Lubricated bolts or not so? Question

[pelissima]

I recently renewed our keel bolts and used 3M 4200 for the large washers but also for the threads. I have seen in relative tables that an oiled bolt need 10% less torque to come to the same state as a dry one. Now the question. A keel bolt with 4200 or Sica when at the start is turned by hand behaves with some resistance. However, I tend to believe that when high torque is applied it behaves more like a lubricated bolt (At least as long as the paste is fresh). So, if the torque applied is say 220, the actual will be 10% more. But is it so?.
Comments welcome.

 

[ghostlymoron]

That's a major drawback of measuring stress indirectly using a torque wrench.

 

[vyv_cox]

What torque values are we talking about? The figure needed to pull a typical keel bolt, maybe 25 mm diameter, to 70% of its yield strength is pretty high. An M24 bolt in 5.8 material needs 525 Nm, for example. In many cases the recommended figure for keel bolts is one that will not crush the GRP and is a lot less than it would be if it was in a machine. In that case I doubt if the 'lubricant' would make a significant difference.

 

[Deleted member 36384]

Most lubricants should have a friction coefficient stated somewhere for this use. In many cases it is actually 1.0 which means that there is no change to the actual torque value to be applied, I have found these lubricants to be thick pastes, the ones with 1.0. Next most common that I have come across is 0.9 which is where you get your 10% from of course. If in doubt check with the manufacturer of the product as they probably have the data even if they do not print it on the label. My experience is from the oil and gas industry from critical flanging operations.

So, if the torque applied is say 220, the actual will be 10% more. But is it so?. ... Comments welcome.

No, it is not so, the torque applied is what the machine says it is, there is no hidden torque value just because of some lubricant. If the lubricant is designed for high pressures then the friction coefficient remains the same so you cannot assume that at high pressures the friction coefficient changes, but it could if it is all squeezed out or not designed for the pressures. I think what you are trying to say, I apologise if I am wrong here, is that if the torque arrives sooner, then the tension in the bolt will be less (imagine a burr on the thread which causes high torque before the nut is fully down). In some applications torque v turn analysis (JAM - joint Analysed Make Up) is used to measure the number of turns required to reach a certain torque value and rate of change of torque. Again this is an indirect measurement of tension based on the expected mechanical response of known materials.

vyv_cox's comments are really relevant though. I assume the torque value that you are using is the one specified by the manufacturer / designer of the yacht and not taken from a general table of values for your bolt material. If not, you should try and obtain them from the designer, owners association.

 

[danielefua]

No, it is not so, the torque applied is what the machine says it is, there is no hidden torque value just because of some lubricant. If the lubricant is designed for high pressures then the friction coefficient remains the same so you cannot assume that at high pressures the friction coefficient changes, but it could if it is all squeezed out or not designed for the pressures. I think what you are trying to say, I apologise if I am wrong here, is that if the torque arrives sooner, then the tension in the bolt will be less (imagine a burr on the thread which causes high torque before the nut is fully down). In some applications torque v turn analysis (JAM - joint Analysed Make Up) is used to measure the number of turns required to reach a certain torque value and rate of change of torque. Again this is an indirect measurement of tension based on the expected mechanical response of known materials.

I am interested in the topic and your long explanation which I understand but I am puzzled. As far as I know, the quantity of mechanical interest is indeed the longitudinal stress along the bolt; the torque applied to the fastening nut is just an indirect measurement which involves the assumption of several other parameters, and the first I can think of are the pitch angle and, possibly, the static and/or dynamic friction coefficient between the two metals. Only after knowing these parameters one can state a functional relation between torque and longitudinal stress. Is it so? If yes, the original question of the OP, with slighty more precise technical phrasing, could be indeed quite relevant. I would be very glad to learn more.

Daniel

 

[Cafran]

Time tested system used to be, up tight then a quarter turn, never lost a keel yet!!

 

[pelissima]

The bolts in my case are M20 8-8 quality steel galvanised, and the thread was triggered only out of curiosity.
The recommendation from a Beneteau USA document is for 8-8 quality steel bolts when used in cast iron keels and S/S bolts for lead keels. If S/S bolts are used in cast iron keels they recommend a yearly check! Also, the document advises to add sealant to bolt heads and washers but NOT in the threads as I did myself before reading this document. As to the torques the recommendation is min 13 m.kg (127 N.m) which corresponds to 1/3 of elastic limit and max 27 m.kg (265 N.m) where it reaches 2/3 of its elastic limit. I stopped at 220 N.m but let’s say I had used the max value that of 265 N.m. Now my original question –of little practical value of course- is as follows; by adding sealant in the threads, am I at the 2/3 of elastic limit, or higher? Or lower?

 

[danielefua]

The bolts in my case are M20 8-8 quality steel galvanised, and the thread was triggered only out of curiosity.
The recommendation from a Beneteau USA document is for 8-8 quality steel bolts when used in cast iron keels and S/S bolts for lead keels. If S/S bolts are used in cast iron keels they recommend a yearly check! Also, the document advises to add sealant to bolt heads and washers but NOT in the threads as I did myself before reading this document. As to the torques the recommendation is min 13 m.kg (127 N.m) which corresponds to 1/3 of elastic limit and max 27 m.kg (265 N.m) where it reaches 2/3 of its elastic limit. I stopped at 220 N.m but let’s say I had used the max value that of 265 N.m. Now my original question –of little practical value of course- is as follows; by adding sealant in the threads, am I at the 2/3 of elastic limit, or higher? Or lower?

According to my understanding of the technicalilties, if the sealant in the treads lowers the friction coefficient between bolt and nut - very likely, I guess -, you set the longitudinal stress on the bolt to a value higher than intended, i.e. closer to the elastic limit. If you are interested in knowing by how much, I am afraid I have no idea. Just be optimistic and assume that, stopping at 220 Nm, you did not exceed the 2/3 limit!

Daniel

 

[gordmac]

If you are interested in fasteners and torquing them worth a look around this site: http://arp-bolts.com/