Coating fasteners changes more than just their looks. The coating affects friction and it is the friction under the bolt head and on the thread flanks of the bolt that determines how much of the applied torque is transformed into clamp load.
How the Coating Changes Friction
The Zinc Nickel alloy is harder and smoother than plain zinc and therefore has a lower friction coefficient. This low friction under the bolt head and on the thread flanks of the nut means that a greater proportion of the applied torque will be transferred as clamp load as opposed to being wasted as heat or surface contact resistance.
Reduced Torque Scatter on the Assembly Line
One benefit to the lubricity of the zinc nickel coating is that it produces consistent results. The coating provides a very consistent surface, thus the variation between the minimum and maximum clamp load using a batch of fasteners is greatly reduced. On a production line where fasteners are tightened one at a time by hand with a torque wrench, that coating provides the ability to set the wrench to a single value and produce reliable results every time.
Hydrogen Embrittlement and Post-Plate Baking
As you increase the strength of fasteners, you also increase their susceptibility to hydrogen embrittlement. If the plating process introduces hydrogen into the material, it can cause it to become brittle and potentially suffer a sudden fracture some time after installation. To prevent this, parts that have been plated are baked within a few hours of being plated to drive the hydrogen out of the material. You should therefore check with your supplier that they bake parts of this type as part of their process. The relevant standards and specifications for fasteners will detail the requirements for baking for certain material grades.
Recalibrate Your Torque Tables
Most engineers know that to develop a suitable torque table for bolts that have been coated with a material such as zinc-nickel, the figures published for uncoated or zinc plated bolts will not be adequate. The main reason for this is that the coating increases the lubricity of the bolt threads and the contact surfaces of the bolt with the surrounding material, which will increase the clamp load for a given amount of torque. The increase in clamp load can so exceed the yield point of the joint that serious damage may occur. Suitable new figures for use in torque tables for bolts of this type must be developed and published.
Specifying the Coating Correctly
The thickness of the coating on the fastener threads is critical in order to maintain the thread fit specification. Check that the tolerance class for plated parts still allows for full thread engagement after coating. For Zinc Nickel Plating, see www.poeton.co.uk/surface-treatments/plating/zinc-nickel-plating/.
Pay attention to these simple rules and you can avoid easily avoidable joint failures that are difficult to cause and are difficult to diagnose and to correct.
Comments are closed, but trackbacks and pingbacks are open.