faq

Industrial Fasteners Institute's: Most Frequently Asked Questions About Mechanical Fasteners

Q	Why is a maximum tensile strength specified for ASTM A307 - Grade B?

A	These bolts are often used in pipe flanges, and to protect the flange, the bolt is designed and manufactured to break before the expensive pipe flange is damaged through over tightening. Editorial Note:ASTM A307 now requires a manufacturer and a grade marks on the top of heads. Grade identification consists of "307A" or "307B" on each bolt produced to its standard requirements.


Q	What does ASTM A307 require for cold headed fasteners other than those of the hex configuration?

A	The standard requires that these configurations be stress relief annealed to remove cold work effects, particularly at the junction of head to shank. This is a very important requirement for such products as carriage bolts, for example.


Q	Where do you find information concerning bolts suitable for welding?

A	ASTM A307 includes a supplementary requirement, S1, entitled, "Bolts Suitable for Welding".


Q	Where may one find the definition of "alloy steel" as defined by AISI within ASTM F16 specifications?

A	See paragraph 6.1 of ASTM A490. This paragraph includes a note 4 which defines an alloy steel as follows: "Steel is considered to be alloy, by the American Iron and Steel Institute, when the maximum of the range given for the content of alloying elements exceeds one or more of the following limits: manganese, 1.65%; silicon, 0.60%; copper, 0.60; or in which a definite range or a definite minimum quantity of any of the following elements is specified or required within the limits of the recognized field of constructional alloy steels: aluminum, chromium up to 3.99%, cobalt, columbium, molybdenum, nickel titanium, tungsten, vanadium, zirconium, or any other alloying elements added to obtain a desired alloying effect."


Q	What are the basic differences between an SAE Grade 5 bolt in accordance with SAE J429 and a high strength structural bolt?

A	While both bolts have a minimum tensile strength of 120,000 psi, the differences include the following: Marking Controlled grip ranges min/max for each diameter/length combination of A325's A325's only available in heavy hex configuration as defined in ASME B18.2.1 A325's have specific quality assurance requirements A325T is for lengths 4D threaded fully


Q	What is the rotational capacity test?

A	The rotational capacity test is defined in ASTM A325, Paragraph 6.3. It is a test to determine the efficiency of the lubricant required for both hot dipped and mechanically deposited zinc coated nuts. The test involves full size product. The initial tightening of the nut shall produce a load in the bolt not less than 10% of the specified proof load. The nut is then rotated 240 - 420 , depending upon bolt length. Upon removal of the nut, there shall be no shear failure of the threads, torsional bolt failure, or other signs of failure.


Q	How many threads should protrude through the nut for full thread engagement?

A	A minimum of two thread pitches should protrude beyond the nut face following tightening to assure full thread engagement.


Q	What is the customary temperature of hot dip galvanizing?

A	This temperature is about 950°F and exceeds the tempering temperature of A325's by 150°F. It is important not to leave these products in the galvanizing bath too long.


Q	Why use coatings and platings?

A	Usually coatings and platings are less expensive than going to an upgrade of material like stainless steel from a basic carbon steel. Coatings or platings may help to improve appearance, control torque tension, minimize thread seizure, and may serve as product identifiers in addition to simply providing corrosion protection.


Q	What are the general guidelines for coating thicknesses?

A	To ensure thread assembleability of mating fasteners, the specified thread fit allowance in the inch series divided by 6 provides the maximum coating thickness.


Q	What makes cadmium a desirable material for plating?

A	While there have been many issues involving environment, safety, and health, the lubricity characteristic of this plating material has never been completely duplicated over a full range of applications. In salt water, it forms a protective cadmium chloride surface layer which is not sacrificial.


Q	What factors consume the applied torque when tightening a bolted joint?

A	About 90% of the torque applied in tightening a bolt is used to overcome friction - 50% of the torque is consumed by friction of the bearing face of the nut or bolt, whichever surface is rotated; about 40% is consumed by the contact flanks of the threads. The remaining 10% is useful in producing bolt tension.


Q	Where can I obtain a directory of fastener manufacturer head marking symbols?

A	IFI offers for sale a "Manufacturer Identification Symbols - International Guide" containing symbols from producers world wide. Approximately 1300 companies are included.


Q	What's the correct torque?

