Conrod Bolts 2

[swall]

I'm refering to the type of connecting rod bolts that have a reduced section in the shank (i.e. less than the thread minor diamer). Is there a special name for these type of bolts?

 

[unclesyd]

Other than reduced section bolt, as per print, the rod bolt both large and small are them main applications I've seen this type bolt used.

The reduced section fastener is also use to make a bolted joint that is more flexible as with temperature extremes. I've heard this type installation called a flexible connection.

The aero people may have a name for this type fastener.

 

[TVP]

ISO standards such as ISO 4015 refer to them as Reduced Shank.

 

[swall]

"Reduced shank". I guess that's simple enough.

 

[Screwman]

These parts are very carefully designed for the loadings expected for the engine in question. The location and size of the reduced areas are worked out to give the desired balance of clamp and stretch in the fastener, while still providing location for the bearing caps. The radii and surface finish on the reduced sections are critical to preventing fatigue problems.

Dick

 

[unclesyd]

Adding to Screwman's
Other than OEM or replacement parts called out by a part number all the orders I've seen for this type fastener are called out by print.
Every great once a while a order will say by example, the requestor is asked for a print or it is a no quote.

 

[yates]

The main idea of reduced shank bolts is to try to achieve the same tensile rupture load in the shank as in the thread. This produces a lighter bolt, which is extremely important in con-rod bolts since it reduces reciprocating mass and therefore allows a higher maximum engine rotation speed. However the radius down to the reduced portion is extremely sensitive to fatigue failure, and like screwman pointed out, finish is all important, Shot peening or fillet rolling will also help.

 

[swall]

yates--In the reduced shank bolts I am looking, the stress area in the shank is a little less than the thread stress area. If the object is to achieve equal rupture loads in the shank vs thread, am I correct in assuming that one must also take into account the effect of the stress concentration in the first unengaged thead?

 

[diamondjim]

I think the intent is to have the
shank break before the threads strip
which would be really bad news.

 

[yates]

Right on the ball diamondjim. Although any bolt designer worth his salt should design the thread engagement length long enough to ensure that tensile rupture occurs at a lower load than thread strip (i.e. shear of the thread teeth)
Swall - when you say the shank stress area is smaller than the thread stress area, how are you calculating thread stress area, there are several methods for this depending upon the predominant failure mode. 'Stress areas' are not always the same when condsidering hi-temp and low-temp load situations, partly because the residual benefit of cold work in a rolled thread (and all con-rod bolts will have rolled threads) is different in the two cases. What in fact is done for con-rod bolts is actual tensile tests on the bolt, that way the user is sure of designing either the shank or the thread slightly stronger, as he wishes.

 

[swall]

I was using the values published per SAE J1199 (metric), except for 10mmx1.0, which I got from the Bosch Automotive Handbook, which reference DIN 13.Both you and diamondjim have shed a lot of light on this.