Chapter 15 - Design Considerations for Inserted Fasteners
For fastening and assembling stampings and sheet
metal fabrications together or to other components, incorporating nut and stud
inserts can provide wide design flexibility. Commonly referred to as
"clinch" nuts and studs (Figure 1) or "self-clinching" nuts
and studs, these versatile fasteners can be an effective choice, particularly
when a large number is required and automatic equipment is used for
installation. All of these fasteners can be installed economically--except
concealed head studs, because they require machining operations at very high
cost.
Figure 1. Examples of "clinch"
nuts and studs.
Essentially, inserts are special nut or stud
fasteners that are designed to be pressed into prepared holes in sheet metal
parts. Inserts provide captive male or female threads in materials too thin to
tap, when higher strength fastening is required, or when repeated access after
assembly is anticipated. Economically, they are routinely less expensive than
the conventional nut and bolt alternative.
A great number of different inserts are available
in various thread sizes, lengths, classes of fit, materials and finishes to fit
virtually all design requirements. When quantities justify, custom-made inserts
may prove economically advantageous over standard types.
An important initial consideration in insert
selection is access to the prepared hole. If both sides of the sheet are
accessible, a wider range of manufacturers' products is applicable. How-ever,
when access to only one side of the sheet is available because of other design
features or bends already formed, rivet nut inserts (Figure 2) should be
considered since they can be installed (usually with special tools) like blind
rivets. Alternatively, lack of access to the reverse side may also be solved by
inserting fasteners designed for two-side installation, prior to bending or
other forming operations. When such considerations arise, metalforming suppliers
can offer practical advice.
Figure
2. When insertion of hardware in a prepared hole is impeded by another feature,
rivet nut inserts which are installed like blind rivets from one side of the
metal should be considered. Typical type of rivet nut inserts are shown.
Insert Installation
Basically, nut or stud inserts are inserted into
punched or drilled holes, then clinched or crimped into the sheet metal.
Depending on the type of insert selected, it is clinched against the metal
surrounding the prepared hole, typically deforming and flowing the substrate
metal to lock the fastener into position. Knurled flanges and similar features
are often incorporated in the insert design to aid in anchoring the fastener to
the sheet.
Figure 3. Inserts can be installed by standard
presses with a punch and anvil set-up.
Inserts can be installed by standard presses with
a punch and anvil set-up (Figure 3) and by hydraulic and pneumatic tools. With
automatic feeding capabilities, specially designed presses can install inserts
rapidly.
Some types of inserts, like rivet nut inserts,
undergo most of the deformation via clinching, while the workpiece undergoes
minimal deformation. Here, keyed or ribbed fastener heads are sometimes used to
prevent rotation of the insert in the workpiece, and to resist vibration in
service.
These types of inserts are usually installed via
special pneumatic or hydraulic tools. For high-volume production, fully
automated systems including auto feeders and robotics can be utilized.
Cost Considerations
Generally, the more functions an insert performs
and the more exotic the material and finish, the higher the cost. Beyond that,
the cost of nut inserts vs. extruded-and tapped holes is always a controversial
issue. Often the choice depends on the designer's preference and experience. The
economy of one system vs. the other should be discussed with the supplier.
Extruded holes are ideal in stampings when the
part will be tumbled and finished, then assembled with self-tapping screws.
Because extruded holes are created in a punching operation, many stamping
companies advise using this option when additional holes and other features will
also be punched and formed to create the final stamping. For higher production
volumes this approach can be the most economical choice.
Other means of installing threads--weld nuts and
studs, self-locating projection weld nuts, extruded-and-tapped holes--should
also be evaluated on a cost/performance basis. Metal-forming companies can
advise which alternative is most suited for a particular design.
Figure 4. Example of nut inserts installed flush with the sheet.
Types of Inserts
To facilitate a particular part design, inserts
can be installed flush (Figure 4) into one side of the sheet, or nonflush, with
the head protruding. Flush installations usually require a special head on the
insert and/or a countersunk or counterbored hole.
As depicted in Figure 5, additional fastening
functions are achieved by nut inserts with self-locking features (accomplished
by interrupted threads, coatings, and special nut designs, etc.).
"Floating" fasteners provide for mismatch (e.g., ±0.015 in. (±0.38
mm)) between mating fasteners or holes. Blind-end types form a seal against
liquids and foreign contaminants; and, special spring-loaded panel fasteners can
be flush-mounted as a single unit.
Depending on the type selected (and the
manufacturer), inserts can be used to join more than two components together.
For example, rivet nut inserts can join or "rivet" two sheets and also
provide threads to mount a third component (Figure 6).
Figure 6. Here a rivet nut insert is used to join or "rivet" two sheets and
also provide threads to mount a third component, an angle bracket.
Key Design Parameters
Once the insert type is chosen, based on
functional and aesthetic requirements (such as flush mount, self-locking nut or
concealed head stud), other important factors remain to be addressed. Among
them: strength, workpiece hardness and thickness, material compatibility,
finish, distortion, clearance and tolerances.
Figure
7. Strength of inserts is measured by push-out and torque-out values.
Retention strength of inserts, as measured
by push-out and torque-out values (Figure 7), is a direct result of the metal
flow and interlocking that occurs during installation. Consequently, the design
(and material) of the insert and the material into which it is being installed
can have a significant effect on strength. As may be expected, strengths
increase with larger diameter inserts and thicker sheets.
