BOLT TORQUE TIGHTENING

[Bubik]

Hi everyone!!

I have a question regarding tightening of bolts. We use galvanised M27 10.9 bolts for connections of belt conveyor truss in a cement factory.
The required torque for adequate fastening is to be 1650Nm according to the bolt's manufacture without any lubricant (slightly oiled) or 1250Nm with Molykote lubricant. The guys on site tightened the bolts to 1250Nm without any lubricant as it is too hard for them to achieve 1650Nm ( bad equipment they have).Now, they bought the Molykote lubricant (after multiple fights with the contractor) and want to take the bolts out ,lubricate them, re use them and achieve 1250 torque (as required with lubricated bolts). I know that preloaded bolts cannot be reused when plastic region is reached. as the bolt is 10.9 I did simple calculation from BS EN 1090-2 to check when the maximum elastic region in relation to torque tightening is,this would be about 1483Nm.I used 90% of 1000N/m^2,coefficient 0.2 (it is our requirment)and diemeter of bolt(27mm) and I got 1483Nm for maximum elastic treshhold. The equation is :Required torque = F x d x k. Can the bolts be reused then???

THANKs everyone!!

 

[Lomarandil]

Regardless of the bolt elasticity, I doubt your galvanizing would hold up to bolt removal and reuse. If that is important to your connection longevity, I think it may limit your options.

----
The name is a long story -- just call me Lo.

 

[rb1957]

installed as they are, without lub, the friction would be higher so the preload induced by the 1250 Nm torque would be less than if lub'd, and so the stress state lower. but how many bolts are we talking about ? why not install new unless the constructor can prove good (ie not yielded) ... very difficult to do.

Agree with Lo's comment about the galvanising ...I doubt it survives first installation.



another day in paradise, or is paradise one day closer ?

 

[desertfox]

Hi

Unless it costs a fortune to replace all the bolts I would use new ones, you can probably use the old ones however errors involved in torqueing bolts can be as high as +/- 40% in terms of the achieved preload, so its possible that some of the bolts could be permanently set.
We only reuse bolts here if they are torqued to less than 70% of maximum torque allowed and I notice that the figures you have posted are 1250 and 1483. 1250 is 84% of the 1483 so I would change them

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

 

[Bubik]

Thank you for your answers!!

Regarding the amount of bolts..it is very large. I am talking about a few transfer towers for belt conveyors.The contractor is about a dozen million dollars down so trying to save on anything.
It is a cement factory at the sea so protection of elements is incredibly important.

Regarding your post desertfox (Mechanical) the required torque is 1650Nm...sory for confusion. The 1483Nm i got when i pluged in 90% of Ultimate Tensile strength into equation. (bolts are 10.9)


By the way just to make sure..preloading of bolts is supposed to reach the yield point of material not beyond ???

 

[Ron]

Preloading of bolts should reach 70% of yield, not 100% of yield.

 

[rb1957]

sounds like a bad situation if the contractor is "trying to save on anything" 'cause they're in the hole.

I'd've thought the cost of new bolts was small compared to the time to remove them.

I think you can say with some certainty that the bolts have not been yielded, because of the higher friction, but why should you take that responsibility ? If the contractor wants to reuse the bolts he should prove they have been overloaded (lord knows how the'll do that).

another day in paradise, or is paradise one day closer ?

 

[desertfox]

Hi Peter

The most I have seen bolts preloaded too is about 90% of yield but this would be done with a hydraulic tensioner not a torque wrench. The other thing that's often overlooked is the compressive stress induced into the clamped members, it's pointless tightening bolts to 70% or 80% of the bolt yield stress if the compressive stress in the members exceeds their yield stress value because the extra tension in the bolt will be lost as the clamp members yield.

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

 

[Bubik]

Thank you all so much for your answers !!!

 

[klaus.]

Once a bolt is pre tensioned and loose again you are not allowed to use it again




best regards
Klaus