vibration proof fasteners 3

[wobbles]

I work in a foundry and there is a lot of vibration occuring all the time. We are currently using Grade 5 bolts and in the process of changing to A325. I am looking for a solution to stop fasteners from vibrating loose. I need some help to find a good method to use whether using an threadlocker, split washers, safety wire, or anything else that may be out there to use. Im in the process of getting info about spiralock and nord lock fastners feel free to leave some input about these types of fastners

 

[Binary]

CoryPad,

I appreciate you providing the reference. I guess I communicated to you that your opinion wasn't good enough when I what I was looking for were the reasons behind the opinion like, "I say it...because studies have shown that the stiffness of the lockwire is typically less than the torque cross radius blah, blah, blah." I was wondering about what led to your opinion rather than just the opinion itself (which you made very clear). Anyway, I'm pleased that I didn't inadvertently offend you.

In my application, I cannot say for certain that the lockwire prevented the loosening only because we always did it. However, I must presume - given the cost and hassle of using it - that it was implemented in response to a specific problem and that it rectified that problem otherwise why would my predecessors have adopted it in the first place?

Also, in thinking about the physics of the situation I do not see how it's physically possible for them not to prevent loosening.

The fastners certainly had some preload because they were tightened down but I don't recall that we always used torque values on everything (we did some, though).

In my judgement, there are suitable applications for lockwire, funnelguy's being an excellent example (acknowledging the drawbacks that you cited).

 

[smcadman]

I felt reserved about adding to this discussion because of the perceived arrogance of some individuals, but I will add my input anyway. I have not used safety wire myself, but I have seen it used whenever I used to work in an aircraft duct and repair shop. Here is an online reference to add to this debate:

Go to Chapter 7, Section 7, Safetying

http://av-info.faa.gov/dst/43-13/

Flores

 

[Binary]

Flores, great link!!

It clearly shows what I so ineptly tried to describe.

 

[evelrod]

Thanks for the link, Flores.

Binary, I understand what you are trying to say but I am sorry that it just won't wash. I have been doing safety wire on race cars and aircraft since 1958 and have seen various failures due to incorrect installation, that is true. I have also had correctly wired units fail due to other reasons not the least of which was failure due to loss of tension. The link supplied by Flores is a perfect place to go. The very first paragraph on safety wire should answer your questions and solve your doubts as to the abilities of safety wire to prevent loss of torque.

7-122. GENERAL "...These practices are not a means of obtaining or maintaining torque, rather a safety device to prevent the disengagement of screws, nuts, bolts, snap rings, oil caps, drain cocks, valves and, parts."

CoryPad, I was just a teasin' y'all a bit. I agree with you 100%

Rod

 

[Binary]

Mechanically, how can they disengage. It just seems physically impossible. Is it that the lockwire is just not stiff enough or that it yields or that it was just never tight enough to begin with?

Again, I'm not arguing the conclusion, I'm just trying to understand it from more than simply an empirical viewpoint. I like to understand the "why's" behind the "what's".

 

[CoryPad]

Binary,

You are correct regarding stiffness and yielding. The stiffness problem is twofold - material stiffness (elastic modulus) and structure stiffness (wire diameter, path to next part).





Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[abdul]

Hi,

Here is another solution, its called stage-eight locking devices. It has a positive locking system that should resist all vibration.

http://www.stage8.com/Groovelok.htm

Naturally, any bolted joint can lose some preload from thermal cycling, vibration, gasket/joint relaxation. To prevent these factors from affecting the fastener pre-load, do the following:

- Make the length of the bolt between the nut and bolthead, which is actually under tension, as long as possible. This increases the elasticity of the bolted joint, which makes the fastener act like a very stiff rubber band which can resist vibration and thermal loosening (you mentioned that these are in a foundry).

- Load all fasteners to 75% of their proof load.

