SUMMER 2016

8
THE DISTRIBUTOR’S LINK
Bengt Blendulf
Bengt Blendulf was educated in Sweden and moved to the US in 1974 to start a subsidiary for a Swedish fastener
manufacturer. After working as a technical consultant on the faculty of the College of Engineering and Science at
Clemson University, he established EduPro US in 1997 to teach highly rated courses in Fastening Technology in the US,
Canada, Mexico, Europe and Asia. Being one of the founders, Bengt served as the chairman of ASTM F16.96 Bolting
Technology from 1996 to 2006. In 2006 he received the Fred F. Weingruber award from ASTM for “his efforts to promote
and develop standards for the fastener industry.” In 2013 he also received IFI’s Soaring Eagle Award for “significant
contributions to the technological advancement of the fastener industry”. Bengt is the author of an extensive lecture book,
well over 110 articles and “Mechanical Fastening and Joining”, a book published in 2013 by the Industrial Fasteners
Institute. He can be contacted through www.edupro.us or by email bblendulf@yahoo.com.
BIGGER BOLTS OR SMALLER BOLTS?
An American company was faced with a joint problem.
The assembly had some weaknesses they had a problem
to rectify. It looked like a relatively simple case with a
square end plate being bolted onto a machine part with
four ½” Grade 5 bolts. The external load was applied with a
somewhat irregular fashion and had caused some loosening
of the fasteners. This had eventually caused fastener failures
due to fatigue problems. They did request assistance in fixing
the problem and, at the same time, also to make a quick
conversion to metric since the product was going overseas.
This is (approximately) what the end plate assembly
looked like:
THIS IS NOT A GOOD NUMBER
FIGURE 2
FIGURE 1
The external loads of 10 000 N (2200 lb) per bolt is
not particularly high, but since it is applied at an offset, the
calculation must take this into account. From page 144 in
my seminar work book it is relatively easy to assume that
a Grade 5 bolt 1/2UNC x L would be a good choice. When
a more precise calculation reveals that we are OK on most
safety factors except surface pressure under the bolt head
we can say that we are close but not exactly right. The
problem here is also that the external loads are applied at an
offset which will cause the bolts to experience some bending.
CONTRIBUTOR ARTICLE
Looking at the calculation of the joint (from the above)
it is easy to find the potential problem. I have used the
SR1 program to do this since it is, in my opinion, the best
representation of the VDI 2230 (a German design guide
line). From the “Factors of Safety” it is easy to determine
that it is only the “Safety plate surface pressure” that does
not reach 1 or better. Here we find that we have a 0.76
safety factor, which is not good. Any factor must show 1 or
better to satisfy the minimum requirement so here we find
the reason why the company has had problems in the past.
To fix this problem in inch/pound there are several choices,
but for the export version we will now go straight to metric.
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