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Hands, Mind, and HeartWhat started as a handful of passionate enthusiasts has developed into a major force—and a significant component—of the aircraft industry.
Aircraft Hardware - What You Need to Know Part 2
By Ron Alexander (originally published in EAA Sport Aviation, April 1998)
The proper type to use, designations, and appropriate use of all items was outlined. To reemphasize one major point—use only aircraft quality hardware. Don’t be tempted to use anything less. To do so may compromise the safety of your aircraft and most certainly will compromise the quality. FAA Advisory Circular 43-13 along with the Airframe and Powerplant Mechanics General Handbook provide a good reference for identification and use of aircraft hardware. Use the hardware designated in your plans or assembly manual. Only substitute when you know you meet or exceed strength requirements. The kit manufacturer has tested the design and knows what you need in the way of aircraft hardware to hold it all together.
Volumes of material can be written on the subject of aircraft hardware and its proper use. I am going to continue by elaborating only on the highlights of aircraft hardware that apply to the sport aviation industry. This and subsequent articles will present more of what I think you need to know to build your airplane or to complete a restoration. Hopefully, the items discussed will be of practical value to you as you go through the building process. This month, I will discuss in detail aircraft rivets along with more general information regarding screws, turnlock fasteners, O-rings, piano hinge, etc.
Sheet metal construction is very popular within our industry. A number of very high quality designs are available to build using sheet metal. Sheet metal construction of some sort is required on a large number of aircraft including restorations of antiques and classics. Rivets play a major role in this type of construction. Even the best built parts of an aircraft are only as good as what holds them together.
Aluminum parts may be riveted, bolted, or welded together. Riveting is the most common method used. Rivets are used on sport aviation aircraft as well as metal airplanes as large as a Boeing 747. We are talking thousands of rivets on a typical airplane. Rivets are light in weight and cheap in price. Rivets are not only used to hold aluminum sheets together, but they are also used in securing fittings, nut plates, spars and ribs, etc.. Simply defined, a rivet is a bolt without a nut. More precisely, it is a small metal pin that when properly installed provides a very effective and strong bond.
A rivet consists of a manufacturer’s head, a shank, and a shop head (often termed a bucktail). See Figure 1. The shop head is formed during the installation process by squeezing the rivet or by using a bucking bar and a rivet gun. The shop head functions much the same as a nut on a bolt. When the rivet is installed the action of the riveting tool also expands the shank of the rivet slightly thus providing a very tight fit. The two most commonly used rivets are the solid shank and the blind rivet. The solid shank rivet must be installed using a rivet squeezer or driven with a rivet gun. Blind rivets are installed using a puller or a pop-rivet tool. See Figure 2.
Solid Shank Rivets
Solid shank rivets are identified by the kind of material from which they are made, head type, temper condition, and the size of the shank. Rivets are manufactured with a round head, flathead, brazier head, countersunk head, and the most common, universal head. The universal head rivet carries a designation of MS20470 and is used to replace the round head, flathead, and brazier head rivets. The universal head rivet and the 100 degree countersunk head rivet (MS20426) are the most widely used within our industry. See Figure 3. These MS numbers are also interchangeable with AN numbers AN470—MS20470 and AN426—MS20426. Additionally, two types of material are available within these designations. The first type is made out of pure aluminum and has a low tensile strength (usually 16,000 psi). These rivets are used for softer aluminum alloys such as 3003 or 5052 where strength is not a consideration. They use an "A" as identification and are termed soft. In other words, MS20470A. Structural rivets use 2117 aluminum alloy and are heat treated for strength. They typically have a tensile strength of 38,000 psi. They are also highly resistant to corrosion and use the letters "AD" as an identifier (MS20470AD). AD rivets should be used for all structural applications. What if I pick up a rivet—how can I tell if it is soft or hard? Soft or "A" rivets have no markings at all on their head. In comparison, structural or "AD" rivets have a dimple on the head. If you do not see a dimple do not use the rivet for structural purposes. This dimple is also used as an aid to drill out defective or improperly installed rivets.
As an aside, you may have heard the term "icebox rivet." That type rivet is usually manufactured out of 2017 or 2024 aluminum. They are annealed and used for very high strength purposes. They must be kept cold (refrigerated) until they are used. They must be driven within a short period of time after removal from the refrigerator. This type rivet is more common in larger aircraft and is rarely used within our industry. One other type rivet you may encounter is a 5056 rivet that is used for magnesium alloy structures.
To meet AN specifications, rivets must be coated to prevent corrosion. This can be accomplished using zinc chromate, metal spray, or by applying an anodized finish. The latter is usually used. What system is used to identify rivets? Lets look at a typical rivet designation MS20470AD4-8. MS20470 means the rivet has a universal head. The AD means it is a hard or structural rivet. The 4-8 defines the size. The first number is always the diameter of the rivet in 1/32 inch increments and the second number is the length in 1/16 inch increments. So we have a rivet that is 1/8 inch in diameter and ½ inch long. Pretty simple. MS20426 rivets have a 100 degree countersunk head and use the same type identification found above. The length of a rivet is measured from the bottom of the rivet head to the end of the shank on a universal head rivet and from the top of the head to the end of the shank on a countersunk rivet. See Figure 3.
