Engineering Frequently Asked Questions



What torque do I need for a particular fastener?

Torque guides can be found on the Fastenal Engineering & Design Support webpage within the Other Resources category. If your particular fastener is not represented in these torque guides, contact the Fastenal Engineering & Design Support team.

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How do coatings or platings affect the torque-tension relationship?

A coating or plating changes the surface of the fastener. In general, most coatings increase the lubricity of the fastener over the clean and dry condition. This reduces the amount of friction or surface roughness and means that when the fastener is tightened, more energy goes into stretching the bolt (preload) than is lost to friction between the mating threads or the bearing surface of the bolt or nut. A lower torque will now achieve the same preload in the fastener before the coating was applied. If coatings or platings are employed, the installation torque should use a K-factor appropriate for the internal and external thread conditions.

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What is the K-factor and how do I use it?

The K-factor is derived from the short-form torque equation. When calculating how much torque is required to achieve a certain preload in a bolt, many different factors can have a significant effect. Geometric variables, friction between the threads, and friction between the bearing surfaces and clamp members all play a roll in how much of the input torque is transferred to tensile force in the bolt. It is rare that any of these variables are known for a given application. A simplified formula for evaluating the preload-torque relationship emerged that reduced all the unknown or difficult to determine variables into one value, K. So the equation now appears as T = KDF where D is the nominal diameter of the fastener, F is the desired preload for the bolt, and K is the K-factor. Typically, this value must be estimated or determined empirically. You use the K-factor when you must calculate an installation torque for an assembly.

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Is it better to apply torque to the bolt head or the nut?

In general, there is no difference between applying torque to the bolt or nut. There are, however, situations where it would matter such as:

  • If the through-hole produces interference - then it would be better to apply torque to the nut side of the joint.
  • If the clamp member material is different on one side of the joint - here friction plays a role and you would want to apply torque to the side that has the lower frictional coefficient.
  • If the through-holes have different diameters - this could change the way the bearing face contacts your clamp members (might be better to apply torque from opposite side).
  • If the bolt is very long, it might be better to apply torque to the nut to avoid torsional wind-up effects.

In summary, if you have the same material in contact with the bolt head and nut, the same sized through-holes (no interference) through the clamp members, and the same size/type of bolt head and nut (heavy hex pattern for both, for example), then it makes no difference which side of the joint you apply the torque to.

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Failures and Bolting Problems

What do I do with failed bolts that I want investigated?

Do not attempt to put the broken pieces back together and do not to touch the failure surface. Preserving the failed surface is critical to an accurate failure analysis. If possible, wrap the failed samples to prevent oxidation of the exposed surfaces and contact your local Fastenal store or the Fastenal Engineering & Design Support team for further instructions.

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I have a good bolt and a good nut/tapped hole but I still have assembly problems - why?

It is impossible to manufacture perfect threads. To ensure functionality, tolerances and allowances are given to mating threads. If both the internal and external threads meet their required tolerances and allowances, you should determine the length of thread engagement. Often times, cumulative lead error or high frictional coefficients can cause assembly problems. Ensuring that the nominal length of thread engagement (1 to 1.5 diameters) is not significantly exceeded can help prevent assembly problems. Lubrication is encouraged in any assembly to achieve more consistent and accurate preloads.

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Stainless Steel

Why are some stainless steel fasteners magnetic?

Stainless parts that are cold worked (plastically deformed below the recrystallization temperature) will pick up some magnetism. This is not an indication of the chemical composition of the part. It is merely an unavoidable consequence of the cold working process these bolts go through during their manufacturing. Examples of cold working processes include drawing down of the bolt's body to the pitch diameter for thread rolling, the thread rolling itself, and forming of the head. Some parts may be more magnetic than others, but every part that has been cold headed or had threads rolled will have some degree of magnetic permeability. You can read more about this by viewing the article "Magnetism in Stainless Steel Fasteners."

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How does one prevent galling?

Though galling cannot be fully prevented, it can be minimized via a number of different methods: lubricating threads, adding a PTFE coating, reducing the class of fit, reducing the rotational speed during installation, avoid using prevailing torque locknuts, (typically) rolled threads provide better gall prevention then cut threads, and use proper installation torque. Also of note, the use of dissimilar alloys has a lower chance of galling but the trade off is corrosion properties. See the "Galling" article on the Fastenal Engineering & Design Support webpage for more information.

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What is the difference between 18-8 stainless steel and 316 stainless steel?

The 316 stainless steels have 2-3% molybdenum added while the 18-8 series stainless steels do not. While it has comparable strength characteristics, 316 and 316L stainless steel have a higher degree of corrosion resistance due to the addition of the element molybdenum.

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My customer is asking for 304 stainless steel. Is this the same as 18-8 stainless steel?

18-8 stainless steel contains approximately 18% chromium and 8% nickel. 304 does fall into this category, but so do a number of other stainless steel grades such as 302, 302HQ, 303, 304L, and XM7. Generally, our hex cap screws meet 304 s/s. The other stainless steel products such as socket heads, machine screws, nuts, etc., are likely not to be 304. They are typically 302HQ, which has better properties for cold heading.

