Stretch vs Non-stretch fasteners - TDIClub Forums

I saw this post, and had to comment here. CAUTION - lengthy techno-geek discussion follows -

At work, i frequently design items with bolted joints, so i'm sort-of an expert in this subject.

Considering i work in the aircraft industry, seemingly simple things like bolt torques are not taken lightly, and involve calculations for all but the simplest joints.

Most of what has been posted above is correct, but there are a few items i want to elaborate on.

Torque To Yield (TTY) is indeed used for consistent preload, but it is also used to increase the clamping load slightly for a particular size and strength of bolt. It's hard to explain, but it has to do with the material strength curve. Simply put, the max strength of a material is at a point slightly past where it has yielded. (the high-point in the stress-strain curve) It allows the engineer to use a slightly-less capable (and therefore less expensive) bolt in a particular application.

The disadvantage to TTY hardware is, as we see here, not all mechanics understand the importance of the fastener, and will either ignore the special procedure to install, or will reuse the bolts, which will be
weakened when they are yielded a second time. In the aircraft industry, this can become a danger, and therefore we virtually never use TTY, in fact, i can't think of a single TTY aircraft application.

Angle-of-turn is most commonly used for any critical hardware. The info regarding angle-of-turn, or "angle-controlled-tightening" on that link above is not really correct, at least not in our industry. A.O.T. does not normally tighten the bolt beyond the elastic range, and the bolts are reusable. When applying AOT the bolt will first be tightened to a regular torque, but a very-low value. This ensures that the parts are fully seated, and the low load ensures frictional variability will be small. The bolt is then tightened a particular number of degrees, which has been calculated based on the pitch of the thread, and the strength of the bolt material, to be somewhere below the yeild strength. Because the torque is not measured, for all practical purposes, friction does not matter. Using this, the tip of the bolt moves a fixed amount based on thread pitch and angle, which puts on a much more consistent preload than torquing.

The above paragraph is the really the same principle as the bolt length change method, which is one of the most accurate methods available, but which can only be used where both ends of the bolt are accessable, and where precision measuring tools and the knowledge to use them is also available, but the advantage is the length change method is more accuracy. This is where the length of the un-tightened bolt is measured very accurately, and the bolt is tightened until the bolt is a calculated amount longer than it was before. This is more accurate than AOT, because the reduced length change based on "winding up" or twisting of the bolt does not come into play here. However, this is variable too, based on accuracy of length measurement, and is only applicable to bolts where both ends are available for measurement.

Replacing stretch bolts with non-stretch bolts is possible, however the replacement bolt must be a higher-strength bolt, and a new torquing procedure must be calculated, since the old one will not apply
anymore.

That only solves the strength and preload problem, the consistency of preload, which can be important when things like crush-style head gaskets are involved, had not been improved. Using an extreme-pressure
bolt lubricant like anti-sieze (not motor oil) does improve consistency of preload, but since anti-sieze has a lower (but more consistent) friction coefficient the proper tightening torque is noticably lower with a bolt using antisieze.


To sum it up, you could replace the bolts, provided you can find a higher-strength bolt, and you could calculate the proper torque (or AOT), but i wouldn't reccomend it, and remember, this is coming from someone who could fairly easily recalculate it. Just believe that the factory engineers know what they are doing, and trust them on it. Head bolts are probably the highest-stressed bolts in the entire car, and believe me, they didn't make them TTY just to get you to buy more parts.

mailman said: I spoke with my local dealer, and apparently they reuse the torque to yield fasteners. Boneheads. When something yields plastically like these fasteners are designed to do, it permanently deforms and should NOT be reused. ALWAYS replace them.

But this is a dealer with factory trained techs, should they not be just taken for what they say to be true?

That's pretty scary but at the same time normal to hear from a dealer as the first TB i did on my car I had to special order these bolts as it's not a stock item, and yet I saw at least 3 tdi's in the lot that were getting TB's done.....

I mean when you use the TTY and stretch it, why not reuse it and stretch it even more.......

And Mike994 that's a great explanation, and for the most part as you stated in my experience there are not many if any TTY in the aircraft industry, just the collars the break at the correct torque and it's done. Or if something is torqued like a mini nut it's done, then re torqued by someone else to make sure it's done, as I did yesterday when doing a small part of the install on a winglet project on a 757.

Standard fasteners and OEM fasteners have distinct characteristics that set them apart. Below, we look at some general key differences between the two.

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Specification and Design

Standard Fasteners: These fasteners are the common types you'll find in most hardware stores. They are produced according to universal standards like ISO (International Organization for Standardization), DIN (Deutsches Institut für Normung - German Institute for Standardization), or ANSI (American National Standards Institute). For example, a standard M8 bolt adheres to specific dimensions and thread patterns that are universally recognized. This means you can buy a standard M8 bolt from any manufacturer, and it will have the same specifications.

OEM Fasteners: In contrast, OEM fasteners, or custom made fasteners, are tailored to fit the unique needs of specific equipment or products. A classic example is the fasteners used in automotive engines. These fasteners may be designed with unique head shapes or threading to match the specific requirements of a car model, enhancing performance, or fitting into restricted spaces where standard fasteners cannot.

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The Role of Fasteners in Vehicle Manufacturing

Availability and Accessibility

Standard Fasteners: One of the biggest advantage of standard fasteners is their availability. You can easily purchase them from multiple manufacturers or local hardware stores. They are the go-to choice for general applications, repairs, and maintenance work.

OEM Fasteners: On the other hand, OEM fasteners are not as readily available. They are often only sold through the original manufacturer or specific dealers. For instance, if you need a replacement screw for a specific brand of laptop, you might have to source it directly from the laptop manufacturer or its distributor.

Cost Implications

Standard Fasteners: Generally, standard fasteners are more affordable. Their mass production and universal design mean lower manufacturing costs, which translates to a lower price for the consumer.

OEM Fasteners: OEM fasteners can be more costly due to their specialized nature. The design and manufacturing process for these fasteners are often more complex, leading to higher prices. For example, a specialized bolt designed for aerospace applications, with specific alloys for high strength and corrosion resistance, will be significantly more expensive than a standard bolt.

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Understanding Pricing: The Economics of Importing Fasteners to the U.S. 

Compatibility and Interchangeability

Standard Fasteners: The beauty of standard fasteners lies in their interchangeability. They are designed to be compatible with a wide range of products. A standard nut or bolt can be used in various applications, making them highly versatile.

OEM Fasteners: OEM fasteners, however, are often designed for a specific application and may not be interchangeable with other equipment. For example, a fastener used in a particular model of a motorcycle may not fit in a different model, even from the same manufacturer.

Quality and Performance

Standard Fasteners: While standard fasteners maintain a consistent quality and adhere to specific standards, they may not meet the unique performance requirements of specialized equipment. They are reliable for general use but might not have the specific properties needed for more demanding applications.

OEM Fasteners: OEM fasteners are often manufactured to meet higher performance standards. For instance, a fastener used in marine applications might be designed to withstand saltwater corrosion, something a standard fastener may not be capable of.

The choice between standard and OEM fasteners boils down to the specific needs of the application. Standard fasteners offer versatility, affordability, and easy availability, making them suitable for a wide range of general uses. OEM fasteners, while more expensive and less accessible, provide the necessary customization and performance for specialized applications. Understanding these differences ensures that the right type of fastener is chosen for the job, leading to better performance, safety, and longevity of the product or equipment.

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