Torquing Nuts & Bolts

SELECTING AND TORQUING NUTS & BOLTS

Story & Photos By Jim Clark (The Hot Rod MD)

As a shade tree mechanic I assumed that when tightening nuts or bolts that “tight is right, too-tight is even better, even tighter is broken.” This works for fasteners in non-critical areas like sheet metal attachment but not for more critical fasteners like head bolts or nuts on studs. For these you need a more accurate way of tightening them because they are not held in place because of friction between the bolt and mating surface.

For a fastener to function properly it must be “stretched” a specific amount. The material’s ability to “rebound” like a spring is what provides the clamping force. Different materials react differently to these conditions, so each fastener must be engineered to operate within specific ranges.

If a fastener is over torqued and becomes stretched too much – you have exceeded the yield strength and it’s ruined. If when stretched the fastener is longer than manufactured – even if it is only .001, it is in a partially failed condition. Therefore, fasteners must be engineered with the ductility to stretch a given amount and rebound for proper clamping.

The problem is magnified by the expansion and contraction of the item being held together as it heats up and cools down. Heat, primarily in aluminum, is another problem area. Because the thermal expansion rate of aluminum is far greater than that of steel it is possible to stretch a fastener beyond yield as the aluminum expands under heat. An effective way of counteracting material expansion is through producing a more flexible bolt.

There are three methods that can be used to tighten fasteners to a specified (torque) tension. They are: 1-using a torque wrench, 2-measuring the amount of stretch with a stretch gauge or 3-torque angle (turning the fastener a pre-determined amount). The torque wrench is the most versatile method and one most commonly used.

What Is Torque?
Torque, by definition, is the result of a force applied to an object through a lever arm, thus tending to rotate the object.
The formula used to measure torque: T = F x L
T— Torque
F— Force
L— Lever length from the center of rotation to, and at 90� to, the direction of force.

The most common units of measurement for torque are: inch pound (in-lb or lb-in), foot pound (ft-lb or lb-ft), meter kilogram (mkg) and Newton meter (N�m). When torque is applied to a threaded fastener it produces a clamping force that holds the components together. Too much force, and the fastener will break. Not enough force and the assembly will not stay together. By controlling the amount of torque, the clamping or holding force is controlled.

How A Torque Wrench Works
There are two commonly used types of torque wrench.
1-Flexible-beam torque wrench.
The flexible-beam type torque wrench has a pointer equal to the length of the wrench that remains fixed as the wrench is rotated, tightening the fastener. A graduated scale, located perpendicular to the handle, indicates the degree of rotation as the pointer slides across the scale. These were the standard in years past but are not as accurate as the newer pressure-releasing torque wrenches




		This a flexible-beam type torque wrench. It has a handle with a pivoting grip and a pointer that indicates the torque reading when the bolt is tightened



	The scale on the flexible-beam type torque wrench reads from zero at the center to the top torque rating for the tool in each direction to accommodate tightening both right and left-hand threads



When the fastener is tightened the torque wrench handle deflects while the indicator needle remains straight, passing across the face of the scale thereby indicating the degree of torque being applied to the fastener.

2-Pressure-releasing Torque Wrench.
The pressure-releasing torque wrench indicates when the pre-set torque value has been reached by releasing the handle for a few degrees of free travel. This release or “give” is usually accompanied by an audible “click” signal and tells the operator to stop applying pressure. The torque setting is adjusted by unlocking the handle and rotating the handle to line up the marks on the handle with the desired setting engraved into the barrel. There are usually two scales engraved into the barrel: one for English torque units (inch pounds or foot pounds), the other one for metric torque units (Newton meters). They are usually equipped with a reversing ratcheting head so that they can be used for right-hand or left-hand threads.


	Here are two sizes of pressure-releasing torque wrenches; one graduated in foot pounds/Newton meters and the other in inch pounds/Newton meters


	The larger torque wrench has a barrel that is longer to provide the needed leverage and is rated at a maximum of 250 Ft.-Lbs.


	The shorter torque wrench is graduated in inch pounds/Newton meters to accommodate tightening lighter-duty fasteners where more precision is required

Major foot pound settings are engraved into the top face of the torque wrench housing and the fine settings are engraved on the handle. Rotating the handle moves the fine scale through ten segments on the major scale thereby adjusting the torque setting up or down. Major Newton meter scale is engraved on the opposite side of the barrel and is adjusted using the fine Newton meter setting numbers engraved on the handle.

