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Area 51 fastener locking device rant

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Area 51 fastener locking device rant Powered By Docstoc
					The Selection of Locking Devices for Threaded Fasteners

Preamble
             Take this as friendly advice! No warranty is expressed or implied.
                                          -Stephen.

Threaded fasteners are endemic to the industrial age. A nut and bolt offer controlled clamping
force to hold two components together to enable them to work as one. In static and near-static
applications no form of locking device is required to prevent loosening of the fastener. When
the bolted joint is subjected to vibration, such as from a machinery imbalance or resonant
operating frequency, fasteners magically loosen.

This phenomenon has been a challenge to engineers, millwrights, and mechanics for well over
a hundred years. Among the solutions that have been used over this period are:
    • Safety wire
    • Spring washers
    • Nyloc nuts
    • Threadlocking adhesives

Some of these are even used redundantly.

How is a young designer to select an appropriate fastener locking procedure when he/she
wants to deliver a reliable product that won’t fall apart during normal (or even abnormal)
operation?

To properly evaluate the use of locking devices a thorough understanding of fasteners, fastener
theory, and their application is required. This article will first cover fastener theory. It will then
discuss modes of failure, and then look in depth into the mode of failure which is the concern of
this report, fastener loosening. Methods of counteracting the loosening effect are then
discussed and evaluated.

I have been strongly influenced in my beliefs on this topic by two books:
    • An Introduction to the Design and Behaviour of Bolted Joints, by John L. Bickford
    • Loctite Worldwide Design Handbook
As a further qualification, I have owned and operated many British motorcycles!



Fastener Theory
Structurally, a bolt serves two purposes:
    • It can act as a pin to keep two or more joint members from slipping relative to each
       other.
    • It can act as a heavy spring to clamp two or more pieces together.
                                                                                                            


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In the vast majority of applications the bolt is used as a clamp. In either case the bolt must be
tightened properly if it is to perform its intended function. When the bolt is properly tightened it
is stretched a small amount (preload). This stretch is what provides the clamping force.

Fastener loading is a combination of the following stresses:
   • Tension
   • Torsion
   • Shear
   • Bending

Tension is the preload stress of the fastener, and also subsequent loading of the joint. Torsion
is caused by the tightening of the fastener. Residual torsion always exists (when using common
bolt torqueing procedures) constrained by the friction between the face of the bolt head and the
surface of the joint (or washer). Shear and bending stresses can occur in addition to tension as
the machine containing the fastened joint performs its designed function.

Fastener applications can be ranked as:
   • Critical
   • Non-critical

Critical applications are those in which failure will result in catastrophic damage and/or danger
to personnel. Fasteners which can only be serviced after considerable disassembly to access
the fasteners (e.g. connecting rod bolts on an automobile engine) should be classified as critical
applications.

Joints with negligible loading across the joint, such as the valve cover of an automobile engine,
only require sufficient compression to seal the gasket. Satisfactory seal is the only requirement
of this joint. Although failure of this joint may be classified as catastrophic (loss of lubricating oil)
there is no practical possibility of this occurrence. Oil leakage would be noticed long before the
loss of the multiple fasteners that secure the valve cover. This joint should therefore be classed
as non-critical.

I
Fastener Failure and Preload
Preload is the installed stretch of the fastener to provide clamping force, usually accomplished
by tightening with a wrench. Problems created by incorrect preload:
     • Static failure of the fastener. With too much preload the body of the bolt will break or
         the threads will strip.
     • Static failure of joint members. Excessive preload can crush, gall, warp, or fracture joint
         members such as castings and flanges.
     • Vibration loosening of the fastener. No amount of preload can fight extreme
         transverse vibration, but in most applications proper preload can eliminate
         vibration loosening of the fastener.
     • Fatigue failure of the fastener. Most bolts which fail in service do so in fatigue. Higher
         preload does increase the mean stress in a fastener, and therefore threatens to shorten
         fatigue life. But, higher preload also reduces the load excursion percentage seen by the
         fastener. The net effect is that higher preload nearly always improves fatigue life.
     • Stress corrosion cracking. Stress corrosion cracking, like fatigue, can cause a fastener
         to fail. Stress in the bolt caused primarily by preload will encourage SCC.
                                                                                                       


