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Fastener Tech – The Nuts and Bolts of It

By Bill Ansell (Billavista)

Photography: Bill Ansell

Technical Drawings: Lonny Handwork

Why an Article on a Topic as “Simple” as Nuts and Bolts?

Because an understanding of fastener tech forms a solid foundation for all extreme off-road tech. It’s a great place to start – from the ground up. Much of what we do as builders and maintainers of off-road vehicles is strictly custom, often one-off, stuff. We don’t have the luxury of detailed instructions or the benefit of a major manufacturer’s years of engineering and research. In short, we’re on our own so we need to have a good solid understanding of fastener tech in order to answer questions such as: What size and type of thread should we use to attach our custom suspension links? What type of locking mechanism should we use on our beadlock wheels? Should we use studs or bolts to attach our steering arms to the knuckles?

Bolts

The basic parts of a bolt are:

Head – commonly sized 4/16ths larger than the nominal size of the bolt (diameter of the shank). For example, a ½ inch bolt has a head that takes a ¾ inch socket.
Bearing Surface – machined true and perpendicular to the shank, the bearing surface is the area through which the bolt is loaded in tension.
Shank – unthreaded portion of the bolt. Its diameter is the nominal size of the bolt (equal to major diameter of thread).
Male Threads – the threads on a bolt, screw, or stud are known as “male,” those on a nut or tapped hole are “female.”
Point – the extreme end of the threads, often chamfered for easier thread starting.
Grip Length – the length from the bearing surface to first complete thread.
Thread length – how much of the shank is threaded from point to last complete thread.
Length – the total length of the bolt (the dimension you specify when purchasing) is the total of the grip length and the thread length.

Figure 1 – The basic parts of a bolt

Figure 2 – The parts and dimensions of a thread

The basic parts and most important dimensions of a thread are illustrated in Figure 2. A male thread is depicted, but the terms apply equally to female threads. The thread pitch is the distance from a point on the thread to a corresponding point on the next thread measured parallel to the bolt’s axis (equal to 1 divided by the # of threads per inch). The major diameter is the largest diameter of a thread (measured over the crests of the thread) while the minor diameter is the smallest diameter of a thread (measured over the roots of the thread).

Nuts

Compared to a bolt, a nut is a fairly simple beast. It is really nothing more than a chunk of steel into which is cut appropriate internal threads so that it may be screwed onto a bolt. The flat area of the nut that contacts the joint when it is tightened is known as the “nut face”. Because the only practical way to form the internal threads is to cut them into the nut, these threads are always weaker than the rolled threads of a quality bolt or stud. Selection of an appropriate nut consists of choosing the correct grade and thread to match the bolt used. The only other concern is whether or not to use some sort of “locking” nut. Unless an assembly sees very little load and must also be frequently disassembled, it is best to always use some sort of locking nut – selection of which is covered later in this article.

Bolt, Screw, or Stud?

The choice between bolt or screw is really just a naming convention. A bolt is an externally threaded fastener intended to be used with a nut. It is tightened or loosened by turning a nut on the bolt’s threads. A screw is an externally threaded fastener designed to be threaded into a tapped hole in a part. A screw is tightened or loosened by turning it by the head. In practice, most people call both bolts and screws, “bolts” – in the majority of this article the terms can be used interchangeably.

A stud is an externally threaded fastener that has 2 threaded ends with a non-threaded shank between them. It is designed to have one end threaded into a tapped hole while the other end uses a nut. Most often one end is coarse thread, for threading into a tapped hole, and the other end that takes the nut is fine thread, so that the benefits of both fine and coarse threads can be utilized – these differences will be discussed later. In the manner of operation, a stud is no different than bolt, they are both clamping devices, and neither should really be used as locating dowels or bearing trunnions. The advantage to using a stud occurs when you have a piece that needs to be fastened to a large, cast part that requires semi-frequent disassembly. By using a stud, the assembly can be disassembled leaving the stud in place, reducing the chance of fouling or stripping the internal threads in the cast part, which would be difficult to repair. Using studs to hold a steel steering arm to a cast or forged steering knuckle is an excellent example of this principle.

The Unavoidable Physics

In discussing fastener selection and joint design we must make use of a few engineering terms. Stress is a force or load applied to a part, divided by how big the part is, in other words force per unit of cross sectional area, commonly measured as pounds per square inch (PSI). Strain is a change in shape or dimension in response to a stress. The concept of strain allows us to describe how a part or material responds to an applied force or load. There are 3 things that can happen when a bolt strains:

It can change shape temporarily, “springing” back to its original shape when the stress is removed. This happens when the bolt is stressed below its yield point, and is called, appropriately enough, “elastic deformation.” Note that this is the case, even when the strain is so small it cannot be seen with the naked eye.
It can change shape permanently, taking a “set” even after the load is removed. This is called “plastic deformation” and occurs when a material is stressed beyond its yield point.
Thirdly, if stressed beyond its “ultimate strength,” it will rupture. This is called bad; very, very bad!