Using A Torque Wrench
When using a torque wrench there are certain factors that should be taken into account. Here are some guidelines from DMP Fasteners/ARP. ARP research has verified the following “rules” pertaining to use of a torque wrench:

1-The friction factor changes from one application to the next. That is, the friction is at its highest value when the fastener is first tightened. Each additional time the fastener is torqued and loosened this value gets smaller. Eventually the friction levels out and becomes constant for all following repetitions. Therefore, new fasteners should be tightened and loosened through several cycles before applying final torque. The number of times depends on the lubricant. For all situations where ARP lubricants are used, five cycles are required before final torquing.

2-The lubricant used is the main factor in determining friction, and therefore, the torque for a particular installation. Motor oil is a commonly used lubricant because it’s readily available. If less friction is desired in order to install the fasteners with less torque, special low friction lubricants are available. With special lubes, the required torque can be reduced as much as 20 to 30 percent. It is important to keep in mind that the reverse is also true. If the torque value has been specified for a particular fastener on the basis of low friction lube, installing the fastener with motor oil will result in insufficient preload; the torque has to be increased to compensate for the extra friction caused by the motor oil.

3-Surface finish is also important. For example, black oxide behaves differently than a polished fastener. It is therefore important to observe the torque recommendations supplied with each fastener. NOTE: It is possible for even the most expensive of torque wrenches to lose accuracy. We have seen fluctuations of as much as ten (10) foot pounds of torque from wrench to wrench. Please have your torque wrench checked periodically for accuracy.

Torquing tips
To obtain the correct amount of clamping force a fastener should actually be stretched a measured amount. A properly used fastener works like a spring!

When tightening many fasteners holding one component (engine head, pipe flange, etc.) follow manufacturer’s recommended procedures and tightening sequences. If such procedures are not available, torque in a criss-cross manner, first to 60-70% of the desired torque, then to the final torque.

Nuts and Bolts
Nuts and bolts are the most common fasteners used when building a vehicle. Anyone that has ever worked on one has a basic knowledge of how these fasteners are utilized. However, selecting the proper fastener for the application and installing it properly requires more than a simple understanding of how they work.

Nuts and bolts are created from a variety of different materials, in both standard and fine threads and in both USS/SAE and metric sizes. Selecting the proper one for a given application requires the consideration of a number of factors.

Fastener Sizes
Nuts, bolts and studs used in automotive applications come in USS/SAE or metric sizes. Standard fastener sizes are designated USS for coarse threads and SAE for fine threads. All nuts, bolts and studs, whether standard or metric, are sized according to diameter, thread pitch and length. A standard 1/2-13x1 bolt is 1/2-inch in diameter, has 13 threads per inch and is 1-inch long. An M12-1.75x25 metric bolt is 12 mm in diameter, has a thread pitch of 1.75 mm (the distance between threads) and is 25 mm long. Because the bolts are nearly identical in size it would be easy to mix them up, but they are not interchangeable.


	Hex head bolts are the most common fasteners found in automotive applications. The one on the left is commonly referred to as fine thread, (SAE) while the one on the right is a course thread (USS).


	Standard and metric bolt and nuts also differ in that the distance across the flats on the head of a standard fastener is measured in inches while a metric fastener is measured across the flats in millimeters. So a standard wrench should not be used on metric fasteners and a metric wrench should not be used on standard fasteners.

Washers are used to spread the load on mating surfaces and to assist in preventing fasteners from coming loose. The first two are flat washers, a fender washer that provides additional support for things like sheet metal and a standard washer that provides protection for the mounting surface on the component. Flange nut at the lower left has serrations that act as a built-in lock washer. The others are lock washers that prevent a fastener from loosening. Split locks dig into the surface of the fastener and component with spring-loaded edges while the star washers grip both surfaces with many small raised teeth.

Fastener Materials
In addition to the differences in diameter, thread pitch and length, standard and metric fasteners come in different grades. Standard bolts used in automotive applications come in grades from 0 thru 8. They are usually identified by slashes radiating out from the center on the head of the bolt. Grades 0 thru 2 have no slashes on the head, grade 5 has three slashes and grade 8 has six slashes. Standard nuts have slashes or dots placed around the outside to denote grade.


	Bolts on the left are both grade 5 indicated by the three slashes on the head. The next two are grade 8, one indicated by six slashes on the head, the other by an 8 on the head. The 8 should not be confused with a metric grade that would be marked with an 8.8 grade indicator. Bolt on the right has letters on the head but is just a 0 thru grade 4 bolt because it has no slashes on the head.

Nuts have grade markings on the face of the nut or around the flats. Nut in the upper left is a lug nut with a taper on one face and no grade markings. The next one is a nylon-insert lock nut with six slashes indicating that it is grade 8. Upper right is a castle nut with slots for inserting a cotter pin and has no grade markings. Lower left is a toplock hex flange nut with 6 dots indicating that it is grade 8. The next two are hex nuts, both grade 8, one marked with six dots, the other with six slashes. The last one is a low grade hex nut with no markings.