- Minimize the number of surfaces in a joint. For example, if your bolt rests on a washer, and on the other side of the flange, the nut is also resting on a washer, then you have a total of five surfaces sandwiched in your joint; the bolt head to washer-> the washer to flange-> the flange to flange-> the flange to washer-> and the washer to nut surface. Solution: Purchase what are called flanged bolts, and flanged nuts(these are standard fasteners). These have a built-in washer integral with the bolthead and nut. Result: Only three surfaces remain, which increases joint integrity, since there are less surfaces to slip/imbed.

- Increase fastener diameter to the maximum possible, as this allows a greater preload.

- Eliminate all non-metallic gaskets in the joint(if any), and if necessary, apply O-rings or soft metal gaskets instead. This allows a metal to metal joint, which greatly resists loosening as compared to a gasketed joint with pliable gaskets.

- If separate washers must be used, these should be of hardened steel, and have a minimum thickness of 3/16 inch, for fasteners below 1.0 inch diameter. For fasteners of greater size than this, the minimum washer thickness should be 5/16 inch.The washer diameter should be at least 2x fastener diameter.

- The fastener outer diameter,should, as much as possible, be fitted snugly into the joint hole, if the main problem is vibration.

-Increase the thread engagement, by using nuts with a height at least equal to 1.5 x fastener OD. The increased threads spread out the preload more evenly, which reduces the internal thread deflection, thus maintaing more thread surface area under load. The same applies for blind threaded holes.

- Confirm the flatness of all joint surfaces to be joined. Nonflat surfaces introduce bending loads in the fasteners, and reduce the actual forces holding the joint together. Same thing goes for the surface quality of the joint surfaces; rough surfaces allow embedment, which produces a loss of preload. Misaligned flanges are another bad actor application.

- When torquing, apply antiseize under the surface of the nut or bolt head , including the threads. This increases the amount of bolt torque which actually ends up as pre-load; otherwise much of the torque will be lost in friction. If there is a vibration problem ==> do not torque with anti-seize or oils, only use locktite style adhesives, as these will act as a thread lubricant, and give chemical bond locking afterwards.


Abdul

 

[mechanisms]

How effective are tab washers in preventing vibrational loosening? I know they are ineffective against loss of clamp load? I would like to know what are the limitations of tab washers against loosening? How do they compare with anaerobic adhesives or nord-lock washers?

 

[CoryPad]

Tab washers are good at preventing a nut/bolt from completely rotating (on or off) if they are design correctly to interact with surrounding pieces (e.g. frictional or direct contact with a nearby piece). They definitely do not retain joint preload. They do not compare with adhesives/Nordlock because they perform different functions.





Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[evmundo2003]

pre-load is the key here, you really need a tensioning device to ensure correct preload/ torque has been applied, consider hydraulic tensioning systems, or if two expensive, consider Huck bolts or Rotobolt or variations on a theme of these two.

http://www.rotabolt.co.uk/

Of course as already expressed by the engineers above, it is important that you use a bolt/screw that is man enough for the task in hand, so look at optimum bolt size, length, minimum nuber of joint faces (components) and especially bolt grade 10.9 or 12.9 are ideally what you require for heavy fixing. A correctly specified joint and installed fastener will not come loose.

evmundo2003

 

[Sciscoe]

Wobbles, I would try stepping up to a grade 8 bolt with a high pre load and see if this will handle your problem. You may want to try it out on a convenient trouble location to see if this low cost fix will work in your situation and prior to installing solutions with higher labor and expense. Good Luck

 

[JimMetalsCeramics]

Relevant link:

http://www.apmhexseal.com/selfseal/catalog_pdfs/apm_sf-200.pdf

 

[JimMetalsCeramics]

On an assembly we once prepared, we spring-loaded the washers. However, what we particularly found useful was to use a right-handed spring and a left-handed thread on the nuts and bolts so that when tightening down on the bolt, the spring would compressively pre-load. This should also be doable using a left-handed spring and right-handed fasteners, of course.