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BENGT BLENDULF SELECTING FASTENERS FOR HIGH WORK LOADS WITH HIGH SERVICE LEVELS from page 8
The most obvious types would be:
¤ Hex head (cap) screws and bolts
¤ Flange Screws
¤ Socket Screws
Those categories of fasteners are suitable for higher
load duties, but each has subgroups for specific usage.
What they all have in common are that the driving
systems, head styles, etc., are based on vertical walls,
meaning the walls are parallel to the fastener axis. That
way, the “cam-out”, or the risk of the driving tool being
pushed out while tightening, is not a factor. The socket or
hex wrench does not need an additional axial force to do
the tightening. Ergonomically, this is of great advantage
since the assembler does not have to push down on the
driving tool.
Before we decide whether we should use an external
(i.e. Hex Cap Screw) or an internal (i.e. Socket Head)
driver, let’s keep in mind that there are some alternatives
for the more common and standardized types.
For external drives we have, in addition to the
regular hex head,
¤ Flange Screws
¤ 12-point (flange) head (double hex) screws
¤ 6-lobe or multi-spline flange screws
For internal drives we have, in addition to
regular SHCS
¤ 12-point (double hex) screws
¤ 6-lobe or multi-spline screws
¤ Square drive screws
All those are suitable for higher driving forces (torques).
We will not discuss the specific benefits of the 6-lobe,
12-point in this text, but there are some very valuable
advantages to these designs. Products with these drives
are not commonly available from standard inventories and
may, therefore, require purchasing production quantities.
Let us now focus on selecting external or internal
drive systems.
The external drive (Fig 1A), in this case a Hex
Head, needs a certain amount of space or clearance to
accommodate a driving tool, whether it is a hex socket, an
open end wrench or an adjustable wrench (the latter two
better stay out of production, but they are too often found
in the tool boxes of a service mechanic).
An internal drive (Fig 1B) has its best advantages as
shown here with the head counter bored to be placed
FIGURE 1A - EXTERNAL DRIVE FASTENER
FIGURE 1B - INTERNAL DRIVE FASTENER
below the surface. For rotating machine parts this is
a great plus because the screw head will not catch on
anything. It does not need the clearance for the driving
tool which means that we can avoid bending stresses
more easily than when using external Hex heads. The
down side of counter boring is that we lose some material
thickness in the joint that could be used for clamping,
making the joint less stiff.
Before we all “jump the gun” and replace all Hex
Bolts/Screw with Socket Screws, there are some realities
we need to consider. First of all, socket screws were not
intended to be used as shown in Figure 1a even if we see
them used that way. They were primarily designed so that
they could be counter bored (Figure 1b) or countersunk
(flat heads) to be flush with the surface. This is a very
distinct advantage when we are dealing with rotating
machine tools.
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