The best way to protect a bolt from loosening? 15

[EighthBen]

Hello,

our company is manufacturing machines for wood industry. In certain devices we have a wood log rolling down and hitting the steel wall. At the moment of impact there is a huge amount of energy absorbed. This causes minor movement of the construction elements, and sometimes we get loosened bolts in that part of machinery or nearby.

Therefore now I must research the ways of locking the bolt.
We tried: din 985 nuts, din 127 washers, and Locktite glue. Nothing helps, it keeps loosening. After that we started welding the nuts to the bolts, because it never has to be disassembled, so it is not a problem.

So now I want to familiarize myself deeper with the ways of locking the bolts. I've tried to find the textbook or any other kind of source of information about the usage and differences (in effectiveness of prevailing loosening) of these methods:
din 985 - nut with nylon
din 6923 - nut with expanded bottom and with teeth
din 127 - washer with cut
din 6798 - washer with many cuts
nord lock - from wurth - looks serious, though pretty expensive, haven't tried it yet.

If anybody could advice me where could I find more information on comparison of these locking variants, I would be so grateful. We have other machines with smaller impacts where could avoid welding, but I want to get some theory and lab experiment result before going to "try and hope" routine.

Thank you for your advice

 

[Tmoose]

The Junkers machine that NordLock and others rely on for proof is interesting.
As best I can tell it enforces some amount of sliding between the components clamped by the fastener.
I can not imagine that components that repeatedly slide don't eventually wear themselves and the fastener faces down, decreasing preload.
As material is lostEven A fastener that does not "loosen" (unwind) will clamp less and less effectively, and someday will be likely even loosen.

My preferred strategy to achieve reliability would always be to keep the components from sliding in the first place.
As others have said, a quantity of sufficiently long well-tightened fasteners is often a pretty economical, easy-to-field assemble method to accomplish that.

 

[bdeuell]

I suspect (as many others have suggested) that the bolted joint in the current design is not strong enough for the loads it is subjected to. When you have applied the thread locking adhesive did you see failure of the adhesive bond or did the joint loosen first?

Have you considered trying to dampen the impact forces rather than deliver them directly to your frame structure. If you can add dampened mounts (or even just springs) to this steel wall if could drastically reduce the magnitude of the impact force by spreading the energy out over time.

Also split lock washers are a pet peeve of mine as they preform worse than a flat washer ... at least until the bolt has already become loose.

I have found the NASA fastener design manual to be a good reference, covering the design considerations for bolted connections at a high level.

 

[BillPSU]

Another choice is to use a Dardelet thread. It is a self locking thread. You still may be exceeding the strength of the fastener and stretching, but it won't come loose.

Bill

 

[KENAT]

Re threadlock...

It comes in various grades - make sure you're using the correct one.

Also, relies on an 'active surface' for it to work properly. Doesn't work as well on Aluminum or SST because of the inactive surface. There are primers available that may help.

http://www.loctite.com.au/aue/content_data/133403_LT_4985_Threadlocking_Users_Guide.pdf

I seem to recall that reference has previously been given to a report (NASA?) that points out the limitations of many retaining methods such as Jam nuts and most variations of locking washers. Additionally some locking methods only ensure the nut doesn't fall off completely - but don't install the preload is maintained.

May have been in this forum: forum725
This FAQ also came up when I searched:

http://www.eng-tips.com/faqs.cfm?fid=1257

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Cheetos]

Have you tried tab washers? You can bend the tab over the bolt head.

http://www.roymech.co.uk/Useful_Tables/Screws/Locking.html

 

[Slowzuki]

I'm assuming most of the respondents have not worked in a sawmill environment. Fancy fixes to this problem will not be appreciated by the customer the next time the line is revised or requires maintenance.

I'm assuming the log is falling from a trim saw or sorting machine onto a drag chain or rollers. Many manufacturers use a small ramp to let the log roll past the chain a small distance up the ramp then back onto the chain to reduce the impulse. This can reduce the mass of steel and fasteners needed in the area substantially.

Re the locktite or similar, well prepped bolts with the permanent grades won't likely loosen but require use of a torch to heat them for removal. This can be a problem if the line is inside near the headsaw or planers, trying to clean the floor and machines adequately for hot work isn't fun.

Unless there is a good reason not to, welding with good details to prevent cracks from forming is an excellent option.

Increasing fastener size and switching to fine threads is another option but increasing bolt length and double nutting works well too. Also re: welding nuts to bolts, its better to double nut and weld the two nuts to each other than to weld to the bolt. Much easier to service later, a grinder or sawzall can cut the weld and the relatively cheap nuts removed quickly.

 

[bc1080]

I’ve been there with issues where we just could not get fasteners to stay tight. Honestly, I’d suggest moving away from torque based locking methods and find a mechanical solution (“positive locking”). The match drilled pin or castle nut are example of this. Lockwire is another, it is slightly labor intensive but shouldn’t be very expensive per unit.