Rivets are commonly sold in ¼ pound bags. Obviously, the number of rivets in a bag will vary depending upon the size of the rivet. As an example, using our MS20470AD4-8 rivet, approximately 300-400 would be found in a ¼ pound bag. Supply catalogs should provide you with this information. These rivets are fairly inexpensive—normally $4-$5 per ¼ pound bag.
As you build your airplane you will doubtless encounter areas where the use of a normal rivet will be impossible. Usually this is because the area you are working in will not permit the use of a bucking bar or rivet squeezer. One side may be totally inaccessible or the space so small you cannot get a bucking bar into it. When this occurs, blind rivets are often the answer. A blind rivet is termed such because they are installed in locations where the shop head cannot be seen. Blind rivets are also used to save time in installation of non-structural items such as flooring, interiors, etc.. A blind rivet requires a special tool for installation, usually a rivet puller or a pop-rivet tool. The shop head is formed by drawing a stem through a sleeve. The stem is pulled through the shank of the rivet and a portion of the stem forces the shank to expand thus filling the hole and forming the shop head. The stem itself will snap off as the rivet is installed. Blind rivets may be installed by one person versus driving a rivet which often requires two people.
One type of blind rivet is termed a "pop rivet." These consist of a rivet and a pulling stem resembling a nail. These rivets typically are available in 3 diameters—1/8 inch, 5/32 inch, and 3/16 inch. Three lengths are also available. These rivets are normally used in non-structural applications.
Cherry rivets are another type of blind rivet. The friction Cherry rivet is a self-plugging blind rivet that has a hollow rivet and a stem, much like a pop rivet. They are available with a countersunk head or a universal head. The designation for a countersunk is CR9162 and the universal head is CR9163. The strength of this type Cherry rivet is less than the bulbed type that will be subsequently discussed. You cannot replace a solid shank rivet with the same size of this type rivet. You must use 1/32 inch diameter larger to obtain the necessary strength. A special tool is required to pull these rivets.
Cherrylock and Cherrymax rivets are suitable replacements for the same size solid shank rivet. When these rivets are pulled they form a large bulb that appears very similar to a driven solid shank rivet. See Figure 4. They are also available with a universal head and with a 100 degree countersunk head. Cherrylock and Cherrymax rivets are expensive. They can cost as much as $2 each. Usually they will be in the area of 50 cents to $1 each. So, you will want to reserve the use of these for needed areas only. One advantage in using the Cherrymax rivet is it can be pulled using one tool that will install all rivet diameters.
Two cheaper versions of Cherry rivets are available. These are called Cherry "N" and Cherry "Q". Cherry "N" rivets are non-structural. Cherry "Q" rivets have limited structural use. They do provide a high shear strength. A number of kit manufacturers use the Cherry "Q" type rivet in the construction of their aircraft.
Rivnuts are another type of blind rivet. They are used to install access covers, wing fairing, floors, etc.. Rivnuts are threaded to accept screws or bolts. They are used very much like a nutplate. Basically, a rivnut is a tubular aluminum rivet with internal threads. See Figure 5. A special tool is required for installation. The tool expands the shank tightly against the material being fastened. Rivnuts are available with a flat head or a countersunk head.
Basics of Rivet Installation
The size and type of rivet to use should be included in your plans or assembly manual. Once again, do not substitute. Use what the designer recommends. Also, do not try to save money on tools. A good rivet gun is a valuable tool—get a high quality rivet gun. Use a rivet squeezer whenever possible. Squeezing a rivet is much easier and more effective. A few rules of thumb concerning rivet installation:
- Use only "AD" rivets for structural purposes. These rivets have a dimple on their head.
- Use the proper size drill bits when drilling holes for rivet installation. Use a #40 bit for a 3/32 inch diameter rivet, a #30 for 1/8 inch, and a #21 for 5/32 inch.
- After a rivet is driven or squeezed, the resulting shop head should be checked for proper shape. Basically, the shop head should have a diameter of 1 ½ diameters of the rivet shank and a height of ½ the rivet shank. See Figure 6.
- The maximum distance from the edge of the material to the center of a rivet hole is called the edge distance. The minimum edge distance is 2 diameters of the shank of the rivet being used.
- Distance between rivets in the same row is called pitch. Minimum pitch is 3 diameters of the shank of the rivet. It may be as much as 20D (D=diameter) or more depending on the installation.
- Distance between rows of rivets is called gauge. The distance between the rows of rivets should equal about 75% of the pitch.
- To determine the proper length for a Cherrylock rivet, use a Cherry selector gauge.
