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Blind Rivet Fastening

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 Fastnrs (Pop Rivets collage)What Are They?

Increasingly, blind rivets are found in both blind and non blind applications as replacements for more traditional forms of joining such as welds, screws, adhesives, nuts and bolts and other forms of riveting.

Advantages Of Blind Riveting.

Blind rivets offer low in place costs, fast assembly time and simple, relatively inexpensive tools. They provide strong, reliable, vibration proof fastenings and are tamper-proof.

Blind rivets are also versatile by providing secure fastenings in thick or thin, soft or hard —even dissimilar materials including plastics and other easily damaged material.

Anatomy Of A Blind Rivet

Blind rivets consist of two parts: A setting mandrel and the rivet body.

Pop-rivet-image-2

The second part, the body of the rivet may be open or closed on the end. Most blind rivets used are open ended. Close end rivets seal tight, preventing vapour or liquids from passing through the set rivet. They also provide greater tensile and shear strength than an equivalent open end variety. The body diameter determines the Size of the rivet while the length of the body,  measured under the head like screws, is the grip length. These are two major factors which affect joint strength. Blind rivet bodies are available in carbon steel, stainless steel, copper, aluminum, monel and soft-set aluminum for soft materials.

The mandrel looks a lot like a finishing nail. It has a bulbed head which has a preset breaking point from the rest of the mandrel body. Just after the break point is a slight widening of the body which aids in locking the mandrel to the body during manufacture so they come ready to use out of the box.. The mandrel is the part of the rivet which goes into the pulling tool and a matched to the specific rivet body. Mandrels can be made of aluminum, carbon or 300 series stainless steel.

There is a head at each end. The primary or finished head style is an intrinsic part of the body and is what you see outwardly once the rivet is installed. There are three basic primary head styles: dome, countersunk and large flange.

The secondary head is the one you don’t see; The one formed by the mandrel as it is pulled up into the rivet body.

Larger secondary heads are also available. These rivets are designed to form large folds or wings which provides greater bearing surface and are for use in high strength, structural applications and those which incorporate soft or brittle material.

Pop-rivet-image-3

Once the rivet size and length have been determined, a hole is drilled or punched through the materials to be joined. In the case of fastening into a blind hole, the depth of the hole must accommodate the unset length of the rivet body.How Do They Work?

Usually, the mandrel is inserted into a rivet setting tool and the the rivet body is inserted into the hole until the head sits tight against the material.

The tool is then used to set the rivet by pulling the mandrel head up into the rivet body. This causes the body to expand forming the secondary head and a strong, tight, reliable joint. Once the predetermined breaking limit is reached, the mandrel breaks off and falls away.

Clearances, Allowances and Factor Affecting Joint Strength

Tool Clearance

Designs must take into account the requirements for the tooling to access the assembly position of the rivet. When clearance is not adequate such as with channel and extruded sections, you can set the rivet from the other side, widen the channel or possibly use an extension on the nose of the setting tool.

Edge Clearance

The distance from the center of the rivet to the edge should be not less than 2 times the rivet diameter. Where joint strength is not critical, this can be reduced. For example, a 3/16 (#6) rivet should be spaced no closer than 3/8” from the edge.

Blind Side Clearance

When setting in a blind hole, it is important to have the rivet  long enough to fully form a secondary head. Blind side clearance refers to the length of the mandrel which is inside the inner wall of the fastening. It is “the distance from under the head to the end of the mandrel before setting minus the thickness of the materials being fastened”.

Spacing

The ideal distance between rivet centers, regardless of the type of joint should be three times the rivet diameter. This reduces incidence of rivet or material failure and  can be reduced or increased, depending on joint load.

Rivet Size

The length and diameter of the rivet contribute to joint strength. Provided the fastening is tight, both tensile and shear increase with diameter but are unaffected by length.

Hole Size

Having a correctly sized hole is very important in blind riveting. Too small a hole makes rivet insertion difficult; Too large a hole reduces shear and tensile strength and results in improper setting of the rivet. Punched holes are always more precise than drilled holes and have the greatest fit benefit. If you drill the holes, remove any burrs in and around the holes.

Material Compatibility

Whenever possible use a blind rivet which matches the materials being fastened. Use stainless with stainless and aluminum with aluminum, etc. Doing so prevents shear breaks and the galvanic corrosion that can occur between dissimilar materials. Galvanic protection can also be accomplished through the use of plated, painted or anodized rivets.

