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Rivets


Introduction.... Strength.... Materials.... Stressess>... Assumptions......Failure.... ...Efficiency......Cold Formed rivets.....Hot formed rivets .....Cold Formed rivets
Blind (Pop) Rivets.... Sealed (pop) Rivets.... Peel head (pop) Rivets..... Grooved (pop) Rivets .....Relative stength of pop rivets



Introduction

Rivets are considered to be permanent fasteners.   Riveted joints are therefore similar to welded and adhesive joints.   When considering the strength of riveted joints similar calculations are used as for bolted joints.

Rivets have been used in many large scale applications including shipbuilding, boilers, pressure vessels, bridges and buildings etc.   In recent years there has been a progressive move from riveted joints to welded, bonded and even bolted joints   A riveted joint, in larger quantities is sometimes cheaper than the other options but it requires higher skill levels and more access to both sides of the joint

There are strict standards and codes for riveted joints used for structural/pressure vessels engineering but the standards are less rigorous for using riveted joints in general mechanical engineering.

A rivet is a cylindrical body called a shank with a head.   A hot rivet is inserted into a hole passing through two clamped plates to be attached and the head is supported whilst a head is formed on the other end of the shank using a hammer or a special shaped tool. The plates are thus permanently attached. Cold rivets can be used for smaller sizes the - forming processes being dependent on the ductility of the rivet material...

When a hot rivet cools it contracts imposing a compressive (clamping) stress on the plates. The rivet itself is then in tension the tensile stress is approximately equal to the yield stress of the rivet material





Strength of riveted joint

The notes below are assuming that the plate loads are withstood by the rivets.   In practice the loads are generally withstood by friction between the plates under the compressive force of the contracted rivets.   The calculations provided below are simplified but provide relatively conservative joint strength value.  There is still a need to complete fatigue assessments on joints when relevant

Joint Types

There are two basic types of axial riveted joint the lap joint and the butt joint.

The selection of the number of rivets used for a joint and the array is simply to ensure the maximum strength of the rivets and the plates.  If ten small arrayed rivets on a lap joint were replaced by three large rivets across a plate the plate section area (in tension) would clearly be significantly reduced...


Rivet materials

Rivets for mechanical and structural applications are normally made from ductile (low carbon ) steel or wrought iron.   For applications where weight, corrosion, or material constraints apply, rivets can be made from copper (+alloys) aluminium (+alloys),monel etc.


Design stresses

For rivets used for structures and vessels etc the relevant design stresses are provided in the applicable codes. For rivets used in mechanical engineering, values are available in mechanical equipment standards which can be used with judgement.

BS 2573 Pt 1 Rules for the design of cranes includes design stress values based on the Yields stress (0,2% proof stress) YR0.2 as follows:-

Hand driven rivets ..tensile stress (40%YR0.2) ..Shear (36,6%YR0.2)..Bearing (80%YR0.2)

Machinery's handbook includes some values for steel rivets . I have intepreted these values and include them below as rough approximate values for first estimate.  These are typical values for ductile steel.   Tensile (76MPa) .. Shear (61MPa) ..Bearing (131MPa)


Design Assumptions

In designing rivet joints it is convenient to simplify the process by making the following assumptions.

  • The rivets fail in either pure compression, or pure shear.
  • The shear stress is evenly distributed across the rivet section.
  • The bearing stress is evenly distributed across the projected area of the rivet.
  • The force to cause a rivet to fail in double shear is 2 x the force to cause single shear failure.
  • the tensile stress is uniform across the plate area between the rivets.

Rivet Joint Failure

A rivet joint may fail as a result of one( or more) of a number mechanisms..

  • Shearing through one section of the rivet (single shear).
  • Shearing through two sections of the rivet (double shear.
  • Compressive bearing failure of the rivet.
  • Shearing of the plate(s) being joined.
  • Bearing failure of the plate(s) being joined.
  • Tearing of the plates between the rivets.

Rivet Joint Efficiency

The rivet joint effieciency is simply described as follows

Eff = Max Allowable Force applied to Rivet Joint/ Plate Strength with not holes

The joint efficiency is increased by having multiple rows of rivets.  It is also clear that the efficiency can never be 100% .  The maximum allowable force is the smallest of the allowable shear, tensile or bearing forces.

