Lock Washers on Power Devices - Page 1
Forum section: electronics / projects

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 15, 2016, 11:34:32 pm
content: It's been widely published that split ring lock washers are worse than nothing in terms of preventing bolt loosening, and that tooth washers may or may not be much better. Having used such for 40 years or more, this is tough for me to accept. What I really want to know is are they any more effective when bolting a power device to a heat sink where you get a lot of temperature cycling?I've never liked tooth washers, preferring split rings. My logic is that insulating shoulder washers will compress, and the split ring at least has enough spring range to compensate and maintain some tension. I mostly use tooth washers where I want electrical contact, say to a chassis ground point.Any thoughts, more modern viewpoints or even test data?

author: sleemanj
author_rank: Super Contributor
timestamp: September 16, 2016, 12:49:41 am
content: If the bolt came looser from it's torqued position with the split "lock" washer in place, then all that's going to happen is that the bolt will continue to work itself looser and looser until it falls out, the torqued joint has already failed by the time the compressed flat washer becomes a spring again.As you know, there is no shortage of official comment on split washers in that regard that from respected organisations (a 5 second google will give you references) a split spring washer "seems" like a good idea on the face of it, but when you actually think about it, it's kinda obvious that it's not doing anything really very useful.I would use a toothed washer if any, it has at least some ability to bite against the surfaces (unless it's compressed flat though against a surface harder than the washer I imagine) and prevent rotation, which is after all what this is all about, if that's vibration or thermocycling, it's the bolt or nut rotating you are trying to stop.Anyway, for total locking - that's what locking nuts, loctite, lock wire and split pins (cotter pins to the US folk) are for.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 16, 2016, 01:24:06 am
content: I can't argue with a bit of that, it's just that those solutions aren't very good for power devices, where you want easy disassembly. The Loctite is probably best, but only with careful selection so it isn't a nightmare to get apart. Maybe a spot of lacquer on the threads isn't a bad solution?

author: Phoenix
author_rank: Frequent Contributor
timestamp: September 16, 2016, 01:28:02 am
content: Quote from: Conrad Hoffman on September 15, 2016, 11:34:32 pmAt work we use sprung clips to hold the devices to heat-sinks for our power supplies for TO220 and TO247.E.g.http://www.tarapath.com.au/products.cfm?PRID=3120

author: woodchips
author_rank: Frequent Contributor
timestamp: September 16, 2016, 09:55:23 am
content: Belville washers!Having dismantled far too many bits of electronics the ONLY screwed or bolted joints that have kept tight are those using Belville washers. These are slightly cup shaped with radial serrations on them, when tightened they squash flat and the serrations keep the tension in the joint. The problem with electronics is that we use soft metals, brass and copper, and any type of lock washer creeps and loosens.I don't really understand why manufaturers of components like high current diodes, 200A or so, provide a split lock washer, they are always loose after a few years. Usually why the unit has failed, fried joints. I have found companies that tap a hole in a copper bus bar 1/8" thick, then screwed a hex screw in and seem surprised that it loosens. Even worse are companies that nut and bolt/screw a ring terminal to a PCB, totally forgetting that the PCB is a plastic and will flow, it gets warm and the epoxy starts to melt and it loosens more, a fire is the result.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2016, 10:04:43 am
content: ^^^ONLY conical spring (Belleville) and wave or ripple washers (the stiff ones, not the usual soft ones) have enough force and displacement to keep a joint under useful tension.Consider how much torque you apply while a split washer is being compressed. It's hardly anything! You probably stop cranking when the torque is over 10 times higher than it is while you're still cranking down on the washer. At which point (at 10x full compression), the split washer is nothing more than a badly made flat washer.For power devices, the shoulder washers themselves matter a lot: likely being made of resin, more flexible (and weaker, so thicker too) than a metal washer, so they will compress much farther. The problem is, that force may dissipate over time (cold flow, creep). If you need that assurance, it really just isn't practical to provide it by throwing more metal washers at the problem.Need a good, flexible joint? Get rid of bolts altogether! Use retention clips to hold devices to the heatsink. Some kindsrequireit: "MAX247" for instance. The clip is a huge spring, so it has far more displacement -- more compliance range -- than anything you can thread over a bolt. Even if you don't opt for a screw-less package, using clips with conventional tabbed packages helps, because the force is applied over the body center, not off on a wing.Tim