A	A frequent question, but cannot be reduced to a simple answer or reference table. Many variables determine the correct answer - fastener surface finish, coating, plating, class of screw thread fit, etc. All variables must be taken into account to determine a realistic torque coefficient. Once calculated, check the result in a sample joint to measure clamp load induced from a given applied torque or breaking torque. Then back off an appropriate percentage. But remember that induced variances can be ±20% from a norm. Good reference information can be found in IFI's Fastener Standards, Sixth Edition ($99.50 - U.S. & Canada; $145 - all other countries); and in An Introduction to the Design & Behavior of Bolted Joints by John Bickford, which is also available from IFI ($106 - U.S. & Canada; $126 - all other countries).


Q	Why do bolts loosen?

A	The main reason is insufficient preload, allowing transverse slip of bolt and joint members. Preload, or residual tension, in a tightened bolt means more to assembly strength than actual strength of the fastener itself. In a joint, a bolt torqued to its proper load level can resist a maximum amount of external load without loosening. Designers can take advantage of this fact to ensure correct bolt loading, and at the same time reduce costs. FACT!: Assembled bolts are tightest when stressed as near as possible to their elastic limit.


Q	How is shear strength of fasteners determined? Why don't industry fastener standards include such values?

A	Common practice for steel fasteners is to assume shear strength will approximate 60% of minimum tensile strength. Published data in commercial (non-aerospace fields) does not offer much guidance on shear strengths for bolts, screws, or studs. The first reason is that the number of components loaded in shear is considerably less than for tension, compression, bending, or torsion. The primary reason, however, is the difficulty in obtaining accurate test data. Shear testing inherently involves a number of variables. Therefore, tests are less reproducible than testing for such properties as tensile or yield strength. Most shear testing has been by arbitrary procedures that provide empirical results. The greatest need for shear test data is in structures that are riveted, pinned, or bolted, and also where service stresses are actually in shear. Notable examples are found in the aerospace industry. (A recommended shear test method is given in ASTM B565.)


Q	Is there a technical information source for identifying and solving root causes of bolt sticking problems in high temperature applications?

A	There are several mechanisms contributing to bolt sticking. The Materials Properties Council in New York, New York has a prospectus of planned research to identify remedies and produce guidelines to minimize difficulties in bolt removal. For a copy of the prospectus and list of industry contact references, phone (561) 627-0228.


Q	What is IFI's position on choice of screw thread gaging?

A	Above all else, threads must fit and function to be acceptable. Thread Acceptability System 21 (Method A) is most practical for all threads, except for Class 3A external threads where System 22 (Method B) is recommended. IFI research investigations prove that System 21 (or Method A) GO/NOT GO limit gages can detect minimum material limits. Also, measurement of pitch diameter as a predictor of fastening performance is a meaningless exercise. Chemistry of fastener material, heat treatment, etc. are the key performance characteristics. Measurement of pitch diameter, on the other hand, is of value to fastener producers for analytical purposes relating to the threading process itself. Thread acceptability System 23 (Method C) should not be used as a routine inspection of threaded fasteners. These requirements should be limited to research and analysis as recommended in FED-STD-H28/20A. When higher quality confidence levels are required, sampling plans should be adjusted or statistical process control should be required instead of adding thread characteristic measurements. Fastener buyers should not modify existing procurement specifications to improve quality when supplier noncompliance has been the real issue.


Q	What are the primary conclusions of IFI's research report (IFI RR-2) to determine if pitch diameter size has influence on fastener performance?

A	Study conclusions were as follows; System 21 thread gaging demonstrated the detection of out-of-tolerance pitch diameters in all test lots as well as System 22 gaging. For aerospace nuts, pitch diameter size had no bearing on the fastener's tensile strength performance. It was clear that when the minor diameter was oversized beyond limit requirements, tensile strength was adversely affected. The aerospace nut's pitch diameter size had no measurable effect on joint performance as related to vibration, torque-tension, or fatigue. The pitch diameter size of commercial bolts correlated to tensile strength in that smaller pitch diameters yielded lower strength, but in all cases, bolts exceeded minimum strength requirements. Even the poorest fit commercial bolt and nut combination (bolt undersized by 0.006" and nut oversized by 0.034") passed all tensile and proof load strength requirements. All commercial bolt and nut combinations exceeded minimum torque-tension performance requirements of GM 9084-P.


