| Special Purpose Fasteners Where extra strength is required and weight is a factor, special purpose fasteners are used instead of rivets or nuts and bolts. Some of these areas are: where major structural units join together, where there is high stress, and where the substructures attach to the skin. We will discuss each fastener and its use in the following paragraphs. Rivnuts. Rivnuts, shown in Figure 1-13, are tubular rivets internally threaded and counterbored and used with matching screws. They are applied blind, and they are used where nut plates cannot be installed. An example of such a location is the leading edge on wings where deicing boots are attached. Rivnuts are made in two head styles: flat and countersunk heads with open or closed ends. The keyed rivnut is used as a nutplate, and rivnuts without keys are used for blind riveting where torque loads are not imposed. Closed-end rivnuts are used when a sealed installation is required. The installation of a rivnut is not complete unless it is plugged, either with one of the plugs designed for that purpose or with a screw. A rivnut does not develop its full strength when left hollow. 
Figure 1-13. Countersunk and Flat Head Rivnuts.
Hi-Shear Rivets. The Hi-Shear rivet, essentially a threadless bolt, consists of two parts, the pin and the collar as shown in Figure 1-14. These rivets are classified as special rivets, but are not of the blind type. Access to both sides of the material is required to install the rivet. Hi-Shear rivets have the same shear strength as bolts of equal diameters and have about 40 percent of the weight of a bolt. In addition, they require only about one-fifth as much time to install as a bolt, washer, and nut combination. They are approximately three times as strong as solid shank rivets. The pin is headed at one end and is grooved about the circumference at the other. The collar is swaged onto the grooved end to make a firm, tight fit. Hi-Shear rivets are made from a variety of materials and are used only in shear applications. In addition, they must not be used where the grip length is less than the shank diameter. 
Figure 1-14. Hi-Shear Pin
General Purpose Bolts. The mechanics and appearance of general purpose bolts and their accompanying washer and bolt combinations are not discussed here. They are described in detail in FM 1-563 (Fundamentals of Airframe Maintenance). The discussion here is limited to less familiar special-purpose bolts in general use in Army aircraft. These are Huck lock bolts and Jo-Bolts. Each is described in the paragraphs that follow. Huck Lock Bolt. The Huck lock bolt combines the features of a high strength bolt and a rivet, with advantages over each. The Huck lock bolt is generally used in wing splices, landing gear, fuel cell fittings, longerons, beams, skin splice plates, and other major structural attachments. It is more easily and quickly installed than the conventional rivet or bolt, and it does not require lock washers, cotter pins, and special nuts. Like the rivet, the lock bolt is installed with a pneumatic or pull gun. The most commonly used Huck lock bolts are the pull, stump, and blind types, shown in Figure 1-15. Common features of the three are the annular (circular or ringed) locking grooves on the pin and the locking collar. Each one is discussed in the following paragraphs. Pull-type. The pull-type lock bolt is mainly used in primary and secondary structural members. It is installed rapidly and has approximately one-half the weight of an equivalent general-purpose bolt and nut. These bolts are available with modified brazier, pan, and countersunk heads. A special pneumatic pull gun is required to install this lock bolt. The installation can be made by one man because bucking is not needed. Stump-type. The stump-type lock bolt, although not having the extended stem with pull grooves, is a companion fastener to the pull-type lock bolt. It is used primarily where clearance will not permit effective installation of the pull type. These bolts are also available with modified brazier, pan, and countersunk heads. The stump-type lock bolt installation is made with a standard pneumatic riveting gun, a hammer set for swaging the collar into the pin-locking grooves, and bucking bar. Blind-type. The blind-type lock bolt comes as a complete unit or assembly, and it has exceptional strength and sheet pull-together characteristics. These lock bolts are used where only one side of the work is accessible and generally where it is difficult to drive a conventional rivet. 
Figure 1-15. Huck Lock Bolts
Jo-Bolts. Jo-Bolts are high-strength structural blind fasteners. They are used in close-tolerance holes or where Jo-Bolts may be required for weight-saving advantages. In all cases, Jo-Bolts are always considered to be a part of the permanent structure and primarily subject to shear loads. The Jo-Bolts, installed as a unit, consist of a bolt, nut, and sleeve. These bolts, identified by head types, are discussed in the following paragraphs and illustrated in Figure 1-16. Flush-type. The flush-type Jo-Bolt has a head that fits flush with the surface being held. The bolt generally uses the same size countersink or dimple required for the correspondingly sized standard screwhead. The nut and bolt are made of alloy steel; however, the sleeve is made of annealed corrosion-resistant steel. All components for the flush-type Jo-Bolt are cadmium plated. In addition, a flush-type Jo-Bolt is available that is designed to fit into a countersunk or dimpled hole prepared for a precision rivet. Nut shank size and material specifications are the same as those just described. 
Figure 1-16. Jo-Bolt Head Styles
Hex-type. Jo-Bolts with hexagon-shaped heads (hex type) are made of an alloy steel. Their sleeves are annealed and corrosion resistant. In addition, the bolt and sleeve are cadmium plated. Millable hex-type. The millable hex-type Jo-Bolt usually takes the same size countersink or dimple used for the corresponding size rivet. The bolt is made of alloy steel and the sleeve of corrosion-resistant steel. Both are cadmium plated. The nut is made of an aluminum alloy, and, after installation, the nut head is milled flush. Oversize-type. The oversize-type Jo-Bolt is used in special applications where the hole size has been exceeded and standard Jo-Bolts cannot be used. The head size and material specifications are the same as for the flush and hex types except for the size of the nut shank diameter.
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