	Metric bolts have a property class (grade) number, rather than a slash, molded into the head to indicate bolt strength. In this case, the higher the number, the stronger the bolt. Property class numbers 8.8, 9.8 and 10.9 are the most commonly used on automotive applications. Metric nuts usually have the property class (grade) number stamped into the side.


	The three bolts shown are all priority class (grade) 8.8 and 10.9 as indicated by the numbers on the heads. Unlike USS/SAE English threads, metric threads are not divided into coarse and fine thread, but instead sized by designating the thread pitch (distance between threads).

Fasteners are made from a wide range of materials. Everything from nylon to titanium, and even some very exotic metal alloys for highly specialized applications. Most of the fasteners that we would be purchasing for an automotive application would be produced from either steel or stainless steel.

Steel fasteners are usually stronger than ones made from stainless steel but are much more susceptible to corrosion. That choice has been eliminated now by these two offering from DMP Fasteners/ARP. The stainless steel fasteners and their superior grade of chrome moly have strength properties of nearly the same specs so either can be used in many different applications without sacrificing the necessary strength.

STAINLESS STEEL: Ideally suited for many automotive and marine applications because stainless is tolerant of heat and virtually impervious to rust and corrosion. ARP “Stainless 300” is specially alloyed for extra durability. It’s polished using a proprietary process to produce a beautiful finish. Tensile strength is typically rated at 170,000 psi.

8740 CHROME MOLY: Until the development of today’s modern alloys, chrome moly was popularly considered a high strength material. Now viewed as only moderate strength, 8740 chrome moly is seen as a good tough steel, with adequate fatigue properties for most racing applications, but only if the threads are rolled after heat-treatment, as is the standard ARP production practice. Typically, chrome moly is classified as a quench and temper steel that can be heat-treated to deliver tensile strengths between 180,000 and 210,000 psi. applications. Although Aermet 100 is a maraging steel that is far superior to other high strength steels in its resistance to stress corrosion, it must be kept well oiled and not exposed to moisture.

The following terms help to explain some of the other things that should be considered when selecting fasteners.

Hydrogen Embrittlement: This condition results from the accumulation of hydrogen gas in the atomic structure of the metal. This gas flows to the point of high stress (stress risers) and causes microscopic cracks. The hydrogen then flows to the “new” crack tip and causes it to crack further. In this way the crack moves across the part, because the crack-tip IS the stress riser. Finally the crack gets so large that the section is not large enough to support the load. No hydrogen embrittlement can take place without tensile stress. ARP employs a baking process that purges hydrogen gas from the steel.

Stress Corrosion: This is a special form of hydrogen embrittlement in which the metal is attacked while under stress. Without the stress the crack will not move. But under stress the crack moves and corrosion takes place at the freshly opened crack face.

Preload: The force IN a bolt when it is installed with a torque greater than simply hand tight. Preload can be established by measuring torque or bolt stretch or by the less than accurate “turn-of-the-nut” method.

Clamp Load: This is the force exerted by a tightened bolt and is the same as preload.

Stretch: The increase in length of a bolt when installed with a preload.

Thread Engagement: This refers to the number of threads engaged in a nut or threaded hole. Full engagement, meaning all the female threads are engaged, is a desirable configuration to maximize fatigue strength.

Ultimate Tensile Strength: The maximum stress that a particular material can support without breaking. It is expressed in terms of lbs. per square inch, and is measured by means of a tensile test. The maximum force (lbs.) that a test specimen can support is divided by the cross-sectional area (square inches) of the specimen; the result is ultimate tensile strength in psi.

Yield Strength: The stress at which a given material or component exhibits a permanent deformation (i.e. “takes a set”). When the load that caused the stress is removed, the part will not return to its original dimensions. If you exceed the yield strength of a fastener (tighten it until it feels funny and then back it off a bit) the fastener is ruined and must be replaced.

Summary
Nuts and bolts need to be tightened to a preset torque to prevent them from loosening. That does not mean that tighter is even better. Fasteners are designed to stretch a designated amount, acting like a spring, keeping the connection under a specific load. This is what keeps it from loosening.

A torque wrench is the most commonly use tool to tighten fasteners to the desired torque setting. Two types are available with capacities ranging from fine inch pound settings to in excess of 600 foot pounds. The older style flexible-beam type torque wrenches are not as accurate as the pressure releasing type but are sometimes required in applications where a spark might set off flammable materials. Specific information about their operation is usually packaged with the tool.

Fasteners are available in a wide variety of styles and materials. Choosing the right one for the application is critical. Information about their makeup and use has been included here but much more information and assistance in their selection is available by going to websites or looking in the catalogs of suppliers like DMP Fasteners/ARP.

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