Machine Bolts Failure

Talk to a specialist....

http://www.hi-tensilebolt.com.au/Front%20Page.html

http://www.fastenal.com/web/home.ex

I think you need a way to absorb the shock before it reaches the bolts.

My only experience with something similar, is with paint shakers; they use rubber padding, bushings, washers, etc. Not necessarily all of those, but the holding bolts are isolated from the full impact of the shaking, and the paint buckets are still held firmly in place.

There is a great deal of weight involved much more than a paint shaker. The rubber would bee destroyed in a very short time.

If rubber will not work, how about spring isolators? We have used them to isolate equipment weighing several tons (much like the suspension in a car or truck).

I'd go with a softer bolt. Something that is less brittle.

It's cheap enough to try.

As lyn suggests, use bolts that are less brittle. Are you using torque wrenches and not over-tightening them. You cannot "feel" the yield point on alloy bolts as you can with mild steel bolts.

I was more concerned that the bolts may be under-tightened, and are thus seeing more of the stress changes than they should. Leading to fatigue breakage. It should be possible to "read" the fracture and tell if it was from fatigue.

Larger (diameter) bolts.

There is no information about the size of the existing bolts, their configuration, nor how large are the vibration loads. That makes it hard to give suggestions.

You could suggest he give more information....

He just did in a way, didn't he?

Don't use hardened bolts as they can be brittle and prone to snapping. Use medium to high tensile bolts and torque accordingly.

I've also had success using extra long bolts in place of standard ones with a tubular spacer under the head bearing down onto the bolting surface. The longer bolts are able to elongate a lot more than short ones and can cope with a lot more pain under operation.

Interesting. Sounds good to me, but with deference to AH's admonishment to always put personal safety first.

Like another reply, LARGER diameter Metric Fine thread Bolts in grade 12.9 also apply 680 grade loctite BETWEEN any flat bolted surfaces to eliminate any sliding/sideways movement

1) Bolts have to be separated by springs so that vibrations/oscillations do not transfer to bolts.

2) Seems bolts are not tightened enough. Mis match of nut and bolt pitch- Need to check.

3) Soft bolt material is better - spring steel can be looked into.

4) Check the OEM specificationss first. You might have changed the same especially the vibrations absorption piece- rubber/gasket/spring/washer, material and size of bolts. Any retrofitting/ modification done to originals design??

A look into the causes of vibration should also be on the cards. Why the machine is shaking heavily? Any resonance issue.Have you tried to quantify vibration? It will be interesting to know if it is within limit as per existing standards. A root cause analysis may help . Simply changing bolt material will no help much since if vibration is too high, there will be ultimately fatigue failure.

I agree with Irshad, you need to look at the root cause...maybe you can reduce the feed rate of the fibreglass or slow down you machine then evaluate....It seems that all the discussion about bolts is just a band-aid and if you double the number of bolts and then they start to crack and shear...this may cause a more severe safety concern for your employees.....

Does the original manufacturer realize that you are chopping up fibreglass...what are their recommendations.....

I wonder about using longer high tensile bolts with a high tensile spring on the shank - as used on some Catalytic converters and other vehicle exhaust joints to absorb shock, vibration and movement.

What does the manufacture recommend? Have you contacted them as to why? Where are the bolts snapping at in the shank, threads, or under the head? What do the bolt holes look like, are they worn? Lots of questions, but I would start with the manufacture 1st.

Bolts are not supposed to absorb any real movement between the pieces they connect. Thats a job to be done by the friction between the joined pieces, due to the pressure the bolt applies. And I don't believe ANY vibration level would induce fatigue to a properly tigtened quality bolt due to its inertia only. So problem can be either bad bolt quality, wrong (too high or too low) bolt torquing, machine operation beyond specs, and not to exclude, a possible bad design to begin with. But "broken bolts" as only data is not nearly enough for solutions. S.M.

Tinkering with ways to alleviate the symptoms i.e. changing the type of bolt, is not your best option. The bolts are acting as a fuse that fails when there is a fault and protects the rest of the equipment. Finding a solution to the failing bolts will only move the problem to another component and failure of that item may be dangerous to life and limb.

Without knowing more about the machine it is not possible to offer more than suggestions. Presupposing that the multiple impacts between the chopper blades and the fibreglass is the root cause of the vibration?

1 Change the contour of the blade profile so there is more cutting action and less ripping action. Alternatively if the blades are currently set to give a chop action try angling the moving set of blades so you achieve a scissoring action.

2 Increase the weight further with the addition of a (larger) flywheel that absorbs more of the impact energy so less is available for transfer to the machine frame.

We have lots of suggestions but still don't know the failure mode of the bolts. If it is tensile because of the loads imposed, extra tightening would add to the problem. If it is shear then tightening would help by increasing the friction between the bolted parts. We need to know more.

