Bolts failure in a high vibrating environment

Do you have lock washers?

Yes, it has lock washers and thread loctite additive. We have change our loctite with high strength ones. I just forgot the code. I can say it helps because similar breakdown was greatly reduced. Bolts comes off is not very often as before. But still we encounter same failure. What we are doing now is physically check bolts tightness and re-tight using impact wrench.

Try using a simple dynamic absorber ..

It sounds like you are using single nuts. First, check your lock washers, the standard spring type only work one way. You have to ensure the washer slides against the nut on tightening and the ends bite if teh nut tries to undo.

Try using lock nuts. If you have limited space cut full size nuts in half. method is to tighten main nut down to torque then tighten a second nut on top. This stretches the bolt between the two nuts holding them together and therefore stoping the main nut from coming loose

It is an opinion which is not correct since when you strech the bolt with the second nut you unload the loop of force closing via the first nut so that the situation is not better but some times even worse!

The solution to that is to put the full-size nut on top, and the thin nut under it. I know it sounds backwards, but I've seen the analysis and it works.

Lock washers only add fuel to the fire avoid in vibrating Environment

According to your mail you use impact tools. It could be possible that due to the dispersion of the tools and variation of friction some bolts are under the required pretension and thus become easily loose or even break.

Lock washers are not all of them effective. You should give an indication which type you used.

Some times even if you use Loctite the assembly was wrongly designed from the start and the bolt does not get loose but breaks due to settings and lost of pretension.

You could get a better answer if you give more information about the assembly and the bolts as well dimensions as class and torque specifications.

Do you make any preparation of the surfaces and bolts before assembly?

Other options include safety wire, or castle nuts with kotter pins, but I would suggest that you start with torque.

Rather than use a torque wrench, use a hydraulic stetcher. This allows you to precisely set the preload on the bolt and assures that things are right.

I bolted joints, torque does not matter, bolt stretch is what is important. Torque is just one way to measure stretch, and a lot can go wrong with a torque measurement. That is why hydraulic stretchers are the best bet for this sort of application.

Another option is to use "Supernuts" (you can google that and find them). The advantage of supernuts is that you can measure stretch with a feeler gauge, and you can use a fairly small wrench to pull up a pretty large bolt.

Bolts through rigid joints longer than about 10 thread diameters can be stretched enough by proper torquing to hold their preload in most machinery applications. You need a good bit longer in things like engine headbolts and such. Anything flexible in the joint like a gasket is a trouble maker and source of possible fatigue failure if there are high external forces on the bolts. A good machine design text should cover this in detail.

For common joints you can't go wrong with stainless steel safety wire as long as the environment is not too corrosive. We have 100 years of experience with safety wire in aircraft and automotive racing. It is the preferred method where some of the recent neat inventions like bolts with stretch indicators or special locking devices aren't practical. The nice thing about safety wire is that quality of workmanship in the wire job is easy for a trained inspector to see. The only problem is that it can be difficult in most situations to verify that the fastener is properly torqued. That makes safety wire nice for the bulk of shorter bolt applications where torquing can't stretch the bolt enough to protect it from the effects of heavy vibration and therefore can't be relied upon.

Most other locking methods have their shortcomings as well as their superiority depending on the application as have already been noted.

Ed Weldon

I am "CoronaCameraMan"

Your description, the questions and statements to this point do not address:

1. Loctite (or similar) will only fill a very small gap, thus if the bolts are going into threads that are "worn out, you may need to put in new threads.

2. If the surfaces (clamped faces and bolt head seat) that the bolts are seating against are not flat then the bolt will become loose.

3. Some bolts had a o-ring under the head as a seal and these did not seat correctly if on a rotating shaft and would break.

4. As mentioned you may need to put studs into the existing threads, use a dial indicator on the end of the stud to observe the "stretch" of the stud to determine the pre-load applied to the mating faces.

5. If the mating faces are not perfectly parallel you may need to have the sufaces ground flat and parallel.

6. If the surfaces are being torqued it may be an idea to install straight or taper dowels to maintain alignment and prevent side loading from loosening the bolts or studs. If the dowels are ever found broken it will indicate severe torquing of the parts and that more or larger diameter dowels are required, or that the application requires a different type of capture. Dowels, may also be drilled and clamped into the joint between the mating surfaces.

7. Determine the bolt or stud preload required to clamp the suface faces together always, dtermine the bolt or stud grade required that will be elastic for pre-stress and the loads.

8. Body bolts (where the hole is reamed to the diameter of the bolt or stud with about 1 thousandth clearance) are used on machinery flanges to transmit torque in addition to dowels installed for alignment after the bolts or studs are initially installed. Each bolt/stud is numbered to the hole it fits.

Hope this helps with your problem.

i work in the same industry and the best thing i have found for a problem like that on vibrting machines is a simple cone lock not you can use once them throw them away

quote: "I work in the same industry and the best thing I have found for a problem like that on vibrating machines is a simple cone lock nut you can use once them throw them away." .....bodefino

The voice of experience counts for a lot. The link below shows this type of nut and a number of other types of locking nuts.

http://www.jamesglen.com.au/training/nuts.htm

For those whose situation may make it difficult to order the necessary nuts from a regular supplier note that some types lend themselves to local manufacture in virtually any competent machine shop. This is especially advantageous for maintenance situations in low labor cost areas. In the case of the cone type lock nut some experimentation by the machinist with fixtures on a hydraulic press (it would be nice to have a pressure gauge on it) should lead to a suitable method for producing the appropriate distorted thread.