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    •   Joint separation. Proper preload prevents joint separation; this means that it reduces or
        prevents such things as leaks in a fluid pipeline or cylinder head gasket failure and
        subsequent blowby in an engine.
    •   Joint slip. Joints that are subjected to shear loads at right angles to the axis of the bolt
        rely for their strength on the friction forces developed by joint members, forces created
        by the clamping force exerted by the bolt on the joint. Therefore it is preload that
        determines joint integrity. If preload is inadequate the joint will slip, which can mean
        misalignment, fretting, or bolt shear failure.

How Much Preload?
The maximum possible preload is always desired, but in choosing this, the following must be
considered:
   • The strength of the fastener and of the joint members under anticipated static and
       dynamic loads.
   • The accuracy with which the fasteners can be tightened.
   • The importance of the joint, ie. The safety factor required.
   • The operating environment that the joint will experience in use. (temperature, corrosive
       fluids, shock loads, etc.)
   • The operating loads that will be placed on the fastener in use.

Factors That Affect Working Loads in Fasteners
   • Tool accuracy: the accuracy with which a torque wrench (or equivalent) produces a
      desired torque.
   • Operator accuracy: operator error is a variable.
   • Short-Term relaxation: even if a joint is preloaded accurately it doesn’t usually retain
      that preload. Tightening adjacent bolts can cause previously tightened ones to partially
      relax.
   • External loads: external loads add to or subtract from the tension in the fasteners, and
      therefore the clamping force in the joint. Such loads must be predicted and accounted
      for when the joint is designed, and when the correct preload is chosen. External loads
      are created by such things as hydrostatic pressure, inertia, the weight of other portions
      of the structure, etc.
   • Thermal Effects: differential expansion or contraction in bolts or joint members can add
      to or subtract from the tension in the fasteners and the clamping force on the joint.
   • Long Term Relaxation: long term relaxation can be caused by corrosion, creep, cyclic
      loading, or vibration.
   • Quality of Parts: parts must be of the right size, hardened properly, and in good
      condition. If bolts are soft the expected preload cannot be achieved. If joint members
      are warped or misaligned it may take an abnormal amount of tension in the fasteners to
      create the necessary clamping force between joint members.
                                                                                                      


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Friction
If no other method of locking a fastener is used, friction is what maintains the preload of the
assembled joint. In a static application friction can be adequate if care is taken to maximize the
friction available. In a dynamic application, however, fluctuating tensions (especially overload
tensions causing plastification of the fastener or joint) will negate the friction contribution.
Without the friction component the net forces that work on the fastener will tend to loosen it.
These forces are primarily the residual torque from tightening and the inclined plane of the
thread helix. Even if an application is thought to be static, there can be vibration from other
components which will cause loosening, so it is unwise to rely on friction alone in any critical
fastener application.

Variables That Affect Friction
    • The hardness of all parts
    • Surface finishes
    • The type of materials
    • The thickness, condition, and type of plating, if present
    • The type, amount, condition, method of application, and temperature of any lubricants
       involved
    • The speed with which the nut is tightened
    • The fit between the male and female threads
    • Hole clearance

II
Fastener Failure
Fasteners will fail in five modes:
   • The body of the bolt will break (usually at one of the three stress concentration points)
   • The bolt threads will strip (fail in shear)
   • The nut threads will strip
   • Both threads will strip simultaneously
   • The fastener will loosen

Sudden failure by breakage of a bolt or the stripping of threads is beyond the scope of this rant.
This rant will concentrate on the final item of the above list, loosening.

