Structural Fasteners – Special Purpose Fasteners – Lockbolt Fastening Systems

in Aircraft Metal Structural Repair

Lockbolt Fastening Systems

Also pioneered in the 1940s, the lockbolt is a two-piece fastener that combines the features of a high strength bolt and a rivet with advantages over each. [Figure 4-106] In general, a lockbolt is a nonexpanding fastener that has either a collar swaged into annular locking groves on the pin shank or a type of threaded collar to lock it in place. Available with either countersunk or protruding heads, lockbolts are permanent type fasteners assemblies and consist of a pin and a collar.

Figure 4-106. Lockbolts.

Figure 4-106. Lockbolts.

A lockbolt is similar to an ordinary rivet in that the locking collar, or nut, is weak in tension and it is difficult to remove once installed. Some of the lockbolts are similar to blind rivets and can be completely installed from one side. Others are fed into the workpiece with the manufactured head on the far side. The installation is completed on the near side with a gun similar to blind rivet gun. The lockbolt is easier and more quickly installed than the conventional rivet or bolt and eliminates the use of lockwashers, cotter pins, and special nuts. The lockbolt is generally used in wing splice fittings, landing gear fittings, fuel cell fittings, longerons, beams, skin splice plates, and other major structural attachment.

Often called huckbolts, lockbolts are manufactured by companies such as Cherry® Aerospace (Cherry® Lockbolt), Alcoa Fastening Systems (Hucktite® Lockbolt System), and SPS Technologies. Used primarily for heavily stressed structures that require higher shear and clamp-up values than can be obtained with rivets, the lockbolt and Hi-lok® are often used for similar applications. Lockbolts are made in various head styles, alloys, and finishes.

The lockbolt requires a pneumatic hammer or pull gun for installation. Lockbolts have their own grip gauge and an installation tool is required for their installation. [Figure 4-107] When installed, the lockbolt is rigidly and permanently locked in place. Three types of lockbolts are commonly used: pull-type, stump-type, and blind-type.

Figure 4-107. Lockbolt grip gauge.

Figure 4-107. Lockbolt grip gauge.

The pull-type lockbolt is mainly used in aircraft and primary and secondary structure. It is installed very rapidly and has approximately one-half the weight of equivalent AN steel bolts and nuts. A special pneumatic pull gun is required for installation of this type lockbolt, which can be performed by one operator since buckling is not required.

The stump-type lockbolt, although not having the extended stem with pull grooves, is a companion fastener to the pulltype lockbolt. It is used primarily where clearance does not permit effective installation of the pull-type lockbolt. It is driven with a standard pneumatic riveting hammer, with a hammer set attached for swaging the collar into the pin locking grooves, and a bucking bar.

The blind-type lockbolt comes as a complete unit or assembly and has exceptional strength and sheet pull-together characteristics. Blind-type lockbolts are used where only one side of the work is accessible and generally where it is difficult to drive a conventional rivet. This type lockbolt is installed in a manner similar to the pull-type lockbolt.

The pins of pull- and stump-type lockbolts are made of heat-treated alloy steel or high-strength aluminum alloy. Companion collars are made of aluminum alloy or mild steel. The blind-type lockbolt consists of a heat-treated alloy steel pin, blind sleeve, filler sleeve, mild steel collar, and carbon steel washer.

These fasteners are used in shear and tension applications. The pull type is more common and can be installed by one person. The stump type requires a two person installation. An assembly tool is used to swage the collar onto the serrated grooves in the pin and break the stem flush to the top of the collar.

The easiest way to differentiate between tension and shear pins is the number of locking groves. Tension pins normally have four locking grooves and shear pins have two locking grooves. The installation tooling preloads the pin while swaging the collar. The surplus end of the pin, called the pintail, is then fractured.

Installation Procedure

Installation of lockbolts involves proper drilling. The hole preparation for a lockbolt is similar to hole preparation for a Hi-Lok®. An interference fit is typically used for aluminum and a clearance fit is used for steel, titanium, and composite materials. [Figure 4-108]

Figure 4-108. Lockbolt installation procedure.

Figure 4-108. Lockbolt installation procedure.

Lockbolt Inspection

After installation, a lockbolt needs to be inspected to determine if installation is satisfactory. [Figure 4-109] Inspect the lockbolt as follows:

  1. The head must be firmly seated.
  2. The collar must be tight against the material and have the proper shape and size.
  3. Pin protrusion must be within limits.
Figure 4-109. Lockbolt inspection.

Figure 4-109. Lockbolt inspection.

Lockbolt Removal

The best way to remove a lockbolt is to remove the collar and drive out the pin. The collar can be removed with a special collar cutter attached to a drill motor that mills off the collar without damaging the skin. If this is not possible, a collar splitter or small chisel can be used. Use a backup block on the opposite side to prevent elongation of the hole.

The Eddie-Bolt® 2 Pin Fastening System

The Eddie-Bolt® 2 looks similar to the Hi-Lok®, but has five flutes, equally spaced along a portion of the pin thread area. A companion threaded collar deforms into the flutes at a predetermined torque and locks the collar in place. The collar can be unscrewed using special tooling. This fastening system can be used in either clearance or interference-fit holes.