| Author | Message | ||
     Brian Vogel Grand Master Username: guyslp Post Number: 1385 Registered: 6-2009 |
On another thread, Geoff Wootton brings up thread locker and lock washers and asks for opinions. Having been "down this road before" on the RROC-US forums, I thought I'd post again the pertinent bits from that discussion. I am not "editing for tone" since some of the responses I made were to direct challenges to documented technical data. Suffice it to say that I don't accept anyone's anecdotal data over "hard data" that definitively settles, or nearly definitively settles, an issue. ------------------------------------------------ Portion of initial post: For reference, the easiest summary is the article on wikipedia about washers. Others exist, too, but this one keeps it very simple and to the point. It also notes that there are questions about how effective the traditional split-ring lock washer actually is. The NASA Fastener Design Manual makes for some "good light reading." It immediately dismisses the conventional split-ring lock washer as not having any real locking properties (which is what my gut has always pretty much made me think even before seeing this). Follow-up post after complaints of the "that can't be right" and "you didn't do your homework" variety: Since, it seems, that every time I post something that goes against the conventional wisdom based on hard data (references to which I sometimes post, sometimes not) I get excoriated for it, here are some of the references (there are more): The PDF slides from a presentation in late 2011 from Hill Country Engineering entitled, Helical Spring Lock Washers: Separating Myth From Truth Direct link to the webpage & video referred to in that presentation, Helical Spring Lock Washers A lovely little back-and-forth on the forum eng-tips.com: Studies on the use of lock washers?, and the direct link to one of the FAQ documents there entitled, Is a lock washer an effective torque retention device?, that includes brief quotes from the NASA document, the U.S. Navy, the British Ministry of Defense, and AMSE regarding lock washers (and gives you the documents from which they were taken, if you wish to research those). Another thread on eng-tips.com, including some references, entitled Split Washer Placement. There are others. It's not my job to list them all. A web search on [lock washer effectiveness] or [lock washer efficacy] will turn up what you need to wade through if you want more information. There's lots of trash in those results, too, so pick your sources critically. Just because tradition dictates something doesn't mean that it's necessarily true. I've found that having a healthy skepticism, and a desire to search out as much hard data as I can find, has served me much better than anything else. Viscount Philip Snowden said something that sums up my general approach to life: To quote me the authority of precedents leaves me quite unmoved. All human progress has been made by ignoring precedents. If mankind had continued to be the slave of precedent we should still be living in caves and subsisting on shellfish and wild berries. Addendum to add an additional document several posts later: Addendum: This paper, The Function of Washers in a Bolted Joint, has enough mathematical detail in it to make most people's (definitely mine) heads spin - but there's plenty that's fairly straightforward, too. ------------------------------------------------ Which pretty much sums up why I think split ring lock washers are useless for the purposes of keeping things "locked", though when supplied I still use them as I would a flat washer. It's pretty much the engineering equivalent of the real estate mantra, "Location, Location, Location," -- Torque, Torque, Torque -- that determines whether things are going to hold together or not (which is why I very seldom use thread locker, either). Brian | ||
| Robert Noel Reddington Prolific User Username: bob_uk Post Number: 109 Registered: 5-2015 |
The split ring lock washer is not really a lock washer its job is to apply spring pressure against the nuts thread. The idea is that the nut is easy to fit without spanners but the last two turns are stiff like a nyloc nut. No lock washer design is perfect some are better than others. And maybe thread stiffeners would be a better name. Tab washers and locking wire and split pins are much better. I fit what was fitted by the makers. Because it simply works. | ||
| Brian Vogel Grand Master Username: guyslp Post Number: 1387 Registered: 6-2009 |
