WINTER 2017

94
THE DISTRIBUTOR’S LINK
BENGT BLENDULF A LOGICAL APPROACH TO A BOLTED JOINT DESIGN from page 8
FIGURE 2
accuracy, lubrication, speed and others. We now
have a tool from IFI that can help us if we are
using Torque-Tension method to get the right
preload in the fastener. It is called “Torque
Book for Fasteners”, a product developed by
Joe Greenslade. From this document we can
find nominal friction values for the vast majority
of lubricants and for various surface conditions.
This help would take us much closer to develop
a reasonable accurate torque value. This would,
combined with the rightly chosen tightening tool,
give a solid start for the joint design.
To verify the joint we need to calculate how
all those variables in geometry, forces, materials,
temperatures, tightening, fastener choices, etc.
will fit together.
In Figure 3 below, we can see some of the
mathematics from SR1 (VDI 2230) on the joint.
This is a great tool for joint calculations and it
showed that the starting point in Figure 1 gave
us a too high surface pressure under bolt head
and nut.
So, the “design force” now goes up from using a M6
fastener to a M12 in the 8.8 column or a M10 in the
10.9 column. This type of estimation of bolt size will be
right in most cases, but must also be verified.
So, we are off to a good start, but need more
information to judge the joint. First of all, are the
MATERIALS in the joint compatible? If all joint parts are
made of steel (any type) and we are using steel fasteners
we should have no problems. If, however, the joint is
made of aluminium we must make allowances for that.
Both for the normal elasticities at room temperature,
and any TEMPERATURE (high or low) influences on
elasticities. Any combination of materials in the joint can
have a great impact on its success.
TIGHTENING will also play a major role in the
joint performance (good or bad). This is an area where
we have great problems with many variables with tool
FIGURE 3
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