Riveted assemblies: design and inspection

What a rivet has to do

A rivet clamps two or more parts together and carries load through the joint. It does that by filling the hole, holding the sheets in contact and forming a second head on the far side. The joint works when the clamp is tight, the hole is filled and the material around the hole can take the bearing load.

That is why a riveted assembly is a system rather than a fastener choice: the rivet, the hole, the sheet, the set tool and the operator all contribute.

Rivet types and where each fits

Solid rivets are the strongest and the simplest: a shank with a preformed head, set with a hammer or a press. They need access to both sides and a bucking bar or a die. Semi-tubular rivets have a hollow tail that rolls over, which lowers the setting force and suits thin sheet and high-volume press setting. Blind rivets are set from one side with a mandrel, which makes them the practical choice for a closed section or a tube.

The choice follows access and load, not habit. A solid rivet in a blind location cannot be set. A blind rivet in a joint that sees vibration and a high shear load may loosen if the hole clearance is too large. A tubular rivet in a brittle plastic will crack the boss if the set force is not controlled.

Hole, edge and pitch

The hole sets the joint. Too small and the rivet will not enter without damage; too large and the rivet has to expand to fill a gap, which reduces clamp and encourages movement. A clearance of about 0.1 mm over the rivet diameter is a common starting point for a solid rivet, but the supplier data and the sheet material decide the real value.

Edge distance protects against tear-out. As a starting point, keep the hole centre at least two diameters from the sheet edge, and increase it for thin or soft material. Pitch between rivets is usually three diameters or more, so the sheet between two holes can carry the load without bulging.

Load path and failure modes

A riveted joint in shear can fail in four ways: the rivet shears, the sheet tears out to the edge, the sheet bears and deforms around the hole, or the head pulls through. Each failure points to a different fix. Shear says the diameter or the number of rivets is low. Tear-out says the edge distance is short. Bearing says the sheet is too thin for the load. Pull-through says the head is too small or the sheet is too soft.

Double shear, where the rivet passes through three plates with the load on the outer two, roughly doubles the shear capacity compared with single shear. A joint that sees repeated load also needs a check on hole clearance, because movement in a loose hole frets the sheet and loosens the joint over time.

Setting the rivet

The set force has to be high enough to fill the hole and form the tail, and low enough not to crush the sheet or crack a plastic part. On a press, the stroke and the force are set and monitored. On a hand tool, the operator controls the stroke, which is why a gauge and a training standard matter more than the tool brand.

Record the buck height or tail diameter, the head contact and the clamp. A tail that is too tall means the rivet was not fully set. A tail that is too flat means the sheet may be crushed or the rivet over-driven. Both are visible on a sample and both are missed by a check that only counts the rivets.

What to inspect

Inspection starts with the setup: the correct rivet, the correct hole, the correct tool and a sample set at the start of the shift. Then check the sample for tail height, head contact, sheet clamp and any cracking around the hole. A visual check at production speed will find a missing rivet, but not an under-set one.

Add a destructive check at a defined frequency. A push-out or shear test on a coupon, or a peel test on a sample assembly, gives a number that can be compared with the first article. For a safety-related joint, agree the sample size and the acceptance value before production, and keep the results with the lot.

• Rivet part number, diameter and material against the drawing
• Hole size and edge distance on the first part of each lot
• Tail height or tail diameter within the agreed range
• Head flush or proud as specified, with no gap under the head
• No cracking, distortion or mark on the visible face
• Destructive shear or push-out result at the agreed frequency

Materials, corrosion and mixed joints

A rivet is usually a different alloy from the sheet, so a galvanic pair is created. Aluminium rivets in aluminium sheet are safe. A steel rivet in an aluminium sheet will corrode the sheet around the hole, especially in a wet or salty environment. Choose a compatible pair, or add a coating, a sealant or an insulating washer. In a plastic assembly, a metal rivet can crack the boss, so spread the load with a shoulder washer or a tubular rivet.

What to put on the drawing

State the rivet standard, diameter, material and finish, the hole size and tolerance, the edge distance and pitch, the required set condition and the inspection frequency. Add the visible face requirement and any sealant. If the joint is structural, say so, and supply the load case so the joint can be checked rather than copied.

• Rivet standard, diameter, material and finish
• Hole size, tolerance and edge distance
• Pitch and rivet pattern
• Set condition: tail height, head flush or proud
• Inspection frequency and destructive test method
• Sealant, coating or isolation where dissimilar metals meet