Small Friction Force Research Articles

A thin-film magnetorheological fluid damper/lock

This paper presents the design, testing, and application of a thin-film magnetorheological(MR) fluid damper/lock. This MR damper/lock is designed for the use in a model adaptivefan nozzle system actuated by shape memory alloy wires. The MR damper/lock (a total ofeight in the fan nozzle system) will lock the opening size of the fan nozzle and providesdamping when the system vibrates. For this purpose, the MR damper/lock hasto have the following characteristics: (1) The device is in the locking positionwhen the power is off. (2) The device has a small static friction force (less than1 lbf) when the power is on. (3) The device generates a small kinetic friction forcewhen it slides during the power-on period. (4) Its damping coefficient can beadjusted. (5) It must be compact. To meet these requirements, a new designof a damper/lock using thin MR fluid film is employed. The device consists offive major components: two soft steel bars, two stacks of permanent magnets,two groups of magnetic wires, a soft steel slider, and MR fluids. Utilizing thepermanent magnets, the MR fluid is trapped and the device is always in thelocked position. When the device is powered on, the flux of the electromagnetspartially cancels and re-directs the rest of the flux from the permanent magnets, andthen the slider is free to move. In this design, MR fluid reduces the air gap andincreases the locking force when it is powered off. On the other hand, it alsofunctions as a lubricant to reduce the kinetic friction forces when electromagnets arepowered on. Extensive tests of the MR damper/lock are conducted to reveal itsforce–displacement curves and force–velocity curves under different applied voltages.Utilizing these testing results, the MR damper/lock is applied to the model adaptivefan nozzle system to perform both locking and damping tasks with a feedbackcontrol. Experimental results show that these tasks are successfully accomplished.

A Strange Direction for Static Friction

Except at the Earth's poles and at the equator, a box on a horizontal surface cannot remain at rest relative to the surface unless a very small static friction force acts upon the box. I demonstrate here that in the Northern Hemisphere, the required static friction is in the counterintuitive direction of south to north.

Preparation and nanotribological behavior of PAH/graphite oxide molecular deposition film

Polyallylamine hydrochloride/graphite oxides ultra thin film (coded as PAH/GO ultra thin film) was prepared by molecular deposition (MD) in laboratory. The resulting PAH/GO ultra thin film was heated to convert the electrostatic force towards covalent bond so as to increase the bonding strength of the film. The structure and nanotribological properties of the film were analyzed by means of atomic force microscopy and ultraviolet-visible spectroscopy. It was found that the PAH/GO ultra thin film had a much smaller friction force than the glass substrate and the friction force was dependent on the hardness and appearance of the glass substrate.

Properties of diffusive systems near a saddle point: application to a quartic double well

This paper aims at the analysis of diffusive properties of unidimensional mechanical systems in the environment of maxima and minima of the potential. It begins with a study of the properties of the singular solutions of the Hamilton–Jacobi–Yasue equation in the above-mentioned environment, in both strong or very small frictional forces. For the quartic symmetrical double-well potential, approximate solutions are found for local validity and the diffusion operator is then calculated in the limits of deep wells and small temperature, the regime being supposed to be aperiodic, with high or moderate values of frictional coefficient. This equation is proved to be nonunique. This operator is then reduced to second order by imposing suitable boundary conditions. Thus an appropriate eigenvalue equation is obtained to describe stationary states in the environment of extremal points of the potential energy function. The main interest of this work relies upon the fact that transition times between wells mainly depend upon fluctuations near the saddle point.

Seismic isolation of buildings with sliding concave foundation (SCF)

AbstractIn this paper, a new base isolation system, namely the sliding concave foundation (SCF), is introduced and the behaviour of the buildings using such a system is theoretically investigated. A building supported on the new system behaves like a compound pendulum during seismic excitation. The pendulum behaviour accompanied by the large radius of foundation curvature shifts the fundamental period of the system to a high value (e.g. more than 8sec), in a frequency range where none of the previously recorded earthquakes had considerable energy. This results in a large decrease in the structural responses. Since small friction forces are essential on the contact surfaces, PTFE sheets can be used as sliding surfaces. Although the pure frictional sliding systems have the same efficiency as the SCF, in reducing the responses of the superstructure, the main advantage of the new system is a significant decrease in sliding displacement. The performance of the SCF subjected to a number of harmonic and non‐harmonic base excitations is studied and its ability to reduce the structural responses is examined. Some numerical examples are solved for a single‐degree‐of‐freedom (SDOF) structure and the responses are compared with the responses of the same SDOF structure on a fixed base or a pure frictional sliding support system. The comparisons confirm the effectiveness of the new system. Copyright © 2002 John Wiley & Sons, Ltd.

