Supplier for off the shelf torque-to-yield (TTY) fasteners

[kirrer]

Hi all,

I'm working on a project right now where I need a TTY fastener to attach 2 custom components together. Since the 2 members to be fastened are custom, I can still modify the design around an existing TTY fastener, to save costs on custom fastener manufacturing. Therefore, I'm looking for any suppliers which have these type of fasteners available for purchase.

I can't give out the application for obvious reasons, but the fastener should be appx. M12, fine thread, 10.9 class, preload appx. 75 ±15 kN after yield. The preload force itself isn't as important as a consistent preload - I realize 75±15 kN can likely be achieved with just torque & lubricant, but I need it to be consistent from one application to the next, hence TTY.

Any leads are appreciated, thanks!

 

[Screwman1]

Do you have the installation equipment for Torque to yield or are you going to use torque angle? There is not a special type of fastener for these tightening strategies (although most of the bolts are designed for a specific application). You will need to do a joint design study to determine how much stretch you need in the fasteners and if the bearing surfaces can handle the clamping force.

 

[kirrer]

You're spot on, Screwman1 (fitting name, too!) - we've made a joint study and designed the fastener for the specific joint (thread dia/pitch, material class, length, reduced shank diameter, thread length, etc). A question came up whether we can use an existing TTY screw instead of a new custom screw, and in general, I can make another analysis of the joint to ensure the fastener will work, and modify geometry if necessary. But the problem is that I can only find off-the-shelf parts for screws which are not TTY - ie, Granger, McMaster, etc.

I guess the request for a special fastener is to ensure that there is a reduced shank which will allow for lower fastener stiffness & ensure yielding in the shoulder, rather than in the thread itself.

The goal for torque+angle is for servicability. For series production, yield point sensing equipment could be used to ensure consistent preload regardless of interface friction, but the same equipment could not be used in servicing.

To that end, do you know of suppliers which sell TTY bolts, in the sense that they have this reduced shank for yielding?

 

[Screwman1]

I am not aware of any "standard" torque to yield bolts; everything I have dealt with has been made for a specific design. could you rework a standard HHCS to get the stretch you are looking for? A guy with a centerless grinder could take those parts down in a hurry.

 

[kirrer]

That's what I was afraid of. Even the ones which are mass produced to an existing design are not typically available due to NDA's and proprietary details of the design. And like you, I have also not found any off-the-shelf parts - like you described, the screw design & joint design must fit, and typically the customer has an existing joint and the screw needs to match it, so a custom screw is born.

Thanks for the tip on modifying existing cap screws - that's an option, though I believe I'm going to have to convince the bosses to spend the development cost for the custom screw.

In case anyone else has different experience, please let know - thanks!

 

[1gibson]

No specific reference, but you may look towards automotive for off the shelf TTY. Rocker arm bolts, or head bolts.

 

[CoryPad]

Try ARP

arp-bolts.com/

 

[TVP]

kirrer,

There are a few standards for waisted shank fasteners, but they are aerospace specifications like DIN 65115 or SAE AS3650. Dimensionally they are similar to reduced shank fasteners (ISO 15071 Type R), except that the shank diameter is ~ minor diameter rather than the pitch diameter. As an example, the waisted shank diameter for an M10 would be ~ 7.5 mm instead of ~ 8.9 mm. The transition for shank to thread has a radius almost as large as the thread diameter (e.g. 8 mm for M10) with a 15 degree angle. The waisted shank transitions back to the nominal body diameter under the head, with the shank at least 0.5*d away from the underhead.

 

[kirrer]

1gibson - this is a good tip, but I don't know what size & material bolt is used in what application, other than to buy one & measure it. Do you know of some sort of database / catalog for stock rocker arm / head bolts?

CoryPad - ARP is an excellent recommendation, I started with them actually but they're not selling TTY bolts (at least, in their catalog). I will send an email to my contact there to be sure. The material they use is a bit stronger than a 12.9 though, which is too strong.

TVP - excellent suggestion! The waisted shank geometry would be perfect. I tried searching for suppliers, though, and couldn't find many - additionally, the publicly available information I had found on DIN 65115 suggests it's available in just standard thread (M12x1.75) but not fine thread (M12x1 would be ideal for angle to preload relationship). Do you have access to the standard to know whether it also allows for fine thread, and/or who can supply this kind of bolt in the US? I would imagine there's a better chance of finding a supplier for fasteners according to SAEAS3650, but (without having the standard) I would also imagine these are available in only UNJ thread, and I need to keep it all metric.

I tried googling for about 2 hours after the tips in this thread, but couldn't find any solid leads - hopefully I'm not leaning on you guys too much, just looking for a push in the right direction. Thanks for your help!

 

[1gibson]

GENERALLY - OEM rocker are bolts are TTY. If you find a suitable size, you will want to confirm by checking the OEM's torque specs in the instructions for replacing the rocker arms. You should be able to find install guides via google, but if no other options, you can always check a Hayne's manual.

This might be a start:

http://www.napaonline.com/Catalog/R...eyword&Nty=1&Dn=0&D=rocker+bolt&Dk=1&Dp=3&N=0

 

[TVP]

kirrer,

DIN 65115 specifies MJ threads according to ISO 5855, so the thread size is MJ12x1.25. Manufacturers to consider include Alcoa Fastening Systems (UK, etc.), Johann Maier, and Ribe (Richard Bergner).

With regards to ARP, if you look at their catalog, most of the main bolts and cylinder head bolts are waisted shank designs intended for torque to yield. Under the cylinder head bolt section it even says "Most applications have undercut short bolts that can help eliminate head gasket failures through providing
more “stretch” to compensate for the additional compression of gaskets."

In my opinion, you have two options, depending upon the quantity of fasteners you need. If it is high volume automotive or similar, then you should talk to producers of high volume automotive engine fasteners that are familiar with torque-to-yield requirements. Acument and Kamax are two leaders in this area. If it anything else, then you probably should talk to suppliers of small quantities that will make what you want. Fastenal is one option here. Or perhaps contact a distributor of aerospace fasteners like Atlantic Fasteners (

http://www.afaero.com/)

that will have access to many manufacturers that produce reduced and waisted shank fasteners.