Q	What are some hazards in embracing fastener engineering standards from other countries?

A	A word of caution to any company considering adoption of standards developed by another country: Engineering standards are just a single component in a very complex commercial system. Each country has its own unique commercial system. And, until the total commercial system of a particular country is thoroughly understood, it's not particularly useful to compare your engineering standards with the standards of that country. Example: Any North American company that adopts and applies DIN (German) standards in design, procurement, and manufacturing will experience several difficulties which can run the gamut from "minor" to "serious". The primary concern is a differing view of the engineering standards themselves. Europe and Japan do not share North American definitions and attitudes. One example is the word, "standard". For hex fasteners in European auto engines and transmissions, European designers stated they only used first choice ISO diameters and pitches for standard fasteners. Yet, teardown analysis found several instances when 7mm dia. (second choice) hex fasteners were used. On questioning, the European designers stated that these were not "standard fasteners", but were "specials" designed to perform specific functions. In this instance, Europeans viewed "standard" fasteners as those readily available in hardware stores; whereas "special fasteners" are obtained from authorized parts depots. North American designers would have viewed these same fasteners as standard parts. Hence, the problem of obtaining adequate repair parts "in the field" can be a serious challenge. The repair problem is compounded when nonstandard parts are used in the original design. Differing views of standards create other difficulties. In the early days of metric conversion, a review of DIN fastener standards found several dimensional requirements which North American producers stated they could not meet. As an example, the diameter of the bearing surface on DIN hex head bolts was much larger as a percentage of the minimum width across flats than specified for comparable inch standard hex bolts. Interestingly, dimensional analysis of European-made products found that European producers also could not, and were not meeting these dimensions. However, at ISO meetings, no European representatives wished to change the bearing diameter dimensions because these dimensions were not causing problems to either German producers or users. Result: GM plants in North America rejected all of the initial shipments of DIN hex fasteners due to these dimensional problems. Hence, a word of caution! Before taking the step of adopting standards developed by another country, it is essential that the entire commercial system of the country developing and utilizing these standards be thoroughly understood. Unfortunately, those of us in American standards development work have doubts that it is possible to adequately develop the in-depth understanding of the commercial system of another country required to effectively use the standards of other countries. Therefore, we urge companies to work through and use American National Standards in their design and manufacturing practices. We believe that the statement concerning inter-changeability between American National Standards and ISO standards represents an extremely important tool to achieving true international standards. This statement, combined with open, internationally acceptable objectives by North American standards developers, will result in a worldwide system of interchangeable design. Of course, the long-term goal of ASME Standards Committee B18 has been to develop internationally harmonized standards.


Q	Can various bolt grades be mixed in the same joint?

A	A case example: An audit rebuild schedule for 200-ton capacity, two-story-high trucks for mining operations showed that the electric motors for rear wheel drive power and braking (costing $80,000/axle), were to be assembled with SAE Grade 8 bolts. In fact, most drive motor connections were found to be Grade 5. Some even displayed a random mix of Grade 5 and 8, along with even higher grades - all on the same flange connection. All bolts were installed by an authorized rebuilder for the motor manufacturer. Problem: No matter what standard torque setting the fastener installer chose, it would be incorrect. Mixed fasteners lead to serious problems in tightening, producing incorrect clamp loads. Breakdown of these monster trucks was extremely cumbersome and expensive. The same situation deserves attention for your product as well.


Q	Can a fastener end user standardize on a single stainless steel fastener grade?

A	To achieve economies, some users standardize on a single stainless fastener grade - such as 302HQ (often without referencing a consensus standard such as ASTM F593). The problem is that more than one stainless grade may be needed to meet specific end use requirements. Food processing mixers, blenders, cookers, etc. are examples. Incorrect application could cause liability risk. Unfortunate example: Stainless Grades 303 and 302HQ were substituted for Grade 316L to assemble artificial human hearts. This material substitution compromised compatibility with the human body. Result: Screw heads "popped". Yet, all fasteners were procured under "critical" application requirements requiring both lot and part traceability. Marine hose clamp example: Many marked "stainless steel" are actually low-grade stainless, and/or utilize plain steel screws. In underwater thru-hull boat fittings, the clamps could rust, precipitating water leaks. Test: if the clamp sticks to a magnet, it is not suitable for marine use.