Quantitative Effects of Projectile-Launch Tube Wall Friction on Ballistic Range Operation

Theoretical and experimental studies are conducted on the quantitative evaluation of a friction force at the interface between a projectile and the launch tube wall in a ballistic range. From a mechanical balance relation applied to the projectile, the friction force is expressed theoretically as a function of the net acceleration of the projectile. With this formulation, experimentally measured in-bore projectile velocity proe les obtained using a velocity interferometer system for any ree ector are well interpreted. If the initial projectile motion is signie cantly suppressed by the friction force while the burning rate of the propellant remains signie cant, the base pressure becomes much higher than that with a negligibly small friction force, thereby enhancing the net acceleration. However, if the projectile-tube e t is excessively high, the main function of the friction force is to decrease the muzzle velocity.

Geometric aspects of the theory of fiber tension, storage, and transport mechanisms in the absence of friction forces

The mechanisms that perform drawing, storage, or transport of chemical fibres significantly affect their final properties. The possibility of using the small parameter method is examined for calculating the parameters of this effect: if an accurate solution of the problem is known for some initial values of the basic parameters, then its solutions for other values which differ comparatively little from the initial values can be obtained. The method allows gradually increasing the adequacy of the solution by using new or altered values of the basic parameters in the calculation. An accurate solution corresponding to negligibly small friction forces was obtained for the case where the mechanism consists of two rotating elongated cylinders which the fibre is repeatedly wound around.

Development of a Three-Pad-Slider.

We studied a magnetic recording head of a three-pad-slider which had flat-inclined air-bearing surfaces. We expected a small friction force and a small velocity dependence on the flying height from the configuration. We confirmed these expectations by manufacturing heads with MR elements. We also performed continuous 1000-hour seek tests and read/write tests and found that the variation of signal amplitude was less than ±20 percent. After the tests, there was no measurable wear for three heads at the front contact pads and the rear contact pads among the total four tested heads. There was no measurable wear on the disk surface as well. These results demonstrate the sufficient durability of the head-disk interface.

Rheology of ice at the bed of Engabreen, Norway

AbstractA three-dimensional finite-element model is used to analyze field data collected as dirty basal ice flowed past an instrumented obstacle at the bed of Engabreen, a temperate glacier in northern Norway The ice is modeled as an incompressible power-law fluid, with viscosity , where ΠD is the second invariant of the stretching tensor, and B and n are two parameters. Using measurements obtained in 1996 and 1997, two values of B are obtained, one using the measured normal stress difference across the obstacle, and the other using the measured bed-parallel force over the instrument. These two values are not equal, probably owing to small frictional forces at the bed unaccounted for in the numerical model. Hence, B ranges between 1.9 × 107 and 3.2 × 107 Pa s1/3 in 1996, and between 2.2 × 107 and 4.1 × 107 Pa s1/3 in 1997. These values are smaller than measured elsewhere for clean glacier or laboratory ice. Field measurements of water content, fabric and texture of the basal ice suggest that unbound water between thin sediment layers and lamellae of clean ice may act as a lubricant and significantly weaken the ice. Near-isotropic fabrics indicate that preferred fabric orientation does not enhance the deformation.

Dynamic microforce measurement by distortion detection with a coupled-cavity laser displacement sensor stabilized in a mechanical negative-feedback loop

A small displacement sensor for dynamically measuring small forces is presented. Employing a coupled-cavity laser configuration constructed with a moving external mirror, this sensor provides high sensitivity with a minimum detectable displacement of 0.8 nm, independently of the lasing wavelength, while the lasing state is mechanically stabilized by a negative-feedback loop to maintain a constant external-cavity length. The sensor is shown to be of great use for detecting extremely small distortions in a stiff cantilever, reflecting transient variations in small friction forces in mechanical systems.

UFM observation of lattice defects in highly oriented pyrolytic graphite

Ultrasonic force microscopy (UFM) can be used to image the distribution of elastic modulus up to several tens of GPa, which is not possible by the force modulation mode using a soft cantilever of spring constant less than 1 N m −1. It was shown that by careful design of a piezoelectric transducer, together with a sample with small friction force, deflection vibration of the cantilever without torsion vibration is achieved, even during scanning perpendicular to the cantilever axis. Using this UFM, we propose that we have observed lattice defects reproducibly under atomically flat terraces of highly oriented pyrolytic graphite. Some defect was bent at surface steps, suggesting an interaction between the defect and the steps. The depth of a defect was found to be more than 3 nm on an assumption that it is continuous across surface steps. Contrast at the edge of terraces was explained by an geometrical effect associated with excitation of torsion vibration.

Damper of small vibrations

Vibration damper has no rubbing parts. Thus eliminates even very small static frictional forces and therefore responds to and damps extremely low levels of vibration. Damper dissipates vibration by motion of piston in volume of silicone oil. Coaxial rigid shaft holds upper and lower bellows at constant length while piston vibrates up and down. Although volumes of upper and lower bellows change continually, total volume of bellows assembly stays same.

