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工作機械 課題 原文
Good lubrication produces smooth motion, but dry sliding is usually discontinuous. When a lead slider is used on steel, for example, the tangential force may be steadily increased without appreciable movement occurring, until a limit is reached at the static coefficient of friction, and the slider suddenly breaks away. After moving a short distance, sufficient to release the spring force, it will remain stationary with respect to the lower surface until the tangential force builds up again. This "stick-slip" motion is characteristic of a large number of pairs of different metals and is associated with a falling friction-velocity relationship. The magnitude of the observed vibration is governed by the elastic characteristics of the apparatus.

If the friction does not vary significantly with speed, the static and kinetic values may be approximately the same, and stick-slip motion will not be observed. Usually pairs of the same metal, for example copper on copper, do not show stick-slip , but a rather erratic high friction is found.

Stick-slip may occur under poor boundary lubrication conditions, for example when steel surface are lubricated with paraffins or alcohols. Good boundary lubricants such as fatty acids usually prevent such oscillation occurring, unless the mechanical conditions are such that partial hydrodynamic lubrication occurs during a 'slip', giving a lower coefficient of friction. In most practical operations it is very important to avoid vibration, so a flat , or slightly rising, friction-speed characteristic is advantageous.
[PR]