Surface treatments prolong disc spring life

Conical shaped disc springs, or Belleville washers, can have a number of protective surface treatments, as Alex Martin of Belleville Springs explains.

Conical shaped disc springs were first conceived in the mid-1800s, and the invention was subsequently called a Belleville washer after its inventor.

Subsequent developments have included many new surface treatments, and these have made this simple product even more versatile.

Obviously, the choice of available types of surface treatments is almost endless, but it is absolutely essential to bear in mind the following: do not electroplate disc springs.

During the process of electroplating, hydrogen gas may be absorbed through the surfaces of the disc spring, which in turn may lead to the spring becoming brittle.

Although it is possible that a subsequent heat treatment, referred to as de-embrittlement, may relieve this condition, our experience has shown this to be unreliable.

In phosphating, a zinc phosphate coating usually with subsequent oil or wax treatment.

This treatment is widely offered as "standard" on most stock-range carbon steel disc springs.

The protection offered is sufficient to prevent corrosion throughout storage and normal transit conditions.

It is adequate also for those applications where the disc springs are not directly exposed to the elements.

However, where the application involves a more hostile environment, such as the disc springs being open to weather or marine conditions, chemical or acid laden atmospheres, a superior treatment or material must be considered.

Mechanical zinc plating is a method of depositing substantial thicknesses of zinc on the surfaces of disc springs without the risk of "hydrogen embrittlement" associated with normal electroplating.

The zinc is impacted onto the surfaces by way of tumbling the disc springs in a rotating barrel, together with glass beads, metal powder, and promoting chemicals.

In addition to removing the risk of embrittlement, the "peening" aspect of this process is beneficial in terms of some stress relieving of the components.

There are two forms of subsequent passivation treatment.

Clear passivation prevents oxidation of zinc coating in storage, handling, and transit.

It also assists in maintaining the aesthetic appearance of the zinc plate.

Alternatively, yellow chromate passivation has similar advantages, with the additional benefit of slightly enhanced corrosion resistance.

The only disadvantage is that the "gold" tint is often of a patchy "nonuniform" nature and may prove unacceptable if appearance is critical.

With electroless nickel (Kanigen) plating, as is the case with mechanical plating processes, the risk of hydrogen embrittlement is avoided with this method of chemically depositing a nickel coating.

Although compared with other treatments discussed here, this process is relatively costly, the high degree of corrosion resistance and smooth "satin-like" finish often justify the extra expense.

The Sherardising process again uses zinc, this time in the form of zinc dust mixed with an inert filler which, together with the parts to be coated, is placed in a sealed container.

The container is placed in a special furnace and rotated at a temperature which is sufficient to "fuse" the coating but without risk of affecting the spring properties of the components.

Coating thicknesses from 10 to 50um are possible, which makes for a wide range of protective coatings.

Finally, the delta-tone process involves dipping the components in an organic resin and zinc mixture, the surplus is removed by spinning, and the bonding of the coating is completed at oven temperatures which have no effect on the metallurgical or heat treatment properties of the components.

Salt-spray corrosion resistance tests on this coating can result in a performance equivalent to that obtained with electroless nickel plating.

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