High Strength Fasteners 1

[jerryv]

My company exclusively uses Unbrako fasteners in specific high strength applications.

This practice as you can imagine gives our purchasing department fits when fastener deliveries do not meet expectations.

For this reason I have been searching for an acceptable alternative. I have found two companies who come close in strength but they their published strengths do not match Unbrako.

Can anyone recommend an Unbrako alternative or explain why the Unbrako numbers are so much higher.

Example:

Unbrako:
5/16 - 24 Socket head cap screw has a tensile strength of 11,000 lbs.

Camcar and Holo-Krome:
5/16 - 24 Socket head cap screw has a tensile strength of 9,530 lbs.

 

[hydtools]

Safety Socket Blue Devil:

http://www.safetysocket.com/bd/index.htm

Holo-Krome:

http://www.holo-krome.com/

SPS:

http://www.spstech.com/aero/prod_lit/socket_brochure.pdf

Ted

 

[drawoh]

jerryv,

Is your company after Unbrako's stress values, or their QA program? Anybody can claim their fasteners exceed standards.

JHG

 

[CoryPad]

You have found two excellent alternative sources. You have the wrong value for the non-Unbrako sources: it should be 10,440 pounds. These data are standardized in ASTM A 574 Standard Specification for Alloy Steel Socket-Head Cap Screws. The basic strength level is 180,000 psi. Unbrako guarantees 190,000 psi, which is a 6 % increase. Ubrako provide high-value parts, but this is really a gimmick. All manufacturers target above the minimum value to allow for statistical spread, etc., so you could be getting 190,000 psi (or better) every day.

 

[zigzager]

Your best bet, both from an Engineering standardization and Procurement standpoint is to stick with National Aerospace Standards (NAS,) which are consensus standards developed by the National Aerospace Standards Committee, which is part of AIA. A NAS example is NAS1351 socket head cap screw, rated at 180 KSI FTU, which for the 5/16-24 = 10,440 lbs.

 

[jerryv]

According to the Unbrako/SPS catalog (link above) the tensile strength for a 5/16 UNRF socket head cap screw is 11,000 lbs this is above the NAS standards.
If this is correct the SPS is 5% stronger then the Holo-Krome (10,440 lbs) fastener. If you also look the SPS material tensile strength they claim to be able to achieve 190,000 psi. Holo-Krome is stating 180,000 psi. Again a 5% increase in published tensile strength.
Our customers have substituted SPS with Holo-Krome fasteners and claim to have experienced far greater number of fastener failures resulting in costly machine crashes. In our design the increase in strength seems to make a difference. The problem is that it limits us to only one supplier.

 

[TVP]

jerryv,

Unless you are supplying something to the aerospace industry, there is no reason to specify NAS1351 instead of ASTM A 574. The previous post by CoryPad was a good summary of the situation. Have you tested fasteners from both Unbrako and Holo-Krome? Are they definitely different in terms of proof load, hardness, wedge tensile test, decarburization, etc.? Holo-Krome and Camcar are both excellent suppliers of socket-head cap screws, and their fasteners should not perform significantly different from Unbrako in a properly tightened joint.

 

[hydtools]

jerryv,
Why did your customer need to replace the original SPS faseners? Did they properly assemble the new fasteners to your specifications?
If 5% change makes or breaks your design, I think your design needs to be re-evaluated to allow greater variation without failure. Other customers will replace commercially available fasteners in your product. Or you need to make the fasteners 'special' to be replaced only with your OEM fasteners.
We have successfully interchanged fasteners from above mentioned manufacturers.

Ted