Mean Hertz load

At the end of each interval the average scar diameter is recorded. Two values are generally reported— load wear index (formerly mean Hertz load) and weld point. 

Apart from the frictional effects, two other performance characteristics need to be considered, namely load-carrying capacity and wear. Most of the available information on these subjects has been obtained from the practical use of commercial materials and will be discussed later, but Groszek and Witheridge found that molybdenum disulphide ground conventionally in air gave a small increase in Mean Hertz Load and Weld Load in a Four-Ball Test as 5% dispersions in a mineral oil. However, oleophilic molybdenum disulphide, ground in n-heptane, gave a 170% increase in Mean Hertz Load and a 260% increase in Weld Load. 

The best performance found in the Climax Molybdenum Company test programme was for a lithium hydroxystearate soap-thickened grease with a conventional EP additive and 10% by weight of molybdenum disulphide with a nominal particle size of 7.0 jum. This grease had a Timken OK Load of 23 lbs (10.5 kg), Falex load capacity of 1450 lbs (658 kg). Mean Hertz Load of 90.5 and Weld Load of 630 kg. 

Table 13.7 Increase in Load-Carrying Capacity of a Di-Ester Grease With Molybdenum Disulphide Content (Data from Ref. 173) (Mean Hertz Load figures obtained by ASTM D-2596 the IP239 method will give results between 10% and 20% higher)...

When tested in the four-ball machine, solutions of sulfur in petroleum oils of moderate viscosity or in white oil raise the critical load for the onset of severe, destructive wear, which is designated as "antiseizure" action in the technological idiom of the four-ball test. Davey [54] found a significant increase in the critical initial seizure load from 834 N (85 kg) for a petroleum base oil to 1275 N (130 kg) for elemental sulfur dissolved in the oil. Sakurai and Sato [55] observed a 3.2-fold increase in the load-wear index (mean Hertz load) for a 0.5 weight-percent solution of elemental sulfur relative to that of the uncompounded white oil. The load-wear index is a specialized result of the four-ball test that can be taken as indicative of the average antiseizure behavior of the lubricant. Mould, Silver and Syrett [56] reported a load-wear index ratio of 3.08 for 0.48% sulfur in white oil relative to that of the solvent oil, and also an increase in the initial seizure load from 441 N to 637 N (45 kg to 65 kg) and in the 2.5-second seizure-delay load from 490 N to 833 N (50 kg to 90 kg). 

The work of Forbes and Battersby [46] is an integrated study of the relations among the chemical structures of the dialkyl phosphites, their adsorption on and reaction with iron, and their behavior in four-ball bench testing of lubricant additive effectiveness. The four-ball data in Table 11-17 for solutions of additive in white oil show that both the wear/load index (mean Hertz load) and the initial seizure load are critically responsive to concentration, with a strong effect when the concentration increases from 0.01 to 0.04 molal (0.031% to 0.124% P). The initial seizure load is an uncomplicated criterion with a straightforward interpretation, whereas the wear/load index is contrived, both in concept and performance. The low-load 50 minute wear data show inconsistencies in the influence of additives that have not been explained. 

The influence of the metal ion is seen in Fig. 11-12, which shows low-load four-ball wear data by Allum and Forbes [58]. The results fall into three broad groups low wear levels associated with the ions Zn, Cd" , Ni, Fe " and Ag" " intermediate wear with Pb" , Sn" , and Sb" " " high wear with Bi . Where direct comparison for the effect of alkyl groups are available, they show the ester of the secondary alcohol 4-methylpentanol-2 has a stronger antiwear function than the ester of n-hexanol, except for the nickel salts. No consistent trend on which to base an acceptable explanation for the additive action of these phos-phorodithioates was observed in the data for the wear/load index (mean Hertz load). 

Hertz [27] solved the problem of the contact between two elastic elliptical bodies by modeling each body as an infinite half plane which is loaded over a contact area that is small in comparison to the body itself. The requirement of small areas of contact further allowed Hertz to use a parabola to represent the shape of the profile of the ellipses. In essence. Hertz modeled the interaction of elliptical asperities in contact. Fundamental in his solution is the assumption that, when two elliptical objects are compressed against one another, the shape of the deformed mating surface lies between the shape of the two undeformed surfaces but more closely resembles the shape of the surface with the higher elastic modulus. This means the deformed shape after two spheres are pressed against one another is a spherical shape. 

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