author: tatus1969
author_rank: Super Contributor
timestamp: September 16, 2016, 10:15:03 am
content: According to the construction engineers, split ring and tooth washers have been removed from the list of European standard parts quite some time ago, because they have proven to be unsafe. They told me things like rust entering the bended metal and that they degrade long term. Certainly that does not relate to electronics so much.The problem is always that you need to maintain tension between nut and screw. As soon as you have plastic parts like isolation washers or PCB material it gets problematic. FR4 will slowly flow away over time and temperature cycling.Stacking two nuts is always a solution. Long screws also help, because they add more elongation when tightened with the same torque, compared to short screws.We had the challenge to fasten ISOTOP transistors directly to a PCB, and ended up using spring washers, regular washers (for electrical contact), and thread lock. You can imagine how "easy" later disassembly for repair was. That was the price for not using press-fit bushings.Edit: Belleville and ripple washers were also canceled for that design above, but they got close to acceptable.

author: tatus1969
author_rank: Super Contributor
timestamp: September 16, 2016, 10:19:18 am
content: Quote from: T3sl4co1l on September 16, 2016, 10:04:43 am100% second that.

author: mjkuwp
author_rank: Unknown
timestamp: September 16, 2016, 10:24:04 am
content: I have studied fasteners and locking methods for threaded fasteners, but it was with an eye toward structural joints.here are some good referenceshttps://www.amazon.com/Handbook-Bolts-Bolted-Joints-Bickford/dp/0824799771http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900009424.pdfhttp://www.boltscience.com/The helical split lock washer is an interesting animal and probably is often misunderstood. I don't think it is possible to make a uniform rule about using it. One thing I learned in my review of literature is that one researcher found the trapezoidal cross section to play a roll in making a very rigid, low travel spring. On flat, hard surfaces, the washer will roll out under high tension/torque. From memory, it stated this had the affect of adding about 1in of bolt length.A reference I like to use for structural joints is what I find when I work on my cars (because they are proven to be reliable). It is rare to find a split lock washer on a car. For the most part I find long, skinny bolts often with flanged heads. I have also found critical joints are tightened, not with a torque specification but with a torque + angle of rotation specification. This is because the bolt shank needs to be stretched in order to keep the joint tight. If you joint has a very short bolt there is not much opportunity for stretch and this is where a split lock washer *might* be useful although I have never proved it myself. In my designs (after I did this investigation) I stopped using split lock washers and always try to have designs with long skinny bolts and rigid, flat surfaces to clamp to.And yes, we will use Loctite or similar material if we cannot find other ways to keep a joint tight but this is a last resort. It could also be considered a "belt and suspenders" solution. I don't know if one can use Loctite on an electrical connection. Is it conductive? Are there rules in the electrical standards related to this?That said, I also work with electrical connections and just yesterday removed a ring terminal - terminated wire from an M5 post on an EMI filter. It had split lock washers on it which seemed to be performing the function of keeping the parts in contact by slight spring tension even though the nut was actually quite loose by 'mechanical standards'.not trying to contradict you because to some extent this statement is quite true, but I wonder if you can find some sources for this, I could learn something : )Quote from: Conrad Hoffman on September 15, 2016, 11:34:32 pm**edit, PS: all the responses in this thread so far are quite helpful and I think, correct imho. I wrote my response before/while member tatus1969 was posting so I hadn't read that information prior to composing my message**

author: 2N3055
author_rank: Super Contributor
timestamp: September 16, 2016, 10:52:34 am
content: Elastic clips holders or some type of elastic washers (Belleville or wavy type) worked best for me.I did on a occasion put longer screw and two nuts (old school style), or Loctite ..Thermal dilatation can be only dealt with elasticity...