Bob, I tend to fit "what was there" too, but I know that a lot of it doesn't "work" because of what's there but because the fasteners are torqued enough to "grip tight." If a helical lock washer is applying spring pressure at all then the joint has already failed and it's slowing down the inevitable. The theory you give as to how they work is addressed in a number of the references I cited, and while "that was the theory" it's not true in practice. Most of these sorts of things were invented based upon untested hypotheses that make perfect sense, but that don't hold up under scrutiny. They're also grossly misnamed as a result. Helical spring lock washers neither lock nor prevent loosening. They compensate for it, just a bit, and if they haven't taken set, if it occurs. Brian | ||
| Robert Noel Reddington Prolific User Username: bob_uk Post Number: 110 Registered: 5-2015 |
I have always thought that if the thread is correctly torqued so the bolt in under tension like a spring then it shouldn't come loose. If it does come loose then a split helical washer is not going to stop further loosening. This discussion has arisen this time due to worries about the fan falling off. I think nuts with holes for wire are available. Also the wire is available as well. Important to use the correct wire and spinner pliers. | ||
| David Gore Moderator Username: david_gore Post Number: 1644 Registered: 4-2003 |
RRMC used tab washers where fasteners becoming loose in service would cause problems. I rest my case m'lord  . | ||
| Paul Yorke Grand Master Username: paul_yorke Post Number: 1377 Registered: 6-2006 |
Bob, Vibration can loosed a correctly tightened bolt or nut. As can heat and cooling repeatedly. On a Shadow engine the sump bolts are on a constant mission to escape and need nipping up every year. Just the rear ones you can see between the flywheel cover and the subframe, plus the two that are hidden by the subframe. The two left hand manifold to head bolts ( as you look at the manifold ) are also attention seekers. Many a gasket has been blown just because of loose or missing bolts. I've not seen loose fan nuts, although I've found the aftermarket pumps have weaker studs and can not be tightened like the OE ones. Be warned. | ||
| richard george yeaman Grand Master Username: richyrich Post Number: 313 Registered: 4-2012 |
Paul you are so right about the manifold bolts I was doing manifold gaskets last Wednesday and when I was undoing the bottom bolts of the B side I couldn't help myself from laughing three of the bolts came undone with my fingers. I hope the A side does the same when I do it next week. Richard. | ||
| Robert Noel Reddington Prolific User Username: bob_uk Post Number: 116 Registered: 5-2015 |
I think I put it badly. Bolts that are subject to vibs and a thermal cycle can come loose. Thankfully the really important ones don't. Loose sump bolts are common on other makes. A lot modern gaskets have metal spacers in the holes. This allows the bolts to be tighter and stretch so that there a spring force to stop bolts loosening. Cork gaskets are too soft to tighten bolts enough to give them stretch without squashing the gasket at the bolts location and thus bending the sump flange as well. The manifold nuts could be locked with tab washers but once the engine is in the car the tab washers would be very differcult to release. Even if the bolts never come loose the exhaust manifold gasket could still leak or blow. I understand the paranoia of a fan falling off, the damage doesnt bare thinking about. If the nuts become loose the fan will rattle before it falls off. The fan is not driven by the bolts. The fan is driven by the clamping force between the flanges which comes from the stretch of the bolts. If the fan comes loose then it can shear the bolts off. So the torque on the bolts is important. Too much and the tension on the bolt will exceed its critical point. This means that when the bolt is undone it stays stretched. Once this happens the clamping force reduces greatly and further tightening snaps the bolt. Locite is quite good for this sort of thing. However once locite is set continually checking the bolt over say 5 years could break the bond. Another method I have seen is to paint the exposed bit of stud with paint. And nuts with a slight pitch mismatch which causes the threads to be stiff. | ||
| Randy Roberson Grand Master Username: wascator Post Number: 436 Registered: 5-2009 |
