shearing bolts 2

[PH163RD]

Hi all,
we are curently using M16 X 30 UNC Mildsteel bolts which continue to shear due to a bending moment on the beam and clevis bracket they are connecting.

has anyone got any ideas on bolt upgrades to a more ductile material? and any other helpful information?

regards

 

[rb1957]

doubt you will solve your problem with more ductile material, mild steel is pretty ductile already.

it may be blindingly obvious, but i suspect you need bigger or stronger bolts.

good luck

 

[zekeman]

Why don't you give us a hint as to the configuration and loads you are talking so that we don't have to guess what your shear bolt requirements are.

 

[zcp]

...or more bolts or a larger pattern.

ZCP

http://www.phoenix-engineer.com

 

[CoryPad]

...or the actual characteristics of your fasteners, as M16 and UNC are contradictory...

Regards,

Cory

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

 

[Cockroach]

I completely agree with RB1957, you have a loading issue contributing to failure, not a metallurgical one.

Maybe go with a weld? Are the bolts serving a purpose other than the typical assembly issue?

Good point taken by CoryPad. You're mixing metric and imperial nomeclature.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[PH163RD]

thanks for the feedback.
structurally we are increasing the distance between the bolts to reduce the force on the bolts. although i was curious to whether there were an obvious bolt upgrade i may have missed besides a larger/stronger bolt.

 

[sreid]

Possibly industrial bolts from Unbrako.

http://www.unbrako.com/

 

[rb1957]

by "increasing the distance between the bolts" do you mean that you're increasing the PCD ...

if you've got room for this, i guess you've got room to add more bolts or bigger bolts. if you don't have a budget you could use Inconel bolts.

 

[diamondjim]

Do you mean M16 x 30mm long bolts?
Increasing the space between centers
will increase the load on the bolts
unless as rd1957 points out that you
are increasing the pitch line or
moment radius of the bolts.

 

[PH163RD]

yes M16 x 30mm long bolts we are slightly upgrading to a M18.

we will increase the moment radius through increasing the spacing by moving the upper bolts higher above the area where the stress in concentrated.

 

[berkshire]

PH163RD (Mechanical)
If you are using metric bolts, are they grade 8.8 ? If so you might look at using grade 10.8
B.E.

 

[rb1957]

can you inter-pitch bolts on a smaller PCD ?

 

[dozer]

PH163RD,
Did you actually design this connection? By this I mean determine the maximum force in a bolt and choose a size and material that will carry this force with an adequate safety factor? If so and you're breaking bolts one of three things is happening.

1) You made a mistake in your design.

2) The bolts really aren't as strong as the manufacturer is claiming.

3) You've got a larger load on the connection than you think, like in a case where someone has told you the scoreboard your supporting weighs 5000 lbs but it really weighs 10,000 lbs! (The numbers are made up, but this really happened. I remember reading it in ENR several years ago.)

Point is, you really need to figure out what is going on, not just stuff the holes with stronger bolts until it works. If this is a human safety application (a building, the drive shaft on your car) then this is even more true.

By the way, it's grade 10.9 not 10.8. There is also a grade (technically class) 12.9. I don't know how available it is.

When you say "mildsteel" (sic) that suggest to me that you really don't know what you have. I don't know much about metric standards but you need to be ordering bolts that conform to some minimum strength requirements and have documentation (material certs) to prove it. Not something off the shelf at Ma and Pa's local hardware store.

Good luck

 

[ERT]

Hi all,

dozer has several good points here.

To add to this, it is important to do an evaluation of the entire joint. If your bolt sizing is marginal, you must consider the stiffness of each component in order to determine the load in each bolt. To do this you must consider the bolt preload and understand what fraction of the external load increases the bolt load, and the remainder will relieve the preload at the mating faces. This procedure will also allow you to determine if you have enough preload to ensure the joint will not separate. It is essential that joint separation be avoided - especially if your joint experiences shear load. If you maintain good joint clamping, this friction will help carry the shear load instead of shearing the bolts.

Any good machine design textbook will have a section on bolted joint calculations.

Regards,
ERT

http://www.akeng.com

 

[PH163RD]

thank you all for the feedback. we are currently using grade 8.8 bolts but will upgrade to a 10.9/12.9.

the problem in the design is no specific calculation was made for the bolt loadings, it was simply stated that M16X30 bolts are to be used. we carry these bolts in stock in the stores although it is my impression the fitters MAY simply use a M16X30 bolt which is not of an acceotable grade.