author: tatus1969
author_rank: Super Contributor
timestamp: September 16, 2016, 10:53:50 am
content: Quote from: mjkuwp on September 16, 2016, 10:24:04 amNot that I am aware of. For the ISOTOP that I mentioned it is not relevant, but we had to make sure that no Loctite could be spit when inserting the screw. For this reason we used ready-made screws with dry thread lock paste on them (don't know how that is called exactly, you see these everywhere when opening laptop / smartphone casings). The dip consists of small coated thread lock "balls" (like M&Ms :-) that crack open when fastening the screw.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2016, 11:04:37 am
content: Quote from: tatus1969 on September 16, 2016, 10:53:50 amScratch'n'Sniff(R) -- Now with Cyanoacrylate!Tim

author: tatus1969
author_rank: Super Contributor
timestamp: September 16, 2016, 11:16:16 am
content: Quote from: T3sl4co1l on September 16, 2016, 11:04:37 amlike this?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 16, 2016, 11:58:31 am
content: Good stuff! Now, can I infer from this that there are times when smaller hardware is better, as one can get more stretch from a smaller diameter bolt than a big one, assuming either is strong enough for the application. Or, that several small bolts will be better than one large one for the same reason?I've used Belleville washers for various things, but have never seen the serrated ones like Nord-lock. They seem limited to #6 and larger- I use a ton of #4 and smaller hardware (sorry rest-of-world, the USA is still mostly lost in antiquated hardware standards!)

author: tatus1969
author_rank: Super Contributor
timestamp: September 16, 2016, 12:37:06 pm
content: I haven't thrown Nord-Lock into the game, as these fully rely on the bolt maintaining its tension. If not, they'll just fall apart because they don't provide elasticity at all.

author: TheBay
author_rank: Unknown
timestamp: September 16, 2016, 03:21:20 pm
content: You wouldn't want anything coming off inside this!

author: jitter
author_rank: Frequent Contributor
timestamp: September 17, 2016, 05:44:20 am
content: Quote from: Conrad Hoffman on September 15, 2016, 11:34:32 pmI have always wondered about the effectiveness of spring washers, especially since most of the stuff we mount on heatsinks at work don't use them at all. And where electrical contact is important, serrated washers or a version of the Belville washer with the teeth that bite trhough oxidation or paint layers (grounding washer?) are used. When I look at my car, especially the exhaust manifold that gets serious heat cycling, serrated flange nuts are holding it on, torqued to a spec not even that high to prevent squashing the manifold gasket. I did loose one of those nuts once, though.

author: woodchips
author_rank: Frequent Contributor
timestamp: September 17, 2016, 09:59:47 am
content: Plenty of info here!Note that a wavy washer and a Belville washer are not the same thing. A Belville washer is actually a shallow cone, so when tightened the outside diameter of the washer is in serious tension, it is this that holds it tight.Different applications require different solutions. For electronics the problems are soft base materials, copper, PCBs, aluminium etc and heat cycling by component dissipation or contact resistance of joints. Not the same as for structural steelwork when the bolt can be stretched to provide the friction needed for the thread not to slip and loosen.Looking in my HPC Drives catalogue they list Belville washers as safety washers, smallest is 3.2mm od. That uses 0.35mm thickness steel.

author: cs.dk
author_rank: Unknown
timestamp: September 18, 2016, 07:18:04 am
content: Quote from: jitter on September 17, 2016, 05:44:20 amThe nuts on the exhaust manifold is often made from copper, so they don't really hold up to a lot of torque.Nord-Lock washers are quite good. I've used them in ie. hi-speed drum chippers, you don't want a loose bolt in there.