The Delft EHL diagram from a historic perspective: righting a 50-year-old wrong

Since many years nondimensional groups have been used in classical EHL contact theory. The so-called Moes groups have become standard in most of the literature, after its first appearance in Proceedings of the Instn. Mech. Engrs., 1965-66, Vol. 180, Pt. 3B. Three nondimensional groups were established: H, L and M. These groups were to replace the four groups defined by Dowson and Higginson in their publications and book Elastohydrodynamic Lubrication (years 1959-1966), H, U, W and G. Most people nowadays refer to these 3 groups as the “Moes groups”, and even a lot more scientists prefer to use the 4 Dowson and Higginson groups.

The paper starts by developing a useful set of nondimensional numbers for EHL film thickness problems. Leading to a set which contains a minimum number. It is proven that Blok has been using a set of nondimensional groups as early as 1957 or before that date. Blok freely shared his research results with his colleagues, one of them being the German scientist Peppler, who published them (with proper credits). These nondimensional groups were essentially the same as the groups used later by his students Koets (see Dowson and Higginson, (1966)) and Moes (1966). Blok preferred to designate the EHL film thickness chart with these nondimensional groups as the Delft Diagram. After 1957 Blok replaced the letter ‘N’ for the load number by the letter ‘M’, to honour his Swiss colleague Meldahl, who investigated gears in the first half of the 20th century.