Rivets are initially sized with nominal diameters of between 1,2√ t and 1,4 √t (t = plate thickness)   The diametrical clearance provided for hot rivets is about 1,5mm max.   For cold rivets very tight fits are often provided by using reamed holes.  It is important that the rivets are not positioned too close to the side of the plate or the edge of a plate. m t should be greater than 1,5 d and m a should be greater than 1,5 d. (d = nominal rivet diameter).   It is also suggested that the distance between rivets in the rows (pt) is greater than 3d and the distance between rows (pa )is greater than 3 d.  Uisng these guidance factors the strength calculations are simplified

Rivets calculations are generally completed to check for three failure modes : rivet shear, plate tensile failure, and rivet /plate bearing..These are shown below.   A rivet joint can also fail due to plate shearing (tearing )behind the rivet.   This calculation is not always completed because the joint design should include that the minimim metal land behind the rivet (ma above )is specified ensuring that other failure modes will operate before this mode.

It is also important that the axial pitch (p a ) is maximised (see above note )to ensure that the weakest section of the plate is through a row of holes

Rivet Shear
The rivet shear calculation is

τ = F / ( n p d 2 /4 )

  • τ = Shear Stress (MPa)
  • d = rivet diameter (mm)
  • F = Total Axial Force (N)
  • n = Number of Rivets

Plate Tensile Stress
The tensile stress in the plate =

σt = F / [t (w - n r d 1)

  • σt = Tensile Stress (MPa)
  • d 1= rivet hole diameter (mm)
  • n r = Number of Rivets in a row across the plate

Plate /rivet bearing stress

The Plate/Rivet bearing stress =

σc = F /(n d t)

Plate tearing stress..

The plate shear stress =

τc = F /(2 m a t)








Hot Forged Rivets to BS 4620: 1998

Note: The Standard BS 4620 has been declared obsolescent as it is no longer used for current technologies....

Nom
Shank
dia
Dia
Tol
60 Deg Csk & Raised Csk Snap Hd Universal Hd
Nom dia Raise Ht Nom dia Nom Depth Nom dia Nom Depth Rad Rad
D W D K D K R r
14 +/- 0,43 21 2,8 22 9 28 5,6 42 8,4
16 +/- 0,43 24 3,2 25 10 32 6,4 48 9,6
18 +/- 0,43 27 3,6 28 11,5 36 7,2 54 11,0
20 +/- 0,52 30 4,0 32 13 40 8,0 60 12,0
24 +/- 0,52 33 4,4 36 14 44 8,8 66 13,0
24 +/- 0,52 36 4,8 40 16 48 9,6 72 14,0
27 +/- 0,62 40 5,4 43 17 54 10,8 81 16,0
30 +/- 0,62 45 6,0 48 19 60 12,0 90 18,0
33 +/- 0,62 50 6,6 53 21 66 13,2 99 20
36 +/- 0,62 55 7,2 58 23 72 14,4 108 22
39 +/- 0,62 59 7,8 62 25 78 15,6 117 23
Non-Preferred sizes





Cold Forged Rivets to BS 4620: 1998

Note: The Standard BS 4620 has been declared obsolescent as it is no longer used for current technologies....


Nominal Shank dia Dia
Tol
90 Deg Csk Snap Hd Universal Hd Flat Head
Nom dia Nom dia Nom Depth Nom dia Nom Depth Rad Rad Nom dia Nom Depth
D D K D K R r D K
1 +/- 0,07 2 1,8 0,6 2 0,4 3,0 0,6 2 0,25
1,2 +/- 0,07 2,4 2,1 0,7 2,4 0,5 3,6 0,7 2,4 0,3
1,6 +/- 0,07 3,2 2,8 1,0 3,2 0,6 4,8 1,0 3,2 0,4
2,0 +/- 0,07 4 3,5 1,2 4 0,8 6,0 1,2 4 0,6
2,5 +/- 0,07 5 4,4 1,5 5 1 7,5 1,5 5 0,8
3 +/- 0,07 6 5,3 1,8 6 1,2 9,0 1,8 6 0,9
3,5 +/- 0,09 7 6,1 2,1 7 1,4 10,5 2,1 7 1,0
4 +/- 0,09 8 7 2,4 8 1,6 12 2,4 8 1,3
5 +/- 0,09 10 8,8 3,0 10 2,0 15 3,0 9 1,5
6 +/- 0,09 12 10,5 3,6 12 2,4 18 3,6 10 1,8
7 +/- 0,11 14 12.3 4,2 14 2,8 21 4,2 14 2,0
8 +/- 0,11 16 14 4,8 16 3,2 24 4,8 16 2,5
10 +/- 0,11 20 18 6,0 20 4,0 30 6,0 20  
12 +/- 0,14 24 21 7,2 24 4,8 36 7,2    
14 +/- 0,14   25 8,4 28 5,6 42 8,4    
16 +/- 0,14   28 9,6 32 6,4 48 9,6    
Non-Preferred sizes