We have also used lock plates and tab washers that index to something on the hardware to prevent rotation. Tab washers are bent/deformed around the bolt head/nut after assembly but lock plates usually need to be installed first. Maybe modify the assembly operation and install a lock plate on the bolt side and torque against that, then use a tab washer on the nut to lock it after torqueing. You can also design the hardware itself to lock one end of the fastener by making it captive (have a slot that locks the faces of a hex bolt, etc).

 

[Bales]

A method discovered by Rolls-Royce engineers was to use nylocks in reverse, that is with the nylon bit going on first. The nylon is squeezed tighter by the nut following it.

 

[Sawbux]

Reduce impact by putting rubber behind the back-stops (steel wall).

Load bolts in tension, with long shanks to absorb impact over the length.

 

[alloyboy]

Any weldment ,assembly if subjected to repetitive impacts, will loosen in time !
I have to agree with bdeuell. deceleration of the item (log) before impact, IMHO ,will lessen the chances
of failure.

 

[IFRs]

Ever tried lock bolts? Google Huck Bolts. They are used in the rail industry for putting rails an box cars together, the repetitive hammering over the rail joints messed with bolts. They are used in many other industrial applications, are available in a wide variety of sizes and grades. They have special removal tools that make it easy in the future.

 

[Kowalski]

There have been a couple of responses that are getting more towards a non-brute force method, but what I would suggest is just putting springs under the nuts. The springs will maintain tension on the bolt and hold the thing together when it's repeatedly getting pounded by logs.

You just need to find a fairly heavy spring and don't let them torque them all the way down.

 

[desertfox]

I would be extremely careful using a helical compression spring as this would work on the same principle as a spring washer and the are well documented for loosening under any vibration.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[hydtools]

Springs are very effective keeping side rod and foot bolt nuts tight on paving breakers and mine drills.

Ted

 

[CoryPad]

desertfox,

The helical compression spring can have a much lower spring rate than a helical spring-lock washer, which will make it much less sensitive to vibration loosening. Having said that, I always favor preload as the primary method to avoid vibration loosening.

 

[bdeuell]

I would add a second jam nut to Kowalski's design, currently the nut is only torqued against the spring so there's very little preload to keep the nut from loosening. adding a second jam nut and then tightening them against each other would lock the nuts on the end of the bolt. this wouldn't increase the preload in the length of the bold but the intent of the spring in this case is not to add preload but to increase the time over which the impulse force is applied to the frame thereby reducing its magnitude.

I think it is still important to understand why the failure is occurring to ensure the solution is appropriate, i.e. is the joint failing because the bolts are not strong enough for the impulse forces they are subjected to or is it failing because vibrations are working the nut loose.

 

[alloyboy]

This thread is drawing more attention than it truly deserves !
Without a schematic of the production line, it is difficult to visualize how the log strikes the stop plate.
Perhaps it's time to go back to the proverbial drawing board.

What happens to the log after it strikes the plate ?

Where does the log strike the plate ,top, center or bottom ?

More detail is needed !

Would hydraulic damping be of value here ?

 

[desertfox]

Hi

Unless we know more about the external forces that the joints experience using a spring behind the nut could make matters worse.
The bolt preload is limited by the amount the spring is deflected and its stiffness, so how do you know what preload to apply if the external forces are unknown?
A bigger problem with the spring and nut arrangement in this particular instance is spring ratcheting, this can happen during any impact and the spring as a tendency to rotate, if this occurs the nut could be slowly rotated off the bolt as the spring relaxes it's preload.
The final problem is also one of spring natural frequency because if the logs hit at regular intervals the spring alone could vibrate and again loosen the joint.

Just to give some idea of what might be involved in maintaining bolt preload using a spring I've attached this link to a paper on using beiville springs for the same purpose:-

http://www.sealing.com/fileadmin/docs/Using_Bellville_springs_to_maintain_bolt_preload.pdf

To use a spring behind each bolted fastener seems an expensive and time consuming option if you want to design it correctly and I don't think this application warrants this.







“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[BillPSU]

Drill a hole through the nut and bolt and use a stainless steel tie wire. I've never seen a tie wired nut and bolt come loose.

Bill

 

[MikeHalloran]

I used to think that lockwire was an absolute protection against fastener loosening.

... until I heard a rumor about a union nut on a high pressure ball valve that was so thin that it could be stretched enough to jump a thread (without rotation and without fracture of the lockwire) when the face seal o-ring failed.

( Those of you who work in hydraulics know that when a face seal o-ring fails, the effective area producing a separating force goes from the area corresponding roughly to the o-ring ID, to the projected area of the entire face, in this case, roughly the area corresponding to the inside diameter of the union nut, effectively roughly doubling the separating force. )

Of course, it was just a rumor, as was the rumor that the design persisted despite several known service failures, because an engineering vice president held a patent on the particular stupid valve design.

... like that ever happens.



Mike Halloran
Pembroke Pines, FL, USA