Of course all of this presumes that the troublesome fasteners can be replaced with nut and bolt or possibly nut and stud combinations. If you are dealing with cap screws then another locking method will be needed. This can be a particularly vexing problem if sharp edged locking devices or broken safety wires can pose a hazard of some kind to workers or maintenance people or add to the difficulty of cleaning exterior surfaces of the machine.

Ed Weldon

The bolt must fit in the hole tight no slack. Distorted holes where bolts have broken out before need to be rezied and larger botls used. Tight mean all around tightness where it comes to viberation.

Quote: "The bolt must fit in the hole tight no slack. Distorted holes where bolts have broken out before need to be resized and larger bolts used. Tight mean all around tightness where it comes to vibration." ..........dadw5boys

Tight fits of bolts in holes are certainly ideal in high vibration applications. Among others are the shoulder screws (sometimes called stripper bolts) commonly used in tool and die work involving fitting into reamed holes. Such practice may not be suitable for many applications since shoulder bolts are available in a limited number of sizes and materials and are fairly expensive. Further they are fairly useless for prestressing in highly loaded fatigue sensitive applications since the oversize shank will require far more torque to stretch a useful amount than the threads can withstand without breaking. Note also that common shoulder screws are usually heat treated with hex socket round heads. This discourages the simpler head locking methods like bendable tabs or safety wire drilling and the hardness makes lockwashers less reliable.

Ordinary long US grade 2 and 5 as well as common hex socket heat cap screws have cylindrical shanks above the threads that are a few thousandths of an inch smaller than the diameter of the threads. And this diameter tends to vary some. So while it may sound easy to use ordinary cap screws specially fitted in reamed holes (maybe with the thread crests machined down a few thousandths to clear for insertion) careful thought needs to be given to future maintenance and repairs lest an uninformed worker attempt to use ordinary screws to replace the broken special ones that may look the same after a period of use.

BTW I'm not familiar enough with metric fasteners to be able to comment on whether what I say above applies to them.

Ed Weldon

I agree with you. I always go for best grade bolt I can get. I hate rework. You have broken it down well for him on bolt selection.

I have my experience with thread lockers (including loctite)- these are anerobis sealants/ adhesives and forms a hard, brittle filler in the gap, thereby locking it.

in a vibrating media, the micro movements are going to break it and once the adhesion is broken, it is gone.

Depend only on mechanical locks.

In most of the cases, as earlier mails has asked, go for double nut, wire locks, lock washers (folding type), split pins in the bolt at the end of the nut (bolt will have a small hole to accommodate it) , or now several ratchet type lock washers are available. (Not the normal internal/ external teeth washer)

You can try this also http://www.spiralock.com/ for special type of nuts

You can try this out http://www.spiralock.com

I'm not a professional .. but as I can see, every suggestion you guys have made was all about ho to fix those bolts .. which as you stated will eventually come off .. why don't you work on how to reduce the vibration of the machine itself .. maybe using a dynamic vibration absorber ?!

http://www.bretech.com/reference/Practical%20Methods%20for%20Vibration%20Control%20of%20Industrial%20Equipment.pdf

I feel in such case you mustcheck which strength fastners you have put. Try using grade 12 fastners . These must be stronger than the ones you are using. Also use one plane washer then spring washer and then grade 12 nylock nut. Using all tighten to the best torque.This has to be done with all the bolts at a time.

I hope this will solve the problem.

To all who have shared their ideas and technical know-how in dealing with bolts that comes off in a high vibrating machine...Thank you very much. Since I am new in this type of job, all thoughts will ba taken and I will sort which will be the most effective depending on each technical situation that I will be into. A very fruitfull discussion to all.

I will let you know on my next comment after observing each areas of the machine.

I am currently developing a simple easy to use positive-locking fastener that may be useful for the transverse vibration issue you are experiencing with your machine. Please contact me to discuss possible opportunities and or visit the following sites to learn more. Thank you.

http://www.createthefuturecontest.com/pages/view/entriesdetail.html?entryID=1690 OR

www.phil-lok.com

Phil Grainger

(330)285-8170

Those phil-locks look like a purdy solution.

The goal for this project was simplicity, low cost manufacturing and a positive locking feature. All those factors combined does give it that purdy type appearance.

Thanks for the input anyway.

Try those phil-locks, sounds like a good idea

You may be right but we are going to keep on trying anyhoo. Thanks for the evaluation though.

Hi Phil,

Have you made the standard test with oscillating transverse loads?

If "yes" what is the number of cycles till pre-load loss?

We are in the process of get a junkers vibration test done and an end of turn test done on the slotted bolt. Do you suggest any others? Thanks for your interest.

No, since the Junkers test is the most severe and is accepted world wide as a reference.

Please keep me informed when results are available.

Thanks in advance.

guys the very simple solution is use a nut which is called the hardlock nut, uses standard bolts etc with no need for special tools, its a 2 piece locking nut that can be used many times, it was originally designed for use on the rail in japan to stop the bolts coming out on trains travelling at 400kph, check out the website below and you can see how it performs against all other locknuts

http://www.staytite.com/hardlock.html