Blind Rivets (pop rivets )

Solid rivets require access to both sides of a joint and require some skill for correct installation.   Blind rivets have been developed to solve both of these shortcomings.

Blind rivets are available in a range of body styles such as open , closed end ,multi-grip, and peel with head styles in dome, countersunk and large flange and in materials such as aluminium, steel, nickel copper, copper, plastic and stainless steel.

Blind rivets are normally supplied in sizes up to 8mm and able to clamp two plates with total thickness up to 25mm (normal) 50mm (special).  For steel rivets which are not as ductile as copper /aluminium rivets the size limit is restricted to about 6mm.

Standard Pop Rivet
This is used for all open blind riveting locations for normal materials with no structural or access problems.   The rivet is a hollow tube of relatively soft material with a formed head on one side.   The rivet is pre-assembled on a headed mandrel made from a stronger material than the rivet, the plain end of the madrel projecting a relatively high distance through the head of the hollow rivet.   The rivet shank is inserted through the hole in the parts to be joined such that it projects a set distance out of the far end of the hole.   The mandrel is pulled through the rivet using a special tool causing the projecting end to be upset.  The mandrel is engineered to snap at a set tension resulting in the correct formed head on the far side and the joint being under compression...


Sealed Pop Rivet

This rivet is used where the joint has to be fluid pressure tight.   The end include a sealed end completely enclosing the head end of the mandrel and 100% mandrel head retention.   This feature combined with the fact that the rivet expands radially to fill the hole when set ensures that the rivet will seal at pressures up to 35 bar


Peel type Pop Rivet

This type of rivet is used in soft materials including wood, plastic, glass fibre etc.   It is often used to fasten sheets of soft material to metals sheets.  The peel type rivet has a soft metal body e.g. aluminum with a stronger steel metal.  On setting the the far end of the rivet body splits into a number of petals creating a large blind side gripping area capable of withstanding large pull out forces.


Grooved body type Pop Rivet

This type of pop rivet has been designed for use with thick sections of soft , or brittle material e.g. hardboard, plywood,, glassfibre, asbestos board, brick, concrete etc.   These rivets include a shank with a number of grooves which protrude into the material to be fixed.  On setting these grooves are expanded to grip the material as opposed of the rear face.



Strength of blind rivets (compared to solid rivets )

The strength of blind rivets as given in suppliers tables is generally the ultimate strength at failure.  In comparing the strength of blind rivets with conventional solid rivets (same material ) the solid rivets are generally stronger than blind rivets for the blind rivets where the blind rivet mandrel breaks off below the shear line.   For the blind rivets where the mandrel breaks off above the shear line the blind rivets are generally stronger.

As an example for a 4mm blind rivet the shear strength quoted is 3,1kN (mandrel not in shear plane) and 6,7kN (mandrel in shear plane.   The ultimate shear strength of a solid rivet of 4mm dia (assuming a shear strength of about 336 MPa) = 4.2 kN.

It must be noted that a blind rivet is normally used for lighter structures with thinner section plates.  It is generally accepted that a solid rivet correctly installed is more reliable compared to a blind rivet but considerably more expensive to fit.



Sites Providing Information On Rivets
  1. Saphire Products..UK supplier with dimensional data
  2. Rivets..A very informative short paper
  3. Hanson Rivets..Blind (Pop) rivet supplier -site includes technical information (mostly imperial)
  4. Zygology..Blind (Pop) rivet supplier -site includes technical information (Metric)

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Last Updated 24/01/2014