When attempting to turn a bolt with an wrench, there may be a problem with the size of the wrench in relation to the size of the bolt head. If the wrench that is used is too large for the bolt head, the bolt head may strip or round off. The across-flats dimension of the bolt head refer to the distance between the two opposite flat sides of the bolt head; the diameter of the bolt shank is not the same as the size of the wrench that need to be used to gain even tightening of the bolt.
The diameter of the bolt is the measurement of the shank of the bolt, but the wrench must match the size of the bolt head. The heads of bolts is made wider than the shank of the bolt to provide additional strength to the bolt. Thus, bolts with specific shank size require larger wrench sizes for proper fitting.
Use the Right Wrench Size for Bolts
Bolt sizes is indicated in either SAE (inch) sizes or metric sizes in relation to the size of the bolt. To determine the proper size of the wrench for a given bolt size, the use of a reference chart are required. For instance, bolts that are quarter inch in size require a wrench that is seven sixteenths of an inch.
Three-eighths inch SAE bolts require a wrench that is nine sixteenths of an inch. Half inch SAE bolts require a wrench that is three-quarters of an inch. These sizes are provided as more larger sizes than the bolt sizes to ensure that the wrench seat fully on the bolt head; if the bolt head is seated fully onto the wrench, even force will be distributed to the bolt head; otherwise, the bolt head may strip.
Metric bolts have similar sizes to SAE bolts; however, metric sizes use the size of the bolt head in millimeters. For instance, an M8 metric bolt require a wrench size of thirteen millimeters. Metric bolts, however, can have additional specifications; for instance, metric bolts may use either the DIN or the ISO sizing specifications.
For instance, a metric bolt sized M10 may require either a wrench size of sixteen or seventeen millimeters, depending upon whether it is a DIN or ISO bolt specification. In these instances, measuring the across-flats dimension will help the user to determine the proper size of the wrench that should be used for the bolt; otherwise, the bolt head may strip. Heavy hex bolts are a type of bolt that are often used in structural applications.
Heavy hex bolts have heads that are larger and thicker than standard bolts; the additional area that a wrench or impact tool must grip prevent the head from rounding off the bolt. For instance, a three-quarter inch heavy bolt may have a one and a quarter inch wrench size that is recommended for installation. These bolts are often used in the construction of bridges or other large structural beams.
Structural bolts often have grades A325 or A490; these grades indicate the strength of the bolt. A325 bolts is made of medium-carbon steel and are used for medium strength loads in buildings. A490 bolts are made of alloy steel and are often used in bridges, as these bolts are able to handle higher level of stress.
The proper use of a torque wrench to tighten the bolt will ensure that the bolt reaches the proper preload; otherwise, the structural joint may fail under the loads that is placed upon it. Different types of fastener will require different types of tools to set them properly. For instance, lag bolts have hex heads and coarse threads; they are used in wooden applications and require the use of standard wrench sizes.
Carriage bolts have a domed head and a square neck, meaning that the nut must be turned instead of the head of the bolt. Socket cap bolts require an Allen key tool instead of a wrench tool. Torx bolts require a Torx screwdriver tool.
In each case, examining the head of the bolt will allow the user to determine the type of tool that will be required to install the bolt properly. Additionally, the type of socket that is used to install the bolt will also prevent damage to the bolt head. For instance, sockets with six points of contact with the bolt head are less likely to strip the bolt; twelve point sockets are more likely to cause the bolt head to round off, especially with rust on the bolt head.
Twelve point sockets are more likely to round off the bolt head because the points of the socket is forced into the corners of the bolt head. Additionally, impact sockets are used when an impact gun is to be used; otherwise, the chrome of the socket may shatter. Thus, using the correct size of bolt to wrench and using the correct size of socket will prevent the bolt head from being strip.