Mechanisms of Rubber Abrasion in Unsteady State

Abstract Abrasion of rubber under the action of mechanical stress appears to involve two basic processes: micromolecular fracture and macrodelamination. The formation of ridges mainly results from the effect of tearing and tensile rupture, but the production of small particles is mainly due to the effect of microcutting. Ridge and small particle formation are basic characteristics of rubber abrasion. The physical processes during dry wear are gradual tearing leading to crack growth and also rupture of the tongue tip. A new concept of rupture length per revolution has been introduced, which is itself a function of the number of revolutions that have occurred during the unsteady state. Based on this, a quasi-theoretical relation for the rate of wear has been proposed. It reflects the influence of several factors on the rate of wear in a comprehensive way, and, it was verified by the experimental results. In the unsteady state of abrasion, the rate of wear and the spacing of the abrasion pattern not only depend upon the frictional force but also upon the number of revolutions. The relationship between the spacing and the frictional force is exponential. The influence of the rotational speed on the wear rate could be neglected when the speed did not exceed 18 rev/min. Oily wear was found at relatively small frictional forces (kN/F=0.24−0.41 m). The action of microcutting and the decomposition of the rubber are both prerequisites and sufficient conditions for oily wear to be produced. The same rubber shows two different modes of wear under different frictional forces. But a quantitative condition for the change from dry wear to oily wear is not yet clear.

On the indeterminancy of the problem of stratified fluid flow over a barrier and related problems

The indeterminancy of the problem of stratified fluid flow over a barrier and related problems has in the past been solved either by introducing small friction forces or by solving the problem from an initial state and letting the time tend to infinity. Here the connection between these two approaches are considered, and they are shown to be equivalent.

Instrument for measuring small frictional forces

Instrument for measuring small frictional forces

Instrument for Measuring Small Frictional Forces

A simple apparatus has been developed to measure frictional forces of small magnitude. It provides a continuous recording of tangential forces ranging from below 0.01 dyne to about 50 dynes. Details of the stick-slip characteristics are thereby recorded. The small normal forces required are applied and controlled by passing a current through a meter movement. The basic element for sensing frictional forces is a d'Arsonval galvanometer system. The frictional force applied to a lever arm produces a torque about the axis of the galvanometer system. A dual photodiode-servo amplifier system provides a correcting current to the galvanometer so that the mechanically imposed torque is automatically balanced. The correcting current is directly proportional to the tangential force. This apparatus has been extensively used in friction studies of individual textile fibers and for fine wire samples. It has also been used for measurements of stylus friction on flat surfaces under small loads. This paper includes a deta...

The sorting of balls in an automatic machine with a tapered gauge

Calculations, experimental data and the experience gained in using ball-sorting automatic machines with a tapered gauge lead to the following conclusions. 1. The durability of measuring surfaces in automatic machines with forced propulsion of balls is considerably larger (by a factor of 50) than that of the measuring surfaces in automatic machines with a free movement of balls. This is due to the smaller friction forces produced by the movement of balls over the rulers and by the armoring of their measuring surfaces with hard-alloy plates. The jamming of balls on the sloping surfaces of the left-hand ruler is prevented in automatic machines with forced propulsion of balls, whereas in automatic machines with free rolling of balls and the existing design of the measuring unit the balls often jam in the rulers, thus greatly reducing the reliability of sorting. 2. The elimination of indentations in the measuring rulers and the movement of balls at a constant speed reduces the static and dynamic errors of sorting.

A three-dimensional solution for waves in the lee of mountains

This paper presents a three-dimensional small-perturbation approach to the problem of waves produced in a statically stable stratified air stream flowing over a mountain. The fundamental solution for a doublet disturbance in an air stream in which the parameter l2 = gβ/V2 is constant is calculated, and then is extended to that for a disturbance due to a circular mountain in the same air stream. A simple approximation to the known two-dimensional flow over an infinite ridge is also given. The second (’upper’) boundary condition for the solutions is determined in a rigorous analytical manner, assuming the presence of small friction forces, or, alternatively, of time dependence. It is hoped that this will settle the controversy which exists over the choice of this condition.The results show that the behaviour due to a doublet is peculiar and not truly representative of that due to a mountain. The latter shows waves which decay down-stream and are contained in a strip, the width of the strip being determined by the radius of the mountain. An interesting result is that the circular mountain can give rise to waves which have greater amplitude than those produced by the infinite ridge under the same conditions.In some previous papers the waves produced by the infinite ridge have been neglected, but the present paper shows that in many cases this procedure is not justifiable. The detailed solution for the waves behind a circular mountain has a form which emphasizes the importance for lee-wave production of ’resonance’ between the width of the mountain and the characteristic length l-1 of the air stream.

