depends on ones budget and the application with regard to loading, vibration, any possible breaking from shearing wtha ground engaging implement.

A washer depending on its specific purpose should not be substituted -

You do not want anything other than a socket head capscrew spring lock washer under a socket head capscrew whether it is a socket head capscrew that is countersunk and flush with the metal surface or whether the bolts shouder is mating witht the surface, or whether it has fine or course threads.

A bolts fine or course threads should at least have a brand of locking compound that allows it to be removed if necessary and should be torqued tot eh proper value of the type of bolt and its SAE ratings. the vibration load is also a factor and may require a drilled bolt head to wire two or more bolt heads together to maintain the numerous bolts rated holding ability and to avoid its possibly loosening beyond a 1/64- of an inch. In some instances bolt heads are welded to each other in a loop to secure the entire loop of bolts on some types of mining machinery for example

The wonderful thing about socket head cap screws is thier simplicity as fewer tools are needed, they require less space per square inch of surface area, and have no limits in applications subject to budget and simple clearance issues in machinery design which can be over come when the socket heads actual height is taken into consideration-hint hint hint :0 and hauling around fewer tools in a toolbag is nice when your crawing around mining machinery underground as a set number of allen wrenches and allen sockets is all that is required rather than a huge assortment of spanners in pouches. this also reduces tool costs for the mining operation. ■

Hi Jonathon,

The answer to your question "why do they still make them" is the same as the answer is to "why do engineers still specify bolt torque for the assembly of critical components" :

Because they don't know better.

Industry is awash in outdated technical "urban myths". The use of spring washers is one. The impression that a "torque" value can be used in the context of a bolt's tighness is another. Blind faith in either can lead to equipment failure, loss of production and, perhaps even injury.

Here's a non-commercial link that shouts out loud "Wake up citizens: The Emperor has no clothes!" in regard to this widespread misconception - The fallacy and danger of "torque" ■

Originally Posted by Roland Heilmann

... Surely I rely on correct workmanship on site and it's necessary to consider sufficient safety margin in relation to achievable accuracy of preloading.

... please tell, for this US-device, don't one has to use lenght-calibrated bolts, considering the rather small elongations observed with high grade bolts?... ....rather rough / uneven surfaces on top of the threaded end... ...have you had any experiences with bolts that yielded prematurely or had cracks in the thread leading to elongation? ...

Hello Roland,

On critical joints where the fasteners are subjected to severe dynamic stresses, there is, unfortunately, not much of a safety margin to work with. If the designer had done his job correctly, the specified bolt load must often be controlled to within a tight band.

Notwithstanding the myriad often-uncontrolable mechanical variations that will always throw a torque-tension relationship to the winds, "correct workmanship" is only a nebulous ideal. The reality is that while most mechanical contractors are very good at turning nuts, they are woefuly challenged when it comes to understanding the requirements and dynamics of what they're actually doing. For example...

We were recently engaged at a new mine construction site in Africa, managing the control of the bolting process during the installation of a girth gear on the client's SAG mill. The contractor had already set the sections onto the head/shell joint and installed the bolts. We then monitored the preload as the millwrights applied the full target torque. Before the proper load was acheived, the hydraulic torque wrench "maxed-out" at 10,000 psi. (this was unexpected since the tool conceivably had more than enough torque capacity to generate the necessary bolt load). The fastener was immediately removed and inspected for signs of galling (this happens more than most people would think - it just goes unobserved). All was well. However, it was seen that the contractor had only applied lubricant onto the threads. There was nothing under the bolt head, on the washers, on the spotfaces or under the nut. The Moly-paste people love us because as usual, we instructed the millwrights to apply copious amounts of the goop to all surfaces (in complete contradiction to the spec stating that light machine oil be used for this purpose). We then repeated the torquing exercise. As expected, the proper load was achieved well within the capacity of the tool. Unfortunately, this wasn't an isolated instance: We had to remove and correct every bolt and nut set on the joint. Without such control, the entire girth gear would have been mounted too loosely even though the "proper" (sic) torque was applied (with a calibrated torque wrench even!). Tooth failure and forced shutdown would have been a very likely consequence.

Going on to your next point: There are many things that need to be considered for joint integrity. Ultrasonic control is only one, albeit a very crucial one. It doesn't matter how long a bolt is; UT technology is used to measure the delta L of each fastener. The only way that this can be done properly is by initally measuring and recording the unloaded length of each fastener. Regardless of how good the manufacturing quality, one can't assume that each bolt in a set will have an identical length.

Indeed, for UT technology to be effective, proper end preps are necessary.

There have been many cases where our analysis of the signal responses had indicated "something fishy" with the bolt at hand. As such, we were able to identify and condem questionable fasteners before the equipment was put into service.

The moral of the above is that one can't simply rely on hopes, wishes and assumptions (as advocated by conventional practices and prevailing "wisdom" ) when blindly "torquing" critical joints. ■

Hi

I think that spring washers can give the false impression that they will help the fastener become self locking.

If you look at most spring washer and profile (inner/outer diameter)- and compare to structural grade washers and also consider the thread pitch of smaller bolts...I think that you can summise that the spring washer appears to be dimensional to suit smaller dia. bolts and are not really suited for large preloads.

I did a ball mill job recently and the end wall liners were fitted with spring washers on the outside...I found this surprising and when we tensioned up to 800/1000Nm...we found that the spring washer tended to open up with the frictional torque!!!! Some even broke...!!! These bolts were M36

I have never seen spring washers used extensively for heavy machinery.

Cheers.........James ■

Hello,

Normally I am in favor of Loctite thread locker products when I am concerned about bolted connections coming loose unwantedly. Unfortunately, it takes brute force to get them to move again. I have come across a great lock washer company that offers a great way to tighten a bolt with its proper torque and then not have to worry about coming loose even under severe vibration conditions.

http://www.nord-lock.com/

Good luck,

Ralf ■

Good day all.

Rolf provided a good link. I've always liked these washers for smaller applications. One has to be very careful though, when tightening fastener-sets fitted with these because of the significant degree of extra prevailing torque involved.

It's yet another variable to throw into the cauldron along with eyes of newt and tails of spider when having to guess what the outcome of the torquing process might be on that particular fastener, on that joint, on that day ■

Originally Posted by Roland Heilmann

Does anyone do checks of "sufficient remaining preloading" or re-torquing of bolted connections after regular schedule / at bulk machines?

With all due respect, anybody who considers a break-away torque check as a QA procedure is missing the boat. In fact, the boat may be sailing away as the dock structure which was recently torque-checked falls apart from the prop wash.

Re-torquing and break-aways only serve to prove that a certain torque has been applied. This has NOTHING to do with how much bolt load remains (or rather, how tight the bolt is) !!!!

If a certain torquing effort encounters more resistance than expected (this time let's use the example of a non-concentric relative axis between a hole in a mill trunnion and its mate in the head), the effort will be stopped before the bolt has been tightened sufficiently (because, after all, the specified torque will have been applied ). "Checking" this same bolt later by re-torquing is only going to ensure that the same insufficient amount of "proper" (hah!) torque has been applied. The result of this useless exercise will be a dangerous false sense of security and, of course, likely joint faliure.

*By the way, a significant degree of prevailing torque ( I loathe this term because of its misleading suggestiveness) is encountered when using the washers noted earlier. Each time one of the teeth in the top component rides up the ramp of the lower, the joint is compressed and the bolt is forced to stretch. ■