How Bolted Joints Work

Nuts and bolts are clamps. They work by tightly clamping the parts of a bolted joint together. They are able to do this because of stress and strain. When a nut and bolt is tightened in a joint, the bearing surface of the bolt and the nut face come up against the halves of the joint. If tightening continues, the bolt will stretch slightly – it will strain. As long as it is not stressed beyond its yield point it will try and return to its original length, establishing a clamping force. This bolt-stretch, which creates the desired clamping force, is called bolt pre-load. Establishing and maintaining appropriate fastener pre-load in a bolted joint is the principle on which all bolted joints work and is the chief determining factor in how strong, tight, and fatigue-resistant a bolted joint will be.

Most, if not all, properly designed bolted joints in a 4x4 application will cause the joint and the bolt to be stressed in one of two distinct ways: tension or shear.

Figure 4 – Connecting rod is an example of a bolted tension joint

Tension joints

A joint can be designed so that the bolt will be loaded in tension (Figure 4).The parts are loaded such that they to try and pull apart. In this case the load is applied along the longitudinal axis of the bolt. A connecting rod bolt is an example of a bolt loaded in tension.

Shear joints
A joint can be designed so that it will be loaded in shear. In this case, the load on the joint acts perpendicular to the length of the bolt, and tries to cut, or shear, the bolt in half. Bolts used to hold suspension links in their brackets are loaded in shear. There are two sub-types of shear joint: bearing and friction.

Bearing shear joints

In a bearing joint, it is the very close fit of the fastener in the holes that carries the load. Assembled properly, the bolt will be an extremely close fit in its hole – such that SAE fasteners and drilled holes are not appropriate – they allow too much tolerance or slop. Bearing shear joints should be avoided if possible, unless specialized aerospace bolts with exacting tolerances and precise hole-making methods (machining or reaming) are employed. The exception to this rule is if some additional method is employed to ensure there is an extremely tight fit between the fasteners and the holes. The most common method is to employ a floating, split, conical-shaped washer on the fastener with a matching tapered hole in the part. In this fashion, as the fastener is tightened, the conical washer cinches down in the tapered hole as well as against the shank of the bolt or stud, creating a tight, zero clearance fit and preventing slop, wear, and fatigue. The Dana 44 front axle steering arm attachment is a classic example of this method. Zero-clearance locating dowels are another method that can be employed.

Friction shear joints

The second type of shear joint is a friction shear joint. In this case, the bolt clamps the parts of the joint together so that the friction between the clamped parts carries the majority of the load. When this is the case, the bolt itself is loaded only in tension, as it is designed to be, at least until the load overcomes the friction and the parts slip, loading the bolt in shear. Obviously the in-service load on the joint determines the amount of friction required, which in turn determines the clamping force required, and therefore the correct bolt pre-load, as measured by torquing the bolt to spec.

When shear joints are employed, whether they are friction or bearing, they should always be designed so that the fastener is loaded in “double shear” if possible. As can be seen in Figure 5, the fastener or bracket must fail in 2 places for the joint to fail, making it almost twice as strong as the single-shear joint seen in Figure 6.

Figure 5 – Double shear joint

Figure 6 – Single shear joint. Note the bending load on the fastener

Because shear joints (which are the most demanding on fasteners) are very common and are often highly loaded, as in suspension bracketry, the following rules of thumb can help achieve successful joint design:

Ensure that the parts are a very close fit (preferably machined) so that maximum strength can be obtained from the friction between the bolted parts.
Ensure that the joint is sufficiently rigid, and that the holes are perfectly aligned to minimize possible loosening of the bolt and subsequent introduction of shock and bending loads.
Tighten the bolt and nut to achieve proper pre-load and clamping force, and check frequently.
If you must drill the holes, do so with a drill press and properly sharpened bit to keep tolerances to a minimum.
Use only top quality fasteners, minimum of SAE Grade 8, from reputable manufacturers.
Use double-shear joints for all but the most lightly stressed joints.

There is a third type of load all too often imposed on bolts in 4x4 joints, that should be avoided at all costs. This type of load is called bending load and, naturally, occurs when the forces try to bend the bolt. Bending load is actually a combination of tension and compression load on opposite sides of the bolt. Bolts are not designed for this type of load, and if subjected to it will quickly fatigue and fail. In a bending load, the force is acting perpendicular, or nearly perpendicular, to the bolt, similar to a shear load. The difference is, in a shear joint, the parts of the joint themselves are clamped very closely together, creating friction and supporting the bolt. In a bending load, the force is applied some distance from the support of the joint. As a rule of thumb, bending loads are assumed when the load is applied at a distance from the joint greater than the diameter of the bolt. In the case of the single shear joint in Figure 6, the bending load is caused by a lack of support from the overly-thin joint. It is common to observe bending loads in steering linkages where excessively long spacers have been used between the brackets and the spherical rod ends.