Frictional Properties of Wool Fibres

THE friction apparatus devised by Bowden and Leben1 to examine the friction of metals has been used to study the frictional properties of wool fibres. Some accessories were made to adapt the apparatus to the application of loads of 0·05–1 gm. and the measurement of small frictional forces, but in essentials it remains the same as described by the above authors. The modifications enabled a single fibre mounted in a bow to be carried forward with a speed of 0·01 cm. sec.-1 while pressed against a cylindrical piece of ram's horn with the required load. The horn was mounted on a strip of clock spring, the deflexion of which, recorded photographically by a moving-film camera, was a measure of the frictional force.

Societies and Academies

LONDON. Royal Society, March 18.—S. Chapman, J. Topping, and J. Morrall: On the electrostatic potential energy, and the rhombohedral angle, of carbonate and nitrate crystals of the calcite type. Calculations have been made of the electrostatic potential energy of ionic lattices of the calcite type and for the sodium nitrate crystal, in which the lattice is of the same geometrical type, though the ionic charges are different. It seems probable that, owing to the fourfold positive charge in the C ions, the O ions of the CO3 group will be distorted from their natural symmetrical form, and consequently the electrostatic centre of the O ion will be nearer to the C nucleus than the O nucleus is. The calculations indicate that the electrostatic centre of the O ion is 0-9 A.U. distant from the centre of the C ion, compared with the value 1-25 A.U. given by X-ray measurements for the distance between the O and C nuclei. A similar result is obtained for NaNO3.-G. I. Finch and L. G. Co wen: Gaseous combustion in electric discharges. Pt. I. Steady direct current electric discharges are passed through electrolytic gas at 20-100 mm. pressures, and the rate of steam formation measured for varying conditions. It is found that the rate of combustion is determined only by the current, and up to a certain limiting current, combustion is confined to the cathode zone, depends upon the cathode material, and is directly proportional to the current. On exceeding this limiting current, combustion commences abruptly in the inter-electrode zone and is superposed upon cathodic combustion. This inter-electrodic combustion is also proportional to current, but, unlike the cathodic, is independent of the electrode material, and dependent upon gas pressure and separation of the electrodes. Little or no combustion takes place in the anode zone. Combustion seems to be primarily determined by ionisation of the gaseous medium for the current. The abrupt superposition of inter-electrodic combustion suggests a quantum effect, combustion in the inter-eiectrode zone depending upon a certain limiting potential fall being attained.-o C. N. Hinshelwood and W. K. Hutchison: A homogeneous unimolecular reaction: the thermal decomposition of acetone in the gaseous state. The temperature range used was 506° to.632° C. The heat of activation is 68,500 calories, and the results can be summarised by the equation lnk =34-95 - 68,500!RT. Calculation shows that the number of molecules reacting per second is about io5 times greater than the maximum number that could be activated by collision. The absolute rate at 835° Abs. is the same as that of the nitrogen pentoxide decomposition at 328° Abs.-H. Lamb: On wave resistance. The case considered is that of a solid of dimensions small compared with the depth below the free surface, travelling horizontally under water. Various particular cases of this have been worked out. There appears to be room for a more general investigation in which no assumption is made as to the form of the solid. The device employed for calculation of the resistance is to introduce small frictional forces, and to equate the work done against resistance to the dissipation of energy.-C. E. Eddy and A. H. Turner: The L emission series of mercury.-G. R. Goldsbrough: The properties of torsional vibrations. In order to bring out clearly the nature of the torsional vibrations in reciprocating engine shafts, a simplified model is chosen which emphasises the main characteristics. Even when friction is great, there are two critical speeds of shaft rotation corresponding to each term in the externally applied force.-E. T. Hanson: The theory of ship waves. The oscillations of a fluid under gravity, when the depth is variable or very great, have been worked out in two special cases. The theory is now extended very generally to cover a series of cases in which the fluid may be treated as a sea, bounded by a sloping shore on one side and extending indefinitely on the other. It is shown that, whenever the inclination of the shore to the horizontal is a submultiple of a right angle, the problem can be completely solved, and in each case there is a doubly infinite number of possible modes of motion under gravity. This extension makes it possible to calculate the precise effect of a sloping shore upon the formation of the beautiful wave pattern which often follows in the wake of a ship.-A. O. Rankine: The effect of temperature on the viscosity of air. The values of the viscosity of air obtained by Williams (Roy. Soc. Proc., A, vol. 110, p. 141) in the lower part of the temperature range are seriously in error. The conclusion that Sutherland's law of the variation of viscosity with temperature becomes invalid for air at the relatively high temperature of 250° C is not justified.