Toothed vs Serrated Lockwashers 3

[alexit]

Normally I specify DIN6797 toothed lockwashers when working on structure of machine tools. Recently I have needed countersink screws, so I specify DIN 6798V countersink serrated lockwashers. I see that this 6798 standard also applies to serrated flat lock washers.

I ask what is purpose for difference between serrated and toothed lock washers? Is one better for steel, one better for aluminum? Or was one preferred by Sony and the other by Matsushita like Betamax?

Thanks,
Alex

 

[Tmoose]

If my joints are really structural or I'm trying to develop reliability and full stiffness (machine tools) I'd lose the lockwashers and bet it all on preload. Hardened flatwashers help with proper torquing and preload.


NAVAL SHIPS’ TECHNICAL MANUAL
CHAPTER 075
FASTENERS

"Properly preloading a fastener all but eliminates the tendency of all the fastener components, studs, nuts, and bolts, to rotate relative to each other."

"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."
-----------------------------------
MIL handbook HDBK 60 - page 7 - If practicable, a ratio of joint length to bolt diameter of 4 ore greater is recommended.

 

[evelrod]

TMoose is right on. Lockwashers, star washers, split washers, whatever, are nothing more than a holdover from the dark ages. Many engineers still assume that the little darlings will prevent loosining of a fastener. Maybe it makes them feel better, I don't know. I think their wrong!
Yes, I know there is some seriously loyal split washer fanatics, it's much like "faith based engineering" in my opinion. Proper torque, pre tensioning, chemical lockers, safety wire...all work to one degree or another. Split washers do not. I have not assembled a race engine in many years that utilized a lock washer and I have had zero failures due to NOT having them.

Rod

 

[Tunalover]

Ditto here! I vibration tested equipment with lockwashers and some of the bolted joints loosened and failed. The joints were reassembled with prevailing torque style fasteners and the failures ended. For blind holes, use screws with a nylon patch or pellet. For through holes, use lock nuts with nylon inserts. There are military standards for these.

Does anyone know of a military standard that provides mil spec part nos for the above mentioned screws in SST?




Tunalover

 

[jetmaker]

If the part adjacent to the nut is rotating, then a toothed washer is a must to prevent possible loosening of the nut during service. Furthermore, a secondary locking system such as a castellated nut and cotter pin, or locknut are desireable.

Regards,

jetmaker

 

[Eltron]

I'm with the rest of the guys. I wouldn't rely on lock washers for joint strength. We've been over this in a ton of forums before. Each type of lock washer has very specific torque and material specifications, and if you over / under tighten them, you aren't gaining anything. As per your original question, I'd give the folks at Sony and Matsushita a call, but I doubt they'll have anything like a straightforward answer.

 

[Screwman]

Add one more to the get rid of the washers team.
If a joint and fastener are properly designed (able to carry the required load without yielding) then the best way to create proper assemblies is to tighten the bolts (properly sized, good L/D ratio) to their full load carrying capability. By far the biggest problem that I have seen on machine tools over the years is that the bolts are not tightened to nearly their full capacity. It takes a lot of torque to load up fasteners of significant size.
Also, almost by diffinition you don't want to be using a countersunk head in clamp critical joints. That head design is much most suseptable to clamp load loss and creep than a 180 degree protruding head.
If there is nothing else that you can do to redisign the joint then either use fasteners with locking features (nylon or deflected metal) or use some Loktite on the threads. These are band aids, but sometimes you have to use a band aid to stop bleeding.

Dick

 

[diamondjim]

You might want to use a larger diameter
plain washer under a sockethead capscrew if the
material is aluminum to ensure that you
do not exceed the bearing stresses on
that surface. We have used a serrated
surface type of bevel washers under the
heads of sockethead screws with success.
Forgot the din number that describes this
type of bevel washers.

 

[alexit]

Wow, who then is putting all those lockwashers in every aircraft, auto, bridge, building, and machine tool...is this just old-school thinking?

My joint is D2 HRc65 ground plate 35mm thick with 0.004 flatness attach to 0.8m tall ground u-shape casting, same flatness across tops. I normally use 36x M8-1.25 x 100 SHCS 10.9 grade with tooth lockwasher torque to 30N-m (+/-2). For this new application I have no space for screw heads under decorative cover so I take same plate and add 90° c-sinks (position accuracy +/-0.05) for to use 36x M8-1.25 x 80 FHCS with serrated c'sink lockwasher torque to same.

So I should use patch screws and no washers?

