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It is difficult to consider some of the other features of the Supertec studs without sounding critical and as if I don't like these parts so I would start by saying that in general they appear to be well made and good quality and offer reasonably good value.
As is the case with the material selection, the 'other features' would also bear looking at from an engineering point of view.
1. Longer thread engagement into the case
It is well understood that load distribution along a thread is not uniform and the majority of the load being supported by the first engaged thread.
As a general guide once a thread engagement of more than 5 pitches the thread pitch tolerance accumulation pretty much guarantees that there will be unequal load sharing in the thread engagement.
Also the longer the thread the more difficult it can be to install and the thread as pitch errors can cause seizing.
As a rule of thumb it is not usual to design studs with a thread engagement beyond 1.5 to 2.0D.
If you need to ensure that the root shear stresses are lower than the tensile stresses in the stud so failure always occurs in the stud you should reduce the diameter of the body of the stud hence giving an effectively larger diameter threaded end portion.
It seems that the standard stud meets the criteria of 2.0D stud engagement so it is difficult to see why a longer thread will provide much benefit.
2. Fine Pitch Thread
Fine pitch threads have a larger pitch diameter than a coarse thread and tend to have a higher thread friction torque than a coarser pitch but this is genrally counteracted by the pitch torque change.
This is governed by a simple relationship:
While a fine pitch thread does have a slightly larger pitch diameter, the higher thread friction torque is negated by the pitch torque change. This is a simple relationship:
F = 2•π•M/p
Where
F = Force
π = 3.142
M = pitch torque
p = pitch
Whilst more engaged threads may increase prevailing torque this component is small compared to the total applied torque and for free running nuts there is essentially no difference between a coarse and fine pitch.
Coarse pitch threads have a higher lead angle than fine pitch threads and as the helix angle is a complement to the lead angle the coarse pitcg thread has a have a smaller helix angle than the fine pitch thread.
This means the fine pitch fastener will develop more axial force for a given torque than a coarse pitch fastener. The coarse pitch, however, develops a a more linear displacment for a given angular displacement.
Fine pitches do provide finer adjustment as they advance less per rotation than coarse threads but the higher prevailing friction can make them more prone to allow variation in axial force.
In general the differences in either torque or axial force for a single pitch step is less than 5% so I am not sure there is much practical differences.
3. 12 point nuts with integral washes
I would agree with the reduced space requirement for Twin Plug Motors compared to the standard nuts.
The intergral flange on the nut will increase the friction and hence the required torque for a given load.
I also looks as if the nuts supplied are coated with a dry film lubricant to ensure consistent tightening.
These aspects of the design, along with the fine pitch thread leads me to ask about the difference between the 'Nut Factor' K of this fastener compared to the standard part and the impact this may have on the preload comapred to using a standard nut.
For example an increase of 35% in the bearing area of a nut due to the presence of a flange would typically reduce tha axial force provided for a given torque by around 8%.
I am not sure if there is any difference in the torque recommended for tightening when using these nuts and it would be interesting to strain gauge some of these studs and compare them to the standard components to see if there is a difference in axial force.
I would imagine that carefully ground washers are very helpful in eliminateing any bending moments in the stud and helping to ensure even tension.
galling of nuts onto Dilavar studs is likely to be a function of the coating but also martensitic steels are less likely to gall than Ausenitic Steels due to basic crystallography and surface energy considerations and this observation is a benefit.
Using a Silver Plated Aircraft style fastener or using a suitable dag such as Boron Nitride would eliminate this problem.
4. Guarantee
Is the guarantee against failure of the stud or loss of preload or against pulling out of the case?
My general conclusion is that by simplifying the design of threaded fastener assemblies to a torque-tension relationship, with no mention of rotation angle, displacements, or strains does not fully address all of the relevant issues.
I still believe that the best 'fix' is to install case savers/timeserts and to install the studs in a well controlled environment in a consistent manner.
I am sure that many of the high quality aftermarket studs are manufactured to better tolerances and standards than stock parts and may help eliminate some of the issues.
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