Nuts & Bolts

Building Basics

HAVE YOU E V E R T R I E D TO PULL ',apart two pieces of stuck-to- ^ - : igether duct tape? Tug as you • •may on the ends of the tape,the adhesive seems to stick toitself with a tenacity thatwould stymie a tractor pull.But stick it on a surface andpull vertically and the tape comesoff with comparatively little effort.What you've witnessed is the differ-ence between two stress forces beingapplied to the tape's adhesive: Shear,trying to separate the two pieces bypulling on the ends, and tension,the motion of pulling the piece upfrom the table.

Rivets resist these same forces inways useful to those who want to

RivetsKeeping it all together

H.G. FRAUTSCHY

build structures that will be lightenough to fly but strong enough totolerate the substantial loads im-posed by the forces of flight. A rivetacts as a clamp that holds two ormore pieces of material together, butits main job is to resist shear forces.A properly installed rivet will resisttension to a lesser degree, but its pri-mary job is to transmit loads alongthe piece of material, not at a major

angle away from it.Rivets have been in use since

aviation's earliest days. Untilwelded structures became morecommon, riveting a structure orcomponent was the preferredmethod. When the temperand/or dimensions of an alu-

minum or steel sheet must bemaintained, riveting is still the mostcommon method used to join thevarious components.

Rivet TypesThe two rivets we see most often

are the round universal head MS20470 and the 100-degree counter-sunk MS20426. They are identical tothe older AN426 and AN470 rivets

MOSTEFFECTIVE

INSHEAR

»

-6SDMIN.1.5 D MINI. -

-CLECO-HOLDSMATERIALTEMPORARILY

RIVET GON ANDBOOKING BAR

PNEUMATIC ORHYDRAULICBUNDRIVETTOOL

COUNTERSINKCOTTER

DIMPLE

ILLUSTRATION BY TOM CHIPLEY

82 JUNE 2000

that became the standards duringWorld War II. Before the war, manu-facturers used a bewildering array ofrivets, including the round head(AN430), with a higher-diameterhead than the newer universal head;the brazier head (AN455 andAN456), with a flatter and widerhead; and the flat head (AN442),among others. Most of these typeshave gone by the wayside, but youcan still find them at times, particu-larly when someone is restoring anolder airplane and wishes to main-tain the same look.

And there are "blind rivets," sonamed because they can be installedin places where it would be impossi-ble to get a bucking bar to form thebucked head. (Some people callthem "pop rivets," after the soundthey make when the stem separates.)

A blind rivet is installed using ahand-operated puller or a pneu-matic gun that pulls the rivet's stem.After the rivet is set, the pullingforce separates the stem from therivet at a manufactured weak pointin the stem. The remainder of thestem then stays in the rivet (part ofthe rivet's strength comes from thestem), sealing the rivet. Blind rivetscome in both round head and coun-tersunk versions.

There are other high-strength"rivets" used in airframe building,including the Huck and Hi-Shearrivets, where a collar is installed on amanufactured head and shank usingspecial tools. These close-tolerancerivets are sometimes seen in built-upspars and other highly stressed com-ponents. The line between rivetsand high-strength, close-tolerancefasteners can seem a bit blurred.Think of it this way—if you have todestroy part of the fastener to re-move it, it's a rivet!

Making It StrongAs written back in the late 1930s

edition of Brimm and Boggess' Air-craft Maintenance for the AirplaneMechanic, "The strength of a riveted

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Building Basics

joint depends upon six factors: (1)The diameter of the rivets; (2) thenumber of rivets; (3) the strength, inshear and bearing, of the materialfrom which the rivet is made; (4)the strength in bearing of the mate-rial being joined; (5) the thickness ofthe material being joined; (6) thenumber of planes along which shearis resisted. This list does not includesuch items as improperly headedrivets, holes too large for rivets, etc.,but assumes that the workmanshipis satisfactory and that the diameterof the hole is not more than 5 per-cent greater than the diameter ofthe rivet."

The bearing strength is theamount of force applied to an in-stalled rivet that causes the rivet toelongate the rivet hole. It can "bear"so much load before the rivet holesdeform and the joint fails. Well-de-signed joints will have a bearingstrength close to the shear strengthof the rivets included in the joint.

Another way to increase the shearstrength of a riveted joint in thinnersheet materials is to dimple the ma-terial and use countersunk rivets.Reducing overall drag is the primaryreason for using countersunk rivets,and an added benefit is a strongerjoint—by dimpling the hole, youadd the cross section of the materialto the overall strength of the union.In thicker sheets, the hole is drilledand countersunk using a specialtool. The "microstop countersink"allows the operator to accurately setthe depth of the countersink, soeach hole is consistently bored.

gWorkmanship

Like many building basics, rivet-ing is an acquired skill—one you'llgain with practice. Good workman-ship includes drilling accurate holesin the material to be joined and de-burring each hole so the rivet canrest directly on the material.

There are a few different methodsfor setting a solid rivet. Using a ham-mer and bucking bar is the simplest

method, and it requires a surprisingamount of practice to do well. An-other tool often used is a handheldor bench-mounted rivet squeezer. Asthe name implies, the tool squeezeseach rivet a set amount to upset theshank and set the rivet.

The most common method is us-ing a pneumatic rivet gun with arivet set and a bucking bar. The rivetset (the part that goes in the rivetgun) for a round head rivet set isconcave to ensure that the drivingforce is applied to the rivet's center.For a countersunk rivet the set has aslight convex curve to it, again toensure that riveting force is appliedto the rivet and not to the surround-ing material. A bucking bar (usedwith hearing protection, please)held against the rivet's shank upsetsit and forms the other side of therivet's "clamp."

Riveting is a skill easily learnedand one mastered with practice. Toavoid ruining parts of your air-plane, it's good to practice on scrapmaterial before you start makingparts. With this skill mastered youcan create a beautiful aircraft froma pile of sheet aluminum and a cou-ple of pounds of rivets. Don't beintimidated by the concept—youcan do it! •

ResourcesThe Homebuilders Headquar-

ters section in the MembersOnly area of the EAA webslte atwww.eaa.org has a number ofonline articles on metal aircraftconstruction.

Aircraft Inspection and Re-pair, AC 43.13-1B—F00191:$18.95

Aircraft Sheet Metal—F37118: $17.95

Both these books are avail-able from EAA through itswebsite at www.eaa.org or bycalling 800/843-3612. Pricesdo not include shipping.