Toothed vs Serrated Lockwashers
Toothed vs Serrated Lockwashers
(OP)
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
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





RE: Toothed vs Serrated Lockwashers
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.
RE: Toothed vs Serrated Lockwashers
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
RE: Toothed vs Serrated Lockwashers
Does anyone know of a military standard that provides mil spec part nos for the above mentioned screws in SST?
Tunalover
RE: Toothed vs Serrated Lockwashers
Regards,
jetmaker
RE: Toothed vs Serrated Lockwashers
RE: Toothed vs Serrated Lockwashers
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
RE: Toothed vs Serrated Lockwashers
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.
RE: Toothed vs Serrated Lockwashers
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?
RE: Toothed vs Serrated Lockwashers
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
RE: Toothed vs Serrated Lockwashers
Use Low-head socket cap screws & pins to locate.
Use Loctite, Long-lok fasteners, or saftey wire - skip the lockwashers!!
RE: Toothed vs Serrated Lockwashers
http
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.
RE: Toothed vs Serrated Lockwashers
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.
RE: Toothed vs Serrated Lockwashers
RE: Toothed vs Serrated Lockwashers
The section on helical lock washers calls them "Obsolete and Dangerous".
RE: Toothed vs Serrated Lockwashers
RE: Toothed vs Serrated Lockwashers
RE: Toothed vs Serrated Lockwashers
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.)