Torque reference values 3

[PAL32]

I am trying to put together a reference chart for screw torque values and had some questions. I was hoping there was something online that i could grab but it dosent seem to be the case. All fasteners are metic A4-70 and the work piece is either aluminium or SS.

Is there a universal standard that should be followed? If so which one?
How does loctite/threadlocker affect the K value?
Does the recommended torque value differ depending on application - High vibration, underwater etc.

Thank you in advance.

 

[SwinnyGG]

and the work piece is either aluminium or SS

This is enough to tell you that your values won't be repeated.

 

[RRiver]

Just my opinion but for screws with the given information, torque to yield usually works. Loctite has a lot of information on their website regarding their different products.

 

[desertfox]

Hi
Try these references, the first one gives maximum strength etc of the stainless material you are using, the second gives the formula for calculating torque.
A word of caution though, torque values are highly variable and depend on lots of things within the clamped joint, also when clamping materials which have a much lower strength than the bolt strength, care needs to be taken that you don’t embed the bolt head or nut into the surface of the clamped parts because if that happens the preload you wish to generate won’t happen.

https://roymech.org/Useful_Tables/Screws/Strength.html

https://roymech.org/Useful_Tables/Screws/Screw_loads.html

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

 

[RRiver]

desertfox, exactly the reason for the TTY suggestion. There's 2 types of information available on figuring bolt\screw torque. One tells how it's calculated and the other is about why the first is unreliable. What is usually identified as consistent though is the individual manufacturer bolt\screw specs. Once the OP identifies what he wants it may be more reliable to work backwards from yield strength based on fastener material and class and TTY. The clamping force or preload should also be a spec given.

 

[drawoh]

APL23,

I take it you using the formula T=CFD, where C is the friction factor. According to my machine design textbook (V.M. Faires), the factor is 0.2 for an unlubricated joint, and 0.15 for a lubricated joint.

You make all sorts of assumptions when you do this calculation and generate charts off of it. You need to state these assumptions clearly so that people will recognise when the chart does not apply to their application.

Faires almost certainly assumed steel on steel. Your application of A4-70 stainless screws to aluminium or stainless steel is fairly weird. I think you need to set up experiments to work practical friction factors, possibly for each of your tapped hole materials.

Many threadlockers, including the blue Loctite[ ]242, are classified as lubricating. This gives you better resolution of torque to tension, and a locked fastener. Other threadlockers can be sealing or wicking.

A lot of the torque charts I have seen assume that the tensile force is 90% of the screws yield or proof stress. This is a good way to assure the screw does not come loose. If you remove the screw for any reason, you have to assume plastic strain. The removed screw must be discarded and replaced. If you plan to re-use screws, tighten to 75% of yield or proof load, or less.

There is lots of stuff on screws loosening on Bolt[ ]Science's website. I strongly recommend reading it.

--
JHG

 

[desertfox]

Hi ALP23

The problem with torquing the bolt to yield or to 90% of its yield stress is that you are focusing on the bolt strength, if you attempt either of these,without consideration of the aluminium parts being clamped then your preload will probably be lost. Analysis of the joint and bolt stress after assembly needs considering see below.

https://roymech.org/Useful_Tables/Screws/Joint_Stiffness.html

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

 

[RRiver]

""Important Note: Widely different stiffness values result from different studies. It has also been proved that different loading conditions and surface conditions also affect the resulting stiffness.""

This is where a lack of information can lead to a variety of interpretations. The OP said screw and not bolt. Cap screw or 10-24 phillips head machine screw? Is the question about joining plate or a sheet metal access cover in a high vibration area? Both screws have the same spec parameters with very different values.