Engineers who design products assembled using blind rivets should heed these four rules that are unique to the commonly used fastener:
- Select setting tools carefully.
• Select rivet size and mating joint elements for maximum strength.
• Match rivet material and type to joint materials.
• Match rivet length to joint thickness.
Below are some tips for following those rules.
Selecting Tools and Clearances
Among the main issues to be resolved when selecting a riveting tool include power rating, air-pressure rating, and nosepiece type. Most rivet makers are glad to help and offer advice on these decisions. But once the right tool is chosen, tool clearance should be the next consideration. For example, distances from walls or projections to the rivet centerline should be checked to determine if the tool will fit; that is, if enough clearance (or room) is available. Clearance should also be checked if the rivets are going to be set from the inside of a channel of cylindrical section.
In some cases, the problem can be solved by using a nosepiece extension. It can add up to an inch to the length of the setting tool. A better solution is to set the rivet from the unobstructed side, where the tool can be placed flush with the work.
Note that rivets are installed from the side with the rounded head.
Angles and formed sections can interfere with the setting tool. If possible, rivets should be set from the unobstructed side. Alternatively, the length of the angled section can be reduced or the angle opened up.
Designing for Strength
Companies that make blind rivets usually recommend specific hole sizes for rivets they manufacture. Their recommendations should be followed closely if the engineer or designer wants to get the most strength, maximum pull-up, and best hole fill from the rivet.
If hole alignment is a problem, putting a slot in the joint material facing the setting tool and using a hole of the recommended diameter in the second joint material will give the riveted joint its maximum strength.
Another key factor in getting the most strength out of the joint is to ensure the parts are properly mated. For example, when a tube is joined to a flat sheet or beam, setting the rivet against a section of tube that is flattened provides maximum rigidity. A slightly less rigid, but still satisfactory joint can be made without flattening the cylinder. However, extending the rivet through both walls of the tubing results in a weak joint.
Blind rivets exert high forces when clinched, so rivets should never be set against cantilevered joint extensions. Small gaps between pieces being joined are permissible if both pieces are supported on both sides of the gap. Placing rivets on both sides of the gap creates a rigid join, but it is generally unnecessary.
Combining Different Materials
Blind rivets are most commonly used in aluminum, steel, stainless steel, Monel, and copper. For maximum strength and minimum corrosion, rivets should be made of materials that match the materials of the pieces being joined.
Compressible (soft) materials do not provide a firm base for clinching blind rivets. In such cases, it’s best to add support. One technique is to put a washer against the compressible material, with the rivet head on top of the washer. The washer can often be eliminated by using a large-flange rivet, with the compressible material under the flange.
Match Rivets to Material Thickness
Blind rivets should always have a grip range that spans the thickness of the parts being joined. Actual rivet length is longer that the grip range. If the rivet’s grip is too short, it creates a weak joint. If the grip is too long, the cost of the rivet will be unnecessarily high and the rivet may interfere with blind-side clearance.
Blind-side clearance is the distance from the underside of the rivet head to the end of the mandrel before setting. It does not include the combined thickness of the parts being joined. If there’s not enough clearance, the rivet cannot be properly set.
For the strongest joints in corners, the clearance between the rivet centerline and the edge of the material being joined should be at least two rivet diameters. If joint strength is not critical, this clearance can be greatly reduced, especially when rivets are primarily used for positioning rather than strength.