Proper Use of Spring Lock Washers 2

[Crev64]

I am looking into the proper use of spring lock washers. We have a few applications where we use them, but are having them split open while tightening. One concern is that lock washers are basically useless when torqued down until it becomes flat. Does anyone have any information on this?

 

[DLite30]

From NSTM Chapter 75.

075-5.6.2 HELICAL SPRING LOCKWASHERS. The most frequently used lockwasher is the helical spring(split) type. The helical spring lockwasher (Figure 075-5-11) is flattened when the bolt is torqued down. Once compressed, it acts as a flat washer, contributing normal friction between the nut or bolt and the bearing surface during tightening. If the fastener clamping load relaxes, the spring action of the lockwasher will maintain some load between the threads of the fastener, reducing the tendency of the fastener to rotate. Some helical spring lockwashers have a sharp tooth on each end that bites into each bearing surface and prevents rotation. This type adds friction during tightening and will result in some reduction in preload. Because the helical spring lockwasher diameter is small and it tends to dig in when loosened, it is rarely used on soft materials or with oversized or elongated holes.

You may also want to check to ensure that the lockwashers are the same grade as the fasterns, which may be the cause of them opening up during tightening.

 

[Crev64]

During my testing, I have only been able to recreate the failure by overtorquing the bolt. We normally torque it to 90 ft-lbs but we can't recreate failure in our testing until 130 ft-lbs.

 

[DLite30]

So, I'm guessing you're using a fastener somewhere around 3/4"?

Unless your application is in a thermally dynamice or a varying load environment, and you have enough bolt stretch, you probably don't need lock washers anyway.

 

[Crev64]

The fastener is M16, and we are heading in the direction of being rid of lock washers in these applications. I am just thoroughly looking into the matter before we do so.

 

[IRstuff]

Efficacy of lock washers has been discussed in these forums in the past.
thread108-114051
thread404-230741
faq404-1257


TTFN

FAQ731-376

 

[ShaggyPE]

Many threads on this subject:

http://www.eng-tips.com/viewthread.cfm?qid=161524

and an FAQ:

http://www.eng-tips.com/faqs.cfm?fid=1257

I couldn't find the link to the NASA paper on the subject... but there is one out there.

-Dustin
Professional Engineer
Pretty good with SolidWorks

 

[Yagonyonok]

http://gltrs.grc.nasa.gov/reports/1990/RP-1228.pdf

Page 9 references the lock washers.

Use of system voids warranty.

 

[leisure17]

We had a similar case & we found a little pip on the cut end caused by the shear in manufacture. This caught the nut and deformed it as it was tightened.

 

[Tmoose]

When you remove the bolt from one of those lockwashered assemblies, How do the underside of the bolt head and the mating surface look? Smooth and flat as machined, or gouged and generally torn up?

Split lockwashers were used practically forever by the OEM, and hotrod aftermarket.

http://go.mrgasket.com/ProductImages/500/910.jpg

http://go.mrgasket.com/ProductImages/500/911.jpg

A friend just bought some premium clutch bolts for his 1976 Corvette, made by ARP.
The ARP bolts came with flat washers with a heavy ID chamfer on one side to more than clear the bolt head radius.

http://performanceparts.com/performance_parts_images/arp_130-2201.jpg

I would not really expect a problem with either style washer if torqued to the meager 35 lb-ft usually recommended for those 3/8-16 clutch bolts, but would strongly prefer the flat washers in applications where I was serious about loosening or fatigue from lost preload due to a few thousandths of embedment into a murdered surface (which is almost always).

 

[dinjin]

My guess is that you are crushing the washers exceeding their bearing stresses. Can you use thicker washers with greater surface area by increasing the od of the washer?
What grade of bolt and washer are you using? Can you test the hardness of the washers? I have heard of counterfit bolts but not washers.

 

[Tsiolkovsky]

A Junkers Machine shows evidence that not only are spring washers worthless with regards to locking but they actually contribute to loss of preload and loosening.

 

[Guest102023]

AFAIK here is no 'proper application' of a spring washer. I have done a few fastener failure analyses, including one bolt (incorporating a spring washer) that came inches from killing a worker standing below.