 

[evelrod]

alexit, it's almost "damned if you do, damned if you don't" deal. I certainly can see both sides of this but, even though its been many years since I built an aircraft engine, I did NOT use locking washers of any kind. On the current vintage type engines I build, I limit the use of split washers to OEM stuff, e.g., 50's MG's where the factory counter bored the main studs to contain a split washer thereby almost making it a "flat washer", etc. Model A Fords don't use lock washers but rather a whole bunch of cotter keys on EVERYTHING...something like 522! Henry Ford did't like lock washers because they failed to do their intended job to often, I reckon. Modern race engines I use no lockwashers at all. What can be wired, is. What cannot is locked chemically. I use safety wire and nylon insert nuts (Nylok)where possible and torque all fairly close to yield. I measure stretch rather than rely on a torque number if I can. I use a lot of 1/4"FHCS on window clips, insturment mounts, etc. but with "242" instead of star washers.
A caution on the Nylok type nuts---an awful lot of commercially available stuff is "ungraded" and mostly worthless for critical connections. It's reached the point where I test every batch of G8 hardware I get anymore. I see some strange failures in what is supposed to be quality parts! Easy to get counterfiet fasteners these days, too easy.

When I retired seven years ago, construction grade A308 hardware was still using split washers on hand tightned connections, even on >5/8" fasteners...go figure! It doesn't take much to see that if you cannot tigten the nut properly there might be some benifit in using a split washer. Like I said, I can see both sides but I still dislike using split washers simply because I have had too many fail even under the best of conditions.

Rod

 

[arto]

Also, you don't want to use multiple countersunk/flat head screws on a critical application - unless there's really good tolerances, since the first one will locate & the others will be pulling sideways.
Use Low-head socket cap screws & pins to locate.

Use Loctite, Long-lok fasteners, or saftey wire - skip the lockwashers!!

 

[alexit]

After more searching I find this:

http://www.bossard.com/techinfo/en/extdoc/TT_40-41.pdf

Which is nice description of the function of differnt locking elements, they show no different use for serrated or toothed lockwashers (my original question.) Recommeded serrated flange bolts for high-hardness fastening as serrated and toothed washers are to use with softer fasteners and mateials. Those spring lock only help for stretch deformation (which is maybe for Tmoose joint lenght 4x bolt diameter ratio?)

On parts that move relative we use castle nut and pin always, otherwise it comes loose no matter what.

For this I will source serrated flanged bolts and try counterboring the plate this time.

Thanks for all the replys.

 

[Screwman]

You won't find any lockwahers used on new autos except for special applications. Toothed washers are used to cut through the paint film on grounding screws. A double washer (flat and helical combination) is used on a lot a Japanese interior trim applications to bear up against plastic. The flat washer increase the bearing surface and the helical compensates for the creeep of the plastic to prevent rattles.

To actualy prevent loosening: no way. they don't work.

There are no fasteners that I am aware of any any modern auto which use lockwashers to prevent loosening. The same goes for airframe designs. The parts are designed so that adequate preload is able to be obtained (and maintained) in the jiont that excedes the maximum service load. If that is done, that joints will not loosen.

 

[Heckler]

I would suggest using a bolt/washer/nut of high quality MS/AN/NAS and proper preload. For blind holes use quality hardware and a locking helicoil or loctite. I do not like those fasteners with the nylon pellets they're FOD waiting to happen.

 

[DAVIDSTECKER]

I love the section about "Lock Washers" in a book I have from Bowman Distribution called "Fastener Facts".
The section on helical lock washers calls them "Obsolete and Dangerous".

 

[EngJW]

Toothed lockwashers, that is the internal or extrenal muliple tooth type, can be used for flimsy assemblies where you can't apply a lot of preload. Wire terminals attached to the back of a gauge for example. The teeth embed themselves and apply a lot of friction, so they can keep things from falling apart even if they aren't tight.

 

[vulcanhammer]

I agree with the consensus here. We made pile driving equipment for many years (

http://www.vulcanhammer.net/info/)

and the vast majority of locking schemes were ineffective. Proper torquing, use of Loctite and in some cases fine threads worked the best.

 

[EngJW]

Not using lockwashers on new designs is one thing, but before removing them from an old or proven product one might want to be a little cautious.

1. If you don't have a lot of authority in the company, the boss might call you in for an ass chewing because someone on the line called him and said one of his stupid engineers forgot to include the lockwashers.

2. Customers who know the product well start calling in to the warranty department about missing parts, and they in turn go to the engineering department and raise hell.

3. You remove lockwashers from an aircraft enging connecting rod where they have been used for years. A shop rebuilds the engine and doesn't tighten the rod, causing engine failure and the pilot buys the farm. The company gets sued for several million and the enginer whose name is on the paperwork gets dragged into court for a nasty deposition.

Maybe this is all far fetched, but I think some things are better left alone, especially if we are only dealing with a few pennies per product. (Pennies add up but most companies are not Ford or GM.)