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Mechanical Properties

Do Grade 12 bolts exist?

No SAE Grade 12 exists. On the metric side, there are no grades, only property classes. The highest strength property class is 12.9 which requires a tensile strength of approximately 1200 MPa and a yield strength of about 1080 MPa.

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What's the difference between metric property classes and inch-series grades?

In the metric system, the strengths of different fasteners are broken down into property classes and each class uses the same numbering convention - a combination of two numbers X.Y. The first number represents the approximate tensile strength multiplied by 100 in metric units of MPa. The number following the decimal represents the percentage to multiply the tensile strength by to find the approximate yield strength. For example, for property class 12.9, the approximate tensile strength is 1200 MPa and the yield strength is approximately 90% of this value, or 0.9*1200 = 1080 MPa. The SAE grade numbers don't employ a similar convention.

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Can I replace a B7 bolt with a Grade 5 bolt?

It is never acceptable to make changes without notifying your customer or engineering department. Sometimes products may seem interchangeable, but certain applications may produce the circumstances that highlight the differences. While B7 and SAE Grade 5 fasteners have similar strengths, the B7 fasteners are intended for higher temperature applications. B7 fasteners are tempered at a higher temperature, 1100° F minimum, while Grade 5 bolts are tempered at 800° F minimum. This means that Grade 5 fasteners cannot be used in applications where the temperature can surpass about 450° F due to the re-tempering effects that they may encounter while the B7 fasteners can easily handle this temperature.

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Structural Bolts

What torque recommendations can I use for A325 and A490 bolts?

The structural bolting industry does not recognize torque from a formula as being accurate enough to be used for installation purposes. The reason is that they require A325 and A490 bolts to be installed at 70% of their minimum tensile strength. There is no way that this can be accomplished with any degree of certainty when using calculated torque values.

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What does the X or N in A325X or A325N mean? Is this bolt different than the standard A325 bolt?

The -X and -N designations refer to the usage of the bolt, not a different type of A325 bolt. The X designation means that the threads are excluded from the shear plane while the N designation means that the threads are included in the shear plane. The difference is that by having the threads in the shear plane, the shear capacity of the bolt is reduced. The A325 bolt itself doesn't change.

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What is a Type 3 A325 bolt?

The Type 3 is a weathering steel. It basically offers the same mechanical properties as the A325 with corrosion resistance inherent in the material. Fastenal can supply this type of bolt. You will probably need A563 Grade C3 heavy hex nuts and F436 Type 3 washers as well.

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What kinds of locking nuts or locking washers can be used with structural bolts?

The installation methods that are required to be used when installing structural bolts (A325 and A490) would ensure the bolts are installed to a minimum of 70% of the minimum tensile strength. This would provide greater resistance to vibration that any of the standard lock nuts and lock washers. ASTM A325 and A490 only allow the use of ASTM F436 flat washers or ASTM F959 DTI washers.

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What materials are acceptable for use in ACQ lumber?

18-8 or 316 stainless steel would be perfectly fine without a coating in ACQ lumber. Your other choices would be an extremely durable barrier type coating on a carbon steel fastener. Any of the sacrificial coatings including zinc (electroplated or HDG) are designed to give themselves up to protect the base metal. Unfortunately the chemical reaction between the wood and the zinc means the zinc coated parts won't last too long. There are many dip-spin coatings that are acceptable to use in ACQ treated lumber that can be applied to a carbon steel fastener if stainless steel is not a viable option.

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What is the difference between clear and yellow zinc platings?

Aside from the appearance of these two platings, the clear zinc has a lower corrosion resistance than the yellow, but all Fastenal's clear zinc plated products are RoHS compliant while our standard yellow zinc platings are not. Clear zinc is required to achieve 12 hours before red rust appears in an ASTM B117 salt spray test. Yellow zinc is required to achieve 72 hours before the appearance of red rust.

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What is the difference between a hex bolt and a hex cap screw?

A hex cap screw has tighter tolerances on the body dimensions and features a chamfered end and a washer face under the bolt head. A hex cap screw is often called a finished hex bolt. Hex bolts have a flat end and lack the washer face under the head.

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What types of salt spray ratings do Hot Dip Galvanized coatings get?

The hot dip galvanized coating does not perform well in a salt-spray test chamber. In order for the zinc in the hot-dip galvanizing to perform as it was designed, it needs to develop a patina layer composed of zinc carbonate. This layer is very stable and non-reactive which gives the galvanizing its desired properties, but in order to form this patina, the zinc must go through wetting and drying cycles such as those that would be encountered in a real world environment. In an ASTM B117 salt spray chamber, the environment is kept continuously wet which basically washes off the necessary corrosion resistant products that would otherwise be produced naturally by the galvanizing. Therefore, the hot dip galvanizing appears to have a poor salt spray corrosion resistance, but this type of QC testing isn't really applicable for hot dip galvanized coatings.

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What are the temperature limits of Grade 5 and Grade 8 bolts?

Grade 5 and Grade 8 bolts should not be exposed to temperatures above 450°F or below -50°F. Expect a reduction in yield strength at elevated temperatures.

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