The wrenches most often used in aircraft maintenance are classified as open-end, box-end, socket, adjustable, ratcheting and special wrenches. The Allen wrench, although seldom used, is required on one special type of recessed screw. One of the most widely used metals for making wrenches is chrome-vanadium steel. Wrenches made of this metal are almost indestructible. Solid, nonadjustable wrenches with open parallel jaws on one or both ends are known as open-end wrenches. These wrenches may have their jaws parallel to the handle or at an angle up to 90°; most are set at an angle of 15°. The wrenches are designed to fit a nut, bolt head, or other object, which makes it possible to exert a turning action.
Box-end wrenches are popular tools because of their usefulness in close quarters. They are called box wrenches since they box, or completely surround, the nut or bolt head. Practically all well-manufactured box-end wrenches are made with 12 points so they can be used in places having as little as 15° swing. In Figure 11-6, point A on the illustrated double-broached hexagon wrench is nearer the centerline of the head and the wrench handle than point B and also the centerline of nut C. If the wrench is inverted and installed on nut C, point A will be centered over side “Y” instead of side “X.” The centerline of the handle will now be in the dotted line position. It is by reversing (turning the wrench over) the position of the wrench that a 15° arc may be made with the wrench handle.
Although box-end wrenches are ideal to break loose tight nuts or pull tight nuts tighter, time is lost turning the nut off the bolt once the nut is broken loose. Only when there is sufficient clearance to rotate the wrench in a complete circle can this tedious process be avoided.
After a tight nut is broken loose, it can be completely backed off or unscrewed more quickly with an open-end than with a box-end wrench. In this case, a combination wrench can be used. A combination wrench has a box end on one end and an open-end wrench of the same size on the other.
Another option for removing a nut from a bolt is the ratcheting box-end wrench, which can be swung back and forth to remove the nut or bolt. The box-end, combination, and ratcheting wrenches are shown in Figure 11-7.
A socket wrench is made of two parts: the socket, which is placed over the top of a nut or bolt head; and a handle, which is attached to the socket. Many types of handles, extensions, and attachments are available to make it possible to use socket wrenches in almost any location or position. Sockets are made with either fixed or detachable handles. Socket wrenches with fixed handles are usually furnished as an accessory to a machine. They have a four, six, or twelve-sided recess to fit a nut or bolt head that needs regular adjustment. Sockets with detachable handles usually come in sets and fit several types of handles, such as the T, ratchet, screwdriver grip, and speed handle. Socket wrench handles have a square lug on one end that fits into a square recess in the socket head. The two parts are held together by a light, spring-loaded poppet. Two types of sockets, a set of handles, and an extension bar are shown in Figure 11-8.
The adjustable wrench is a handy utility tool that has smooth jaws and is designed as an open-end wrench. One jaw is fixed, but the other may be moved by a thumbscrew or spiral screwworm adjustment in the handle. The width of the jaws may be varied from 0 to 1⁄2 inch or more. The angle of the opening to the handle is 221⁄2 degrees on an adjustable wrench. One adjustable wrench does the work of several open-end wrenches. Although versatile, they are not intended to replace the standard open-end, box-end, or socket wrenches. When using any adjustable wrench, always exert the pull on the side of the handle attached to the fixed jaw of the wrench. To minimize the possibility or rounding off the fastener, use care to fit the wrench to the bolt or nut to be turned.
The category of special wrenches includes the crowfoot, flare nut, spanner, torque, and Allen wrenches. [Figure 11-9 and 11-10]
The crowfoot wrench is normally used when accessing nuts that must be removed from studs or bolts that cannot be accessed using other tools.
The flare nut wrench has the appearance of a box-end wrench that has been cut open on one end. This opening allows the wrench to be used on the B-nut of a fuel, hydraulic, or oxygen line. Since it mounts using the standard square adapter, like the crowfoot wrench, it can be used in conjunction with a torque wrench.
The hook spanner is for a round nut with a series of notches cut in the outer edge. This wrench has a curved arm with a hook on the end that fits into one of the notches on the nut. The hook is placed in one of these notches with the handle pointing in the direction the nut is to be turned.
Some hook spanner wrenches are adjustable and fit nuts of various diameters. U-shaped hook spanners have two lugs on the face of the wrench to fit notches cut in the face of the nut or screw plug. End spanners resemble a socket wrench, but have a series of lugs that fit into corresponding notches in a nut or plug. Pin spanners have a pin in place of a lug, and the pin fits into a round hole in the edge of a nut. Face pin spanners are similar to the U-shaped hook spanners except that they have pins instead of lugs.
Most headless setscrews are the hex-head Allen type and must be installed and removed with an Allen wrench. Allen wrenches are six-sided bars in the shape of an L, or they can be hex-shaped bars mounted in adapters for use with hand ratchets. They range in size from 3⁄64 to 1⁄2 inch and fit into a hexagonal recess in the setscrew.
There are times when definite pressure must be applied to a nut or bolt as it is installed. In such cases, a torque wrench must be used. The torque wrench is a precision tool consisting of a torque indicating handle and appropriate adapter or attachments. It measures the amount of turning or twisting force applied to a nut, bolt, or screw.
Before each use, the torque wrench should be visually inspected for damage. If a bent pointer, cracked or broken glass (dial type), or signs of rough handling are found, the wrench must be tested. Torque wrenches must be tested at periodic intervals to ensure accuracy.
Calibrating a torque wrench is the process in which the manufacturers of the torque wrench set ensure a precise torque occurs on a standard and consistent basis. Regular torque wrench calibration ensures repeatable accuracy and adherence to standards. A torque wrench is a precision tool and should be treated and maintained like a delicate measuring instrument. A torque wrench must be properly calibrated and maintained on a preventative maintenance and calibration schedule. In order to maintain accuracy, it is crucial that a torque wrench and other measuring equipment be calibrated regularly. Some wrenches or tools may recommend six (6) month calibration intervals, while others may schedule it at twelve (12) months.
The three most commonly used torque wrenches are the deflecting beam, dial indicating, and micrometer setting types. [Figure 11-10] When using the deflecting beam and the dial indicating torque wrenches, the torque is read visually on a dial or scale mounted on the handle of the wrench. The micrometer setting torque wrench is preset to the desired torque. When this torque is reached, the operator notices a sharp impulse or breakaway “click.” For additional information on the installation of fasteners requiring the use of a torque wrench, refer to “Installation of Nuts, Washers, and Bolts” located in Chapter 7, Aircraft Materials, Processes and Hardware.
The strap wrench can prove to be an invaluable tool for the AMT. By their very nature, aircraft components, such as tubing, pipes, small fittings, and round or irregularly-shaped components, are built to be as light as possible while still retaining enough strength to function properly. The misuse of pliers or other gripping tools can quickly damage these parts. If it is necessary to grip a part to hold it in place, or to rotate it to facilitate removal, consider using a strap wrench that uses a plastic covered fabric strap to grip the part. [Figure 11-11]
In certain applications, the use of an impact driver may be required. Struck with a mallet, the impact driver uses cam action to impart a high amount of torque in a sharp impact to break loose a stubborn fastener. The drive portion of the impact driver can accept a number of different drive bits and sockets. The use of special bits and sockets specifically manufactured for use with an impact driver is required. [Figure 11-12]