hadn't heard of this before so I'm curious...Anyone know if this is true? And why?

 

[Heckler]

Depends on the application and industry. For example, aerospace will use locking heli-coils or other locking inserts. Even on ground support equipment I never saw a locking split washer. Locking split washers tend to fail leaving the chance of FOD contamination.

Best Regards,

Heckler
Sr. Mechanical Engineer
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Never argue with an idiot. They'll bring you down to their level and beat you with experience every time.

 

[Peru]

Heckler -

So then, a socket head screw then by itself (no locking inserts etc) could potentially vibrate loose over time when fastened mated with a nut, pem nut, or simple threaded hole. I was curious if there was something about the geometry of the socket head that would explain that statement I had seen.

Mel

 

[swertel]

It probably has to deal with the head diameter of a SHCS compared to a hex bolt and the relation to stock sizes of lock washer, but that is just a guess.

I personally don't trust the split-type lock washer. The spring force exerted by them is not sufficient to maintain the friction coefficient of the preload to prevent the screw from loosening during vibration. Lock washer don't buy much more than what a good torque value will get you.

--Scott

http://wertel.eng.pro

 

[KENAT]

Agree with Swertel.

This can be got around by using extra flat washers between the head and the split washer.

Wave washers are a little better for some applications.

 

[MechNorth]

I suspect that it may have something to do with SHCS often being recessed into counterbores, unlike most hex bolts which tend to be used for surface applications. There usually isn't much clearance in a counterbore to allow for a washer.
Any bolt or screw, with or without a lock washer can eventually vibrate loose. If vibration loosening is the underlying issue, try using a thread adhesive, nylok insert, or a self-locking threadform for the tap (or nut) such as Spiralok.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[eromlignod]

They make special, small-diameter lock washers specifically for use at the bottom of a c'bored hole. We use them all the time.

Don
Kansas City

 

[Heckler]

On our projects we use exclusively MS9556, MS9555 & MS9111 double hex bolts with close tolerance shanks. We use washers due to dissimiliar materials and bearing surface. And locking Heli-Coil inserts into cast aluminum. This is a high vibration appliation, & high/low temp range. Sometimes we use the key-locked inserts (MS51831 type)

Best Regards,

Heckler
Sr. Mechanical Engineer
SW2005 SP 5.0 & Pro/E 2001
Dell Precision 370
P4 3.6 GHz, 1GB RAM
XP Pro SP2.0
NVIDIA Quadro FX 1400
o
_`\(,_
(_)/ (_)

Never argue with an idiot. They'll bring you down to their level and beat you with experience every time.

 

[Ralph2]

We use something called locally a "high shoulder" lock washer. They are made specifically for SHCS
Ralph

 

[Screwman]

The primary reason to not use a lock washer on a SHC is that they don't work. The compressive load required to completely flatten a helical washer and turn it into a "flat" washer is only a very small % of the targeted preload for a SHC. Unless you have lost about 80% of that preload a helical washer is going to act just like a flat washer; and if you have lost 80% of the preload in a joint, no washer is going to help you keep that joint together.
A better solution is to make sure that you tighten the bolts up to their designed load carrying capacity (this may be quite a bit more than you think) and desing the joint so that it doesn't suffer embement and collapse at that load. Make sure that the bearing surface that you are using is hard enough to carry the design load for a SHC (most LCS will yeild if not heat treated).
Also be aware that at the high hardness levels used in SHC there is a very real possiblity of Stress Corrosion Failures if there is any possiblity of corrosion in the joint. I don't like to use PC 12.9 bolts in any application that is exposed to the environment..I have fixed to many SCC failures over the years.

 

[MechNorth]

Excellent points, Screwman. Tks

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[MintJulep]

Seems like we get the "should I use lock washers?" questions every few months.

Like Screwman notes, they don't work.

See faq404-1257

 

[Ralph2]

Of course they do not work... but they give that warm fuzzy feeling to those that do not know better that they "might"

 

[JimLou]

I thought that star lock washers worked by presenting a multitude of angled edges to the bottom of the head and its mating surface. The edges dig into the material so that it takes more torque to go against them and back out the fastener than was applied to tighten it, kind of like a ratchet. Is that another old wives' tale?

 

[dimjim]

We use the starr serrated lock washers under
socket head capscrews even for our military
applications. What does not work is split
lock washers.

 

[unclesyd]

We have literally thousands of H11 SHCS in-service at one time and none have split lock washers. The only washer we use is a flat washer hardened and ground that we use in some components where the thickness of the landing area is approaching minimum.
The information posted above about the hardened SHCS tearing up the landing are is very true. Even though we DAG all the fasteners they will still score the landing area over time to the point that it has to be touched up.

One point about the use of star washers is they can also tear up a landing area in make and break situation especially with hardness below Rc35.

 

[dimjim]

We used Schnorr Washers under the heads
of socket head capscrews. I assume they
are the same as the starr washers that
you were referring to. Search on
Schnorr Washers to see these.

 

[Screwman]

I think that the star washers are what is also known as internal or external toothed lock washers. Once again, these don't work to lock. Because of their thin material, they also get fully flattened at target clamp loads for SHC. If the washers are not fully flattened, then you aren't using the fastener up to full load capability and you either should tighten it more or use a lower strenth fastener. The other thing that happens with toothed washers is that the shape is full of stress risers and they are highly prone to SCC failures in service.
One area where toothed washers work very well is for grounding (earthing). The teeth dig through surface coating and corrosion products and give a good low resistance connection. By the same token, you don't want to use them where surface finish is important, since they dig up any kind of finish on the bearing surface.