Selecting Blind Rivets

There are several factors to determine in order to select the best blind rivets for the specific applications.

You need to determine the tensile and shear values for the application. Consult the specs of the manufacturer of the rivets you plan to use. Usually this will be the single-joint or typical ultimate strength. Shear is when the body of the rivet breaks across the middle. Tensile strength refers to the amount of force required to pull/push the body out of the materials.

Fastnr (Blind Rivets-2ndry heads)

An important factor to determine right off is the total thickness of the material to be joined. A single blind rivet can cover a short range of  thicknesses, this is called the grip range. Select a length that your total thickness falls into. If rivets are too short, the secondary head will not form or perform properly.

As a general rule, the rivet should be of the same physical and mechanical properties as the materials to be fastened to prevent fatigue and corrosion.

Determine what type of primary and secondary heads would best suit your needs and check with manufacturer specs for availability. For most applications, a low profile dome head is suitable. Large flange primary and winged secondary heads don’t provide greater shear or tensile values, they simply offer greater contact bearing surface for fastening soft or brittle materials.

If you need a fully sealed fastening, always use a closed end rivet; for structural applications, look for rivets with the highest shear and tensile values.

Design Considerations For Blind Rivets

There are a few things to keep in mind when designing projects incorporating blind rivets which will make using them easier and efficient to use.

Always consider accessibility for the setting tool. You need to allow enough clearance  to be able to set the rivet with it being flush against the work.

If the height of the head is an issue, or for esthetic purposes, consider using a flat, countersunk head rivet.

When you want to form pivoting joints there are a few possible solutions. The best is to use a rivet which forms a small space between the rivet head and the work. This allows the joint to move freely. Special rivets are available which accommodate this action. If some friction or binding is acceptable, pivoting joints can also be accomplished without the space and with or without a spacer/washer between the two materials.

To make the tightest joints, never use rivets over unsupported spaces. Blind rivets exert considerable force when set, pressure which is spread over an area wider than the rivet head which would cause this type of joint to fail.

In joining tube to plate, maximum rigidity  occurs with the primary head set against the plate and the secondary head inside the tubing. Even better would be if one side of the tube was flat. A blind rivet fastened through both walls of the tubing would be very unstable.

Blind rivets can also be successfully set into blind holes, against a milled slot, intersecting hole or internal cavity.

When joining hard and soft materials together. The best way would be to use a large flange primary head against the soft material and set the secondary head against the hard material. If compressible materials are involved, a washer and a bushing or a flanged bushing should be used to provide a firm enough base for the pressure of the set.

If you have to have the primary head against the hard side, use a large secondary head or a back-up washer against the soft material.

When assembling thick and thin materials, the best joint occurs when the primary head is against the thin material. If you must set the secondary head against the thin material, use a back-up washer.

Single rivets can also be used to fasten multiple layers of material together.

If you are engineering a product and want to use blind rivets, check out the manufacturer’ resources on the ‘net or give your local fastener distributor a call.

In critical situations, actual tests should always be conducted before full-scale production begins.

Blind Rivet Setting Tools.

Blind rivet manufacturers have standardized the product to a point where the setting tools are universal. One brand of rivets can be pulled by another brands tools.

Essentially there are two types of tools, manual and power. Most people have seen the basic hand variety even if they didn’t know what it was. These tools loosely resemble a nut cracker. Two  opposing handles are joined at the setting tip and are squeezed together to set the rivet. Tools for larger rivets resemble bolt cutters it long handles for greater leverage and adjustable strokes. Some manufacturer’s also make specialized hand riveters.

Power riveters are either pneumatic or air/hydraulic actuated. and resemble a pistol in appearance. The main advantages are in productivity, interchangeable pulling heads and decreased operator fatigue. Not all power (or hand for that matter) riveters pull all rivets. Consult the specs. to make sure it’ll do the job you need it to do.

Fastnr (POP Tool prg510)

ECL2760

ECL2730

Web Resources

emhart.com - Manufacturer of POP™ Brand Rivets

avdelcherrytextron.com - Manufacturer of Cherry Blind Rivets.

gesipausa.com - Manufacturer of Gesipa Rivets

marsoncorp.com - Manufacturer of Marson Rivets

imperialrivet.com - Manufacturer of Beryl Rivets

allfastinc.com - Manufacturer of AllMax/AllFast Rivets

 
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