Stribeck
Stribeck, Richard
ACHIEVEMENT(S): Ball bearing studies
NICKNAME/ALIAS: None
DATE/PLACE OF BIRTH: ?
DATE/PLACE/AGE AT DEATH: ?
DISCIPLINE: Mechanical engineer
FIELD: Engineering
LANGUAGE: German
PARENTS: ?
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EDUCATION: ?
MAJOR WORK: ?
LIFE AND TIMES: In the late 19th century, there was no scientific guidance as to the manufacture and use of ball bearings, although advances made by Osborne Reynolds* - whose paper: "On the Theory of Lubrication" (1886) made possible the production of bearings capable of carrying high loads at previously impossible speeds.
When in 1899 the German Small Arms and Ammunition Factories of Berlin found that it needed information on the elastic properties of steel spheres, plates and ball bearings it had to commission its own investigation by Stribeck.
The only work in this field had been done by Hertz* in the late 1870s, who in his 'Contact of Elastic Bodies' dealt with the general case of bodies of any shape pressed against one another.
Hertz defined the following restrictions in the use of ball bearings: that contact must be limited to small portions of the surfaces; that the forces must be normal to the surfaces in contact, and so the surfaces must be perfectly smooth; that the strains must keep within the limits of proportionality; and that the bodies must be homogeneous. From this point, Hertz then derived a series of formulae analysing compressive stress, which he had briefly tested by experiments with first glass and steel lenses, and then plates of various metals.
Stribeck focused his attention on the movement of two or three bodies under compression. Experimenting first with roller bearings, Stribeck found that, to minimize friction, large balls should run in the smallest possible circles, and that the number of balls should be as small as possible.
He then began investigating the frictional balance of ball bearings, testing them at various speeds and loads up to 5000 kilograms. He developed a series of experiments for the investigation of the way in which the deformation of steel balls varied with differences in the applied load and the size of the balls, by measuring the amount by which the centres of two or three hardened steel balls, made from a variety of different types of steel (including un-tempered and oil-tempered steel, and steel hardened in water), approached each other under pressure.
Stribeck’s aim was to calculate the value 'E' for steel most likely to be suitable for the manufacture of ball bearings, and arrived at the value 'E' = 2,120,000 kg per sq cm. In one series of tests, he arranged three balls in a tensile-test machine (which included a five-ton press) and measured the total deflection between two points under the following conditions: with three spheres of the same diameter in contact; with two spheres and a flat plate in between; with two spheres and a grooved plate in between (the spheres resting in the grooves); and with one sphere between the cup-shaped ends of two short solid cylinders.
In these tests, when the load was first applied and then removed, a result in which the two points failed to return to their original positions indicated that the elastic limit of the material had been exceeded at the contact surfaces, and revealed the extent of the permanent deformation (also known as the permanent set, or plastic deformation) associated with each value of load.
Stribeck's main problem was how to determine the maximum permanent deformation possible without rupturing the surface of the ball bearings. This problem could be approached in two ways, either by the calculation of some arbitrary maximum permissible normal or tangential stress in the known pressure area, or by the calculation of a maximum ball-load related to the ball diameter.
Going for maximum ball-load, Stribeck plotted his experimental results on a series of graphs, with the loads producing permanent deformation plotted against the square of the ball size for several different values of the permanent deformation.
From his graphs, it became clear to Stribeck that, given the value of the maximum compressive stress required to produce a given permanent deformation between the two balls, the load required to produce a given permanent deformation is proportional to the square of the ball diameter.
Stribeck's experiments also proved what had previously been suspected by the nascent ball-bearing industry, that the compression was much less when grooved plates were used in the experiments.
The next series of experiments involved tests with complete various sizes and types of ball bearings running on either flat, curved or grooved tracks at various speeds and under a range of radial loads (from 0 to 5000 kg). Stribeck ran the first bearing at speeds of 65, 100, 130, 190, 380, 580, 780, and 1150 revolutions, starting with the lowest speed, the loads being increased step by step, and decreased again.
The bearings running on grooved tracks (or double ring bearings) gave by far the best results for heavy loads, the rolling (and sliding) friction being about 0.0015 for loads above 1000 kg and slightly more for loads below this figure.
When, at around the same temperature, less friction was observed in the descending series than before, Stribeck assumed that the bearing was not yet properly worn in, but he quickly realized that polishing of the roller path influenced friction and the life of the bearing, and decided that if necessary the surface should be both finely ground and polished. The same process was then gone through at higher speeds, and the bearing finally examined.
Stribeck's experiments confirmed that a grooved track provided better support for the balls, increasing the load-carrying capacity, and demonstrated that coefficient of friction of a bearing with grooved tracks was extremely small, and varied only slightly with speed and load.
A major result of his research was the introduction of a generally- accepted basic design for ball bearings, along with the introduction of practical formulae which could be easily understood by working engineers and applied to the design of a whole range of ball bearings.
In modern ball bearings there are three main parts – two grooved, ring-like tracks or races (of the same width, but different diameters, one fitting inside the other) and a number of balls (loosely restrained and separated by means of a retainer or cage), which fill the space between the two races and roll with negligible friction in the grooves.
They have a vast quantity of uses, but one of the most familiar must be in the tip of Biro's* ballpoint pen.
ASSESSMENT: Richard Stribeck was a talented German mechanical engineer who,
through a series of exhaustive experiments in 1899-1901 determined the load-bearing capabilities of different ball bearings, and placed the use of ball bearings in mechanical engineering on a scientific basis.
EXTRA CONNECTIONS: None
ONELINER: Richard Stribeck, ball-bearings pioneer
BIBLIOGRAPHY:
Allan, R.K., Rolling Bearings' 1964
Schafer, George, Rolling Bearings and their Contribution to the Progress of Technology, London: Lewis Books Ltd., 1986
Shaw, M.C. & E.F. Macks, Analysis and Lubrication of Bearings, 1949