Fastener Loosening
There are two methods of failure by loosening
   • Relaxation of tension
   • Self-loosening

Relaxation of Tension
A threaded assembly relaxes when a permanent change in the length of the bolt occurs in the
direction of the axis, or the joint substrate itself relaxes, as in gasketed surfaces. This reduces
the bolt tension and thus reduces the residual clamping force. Permanent reductions in tension
may be produced by:
    • Settling: The rough surfaces of the contiguous parts (e.g. nut, washer) may become
        smoother under the pressure of the bolt tension.
    • Creeping: The surface pressure on the bearing surface of the bolt or nut exceeds the
        compressive strength of the material of the stressed part.
                                                                                                      


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Preventing Relaxation
   • Use bolts with a high L/D ratio (shaft length / bolt diameter). This results in more of the
      shank of the bolt acting as a spring to compensate for minor relaxation.
   • Collar (flanged) bolts and collar nuts used in conjunction with hardened washers that
      reduce surface pressure, and thus, settling on the bearing surface.

Self Loosening
The threaded assembly loosens itself when sliding movements occur between the contact
surfaces. These forced relative movements overcome the frictional forces in the threaded
assembly and the self locking effect of the thread is eliminated. Only when the clamping
force is great enough to prevent such movements can the movement that tends to
loosen the assembly be overcome.

The loosening moment is only dependent upon: the pre-stress force, the pitch diameter, and the
helix angle of the thread. It acts against the direction of tightening and causes the threaded
assembly to loosen when sliding (relative motion at the fastener/contact surface interface,
typically between the underside of the bolt head, the washer, and the surface of the part)
occurs. Sliding movements between the contact surfaces can be caused by:
    • Dynamic load in the direction of the axis. A pulsating axial overload leads to a relative
        movement at the flanks of the thread.
    • Dynamic load at right angles to the direction of the axis. Bending, differential thermal
        expansion rates of the materials, impacts, and vibrations can overcome the forces of
        friction between the bearing parts.

Vibration
Vibration is the most diabolical force that is encountered. The friction of the face of the bolt
head and the friction of the bolt threads are effectively negated by intense vibration. This leaves
the counter torque from pre-stress and the incline of the bolt threads as the dominant force,
which results in loosening of the fastener.

Preventing Self Loosening
   • The use of high tensile bolts allows higher clamping pressures and pre-stress forces that
      may counter and prevent relative motion.
   • Change the design to allow more fasteners to provide clamping force
   • Change the design to incorporate bolts with a higher L/D ratio (historically, according to
      Loctitie Corp, L/D ratio >/+ 6 has been optimum).
   • Friction can be increased by influencing the surface finish and structure on the bearing
      surfaces of the bolt and nut.
   • By applying adhesive, the degree of freedom for lateral movement is eliminated due to
      the fact that the gaps around the thread flanks are filled, and at the same time, thread
      friction is increased by interfacial connection after the adhesive has cured.

NB: Safety wired fasteners can only be justified in positions where loosening fasteners will
result in catastrophic damage and/or danger to personnel. Safety cannot resist loosening of a
fastener adequately to maintain tension of the bolt. Its primary ability is that of preventing the
loosened fastener from completely unthreading and falling out.
                                                                                                    


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III
Threadlocking – Types and Methods
  1. The Settling Method
  The settling method increases the elasticity of the assembly and thus compensates for
  settling in the assembly. The pre-stressed force is largely retained, and relaxation of the
  threaded assembly is prevented. This can compensate for the use of fasteners that are less
  than L/D = 6, which is practically unavoidable in the larger fastener sizes. (e.g. a ¾” bolt
  with L/D – 6 is 4 ½” long)

  Good:
     • Conical spring washers
     • Cup springs of high quality

  Poor:
     • Spring washers
     • Elastic washers
     • Toothed and fan-type washers

  Loctite Corporation states that the “poor” category washers are inadequate and unsuitable
  for securing bolts of Grade 5 or higher.

