[Michael Burbidge]

I was installing the counter-weights on my elevators today and I have to counter-sink the bolt hole a little more so that the bolt head would not protrude and bind on the stabilizer. Countersinking the head a little more made it so that there was not enough thread left to fully tighten the nut. The plans called for 1 washer under the nut. Can I put 2 washers under the nut, or do I have to get a different bolt?

 

[AntiGravity]

No problem, as long as...

...there are still enough threads engaged through the nut. Can't remember the exact number and I'm sure someone here will chime in, but I think it is two threads exposed (though I read just now in another post from Gil the new specs state one thread).

 

[Robert M]

...there are still enough threads engaged through the nut.

If I understand your question correctly, you are having to use an extra washer or two to keep the nut from bottoming out on the non-threaded part on the bolt shank. This is very acceptable provided there are enough threads showing after the nut, especially since this is not a "structural" application of the bolt, nut or washers. Van's uses more than one washer in a few places in the fuselage (that I know of) and I'm jus getting started in the Fuse (RV-9). They even mention it in the instructions to add extra washers if needed.

 

[flion]

IIRC, it is acceptable to use up to three washers on a bolt. I've never had to use more than two. When I do, I try to put one washer under the bolt head and the other under the nut. Another fun washer trick: there's usually a 'flat' side and a 'rounded' side (though the latest washers I'm getting from ACS are manufactured differently and both sides look flat). The flat side goes toward the parts you are bolting together. I got that from the folk at Superior though I don't know how critical it is. In any case, it's not hard to put into practice.

 

[pierre smith]

It's normal.....

......to see most washers with a rounded inside edge and a sharper opposite edge, since they're stamped out by the thousands. The sharper edge can cut into the underside of the bolt head, causing a stress riser. This is the reason you should put the rounded edge under the bolt head.

Regards,

 

[Mel]

It is acceptable practice to use up to 3 washers. If you need more than 3, you should go to the next size bolt.

 

[az_gila]

Washer direction....

......to see most washers with a rounded inside edge and a sharper opposite edge, since they're stamped out by the thousands. The sharper edge can cut into the underside of the bolt head, causing a stress riser. This is the reason you should put the rounded edge under the bolt head.

Regards,

Pierre,

...but if you do this, doesn't the sharp side of the washer then bite into the airframe or engine part?

I would much rather a replaceable bolt head get marked with a permanent "ring indent" than my airframe of engine parts....

A circular stress riser at the periphery of a bolt head would probably be less harmful than one in your structural part. Worse case is that the hex flats of the bolt head would fall off....

Also, in most of our cases, we have steel washers and alumuminum parts, the aluminum will get a much deeper "ring indent" than the steel bolt head.

 

[Mel]

I have to go along with Gil. I always place the "rounded" edge against the airframe/part.

 

[jetblackaircraft]

Washer stress risers

When putting the sharp side of the washer against the bolt head there is only one sharp edge contacting the bolt head. All of the bolt tension is applied to this one edge. When putting the sharp side of the washer against the part to be fastened there are two edges contacting the part, one near the hole and one at the OD of the washer. Also, the washer will deform slightly upon tightening the bolt and the load will be distributed across the whole washer face.

In all honesty it probably doesn't matter. The number of load cycles that most homebuilts go through is small and Van's designs enough factor of safety in so that the fasteners and parts fatigue slowly.

But, I always place the rounded side of the washer to the bolt head. If you are really all that concerned, the arguement could be made that both ways are bad and you should deburr your washers.

(added) Also, if the clearance between the washer ID and the bolt shaft is not enough the sharp edge of the washer might contact the small radius where the bolt shaft meets the head. This would be bad. AN fasteners usually don't have this issue. At least the ones that I buy.

 

[noelf]

In my Aviation Mechanic Handbook (ASA-M-HB2), page 180, section on Bolt Installation, talks about our AN bolts having "a round, smooth, washer-like bearing surface just below the head"...and it continues with the reason why.

Further on..."always place a washer under the nut to provide a good bearing surface and prevent damage to the component as the nut is tightened."

It seems that our task is to use the fasteners in a manner that prevents damaging the underlying surfaces. If washers are used, use them in a manner that does not damage the joined materials.