Why is a minor load applied before setting the Rockwell depth-measuring dial?

Answer 1

1. The tested surface is exposed to the penetrator and the first test load Fo (preload) is applied. A small indentation appears. At this point, the meter is set to zero.
2. Slowly and without shocks the load F1 is applied additionally. Together with the preload this is defined as total test load F. With this load the penetrator enters the material more or less deep, depending on the hardness of the material. This position needs to be kept to reach the final penetration (when testing hard materials the penetration is almost immediate; with soft materials it is necessary to wait for a number of seconds). The penetration procedure can also be observed on the indicator of the meter.
3. When the indicator of the meter finally stops moving, the additional load F1 is removed until the preload is applied respectively. This way, the penetrator remains in the imprint and all elastic deformations, which were caused by the application of the total test load, are eliminated; thus, the meter only shows the remaining penetration depth (as difference between preload and total test load).

The penetrator, preloads, test loads, and the units are standardised in the Rockwell method and can be divided into two groups: standard Rockwell (method N) and superficial Rockwell (method T).

simply

to eliminate the elastic deformation from the hardness.and calculate the permenant deformation only