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September 1, 2000
Steven W. Ells
Mechanical fasteners are an integral part of aircraft and aircraft engine fabrication, repair, and maintenance. There are the fasteners most pilots are familiar with—solid aluminum rivets, stainless steel rivets, structural blind rivets, nonstructural blind rivets, structural screws, nonstructural screws, plain nuts, castle nuts, and steel and fiber self-locking nuts and bolts. Some manufacturers also utilize semipermanent high-strength structural fasteners called Hi-Loks and Jo-Bolts.
Because fasteners play such an important part in the safe operation of airplanes, it's important to understand that proper installation of every fastener is critical. So critical that the manufacturers of every airframe, engine, and accessory have, over the years, expanded the section in the appropriate service manual devoted to specifying the amount of twisting force, or torque, that is required to be applied to fasteners.
When, where, and how to apply the proper torque is probably the most important discipline involved in aviation maintenance. Next to cleanliness, of course.
Torque is the rotational force applied to a fastener. According to an assembler's handbook issued to Lockheed employees who built the L–1011, torque wrenches measure the resistance to turning. To put it in basic terms, torque equals force times distance. For this reason torque values are described by two words; i.e., inch pounds, foot pounds, centimeter kilograms, or meter kilograms. These terms specify an arm, or distance, measured from the center axis of the turning fastener, and the amount of weight (or pull) applied to that arm.
Torquing up a Hamilton Standard feathering propeller onto the prop shaft of a Pratt & Whitney R-1830 is accomplished by having a 180-lb. person hang on the end of an eight-foot-long bar. Following the formula cited above, it might be suggested that the proper torque for this application is eight foot-persons, but this torque is also described in foot pounds. This example shows that the method of achieving a torque can be quite creative if the formula for arriving at a given torque value is understood.
A torque of 20 foot pounds can be applied by using a calibrated spring scale that indicates 20 pounds, as long as it pulls on a one-foot-long wrench. You could also get 20 foot pounds by exerting a 10-pound pull on a two-foot-long wrench. Why all this insistence on torque specifications? Because owners are allowed to perform preventive maintenance on their airplanes to the extent described in FAR Part 43, appendix A. While owners are prevented from replacing structural fasteners, many of the operations that this regulation allows involve removal and installation of torqued fasteners. Therefore, to ensure that pilots get started on the right foot before tackling any preventative maintenance tasks, there are some basic concepts that must be understood.
When torquing a fastener or set of fasteners it's very important to determine how the fastener is loaded. If a bolt application is designed to withstand a shear load—that is, a load where the two halves of the joint are pushed or pulled in opposite directions—the torque applied will be less than if the fastener is intended to compress the joint or load the fastener in tension. When the fastener is designed to prevent shear movement, the fastener is held in place by the torque without applying a clamping force; therefore, the torque applied is less. This can be seen in torque tables.
Most of the fasteners used on general aviation aircraft are standard fasteners, so standard torque tables apply. In FAA Advisory Circular 43-13-1B, titled Acceptable Methods, Techniques, and Practices for Aircraft Inspection and Repair, there is a paragraph stating, "Table 7-1 shows the recommended torque to be used when specific torque is not specified by the manufacturer." In addition to the general torque table in AC43-13-1B, most manufacturers supply general torque tables.
In the general tables of the service manual for Cessna's 182S a 1/4-28 (1/4- inch-diameter shank with 28 threads per inch) bolt loaded in shear should be torqued to between 30 and 40 inch pounds, while the same bolt loaded in tension should be torqued between 50 and 70 inch pounds. This 3-to-5 relationship prevails throughout the torque tables for standard fasteners.
All joints that are loaded in tension—in other words, squeezed together—are designed to be preloaded. In order for the joint to perform as designed, the preload must be greater than the working load. That is the load that is imposed on that joint during normal operating conditions. If the torque applied to the joint is lower than the desired value, the working load may be greater than the preload and the adjoining surfaces may move in relation to each other. The result is wear of the components. In engines, low torque values can result in the fretting of engine case halves or in loss of engine main bearing crush (preload). Loss of the bearing crush may result in main bearing shift (rotation), which will result in engine failure. If the torque applied to the fasteners is greater than specifications, then the threads of the fastener may yield, releasing the joint.
The value of proper torque can be understood by reference to an engine spark plug. Lycoming and Continental, as well as other engine manufacturers, specify spark plug installation torque using a new or annealed (made soft) spark plug gasket and antiseize compound applied sparingly to the plug threads. Champion spark plug gaskets are soft copper and can be annealed for reuse by heating to red-hot and immediately submerging in cold water. If the gaskets cannot be annealed, new ones should be installed prior to installation.
If the torque is soft, or below the specifications, combustion gases may escape between the spark plug gasket-to-head seal. Since these gases are extremely hot, very corrosive, and can attain pressures of up to 800 psi, erosion of the aluminum cylinder head or the copper seal gasket quickly occurs. This further softens the torque. Under extreme cases the spark plug threads or the steel Heli-Coil thread insert installed in the aluminum cylinder head are weakened and the spark plug may blow completely out of the cylinder.