Aluminum inserts in aluminum sheet exhibit lower
push-out and torque-out values than steel inserts in steel. When higher strength
is needed, both carbon steel and stainless steel inserts can be used in aluminum
workpieces. For applications that require optimum strength--such as when
replacing weld studs--high-torque-resistant studs with heavy heads can be
specified to boost pull-through values.
For rivet nut inserts, tensile strength, thread
strength and shear strength, as well as torque-out values can be used in
determining what insert type will resist design stresses. Highest thread
strength is provided by stainless steel inserts, followed by steel, then
aluminum. However, most steel self-clinching fasteners are heat treated and will
then prove the strongest threads.
Hardness of the workpiece is also an
important criterion. In general, inserts are recommended for use in workpieces
up to a specific maximum hardness according to the insert's material type. For
self-clinching fasteners the fastener must always be significantly harder than
the workpieces.
Thickness. Depending on the manufacturer
and insert type, thicknesses from as low as 0.020 in. (.5 mm) up to about 1Ž2
in. (12.7 mm) are suitable for inserts. For nut inserts in sheet metal, typical
workpiece thicknesses range from 0.030 in. (0.75 mm) to 0.125 in. (3 mm),
corresponding to thread sizes from #2-56 to 5/16-18.
For best performance, insert size (diameter and
threads/in.) and shank length for a given type of insert should correspond to
the minimum thickness recommended by the manufacturer. However, shank lengths
recommended for a specific minimum thickness can usually be used in thicker
workpieces so that one "standard" size can be used throughout an
assembly and for similar parts.
Compatible Materials. Inserts are
available in a wide variety of materials, including aluminum alloys, carbon
steel, stainless steel and brass. In general, the insert material should be the
same or nearly the same as the composition of the workpiece material to avoid
galavanic corrosion.
In service environments where this is not a
problem, steel and stainless steel inserts can be used in aluminum to achieve
higher strengths. Steel and stainless steel inserts are installed in aluminum
after anodizing or other finishing operations. Stainless hardware is installed
in steel after plating.
Finish. Various corrosion-resistant
finishes to meet commercial and military specs may be specified. Typical for
steel are cadmium and zinc plating, to which a clear or other chromate can be
applied for additional corrosion protection. Other finish options are available,
but require longer lead times. It is noteworthy that cad plated hardware is
becoming less desirable for environmental reasons, and its use is outlawed for
European markets altogether.
For aluminum, anodizing or color anodizing are
the two options. Stainless steel inserts are usually passivated to enhance
corrosion resistance. Inevitably, standard finishes vary among manufacturers;
custom and unique finishes usually command a premium cost and require extended
lead times.
Edge distortion is hardly ever a problem
if the insert manufacturer's recommendations are followed. Closer proximity to
the edge leads to edge distortion, which may interfere with subsequent assembly
and/or part function. While smaller than minimum edge distances can be used,
they usually require special fixtures to restrain the sheet during installation.
This extra expense is usually cost-prohibitive and often cannot be guaranteed to
prevent distortion.
Figure
8. Proper installation of hardware may be constrained by required clearances for
insert installation equipment, subsequent work to be performed using assembly
tools, and the specified length of the inserts. In this instance, the clearance
between the top of the inserts and the upper flange, and the distance between
the inserts and the inside bend, are insufficient.
Clearance must be adequate. While hole
locations and their distances from other features are usually dictated by design
analysis and the methods that create them (see punched holes and slots in
Stamping Production Chapter), these should be tempered by such factors as
additional clearance for the insert installation equipment, the subsequent
assembly tool, and the length of the insert (Figure 8).
Accessibility should definitely be
considered in the design stage. Particularly important is the closeness of
inserts to bends and formed features. Even if the insert is installed prior to
bending to accommodate a difficult manufacturing sequence, subsequent access to
the insert is a must so that the mating fastener can be installed.
Figure
9. "Recommended" and "not recommended" dimensioning for
accurate positioning of holes.
Tolerance Considerations
Generally, hole diameters for clinch nuts and
studs should not be toleranced by the designer since the supplier implements
nut/stud manufacturing tolerances to maintain insert locations. Usually only the
diameter of the hole should be specified with a reference indicating the insert
type, length, etc.
Typically, the supplier can advise whether the
holes need deburring. Many manufacturers recommend no deburring, since this
extra metal can result in a better clinch. In contrast, manufacturers of
rivet-nut inserts ordinarily recommend clean, burr-free holes.
When accuracy of hole location is important,
holes should be dimensioned from a datum (not chain dimensioned) to avoid
accumulation of tolerances (Figure 9) and resulting misunderstandings. If
tolerances achievable with inserts and punched holes are not acceptable for the
design, then another method of fastening should be considered.
During insert installation, even with the anvil
and punch on center, some movement occurs as the insert is put into the hole and
clinched. As a result of this variable, as well as variations in concentricity
of hardware, and hole location, the accumulative tolerances can range as much as
0.015 in. (0.38 mm) from the design centerline.
If tighter tolerances are required, use of
a fixture can provide more accurate hole location. Perpendicularity of inserts
to the sheet is usually quite consistent as a result of proper installation.
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Excerpt taken from Design Guidelines for Metal Stampings and Fabrications -- 2nd Edition copyright © 1995 Precision Metalforming Association
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