In the following table are given some of the results obtained from Morin’s experiments with unguents interposed.
| Nature of surfaces in contact. | Coefficient of friction during motion. | Kind of unguent. | |||
| Brass upon brass | .058 | Olive oil. | |||
| Cast | iron | upon | brass | .078 | „ |
| „ | „ | „ | cast iron | .314 | Water. |
| Steel | upon | cast iron | .079 | Olive oil. | |
| „ | „ | brass | .056 | Tallow or olive oil. | |
| Wrought | iron | upon | brass | .103 | Tallow. |
| „ | „ | „ | cast iron | .066 | Olive oil. |
| „ | „ | „ | wrought iron | .136 | „ |
Morin’s experiments demonstrated that friction is always proportional to the pressure and independent of the area pressed in contact, providing that the pressure is not so great as to cause the surfaces to abrade in the manner or to the degree commonly known as cutting, which occurs when the area of bearing surface in proportion to the pressure is so small as to press out the lubricating material.
Now, between the degree of abrasion that is sufficient to cause a bearing to heat and the minimum, possibly lies a wide range that is very difficult of classification, and that influences the friction of the bearing and journal. Under any given dimensions of journal area and any given pressure of the same to its bearing, the abrasion, and, therefore, the friction, will be less in proportion as the fit of the journal to its bearing extends over its whole area and with an equal pressure of contact. Under these conditions, and with a bearing area ample for the given pressure, the surfaces of a journal and bearing have a smooth, glossy appearance, with a surface as glossy as plate-glass.
This degree of perfection, however, is only occasionally reached in practice, because of imperfections in the fitting and lubrication.
Now, between this condition of glossy smoothness and the degree of abrasion known to practical men as cutting lies, as already stated, a wide range of degrees of abrasion, and each of these has its own coefficient of friction. This may be readily proved by freely lubricating the bearings of a number of journals working under the usual conditions of practice and smearing the oil just as it passes through the bearings upon a sheet of white note paper, when it will be found to contain fine particles of metal, the number and size of particles in a given quantity of the oil decreasing as the surfaces of the bearings are glossy, and increasing as those surfaces appear dull.
The order of value to resist wear is generally considered in practice to be as follows:—
1st in value, hardened steel running on hardened steel.
2nd (and by some considered equal to the first when the pressure per square inch of area is light), cast iron either upon cast iron, hardened wrought iron, or hardened steel.
3rd, under light duty cast iron upon wrought iron or steel not hardened.