What Grade Bolt to Use?

This is a simple question, despite persistent myths to the contrary. The answer is, SAE Grade 8 from a reputable national manufacturer. Un-graded and Grade 2 fasteners have no place whatsoever on a 4x4 as they are weak and unreliable, and while Grade 5 fasteners may exhibit the necessary strength in some applications, in others they do not, and the lower strength and possible misuse of them is simply not offset by the marginal cost benefit. Grade 8 bolts exhibit greater tensile, yield, and shear strength as well as greater fatigue resistance and, just as important, are capable of greater torque specs and therefore much greater pre-load and clamping strength.

There exists an often quoted myth, that Grade 5 bolts are better in shear than Grade 8 since they will bend before breaking. Not true. Shear strength of alloy steel is approximately 60% of its ultimate tensile strength. Reference to Table 1 shows that the yield strength of a Grade 8 bolt is higher than the ultimate strength of a Grade 5 bolt. The Grade 5 bolt will therefore always fail first whether in tension or shear. The only “gotcha” with the Grade 8 bolt is that, being harder, it is more “notch sensitive.” This means it is more sensitive to build up of stress concentrations caused by notches, nicks, and gouges leading to fatigue and failure. This becomes a non issue if good quality new fasteners are always used and periodically inspected.

Different grades of fasteners can be identified by the markings on their heads. Of course, matching grades of nut and bolt/stud must be used together.

Table 1 - SAE AND ISO GRADE MARKINGS AND MECHANICAL PROPERTIES FOR STEEL FASTENERS

Identification Markings	Specification	Material	Nominal Size Range (in.)	Mechanical Properties
Identification Markings	Specification	Material	Nominal Size Range (in.)	Yield Strength Min (psi)	Tensile Strength Min (psi)
No Markings	SAE J429 Grade 2	Low or Medium Carbon Steel	1/4 thru 3/4	57,000	74,000
No Markings	SAE J429 Grade 2	Low or Medium Carbon Steel	Over 3/4 to 1-1/2	36,000	60,000
	SAE J429 Grade 5	Medium Carbon Steel, Quenched and Tempered	1/4 thru 1	92,000	120,000
	SAE J429 Grade 7	Medium Carbon Alloy Steel, Quenched and Tempered ⁴	1/4 thru 1-1/2	115,000	133,000
	SAE J429 Grade 8	Medium Carbon Alloy Steel, Quenched and Tempered	1/4 thru 1-1/2	130,000	150,000
	ISO R898 Class 8.8	Alloy Steel, Quenched and Tempered		92,000	120,000
	ISO R898 Class 9.8			105,000	130,000
	ISO R898 Class 10.9			130,000	150,000
	ISO R898 Class 12.9			156,000	175,000

What Size?

The diameter of the fastener used will often be pre-determined by the joint design, as in the size of the collar in a bushing. If not, the diameter will have to be determined by a careful analysis of the clamping force and/or shear strength required. Tables 1 and 2 can be used for guidance with the knowledge that an SAE fastener’s shear strength is approximately 60% of its tensile strength. The length of the fastener should be carefully chosen so that there is sufficient grip length to hold the joint securely at the required torque setting without bottoming the nut between the shank and threads and without using washers as shims; while simultaneously having sufficient thread engagement in the nut or tapped hole, without excessive threads protruding from the nut. As a rule of thumb a bolt should protrude through a nut by at least 3 full threads. The reason for this is because often the first 2 or 3 threads of a bolt are poorly formed, usually due to a chamfer on the end of the bolt for easier starting. Thus they will not produce the full strength of the fastener. A screw should thread into a tapped hole at least 1.5 times the diameter of the screw, to ensure the screw will break before stripping the internally tapped threads. Formula for more precise calculations are published in the Machinery’s Handbook.

Check out Part 2 for Fastener Torque and the Importance of Nuts and Washers!

Sources:

Smith, Carroll. Engineer to Win: The Essential Guide to Racing Car Materials Technology, 1985 (Motorbooks International)
Gren, Robert E., Oberg, E., Jones, F.D., Horton, H.L., Ryffel, H. H. (Editors). Machinery’s Handbook, 24^th Edition, 1992, (Industrial Press, Inc.)
Aird, Forbes. High Performance Hardware: Fastener Technology for Auto Racers and Enthusiasts, 1999, (Berkley Pub Group)
Smith, Carroll. Nuts, Bolts, Fasteners & Plumbing Handbook, 1990, (Motorbooks International)

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