Dill Lok-Skrus and Dill Lok-Rivets
Dill “Lok-Skru” and “Lok-Rivet” are trade names for internally threaded rivets. They are used for blind attachment of such accessories as fairings, fillets, access door covers, door and window frames, floor panels, and the like. Lok-Skrus and Lok-Rivets are similar to the Rivnut in appearance and application; however, they come in two parts and require more clearance on the blind side than the Rivnut to accommodate the barrel. [Figure 5-64]
The Lok-Rivet and the Lok-Skru are alike in construction, except the Lok-Skru is tapped internally for fastening an accessory by using an attaching screw, whereas the Lok-Rivet is not tapped and can be used only as a rivet. Since both Lok-Skrus and Lok-Rivets sions for the Lok-Skru also applies to the Lok-Rivet.
The main parts of a Lok-Skru are the barrel, the head, and an attachment screw. The barrel is made of aluminum alloy and comes in either closed or open ends. The head is either aluminum alloy or steel, and the attachment screw is made of steel. All of the steel parts are cadmium plated, and all of aluminum parts are anodized to resist corrosion. When installed, the barrel screws up over the head and grips the metal on the blind side. The attaching screw is then inserted if needed. There are two head types: the flathead and the countersunk head. The Lok-Skru is tapped for 7-32, 8-32, 10-32, or 10-24 screws, and the diameters vary from 0.230 inch for 6-32 screws, to 0.292 inch for 10-32 screws. Grip ranges vary from 0.010 inch to 0.225 inch.
This rivet is a high strength blind rivet used on late model aircraft. It has a minimum shear strength of 75,000 psi, and can be installed by one person. The Deutsch rivet consists of two parts: the stainless steel sleeve and the hardened steel drive pin. [Figure 5-65] The pin and sleeve are coated with a lubricant and a corrosion inhibitor.
The Deutsch rivet is available in diameters of 3⁄16, 1⁄4, or 3⁄8 inch. Grip lengths for this rivet range from 3⁄16 to 1 inch. Some variation is allowed in grip length when installing the rivet; for example, a rivet with a grip length of 3⁄16 inch can be used where the total thickness of materials is between 0.198 and 0.228 inch.
When driving a Deutsch rivet, an ordinary hammer or a pneumatic rivet gun and a flathead set are used. The rivet is seated in the previously drilled hole and then the pin is driven into the sleeve. The driving action causes the pin to exert pressure against the sleeve and forces the sides of the sleeve out. This stretching forms a shop head on the end of the rivet and provides positive fastening. The ridge on the top of the rivet head locks the pin into the rivet as the last few blows are struck.
When securing nutplates in pressurized aircraft and in fuel cells, a sealing nutplate is used instead of the open ended variety previously described. Care must be taken to use exactly the correct length of bolt or screw. If a bolt or screw is too short, there will not be enough threads to hold the device in place. If the bolt or screw is too long, it will penetrate the back side of the nutplate and compromise the seal. Normally, a sealant is also used to ensure complete sealing of the nutplate. Check the manufacturer’s specifications for the acceptable sealant to be used for sealing nutplates.