Appropriate use of washers in metallic joints

[Aerodesign]

We have a number of metallic-metallic joints clamping sections of spanwise sub-spar comonents together with spreader plates. Loads are predominently in plane with minimal pull-thru loads (ie in axial direction of the bolts). We do not usually use washers in these joints.

Is anyone aware of aerospace guidelines on the use of washers in shear and tension joints? What is common practice in the opoption of washers?

 

[sbi]

areo ? from industrial engines and casting I was taught that a flat washer has two sides, if you look at it you will note on hte out side , one edge is sharp and square teh other is rounded.
the rounded go toward the work peice the sharp square gos to the bolt head, lock washer or nut. this eleminates the risk of the washer diging in the the base metal and causing a flaw.

SBI
Central Ne.,USA

 

[WKTaylor]

aerodesign...

Common "round" washers are a fastener with many unique purposes, such as:

Isolate structure surface (surrounding hole) from the "nut-torque" scoring and dissimilar metals. Scoring does (2) bad things: score marks are essentially crack-starters; and scoring embeds a rough/bare surface under the nut or bolt-head, resulting is fastener loosening and bare-metal corrosion... even with self locking/coated nuts. Note: if bolt or screw head is torqued, then a washer can be even more important in this function.

Washers allow protrusion of the bolt shank thru the structure 100%, so that non-counterbored, light weight, low-height HS nuts can be installed without bottoming on threads. Varying washer stack-height allows for structural thickness variations with a slightly long bolt AND still have shank protrusion [no threads in bearing].

Increases bearing foot-print against the structure [especially where mini HS nuts are installed.]

Under head of tension-rate bolts with generous head-to shank fillets, chamfered ID washers allow bolt to sit hard/flat without having to deeply chamfer the lips of the mating hole. This increases shank bearing area in shear [break sharp edge of hole, VS chamfer for fillet-relief] and head bearing around hole.

Hi-lok collars come in (2) versions: as a plain collar and with an integral washer. Plain collars can be installed against "similar material" structure with little/no damage [IE: aluminum against aluminum, steel against steel, etc]. Steel and CRES collars with integral washers can be installed on soft aluminum without scoring damage and dissimilar metal isolation [usually platings or coatings].

Caution: misuse/excessive of washers can have "bad" results.

Washers must have similar hardness/strength [compression and bearing] relative to the nut or bolt-head. If a lot weaker, then the washer is in risk of being crushed and deformed... and will eventaully allow the joint to loosen in-service under cyclic loading.

Washers MUST be compatible with envrionmental conditions and with the mating parts [nut, bolt structure]. Significant dissimilarity without isolation can aggravate corrosion [bare CRES against aluminum]. Also temperature variations and chemical exposure must be accounted for.

Note: Too liberal a use of washers can add significant weight and cost without value.



Regards, Wil Taylor