Trying to simulate a clip critical connection with a bearing condition

[DRW75]

Hey folks,

Is it legitimate to 'undersize' bolt holes (not actually undersize, but match drill the connections for a snug fit) so that a bolted connection resists forces like bearing, but since it is somewhat snug, it moves a relatively small amount - hence behaving more like a slip critical connection??

I know all the bolts won't be perfectly snug, but certainly our effort would be to minimize the slop in the connection bolt holes.

We're basically trying to do this to minimize the number of bolts required to get something close to a slip critical connection as the proper number of bolts will cause clash problem with other items.

Thanks for any thoughts.

DRW

 

[dik]

It's possible, but depending on erection, it may be quite difficult in not having any or minimal tolerance. For bolted connections where a similar to slip critical connection is required, I've welded square washers with reduced diameter holes to provide a non-slip joint and designed the bolts accordingly.

First thing to evaluate is if you need a slip critical connection in the first place.

Dik

 

[DRW75]

The job we are looking at now requires cutting out existing steel beams (plus providing temp shoring), and reinstating the steel with a different connection which will facilitate new equipment. The amount of beam left to connect to will be fairly limited, and hence putting in a lot of bolts will not be possible. Since this is an existing structure, any new deflection will be carried-on to other equipment which is already resting on this steel.

and no... welding is not an option...

so slip is 'somewhat' critical ... lol. there will be some obviously, but it will be preferably minimized.

I wouldn't have thought that the fit-up would be excessively bad by using match-drilling (instead of predrilling and oversizing)

DRW

 

[dik]

Can you provide temporary bolting and use welding for fastening the connection? or a temporary connection that can be used to pry against to bring tighter toleranced holes into alignment? Have you contacted a steel fabricator to determine what sort of tolerance he can live with.

Dik

 

[DRW75]

The location in which the work is being done is a hydrocarbon rich environment, hence any flame hotwork (welding) will not be allowed (existing steel will be cut with a reciprocating saw).

The new splice steel will be located up against cut steel, and splice plates will be through-matchdrilled. Nominal diameter holes will be drilled for the bolts (i.e. 1" hole for a 1" bolt), and each of the bolts will be 'encouraged' to fit.

We are an engineering and construction company, so we will have our personnel for the fit-up. The fabrication will simply be making the splice steel without pre-drilled bolt holes. The existing in-situ steel will obviously not have bolt holes, and hence we will already have to drill on site - thus it won't be a lot more work to through drill instead of pre-drilling.

DRW

 

[HgTX]

Just a stupid clarification question--you can't do a real slip-critical connection because you can't get enough bolts in?

Hg

Eng-Tips policies: faq731-376

 

[DRW75]

Thats right... theres simply not enough boltable steel left after the cut... there is just enough for a pretty clean (and compact) bearing connection however.

 

[dik]

There are also structural epoxies for 'filling' metal; I don't know what their properties are and haven't used the material.

Dik

 

[mog69]

If you pre-drill the bolting pattern to nominal bolt size, I think accumulation of errors in positional tolerance would give you real difficulty in "encouraging" the bolts in without damaging the threads. The first bolt in will fix the relative positions on the new and old steel, and the other holes will invariably have some degree of misalignment. If you were to pilot drill the existing structure and new steelwork perhaps 30 thou undersize, would you have room to get a reamer in and ream through both pieces in situ to give true alignment? The fasteners themselves would perhaps be better if made as shoulder or barrel bolts, ie accurately sized barrel diameter of length sufficient to pass through bolted thicknesses and just bigger than nominal thread - say 26 mm barrel, M24 thread. This reduces the chance of thread damage on fit and gives close control of sideways movement of joint. The Bolts could also be cooled (dry ice) to shrink the diameter and increase clearance whilst fitting. We use this and also liquid nitrogen but you need to be careful about material selection if you go with nitrogen.

 

[DRW75]

I just realized that i miss-spelt the topic as 'clip critical'... lol.

dik - I'm not sure how a filler type material would work, but an epoxy would be conceivable - but without any experience with this material/adhesive, the client (and myself) would be pretty shy about using it in this application as the consequences of it failing would be disastrous.

mog69 - I was thinking that the match drilling would occur one bolt at a time. as the bolt hole is drilled, the bolt would but put in place and tightened, and then move onto drilling the next hole. The notion of providing undersized holes, and consequently reaming out to match for alignment is intriguing as the reaming process would be quicker than full diameter through bolting...

I never heard of barrel bolts pre se - but i guess you are just suggesting using a machined bolt with a died (sp?) smaller thread diameter.

The use of liquid nitrogen would likely be out of the question, and even dry ice may not be too popular for this site...

Thanks for the ideas!

DRW

 

[DRW75]

Dik - Ahh... you're talking about injection bolts then? to fill the slop of the hole.... hmmm... looking into this...

 

[mog69]

You're right, a barrel bolt is just a term for a bolt with the parallel unthreaded portion between the head and the beginning of the thread turned to a close tolerance , then stepped down to a smaller thread diameter. An alternative would be Supergrip Bolts from SKF, so long as the holes are pretty accurate to size and are aligned. These bolts are made in two pieces and are designed to insert easily with a clearance fit at ambient temp, then expand when being tightened until they fit the hole (achieved by drawing the two halves together onto opposing tapers). More details on the SKF website. And no health and safety nightmare with dry ice etc either!

 

[dik]

or a steel sleeve to suit the bolt difference?

Dik