Balancing Pre-load

[Gymmeh]

I am currently working on a connection for between a tubular steel member and an I-beam The design is major over kill for the current application which uses four B7 -5/8Dia. alloy threaded rods to clamp and only needs to be rated for ½ Ton, however I am hoping if I design it correctly it could be rated for 2Ton or more.

Since the connection is square the situation easly simplifies to a threaded rod. Its 5/8 Dia. B7 Alloy ASTM A193. (I attached a CAD of the bolt) The Pre-Load would be 85 kpsi, (ASIC table J3.2 -making the 250lbs load negligible :-D).

Since there are three nuts, If you pre load one nut and then tighten the other nut to preload, the preload condition of the first nut will change.

I am wondering if there is a practical way to pre-load both nuts correctly? (IE: how would a worker do this?)


Thank you for your help.

 

[GregLocock]

Ideally you'd work around the nuts in sequence once they are snug, tightening each by half a turn each time. That's more or less how cylinder head bolts are tightened.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Gymmeh]

How do you know the accuracy of the pre-load?

I am not worried about it with 1000lb.
But Ideally I would like to use the same connection for 3ton.

Or would you still considered 3tons insignificant, compared to the approximately 85kpsi pre-load? ie: not worry about it to much.

 

[rb1957]

it sounds like you want to apply 3 tons tension onto 4 5/8" threaded rods (= bolts) ... about 1500 lbs per ... not much of a load. i think you can allow your preload to prevent gapping at this load. you can control the preload all sorts of ways, the simplest (and most insensitive) is torque control, where as greg writes, you torque each rod a proportion of the load increasing up to the required load and finally check that each rod has the same torque.

personnally, i think you're worrying too much (the distribution of preload between the 4 rods) about a small detail of the design.

 

[Gymmeh]

Thank you,

I assumed I was being over worried, but thought it would be best to get second opinion.
(there are no other engineers in my office)