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Thread tolerance

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SURFACE CORROSION

Coating requires space. Fasteners such as nuts and bolts that are coated for the purposes of corrosion protection need to have slightly altered dimensions. If a coated bolt is to have exactly the same dimensions, it needs to be designed to be slightly thinner. For a nut with coated thread it is necessary to enlarge the inner diameter slightly.

THE PROBLEM IS OLD

100 years ago, when the first bolts were hot-dipped galvanized, they no longer assembled into their respective mating nuts. Conversely, hot-dipped galvanized nuts no longer assembled onto the bolts. Since that time, the initial dimensions of fasteners that are to be coated has been adjusted by manufacturers. This is currently a significant issue in the area of lightweight construction.

Calculation of the initial dimensions of fasteners coated with zinc flake is detailed by the International Organization for Standardization (ISO) in standard ISO 10683. The current version dates from 2014. The data published in the standard refers to bolts with metric dimensions and a flank angle of 60°.

Theory regarding size and manufacture of bolts and screws follows below:

  • Bolts and screws are categorized according to the outer diameter of the thread. A M6 bolt has an outer diameter of 6 mm whereas a M10 bolt has an outer diameter of 10 mm.
  • Thread tolerance categories refer to the manufacturing precision of fasteners.
  • This category designation can be supplemented by the lowercase letter “h” for externally threaded fasteners such as bolts (the large case letter “H” is used for internally threaded fasteners such as nuts).

The letters “g” and “e” added to a thread tolerance category indicate how many micrometers a bolt or screw has been designed in order to create space for a protective coating.

AN EXAMPLE

M10 bolts of the tolerance category 6h have a tolerance of up to 0.236 mm. This tolerance is subtracted from the zero line, (i.e. the outermost dimension). The outer diameter of such bolts is therefore between 9.764 and 10 mm.

If such bolts are coated, their thread is larger at the root, the flanks, and the crest. The counterpart to the bolt, such as a nut, is tighter. It is possible to calculate just how tight. In this determination, the most important characteristic is the thickness of the protective coating on the flanks of the thread.  For each micron of applied coating, the diameter in the threads is increased by four times respectively. For example, if the coating thickness of a M10 bolt is 15 µm, the diameter in the threads is increased by 60 µm. The counterpart to the bolt (i.e. the nut) would consequently have 60 µm less space available.

THE TECHINCAL SOLUTION IS SIMPLE

  • Bolts that are intended to be coated are designed in a way that provides space for the protective coating.  These fasteners have slightly reduced thread diameters, but in turn offer sufficient room on the thread flanks when coated. Bolts such as these are manufactured for very thin and slightly thicker coatings.
  • The thread diameter of M10 bolts of the tolerance category “6g” is 32 µm lower than “6h”.  As a result, the thickness of the protective coating may be applied up to 8 µm. The outer diameter of the bolt is approximately 16 µm smaller.
  • The thread diameter of M10 bolts of the “6e” tolerance category is 67 µm lower than “6h”. As a result, the thickness of the protective coating may be applied up to 16.75 µm. The outer diameter of the bolt is approximately 33 µm smaller.

For each individual bolt size and tolerance category there are different values for g and e. These are compiled in tables.

Protective coatings are usually unevenly distributed in threads, for example, when zinc flake coatings are applied to bolts or nuts using the dip spin process. The coating is somewhat thicker on the root of the thread than on the flank. In contrast, on the crest of the thread, the coating is slightly thinner than on the flank.

The correct design of the thread can be checked using ring gauges for bolts and plug gauges for nuts. With respect to plug gauges, the German Fastener Association in 2012 passed the special guideline “Inspection of thread ability of nuts with zinc flake coatings”. Due to their part geometries, nuts tend to have an increased coating thickness by comparison to bolts. Plug gauges are designed in such a way that they take account of the thicker coating thickness at the root of the thread. Historical evidence demonstrates  that the assemblability of the nuts is not influenced by the thicker coating in the root of their threads.

THREE THREAD DIAMETERS

  • The outer diameter is the distance between the outer points of the thread.
  • The core diameter is the distance of the inner thread points.
  • The thread diameter is the distance between two opposing thread flanks, measured from the middle of one flank to the middle of the other.

All of these distances are measured vertical to the axis.

Glossar

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Technical terms cannot always be avoided. As corrosion experts, we not only want to give you comprehensive advice, we are also interested in making you a corrosion expert yourself.

The variety around the topic of corrosion and corrosion protection is also in our glossary at home: explanations from A as in Adhesion to T as in Thread tolerance. Have fun clicking through!