Hex Cap Screws - What is the intent of the bearing surface? 3

[renasis]

Hello,

I have been investigating the differences between a hex bolt and hex cap screw. The hex cap screw has a round bearing surface underneath the head. The height of this bearing surface is .015" to .025" from the bottom of the head. The circular runout this surface needs to be within a specified FIM reading. What is the intent of this surface? Why is this a feature of hex cap screws and not on hex bolts?

Here is a link describing some of the differences between these types of screws, but it still is not clear to me why the bearing surface is needed.

http://www.portlandbolt.com/faqs/hex-bolts-vs-hex-cap-screws

Thanks,

-ren

 

[JohnRBaker]

If I read my Machinery's Handbook (25th Edition) correctly it seems to suggest that it all comes down to which end gets torqued, which seems reasonable once you stop think about it (which I have to admit that this is the first time that I've ever done so)...

In the case of a 'Bolt' the assumption is that it's used to hold 2 or more items together via a common set of through holes and with a Nut added to complete the fastened joint and that the head of the 'Bolt' is held secure while the Nut is 'torqued' (this assumes of course that the Nut will have a toleranced bearing surface). This is common in construction type situations where parts are pre-drilled and assembled using 'Bolts', which is why the larger examples of these are sometimes referred to as 'Structural Bolts'.

Whereas, a 'Cap Screw' is more often used for assembling machinery where they are often, but not exclusively, used to hold a component in place by passing through an existing clearance hole in the component(s) and then being 'screwed' into a pre-formed threaded hole in the adjoining part. Therefore it can ONLY be torqued via the head of the 'Cap Screw' thus the head needs the toleranced bearing surface so that the proper torque can be applied with some degree of confidence that it will be correct.

Now back when I was designing machinery for a living, we only used 'Cap Screws' even though most of the fastened joints were done using nuts, but there were still many cases where they were used with holes threaded into castings and large fabricated parts, so I guess we just decided to make it simple by using only a single standard to cover all cases. And as alluded to earlier, I never gave it a second thought as to there being any deeper rational behind it.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
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To an Engineer, the glass is twice as big as it needs to be.

 

[renasis]

John,

Thanks for the response. Yes, that makes sense. You would want a tightly tolerance surface to ensure consistent torque values.

I still wonder why the height offset .015" to .025" is needed for this surface, why not tightly tolerance the head?

Thanks,

-ren

 

[JohnRBaker]

If there was no raised area then it's possible that the edges of the hex head, particularly at the points, could gall the mating surface which would introduce a condition which again might cast doubt as to how reliable was the measured torque that was being applied to the fastener.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

 

[dvd]

I believe that hex head cap screws have the round bearing surface to prevent the vertices of the hex intersections from digging in and affecting the torque.