A story from my past had a similar problem. I worked for a while on gimbal mounts for naval radar. The radar array weighed about 4 tons & rotated constantly on the gimbal mount which kept it level as the ship bobbed around beneath it. One of these failed & the array fell to the deck having sheared 32 1/2" high tensile bolts. The investigation revealed that the ships captain was excessively tidy & rather than letting the radar run down to a random position, liked it to be parked in line with the ship. He had a rating stabbing at the 'inching' control until he was happy. The shock loads imposed had caused the failure.

GA for asking if the failure mechanism is in tension or shear.

_{The other bit's also interesting and worthy of note, but, I thought it was worth emphasising that we haven't established the failure mechanism yet.}

Increase the number of bolts so there is less space between each bolt in the failure zone. Dampen the entire machine. Balance the choppers

Assuming that your hardened bolts are of good quality steel, and that you are not prepared to redesign the machine to absorb or otherwise reduce the stresses on these bolts, then I will recommend that you try a custom stainless steel bolt set using 17-4 PH material. The bolts should be further heat treated to H1300 condition. THis steel is a "spring" steel by loose definition but I have built many things with it from farm implements to injection and blow molds and tooling. It is extremely strong but has the ability to be heat treated to enormous toughness. Do not confuse this heat treatment with a "Hardening" heat treatment. The corrosion resistance is also very good so it will not lose strength and toughness over time like a steel which might rust during it's working life span. Another choice would a "Grade 5" bolt. This is a pretty tough bolt and unlike it's more commonly known partner the "Grade 8" bolt it is not harder, but tougher, which in my opinion, is what you need to solve your bolt failure issue. Sure there are many other material options but these are materials I have actually used my self for many, many years. Also, if you are going to actually make these bolts yourself then please be mindful to include a maximum radius permissable in the corner under the bolt head as well as blend the thread out rather than cut a relief groove at the end of the thread. This will also maximize strength.

Here is one of many sites providing machinery isolation mounts. Assuming that the top is not deflecting cyclically and heading for a stress failure of it's own, then you can lsolate the motor. Many seem to have mentioned this. I did not, however, see links to suppliers of machinery isolation mounts. Google will pull down any number of these in the responses.

Here is one such supplier, pulled from the top of the list on a google search.

http://www.vibrationmounts.com/Products.htm?gclid=CPCTidTDlbcCFegWMgodek4AwQ

As several have mentioned - going with "harder" bolts is going the wrong way.

Good luck.

You really should correct the vibration, as mentioned elsewhere.

Some years ago the company I was with built a machine to cut fiberglass scrap that had not been processed beyond the initial winding - the cardboard core was removed and the mass of wound glass fiber was thrown into the machine to be cut into smaller chunks by rotating blades.

We initially had major vibration issues due to the main shaft being run too close to it's critical speed. We changed the speed and shaft diameter to correct that, but over time the glass would accumulate on the rotating parts and throw the balance out, bringing the vibration problem back. From my experience, there is nothing you can do about glass dust and fiber accumulating in your equipment other than manual cleaning.

I suggest you check the critical speed of the rotating group against your operating speed, maybe an adjustment is needed. And, clean the accumulated glass dust and fiber from the rotating group more frequently to maintain the balance.

And now for the "sitting in the sun and enjoying a cold one" Fix.

Automotive Valve Springs. These are made to be beat to death and should survive your application just fine when assembled with correct washers and spacers. Not to mention readily available from most junk yards for pennies so even if they fail monthly, unless there is a "line of fire" safety concern for your workers, they can be replaced as often as necessary economically.

Hey, and your recycling too!

First, bolts should not just be hardened! They become like glass and break. Proper annealing has to be done so they are also tough. Make sure of this before doing other changes. Properly annealed the bolts should normally work) Use high tensile spring steel material if the bolts are custom made.

If 12.9 type bolts don't work use a design where the bolt is longer with a section slightly undercut (Slightly smaller than the thread diameter(no sharp edges)). This allows the bolt to act a bit like a "spring" and has more give and it doesn't get overloaded). That's a standard solution for heavily stressed bolts.(It's used often on engine connecting rods) You can use a spacer when you increase the bolt length. A bigger diameter bolt obviously would work too if possible. (Don't just harden the bolt - the high strength bolts are hardened and then annealed to correct specs)

Don't recommend bronze - it's a lot weaker than steel.

Either way make sure you have the correct why for the failure. I don't know all the details and many things could be the cause of this - including inferior materials used or wrong treatment of it.

Hope this helps.

Josef

I would like to thank everyone for the great information. I think I have enough info to move forward.

Thanks for coming back and letting us know. If you can updated this thread after a month or so and let us know what your solution was - even better.