  2. Loss Prevention Devices
        • Crown (castellated) nuts c/w cotter pins
        • Safety wire
        • Bolts/nuts with thread inserts made of metal or plastic (e.g. Nylocs)

  These techniques often avoid the loss of the fixture but are ineffective in maintaining
  clamping load.

  3. Self-Loosening Prevention Devices
  Self-loosening prevention devices prevent threaded assemblies from loosening themselves.
      • Modified female threadform (Spiralock, produced by Detroit Tool)
      • Bolts and nuts with locking teeth
      • Ribbed flange bolts
      • Adhesives

  Modified Female Threadform (Spiralock)
  Spiralock is a unique, proprietary preload locking threadform. It is exceptionally resistant to
  self-loosening due to transverse vibration, which is the major cause of fastener loosening.
  Advantages
      • Works with all currently used bolts and studs
      • Does not require any secondary methods of threadlocking. (lock washers, thread
          adhesives, crimping, etc.)
      • Eliminates radial movement of the bolt because the geometry of the female
          threadform changes the vector of the thread force, introducing a significant lateral
          component to the force. (hence its ability to resist transverse vibration)
      • The bolted joints improve their tenacity with re-use. (reusable up to 50 times with no
          loss of holding power)
                                                                                                   


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       • Spreads the load of fastener tension over the first five threads equally. (34% of
         tension load is on the first thread of a conventional threadform)
      • CNC threading taps are available that can be used in place of conventional threading
         taps at little or no additional cost of production.
   Disadvantages
      • Proprietary threadform, therefore not available in remote areas. (although Spiralock
         flanged nuts are available and may be used)
      • The flanged Spiralock nuts are more expensive than ordinary nuts.

   Threading taps and flange nuts are both available from Detroit Tool.

   Bolts and Nuts with Locking Teeth (and Ribbed Flange Bolts)
   Nuts and bolts with locking teeth and ribbed flange bolts are primarily used in applications
   where they will never be disassembled because they tend to damage the flange face of the
   mating part.

   Adhesives

   The Adhesive Joint
   Adhesives are bridges between substrate surfaces. The bonding mechanism depends
   upon:
      • Adhesion: the bonding strength of the adhesive
      • Cohesion: the strength within the adhesive


The adhesive and cohesive forces in a bonded joint should be about equal.

Advantages
   • Eliminates radial movement of the bolt. (hence its ability to resist transverse vibration)
   • Fasteners can be reused if properly cleaned of old adhesive and grease.
   • Readily available (Loctite)
   • Although product is expensive, very small amounts are required, hence per-fastener cost
      is low.

Disadvantages
   • Product will not work if male and female threads are not properly degreased and
      prepared.
   • Adhesives typically “set” in 5-15 minutes. Further tightening of the fastener after setting
      will ruin the bond.
   • Difficult to achieve successful rebonding under field conditions.
                                                                                                          


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IV
Conclusion
The thoughtful selection of fastener locking methods can be of great benefit in machine design
and modification, primarily resulting in more reliable operation.

If I ruled the machine world:

Critical applications:
    • Spiralock female threadform
    • Belleville-type heavy duty class cone washer

The heavy duty cone washers flatten out at approximately 90% of proof tension, size for size,
using grade 8 fasteners. This flattening would give a visual confirmation that the fastener is
properly torqued. The cone washer would also maintain a high percentage of preload tension if
there were any settling of the joint.

Non-Critical Applications
  • Spiralock female threadform
  • Hardened flat washer

The hardened flat washer will provide more consistent contact for the bolt bearing face and will
eliminate local plastification of the bolt and joint faces due to local asperities of the joint face.

The use of safety wire should be the result of an informed decision. It is not able to maintain
tension in a loosening fastener. It merely prevents it from falling right out of the joint. It is false
security to believe that a joint that depends on pre-stress tension of a number of Grade 8
fasteners will maintain its integrity after the loosening that safety wire will permit has occurred.

Stephen F. Scott, P.Eng
Area 51 Machine Design

				
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