Load carrying capacity of threads on nuts

It depends on the type and size of thread used to carry the load

You got to help me out here. What do you need to know when you ask the type of the thread? Could you point me to a site where I can find a way to calculate the load carrying capacity? One other question- to prevent a nut from coming loose, we provide a another nut as a check nut. Is that a standard practice? Is a star washer a better alternative?

Type of the thread : Metric,...etc

its a standard method of usingcheck nut

In my experience, the second 'check nut' is the most reliable under extreme conditions of stress and vibration. It is not communly used probably because of the extra expense of the second nut and/or extra assembly proceedure needed.

How rusty are these nuts?

It will depend on the material these components are made of and how coarse the threads are "The TPI threads per inch." or metric equivalent. Having a lock nut will help because you have a greater surface area engaged. It also depends on the thickness of the nuts.

Am I missing something here. We're talking a 300kg load. At a pinch you could dangle that from a 1/4" bolt (mind you I wouldn't stand underneath!!!!). Is the capacity of the threads an issue unless these are really skinny nuts.

Lock, or check, nuts are an excellent way to stop things unthreading but if you want to be 100% sure, wire the nuts.Washer systems were invented because a washer is cheaper than a second nut!

Yes, one thing I should mention is that the suspended load is an agitator.. running at 100 rpm and will be subject to some differential loading depending on the process conditions. What I am looking for is a way to do the calculations to conform that the threads on the coupling and the nuts are good to take the load.

What does the original equipment manufacturer's calculation say? The manufacturer will be able to back up the equipment supplied with calculations showing suitability for use on the material, in the vessel, at the temperatures the equipment is to be used, thereby confirming that the design is correct as despatched. Of course, should local expedient repairs have taken place...

Why not tell us the bolt dia, then it would be easier to help. Also a bit more description to visualise the system. Eg 135mm coupling - what sort of coupling, where is the 135mm measured?

As others have pointed out, the bolt strength is likely to be limiting factor.

Codey

The bolt tension is calculated by the well known formula:

T = kDW where: T = torque, k = friction factor, D = bolt diameter, W = tension load

Typical values of k are 0.11 for lubricated steel, 0.15 for plated steel, 0.20 for unplated steel, 0.30 for stainless steel.

The tensile stress can be calculated from the tension load divided by the tensile stress area of the thread (which are published and readily available). Those tables also usually include the thread stripping areas in area per length of engagement for internal and external threads of various classes. Publications include "Fastener Standards", published by the Industrial Fasteners Institute (IFI) in the U.S.

Since threads fail in shear, the calculated tensile stress is compared to the shear strength of the fastener material, appropriate safety factors are applied, and the threaded joint is designed.

Jam / lock / check nuts are effective to prevent backoff but only when installed after the correct bolt preload is established by the primary nut. The correct preload stretches the bolt slightly and results a properly tightened threaded connection. The check nut then backs up that connection.

Flat washers are used to close gaps and establish a controlled surface for nut rotation (preferably). Split or spring lockwashers are not effective in creating a vibration resistant connection. In fact, for all practical purposes, they are nothing more than flat washers with a split. Tooth lockwashers are only slightly better but the only effective vibration-resistant washers are Belleville types.

There are lots of other ways to create a vibration-resistant, properly tightened threaded connection; including various types of locknuts and thread-locking compounds that can be applied before or after tightening.

try www.boltscience.com

Being a heavy equipment mechanic myself. I came across this question many time and the most apropriate answer I got was:

If you can tread a nut all the way, Witch in your case seem to be possible since you can even put two, then the surface of the tread is greater than the bolt itself (for all standard or metric bolt) so what you have to worry about is your bolt strengt. Wich can be calculated by knowing the strengt of the material and the size of bolt.

Excellent analysis. Strength of Bolt is the real issue. SAE J 429 gives mechanical and material requirements for Externally threaded fasteners. However, if thread percentage is on the low side, might be an issue.

By the way, if it absolutely positively has to stay on, a small hole through bolt and nut or through bolt only past nuts will allow a clevis or similar device to assure retention. Similar to what US OSHA mandates for high pressure couplings.

milo

As you say the load is an agitator I would think that the vibration would be your main area of concern. Either locking wire or a split pin would help here. The load factor concerns both the nuts and what ever they are threaded onto. The materials in use must there for be chosen to suit the job at hand. High tensile steel would be best used. (Unless of course some one knows better)? Well do you feel lucky Punk?

Cross-pollinating with ideas from other industries: the Nord-lock washer?

http://www.railway-technology.com/contractors/rail/nord/

When using 2 nuts (nut and counter-nut) the biggest problem is the torque between the 2. If you have ONLY axial loads the getting loose is rather unprobable. If you have the assembly loaded with variable torques or transverse forces (shear) it is much more probable the preloading will go down.

Ther is a principle which unfortunately is neither well know nor rightly used: the kind of safety used against preload loss MUST be independent of preload itself. The washers you mention are in their action (blocking) related to preload so that there is a negative evolution as soon as for any reason preload decreases. In tests done those washers had the worse behavior.

Second the higher the stiffness of the blocking the higher the risk.

If you have axial forces it should be taken care of what happens when the nuts are tighten one against the other. This is important to choose where you place the thinner one (mostly the 2 nuts are not the same). In case of erronate asembly the threads could be sheared off.

To compute the carring limit of threads you should consider the shear sections for the thread type you use as well for the nut as for the bolt. Consider also that the first thread carries between 30 to 60% of whole load. The shear limit can be approximated by τe=Re*3^-0.5. The section is H*D*π for a thread. Fmax= H*D*π*τe/(0.3...0.6)

The higher coefficient is for stell/steel pairs the lower for steel/aluminium.

It is also possible to assume that at destruction the whole threaded zone is loaded at elastic limit so that z*H*D*π*τe > Cs Fmax z is the number of loaded threads. The length of threade engaged zone will be P*(z+2)

I don't know actual wieght ratings. This may help a little.

Bolt Diameter + Bolt Radius = Standard Nut Size

Bolt Diameter + Bolt Radius + 1/8" = Heavy Duty Nut Size

I don't know the metric conversions. But there probably is one.

As a general rule, the Nut's Wieght Retention = Tinsle Strengh of bolt (Providing they are the same classification)

Finer threads allow greater torque to be applied, not neccesarily greater wieght capacity. There is a piece of literature called "The Riggers Bible," that has bolt characteristic charts. I'm havivg a little internet trouble so I can't help you with a link. Sorry (Mabe Google "Bolts")

Also "double nutting" prevents the stress nut from loosening due to vibration or heat fluxuation. As far as I know it does not add to the strength rating as 1 nut is designed to oppose 1 bolt. from past expierience I find a split spring washer works just as well, is less expensive, and easier to work with. Hope this helps.

If you want it to stay done up and there's a lot of vibration I would avoid any washer other than a quality flat one. Too often the washer cracks and works it's way out. I would also investigate the various Loctite or similar products.

I also agree totally with the post above that if you've got a full nut on it'll be the bolt that fails first. I've had quite a bit to do with situations where fasteners are used to the point of destruction (it's amazing how strong they actually are!) and I've never seen a nut "pulled" off a bolt. The nearest I can think off was the complete destruction of a D shackle where the pin bent and then ripped out of the threaded end but that was a MS shackle and the threaded section split apart. The motivating force was a D7 so it had some excuse!