If the torque applied to the spark plug is above the specifications (excessive preload), then the load imposed on the threads in the cylinder head or the threads of the spark plug may damage the cylinder head.
Torque is applied by special tools, with the torque wrench being the one most people are familiar with. According to FAA regulations, torque wrenches must be calibrated at regular intervals. In spite of controls on the tools, torque application is an inexact science. According to information in the second edition of John Schwaner's Sky Ranch Engineering Manual, the actual tension applied to a bolt when a specified torque is applied may vary from the desired tension by values as high as 30 percent. Reasons for this variation may include improper lubrication applied to the fastener threads, dirty or damaged threads, and burnished threads on the stud, bolt, screw, or nut.
The coating on the fastener is also important. Fastener torque values in airframe maintenance manuals are for cadmium-plated fasteners. Aircraft-quality hardware has to be cad-plated for it to conform to manufacturing specs. This is another reason why fasteners from the local hardware store shouldn't be used in aircraft applications. Even though nonaircraft hardware can conform to the strength standards of aircraft-quality hardware, it often is not cad plated, so the torque tables won't apply.
It's critical to determine the proper lubricant. Torque tables, service manuals, or service bulletins will specify a lubricant if the manufacturer requires it.
Continental Service Bulletin 96-7B is titled "Torque Limits." This bulletin lists the torque values for fasteners used on all Continental engines. A paragraph under a bold warning flag on the first page of the bulletin says, "Proper torquing practices cannot be overemphasized…. If the fasteners are not properly plated, the fastener threads are not clean and free of deformation, or are not properly lubricated, the correct fastener preload will not be achieved even though the given torque value is reached."
Leaving nothing to chance, a new paragraph was added when this bulletin was revised in January 1999 specifying in capital letters that "Torque values listed are for use with clean 50 weight aviation oil applied to the threads unless otherwise specified." Lycoming's torque charts also specify lubricated threads.
Applying the wrong lubricant prior to torquing will affect the joint strength. Schwaner's book has a table showing that the stress on a cad-plated 3/8-24 bolt (3/8-inch shank with 24 threads per inch) varies from approximately 55,000 psi when the threads are dry to approximately 90,000 psi when the threads are lubricated with engine oil. In each case 600 inch pounds of torque was applied. The message here is obvious—inspection of the condition of the fasteners, use of only aircraft-quality fasteners, and use of the manufacturer-specified lubrication are critical when assembling torqued joints.
The first step to understand is that bolt sizes in the torque charts are the size of the bolt shank, not the size of the wrench that fits on the bolt head. In its torque charts, Continental lists the fastener sizes in decimals. This makes it easy for a technician to measure the shank of the bolt with a micrometer or vernier caliper.
Fasteners are torqued dry unless the manufacturers specify otherwise, so it's important to obtain all applicable manufacturer's data before starting work. Often the torque data is available as a service bulletin and the cost will be minimal.
Determining if the manufacturer has specified a torquing sequence is also critical. When a cylinder change or top overhaul is done on an engine the manufacturer will specify not only the torques, but also the proper sequence of torquing the fasteners. This is especially true when torquing cylinders and/or engine through bolts. Engine case-half through bolts, which are misnamed since they don't actually have a typical bolt head, are shafts that are threaded on each end. These high-strength shafts are long enough so that when they are installed through the engine case halves a threaded end projects out each side of the case through opposing cylinder hold-down flanges. Close reading of the Continental torque bulletin requires that both nuts on opposite ends of a through bolt be retorqued when the nut on one end of the through bolt is slacked off for a cylinder removal.
Free running, prevailing, or friction drag torque is the amount of torque it takes to turn the nut on the threads. This value will be very small for a plain nut and somewhat higher for a self-locking nut. Regardless of the number, this torque value must be added to the desired torque to achieve final torque.
The most common types of torque wrenches are the round beam or deflecting beam type, and the micrometer adjustable (breakaway) torque wrench. The least expensive is the deflecting beam type. When a torque is applied the round beam supporting the handle and torque indicating scale bends, which causes the needle to deflect on the scale. This type of wrench is simple, relatively inexpensive, and the only calibration needed is to center the indicating needle on the scale.
The micrometer adjustable, or breakaway, wrench is the choice of professional mechanics for most applications. By setting the desired torque on the micrometer scale, the breakaway point is selected. This point is easily felt as the force applied to the wrench suddenly slackens, giving the operator an unmistakable signal that he has applied the desired torque. Because of the internal release mechanism, these wrenches must be calibrated regularly.
Know the size of the fastener you're torquing; determine whether the tension or the shear torque tables apply; determine the running torque to calibrate for final torque; ascertain whether the fastener is lubricated, and if so, what type of lubricant is required; and apply the torque in a slow, gradual manner. Following these suggestions will help to insure successful fastening. And remember to ask for the guidance of a certificated A & P mechanic if you have any questions about an aircraft maintenance task.
To obtain the Sky Ranch Engineering Manual, contact the Sacramento Sky Ranch at 800/433-3564 or visit the Web site ( www.sacskyranch.com). AC 43.13-1B is available from the Government Printing Office or an online aviation bookstore, such as www.asa2fly.com or www.airportshoppe.com. E-mail the author at [email protected].
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