LUG STRENGTH, SKEWED FASTENER

[StressMan2506]

Fellow stress engineers:

I am a concessions engineer faced with a fastener 2.2 deg off perpendicular. Because of geometrical constraints, the proposed solution is to fit spot-faced support washers on either face of the joint. Does anyone know of a method/approximation for assessing lug strength in such a case?

Louis

 

[SWComposites]

Is this a single pin lug, or a bolted joint?
Is the hole off perpendicular, or is the fastener tipped in the hole?
Any bushings in the hole?

 

[StressMan2506]

Thanks for responding, SW. The problem is that one hole of a bolt group was drilled off line. The subject hole is close to the radiused corner of one of the connected pieces, hence my wish to investigate lug strength.

 

[jetmaker]

Have you considered oversizing the suspect hole to straighten it? You could then install an oversized bushing, or oversized bolt, to compensate for the larger hole ID. Suggest checking the SRM for allowable rework limits

Otherwise, it is difficult to assess the impact of a single misdrilled hole. For the most part, if it is a single fastener in a sufficient quantity, I would assess the joint strength without the suspect fastener. If good, then I would not worry about it. Additionally, I believe misalignements of 2 deg. are allowed with Hi-Loks per spec. Since you are slightly over that number, I would use self-aligning washers to allow the bolt proper clamp-up.

From a fatigue issue, the misalignment will cause a slight decrease in fatigue capability.

jetmaker

 

[plasgears]

A skewed bolt combined with a soft thick washer [mild steel] can effectively take out some of the skew. A spherical washer set would be ideal for misalignment.

 

[StressMan2506]

Hi Jetmaker & Plasgears:

Oversizing the hole would result in a horrendously low edge distance and we have a skewed hole, rather than a skewed bolt in a perpendicular clearance fit hole...

Louis

 

[ieaz123]

Why not spotface the head side and use a self-aligning nut/washer combination on the nut side? The structural effect of the fastener misalignment would be considered negligible in my opinion.

The only strength effect would be due to the spotface.

 

[StressMan2506]

Hi jeaz123:

Thanks for your response, but the hole was drilled skew at short edge distance. It is clearance fit, so there is a bit of play in the hole, but I am interested in assessing the slightly elliptical hole at low edge distance.


Louis

 

[rb1957]

i'd've thought that an interference fit bush would have helped with the short eD that results from opening up the hole; tho' you have to watch the stresses in the ligament due to the interference.

short eD is critical for loading towards the edge, if the majority of the loading is parallel with the edge, it might be acceptable.

i'd worry more about the head of the fastener; being perpendicular with the fastener diameter, it's now not parallel with the outside face. there is a small amount of misalignment allowed with most fasteners, if you're in this range you probably don't have a prolem with the hole. if you're outside this, as well as thinking of the effect of the hole, i think you'll need tapered washers.

 

[StressMan2506]

Hi rb1957:

Thanks for your contribution. Our proposal is to fit spot-faced support washers which will allow good seating of nut and bolt-head, but I am still left with assessing the static strength and F & DT with skew and short-edge distance; hence my original question, namely does anyone know of methods/approximations for considering lug strength where the hole is skewed.

Louis

 

[rb1957]

i'm sorry, i sould have read your post better (particularly the first line) ... i'd have thought that a 2.2deg inclination was negligible. i'd've thought that most heads could absorb that, but there might be special applications (i'm thinking of large single pins, like on undercarriages) that might need a little thought. actually that makes me ask, is the fastener fixed (structural) or moving (like a pivot pin) ?

 

[der8110]

I am still having trouble visualizing the problem. Cosine of 2.2 degrees is 0.99926; so this must be an unusally thick stackup or e/D was initially non-existent.

Analytically, using a larger diameter encompassing the tangent points at the top and bottom surfaces will give you adequate conservatism both as a static lug (if it really is) and for damage tolerance. For damage tolerance, the input stress levels and stress intensity factor solutions are nowhere close to 0.99926 accuracy.

From a damage tolerance perspective, you are probably going backwards by spotfacing, unless it is well blended.

 

[jetmaker]

rb1957,

Your comment about the short eD is correct for ultimate loading, but not accurate for fatigue as a load parallel to the edge would present a big fatigue concern.

jetmaker

 

[StressMan2506]

Hi rb 1957 & der8110:

Thanks for your contributions. The subject fastener is one of a structural joint, not a pivot pin.

The hole was drilled at shorter-than-design edge distance as well as skewed and the proposed spot-faces are in additional support washers and therefore do not contribute to fatigue sensitivity in load-carrying components. I probably will treat the hole as perpendicular and of a diameter slightly larger than nominal to account for the skew.

Louis

 

[rb1957]

sounds reasonable to me

 

[RPstress]

In the past we have happily used the the total areas available for static lug strength calcs. This would also generally be done for fatigue. However, for any crack growth calcs the minimum edge distance would be used.

For two degrees we wouldn't have bothered to take the angle of the pin into account for the pin forces. (Nominally as the pin is canted over it will wind up taking a bit of tension as well as shear.)

Since this is part of a group and will be torqued up, there's usually a bit in hand.