Q	When a bolt is overloaded in fatigue, where can I expect failure to occur?

A	Three stress-concentration locations lead to most fatigue failures in common bolted joints: 65% of failures - 1st thread to engage the nut 20% of failures - at thread runout of bolt 15% of failures - fillet junction of bolt head to shank


Q	What's the difference between hydrogen embrittlement and stress embrittlement?

A	Hydrogen Embrittlement: A Close Look If you think hydrogen's detrimental effect on fasteners is just induced from "processing", think again. It could be the "environment". Failure mechanisms often viewed as synonymous are stress corrosion cracking, hydrogen embrittlement, and hydrogen-assisted stress corrosion. Reason is understandable. Cause and effect similarities outnumber identifiable differences. Reality: Only stress corrosion cracking and hydrogen-assisted stress are corrosion related. All cause failure - actual breaking of the part. But, the fracture is delayed. Sometimes it occurs within hours after load is applied. Sometimes not for months, even years. But, when failure occurs, it's sudden, with no advance warning. Failures occurring in service can be serious, costly, even catastrophic. Hydrogen embrittlement is associated with carbon and alloy steel fasteners. Cause : absorption of atomic hydrogen into the fastener's surface during manufacture and processing - particularly acid pickling and alkaline cleaning prior to plating. And then, during electroplating where the deposited metallic coating traps hydrogen against the base metal. If the hydrogen is not diffused out by post-baking, the gas migrates toward points of highest stress concentration when stress is applied. Pressure builds until strength of the base metal is exceeded and minute ruptures occur. Hydrogen is exceptionally mobile. It will quickly penetrate into any newly formed cracks. This pressure - rupture - penetration cycle continues until part failure. Hydrogen embrittlement is non-corrosion-related. It can be neutralized by proper processing before the fasteners are released for service. And, while hydrogen can be baked out before it embrittles, it's not possible to bake out the micro cracks once formed. Stress embrittlement is similar to hydrogen embrittlement - with the generalized exception that the presence of offending hydrogen is chemical-reaction induced through the service environment - not because of in-plant processing. Example: Caustic materials (such as soaps, detergents), in contact with nitrates and silicates, chemically react to release hydrogen which can diffuse into the surface of non-coated fasteners. Steels with high-carbon contents, and heat treated to high strengths, are most susceptible to stress embrittlement. All hydrogen embrittlement failures are intergranular - but not all intergranular failures can be attributed to hydrogen embrittlement. Note these examples: Three self-drilling screws fastened a curved plastic sill plate to a car door liner. The middle screw frequently failed in factory assembly. Analysis showed misalignment where the sill met the curved metal frame under the middle screw. Also, a factory wash test allowed moisture to hit the misaligned screw, acting as an electrolyte in a galvanic corrosion cell that then generated hydrogen. The hydrogen gas migrated to the over-stressed (due to bending) middle screw, causing a hydrogen-assisted stress corrosion failure. Case hardened fasteners used to hold structural aluminum members to steel beams in a U-channel configuration. These channels, used to retain glass windows, had drainage holes in their tracks. The fasteners were coated with a zinc-bearing, corrosion-resistant, organic compound. Environmental moisture from the glass collected in the tracks, causing a galvanic reaction which generated hydrogen and subsequent fastener failure. Structural aluminum stadium bleacher seats bolted to concrete. During rain, calcium from the concrete became the corrosion agent. Screws holding turn signal lights on auto right rear quarter panels were failing in the factory prior to car shipment. Cause: Assembled cars were tested for water leaks with high pressure jets from an enclosed circulating system. Bacteria/sludge problems in the wash were controlled by adding two gallons of sodium hypochlorate daily. But, concentration levels weren't monitored. After 3 months, the concentration built to a level high enough to cause stress corrosion cracking of the fasteners. Car design was such that the pressurized wash could not penetrate signal light joints on the left car side, hence only failures on the right. Interestingly, the fasteners were mechanically galvanized to guard against hydrogen pickup during manu-facture. But, this could not guard the joint from environmental hydrogen pickup.

Hydrogen Embrittlement: A Close Look