Thickness requirements for threaded fastener surface coating
Electroplated layer
According to the requirements of the ISO 4042-2018 Fasteners-Electroplated coating systems standard, the nominal coating thickness of the electroplated coating is 3μm-30μm. In order to reduce the assembly interference caused by the coating thickness of threaded fasteners, the coating thickness of threaded fasteners should not exceed the basic deviation of 1/4 thread. The following table gives the recommended coating thickness values of threaded fasteners with different specifications, taking the common tolerance G of internal thread and the tolerance g of external thread before plating as an example. (From ISO 4042). The tolerance zone after plating of the internal thread is H, and the tolerance zone after plating of the external thread is h, and the assembly of the thread is checked by using a verified stop-stop gauge.
Non-electrolytic zinc sheet coating
According to ISO 10683-2018 Fasteners-Non-electrolytically applied zinc flake coating systems (Fasteners) standard requirements, The relationship between nominal coating thickness and corrosion protection time of non-electrolytic zinc sheet coating is shown in the following table: (from ISO 10683).
Hot dip galvanizing layer
According to ISO 10684-2004 Fasteners-Hot dip galvanized layer (fasteners - hot dip galvanized layer) standard requirements, the local thickness of the hot dip galvanized layer should not be less than 40μm, the average thickness of the coating batch should not be less than 50μm. To accommodate such a thick coating, the manufacture of threads also requires special limit sizes.
Two different methods and basic deviations are specified:
In the first method, after plating, the nut is tapped with a tap of 6AZ or 6AX(increased tap size) to meet the requirements of fitting with bolts or screws at the tolerance zone g or h of the thread before plating.
In the second method, 6az(reduced thread size) bolts or screws are used before plating to meet the requirements of fitting with nuts with the tolerance zone of the tap thread H or G after plating. For threaded fasteners smaller than M8 and pitch less than 1.25mm, hot-dip galvanizing is not recommended as a surface treatment coating. The following table takes the commonly used internal thread tolerance H and external thread tolerance g as examples to give the recommended coating thickness values of fasteners with different specifications of thread. (from ISO 10684).
The effect of coating thickness on thread fit is shown
Geometric relationship between coating thickness and basic warp of thread
When the coating thickness t is required to achieve a specific corrosion resistance, the basic thread length will be increased by 4t. See the following thread model calculation procedure:
Electroplated layer
The plating process relies on the principle of electrolysis to achieve deposition of the coating, and the overall thickness of the coating is not high. When the tolerance zone of the internal thread is G before plating and the tolerance zone of the external thread is G, the plating requirements of the internal thread tolerance zone of H and the external thread tolerance zone of h after plating can be met, and the thread assembly will not be affected. It should be noted that when higher corrosion resistance is required to increase the thickness of the coating, special adjustment of the thread clearance is still required in accordance with the standard requirements.
Non-electrolytic zinc sheet coating
Non-electrolytic zinc sheet coatings are usually coated using the dip-rotation process, which results in uneven coating thickness. The immersion-rotation process can result in a significant local thickening of the surface coating, generally exceeding the coating thickness by 1/3 to 1/2. The following table gives the minimum fit clearance of fasteners with different specifications of thread by taking the common internal thread tolerance G and external thread tolerance g before coating as examples. (from ISO 10683)
Sample coating thickness selection for non-electrolytic zinc sheet
Example 1 -- Specification M12, 1.75mm pitch, 6g tolerance bolt
● Corrosion resistance: 480h
● Reference coating thickness: 5μm
● Maximum possible thickness: 8μm (5+2.5, reduced to 8)
● Basic diameter thickness of thread: 32μm
● Allowable fit gap (6g) : 34μm
● By calculation, the coating thickness is less than the thread matching gap, and can be assembled normally and meet the requirements.
Example 2 -- Specification M6, pitch 1mm, 6g tolerance bolt
● Corrosion resistance: 600h
● Reference coating thickness: 6μm
● Maximum possible thickness: 9μm (6+3)
● Basic diameter thickness of thread: 36μm
● Allowable fit gap (6g) : 26μm
After calculation, the coating thickness is greater than the thread matching gap, can not be assembled normally, does not meet the requirements.
When small specification bolts, screws or nuts (specifications ≤M6) are coated with non-electrolytic zinc sheet coating and require higher corrosion resistance time, it is necessary to measure whether the coating thickness, corrosion resistance requirements and thread clearance match, so as to avoid assembly failure after coating.
Hot dip galvanizing layer
The thickness of the hot-dip galvanized layer is much higher than the conventional H/g thread matching gap, so the screw thread of the nut or bolt must be specially treated according to the standard requirements during plating, and the fasteners of the hot-dip galvanized layer must be selected to avoid the situation of assembly.