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At this juncture, it is appropriate to introduce the Stribeck curve.
Let us do this with a tribological thought experiment: We are waterskiing.
You, the reader, are standing on wood water-skis which rest on the bottom of a lake only 20 cm deep. In your hands, you grip a handle attached to a rope,
and a long way off is a motorized rope winch.
Fig. 3.01: Thought experiment with a water-skier
The muscles of your upper arm are the tension sensors, and you estimate speed by the wind in your face as you are pulled.
Here we go! The winch pulls you slowly. The static friction between the skis and floor of the lake is high, and your arms are being stretched. After you first start, the skis slide along
under water on the floor of the lake. Water is pressed under the front curve of the skis and the gliding surface of the skis. Since there is a limit to how fast the water can flow off to the
side, the water pressure increases under the sliding surface until it is higher than the static pressure. This water pressure reduces the normal force between the ski and lake bottom,
and the friction decreases. Your arms relax a bit. The rope winch starts pulling faster. The water pressure under the ski increases steadily, and the tension in your arms decreases.
Once the buoyancy increases enough, the skis lift off of the lake bottom. The skis no longer contact the bottom of the lake. You have lift-off.
In this state, you can water-ski with just one arm since the tension has become smaller.
The winch gradually accelerates even more, and your speed increases. Your hair flutters in the wind, the waves cut by the skis hiss.
Depending on how much time you have spent at the gym (the strength of your biceps and triceps), you will have to let go sooner or later. The increasing tension becomes too much to handle.
And why is that, specifically? Well, because of the water resistance, of course. It increases with the speed.
This experiment only works when there are not any rocks sticking out of the surface of the water (ouch).
Analogies to hydrodynamics in lubricated friction bearings: The rotating shaft presses oil onto the sliding surface. Oil pressure arises there that reduces the normal force between
the shaft and bearing. If the bearing is correctly dimensioned and is not too rough, the shaft floats at a specific sliding speed (see p. 122).
Professor Stribeck who spent a great deal of time studying friction bearings describes the amount of friction that depends on the sliding speed as hydrodynamic friction. The following
diagram is named after him:
Fig. 3.02: Stribeck curve
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