Right: just as the wheels are not driven nor resist braking forces by the bolts, but by the friction between the wheel and hub caused by the clamping force created by the bolts. A bolt is a spring: the bolt needs to stretch more than any possibility of the joint giving; then it will stay tight. Say the bolt is stretched 15 thou and the gasket gives 3 thou; the bolt stays tight. say the gasket gives 18thou; ooops, loose bolt. That's why sometimes a smaller bolt will stay tight and a bolt too large won't: a bolt too large won't stretch enough. Very brief and simplistic, but believe me: in oil and gas, we pay attention to bolts and bolted joints. When the genie gets out of the bottle the news people show up, right behind the ambulances and fire trucks. We routinely work in my part of things to about 4000 psi; refineries go higher and hotter; chemicals and plastic manufacture work to the range of 50,000 psi and with some bad stuff too. they make it look easy, but far from it. | ||
| Geoff Wootton Grand Master Username: dounraey Post Number: 765 Registered: 5-2012 |
Very interesting. It appears Engineers know full well that spring washers have nil effect on preventing bolts loosening, otherwise they would use them on cylinder head bolts and crankshaft journal housings. As we know, generally, the former use flat washers and relies on the spring in the thread and the latter use tab washers. Strange the practice of using spring washers continues. I guess I will take my part in perpetuating the myth by still replacing bolts that already have spring washers, but mainly now as they have the secondary function of spreading the load of the bolt or nut. Geoff | ||
| Robert Noel Reddington Prolific User Username: bob_uk Post Number: 117 Registered: 5-2015 |
A comment I read from the designers of the RR V8. The big end nuts, providing the nut is square to its seat and is correctly torqued then no locking is needed. As Paul said he's never seen loose fan nuts. The design of this bit is used on many cars. I haven't worked on many Shadows but worked on cars so fitted and never seen a fan come loose. Unless its been left loose. Red Hermitite is good at stiffening threads. However if the washer was fitted at Crewe then refit them. One can sometimes over think things and get visions of bits falling off. Us engineers often over think things. The most dangerous nut is the one at the steering wheel. | ||
| Patrick Lockyer. Grand Master Username: pat_lockyer Post Number: 945 Registered: 9-2004 |
For those that have no trust in what was and was not fitted OEM, maybe time to use these : http://www.nord-lock.com/ and with stretch bolts if you know the torques. All thees writings I wonder if many repairs etc are in fact getting done by the postings by the so called DIY owners etc. It seems many are just parked up for months on end!!!!!!!!!!!. | ||
| Brian Vogel Grand Master Username: guyslp Post Number: 1390 Registered: 6-2009 |
Patrick, I don't think it's a matter of having "no trust in what was and was not fitted OEM," so much as a recognition that what was and was not fitted was done so under the prevailing hypothesis of its time and to realize that the hypothesis was and is not correct. Split-helical washers serve perfectly well as an alternative to flat washers to spread load, but they don't "lock" anything. Far too many believe that their name is meaningful as to their function, and it is not. There are two particularly interesting comments in the Split Washer Placement thread on the eng-tips.com forums come from poster evelrod [4 Feb 05 14:19] who notes the strength that convention plays in the continued use of helical lock washers (among other things). The other, from poster RestoGuy [13 May 05 12:12], notes the role that originality concerns, particularly for those who enter their cars in competitions, plays. Many of our cars do end up "parked up for months on end," particularly for revivals from the dead, complex repairs, shipwright's disease, or simply other life priorities interfering. That has little or nothing to do with whether a specific repair has been completed. 'Twas ever thus. Brian | ||
| Jan Forrest Grand Master Username: got_one Post Number: 811 Registered: 1-2008 |
In my experience most 'helical' lock washers are nothing of the sort. As many have said: They don't lock. But neither are they truly helical. They tend to have each free end vertically stacked with no overlap. If they were significantly overlapped, and the overlap could be maintained while torquing, then the ends might just dig in to the underside of the bolt head/nut and the face of the item being bolted on. However, unless held laterally, the odds are that they will just spread open - possibly enough to pop back over the bolt head/nut. On lighter loaded, vibration prone joints the application of a dab of quick drying oil based paint can be as good as anything else, with the advantage that, if carefully applied, can indicate that there has been a rotation of the nut. |
