Friction in instruments

Abstract

Friction is defined as a force acting against the direction of motion and resulting in a dissipation of energy in an irreversible manner. The only frictionless motions in nature exist if particles or bodies move on a selected geodesic curve (a geodesic curve is a curve in which there are no forces acting on the particle) which does not give rise to a space dependent change of the field. Coulomb friction is independent from the speed of motion while viscous friction is proportional to the speed of motion. An instrument is defined as a device which transduces one physical quantity into another. Instruments may be classified into control - or measuring instruments. A better way to classify instruments is according to the physical qualities which are transduced. This thesis restricts itself to instruments in which a mechanical motion is present. An ideal instrument is defined as giving accurate and instantaneous readings. In such an instrument the directive force acting on the pointer should be proportional to the second time derivative of the quantity to be measured. Such an instrument has cumulative errors and is therefore impractical. The practical instrument requires a restoring force when the pointer is off the desired point. Such a restoring force causes vibrations of the pointer. Therefore, it is necessary to add a dampening force. The ideal dampener would bring the pointer to rest on the desired point in a minimum time. It would require a complicated computer. In practical instruments a dampener using viscous friction is used. The optimum value of the viscous dampener depends on the instrument error, which is caused by other effects. A formula relating optimum viscous dampening and instrument error is given. (See graph, Figure 5) The static errors of instruments caused by Coulomb friction are shown. The effect of Coulomb friction on gyro-gimbal suspension is discussed. Empirical facts about friction in bearings are collected. They are classified into the following categories: oil film separation, boundary layer lubrication, dry surfaces and ball or roller bearings. The effect of surface grain structure and grain direction on the sliding friction is demonstrated in practical examples. The mechanism of sliding friction with oil film separation is analyzed using hydrodynamic theory. The mechanism of air bearings is treated using thermodynamic theory. Oiling devices to avoid dry friction are described. A hypothesis for the mechanism of friction in surfaces with boundary layer lubrication is made, according to which such friction is caused by a vibration of the crystal-lattice. This hypothesis could be tested experimentally by X-ray diffraction or by artificially excited vibrations with magnetostriction oscillators which should effect the coefficient of friction. It is shown that the major part of rolling friction in bearings is caused by sliding friction. Designs for low friction ball and roller bearings are suggested. The effect of centrifugal force on high speed ball bearings is discussed. Instruments without bearings are described. The high sensitivity of optical and electrical instruments is pointed out. Mechanical instruments using fluid suspension, magnetic suspension and elastic suspension are described. A servo follow-up motion as a means to reduce friction is described. The accuracy of a null point system of measurement is pointed out and its application in conjunction with servo systems shown.