- Special tools must be used for installing some Cherrylock rivets and for Rivnuts.
Screws are probably the most common fastener found on a light airplane. Most are made of a lower strength material than bolts. However, structural screws are made of the same material that you will find in a bolt. Three types of screws are usually found on an aircraft: machine screws, structural screws, and self-tapping screws. Drive screws are also used but they are actually driven like a nail rather than being threaded like a screw.
Machine screws are for general purposes and they are available in low carbon steel, aluminum, stainless, and brass. They are manufactured with a roundhead, washer head, pan head, truss head or countersunk head. They can be fine thread or coarse thread. The designations used in screws are many and varied. AN526 is a good example of a common machine screw. It is a cadmium plated truss head screw. An AN526C ("C" designates corrosion resistant) would be a stainless screw. A MS24693 is a 100 degree flat head screw. The list goes on and on.
Structural screws are made of steel alloy and are heat treated. They may be used as structural bolts. They have a grip length similar to a bolt and they will have the same strength as a bolt of a similar size. They too are available with different types of heads. Structural screws will not be threaded over their entire length versus a non-structural screw. See Figure 7.
This type screw is used for attaching removable parts and in parts where the screw can cut its own threads. Self tapping screws should not be used to replace standard screws, bolts, or rivets. They are made for non-structural purposes. They are available with a round head, truss head, oval head, or flat head. They also have two different shank types: Type A, that is coarse threaded with a sharp point and Type B, that has a blunt point. Type A is used with sheet metal and Type B in conjunction with Tinnerman nuts.
I am sure the majority of you have encountered a turnlock fastener. They are sometimes referred to as quarter-turn fasteners. This type of hardware is typically used on inspection doors, cowlings, fairings, and other types of removal panels. They allow easy opening and access for areas needing inspection or maintenance. They have a very positive locking feature and are easy to both open and to secure. The most popular types of turnlock fasteners are Southco, DZUS, and Camlocs. Southco ¼ turn fasteners are found on a large number of Cessna aircraft cowlings. DZUS fasteners have been used for many years. You will find them on a number of the older military aircraft, antique aircraft, and on amateur-built aircraft. Camlocs are also very popular fasteners.
Three types of DZUS fasteners are the most popular—wing type, oval head, and flush type. A typical assembly consists of a stud, retainer, and a receptacle. See Figure 8. The grommets are made of aluminum and act as a holding device for the stud. A steel spring is used to lock the stud in place—a quarter of a turn of the stud locks the fastener. Special tools are required for installation.
Camloc fasteners consist of a stud assembly, grommet, and a receptacle. The receptacle is usually riveted to the structure of the aircraft. Once again, ¼ turn locks the fastener. Three types of Camlocs are available: 2600, 2700, and 4002. The 4002 series is stronger than the 2600 and 2700 series. All are made of steel and rated to 450 degrees F. Once again, special tools are required for installation.
O-rings are used as a packing to prevent leakage. The O-ring used should be compatible with the fluid being used and with the operating temperature. Certain chemicals, such as hydraulic fluid, must have the proper type O-ring. You will usually receive O-rings in a sealed package. They should be inspected for small cracks or irregularities that might allow leakage. They should also be lubricated prior to installation.
These gaskets are made of copper and asbestos. They are suitable for temperatures up to 500 degrees F and pressures up to 200 psi. They are designated as AN900 or MS35769 gaskets. The dash number following this designation indicates the inside diameter of the gasket in inches.
These flexible grommets are made of black synthetic rubber and they are resistant to hot oil and coolant. They are normally used to protect control cables, tubing, and wiring where they pass through bulkheads and firewalls. They have the designation of MS35489 with a series of dash numbers indicating their size.
MS21919 cushioned clamps are made of an aluminum alloy. They have a cushion bonded to the inner surface and edges of the clamp. The cushion assures a tight fit and eliminates vibration, thus preventing line abrasion. They are normally used for tubing or conduit.
AN742 plain clamps are available to secure tubing. They are similar to cushioned clamps without the cushion. A variety of other clamps are available for use on fuel lines, oil lines, etc.. One of these is the popular Breeze Clamp that are a worm-screw type clamp allowing uniform clamping pressure.
Piano hinge? Yes, we do use piano hinge in building our aircraft. Piano hinge is used as a simple hinge for inspection doors, cowlings, for a quick disconnect fastener, etc.. Two types of piano hinge are available. The first is MS20257 that is made of aluminum and has bent hinge loops. Two half-hinges are held together with a hinge pin. The other type is MS20001 that is an extruded hinge made of stronger material than the MS20257. This hinge has a pin but cannot be pulled apart. This is the type recommended for structural use.
Subsequent articles in this series will focus on control cables, fluid lines, and electrical connections. A significant amount of time in the building process will be used selecting and installing hardware. The more familiar you are with the proper selection and installation of hardware means less time consumed in the building process. Moreover, you will be assured that the assembly of your project is both safe and of the highest quality.