Coefficients of Friction: Static Versus Dynamic

Static Versus Dynamic ◽

New Discovery

Abstract This paper deals with the problem of static and dynamic (or kinetic) friction, namely the coefficients of friction for the two states. The coefficient of static friction is well known, and its theory and practice are commonly accepted by the academia and the industry. The coefficient of kinetic friction, however, has not fully been understood. The popular theory for the kinetic friction is that the coefficient of dynamic friction is smaller than the coefficient of static friction, by comparison of the forces applied in the two states. After studying the characteristics of the coefficient of friction, it is found that the comparison is not appropriate, because the inertial force was excluded. The new discovery in the paper is that coefficients of static friction and dynamic friction are identical. Wheel “locked” in wheel braking is further used to prove the conclusion. The key to cause confusions between the two coefficients of friction is the inertial force. In the measurement of the coefficient of static friction, the inertial force is initiated as soon as the testing object starts to move. Therefore, there are two forces acting against the movement of the object, the frictional force and the inertial force. But in the measurement of the coefficient of kinetic friction, no inertial force is involved because velocity must be kept constant.

The Coefficients of Friction between Rubber and Other Materials. Frictional Grip of Rubber-Tired Wheels

Rubber Chemistry and Technology ◽

10.5254/1.3535466 ◽

1930 ◽

Vol 3 (1) ◽

pp. 67-73

Author(s):

R. Ariano

Keyword(s):

Coefficient Of Friction ◽

Static Friction ◽

Systematic Difference ◽

Road Surface ◽

Dynamic Friction ◽

Simple Relationship ◽

Inflation Pressure ◽

Individual Design ◽

Increasing Load

Abstract (i) The coefficients of friction (ƒI and ƒnI) of rubber tires on dry non-dusty surfaces are practically independent of the load on the wheel, and (with pneumatics) of the inflation pressure; on muddy surfaces the coefficients (especially ƒnI tend to decrease with increasing load. (ii) Dust, mud, or water reduces the friction with rubber tires, but not with iron tires. (iii) The tread pattern reduces the friction on dry surfaces, but increases it on muddy surfaces. (iv) There is no systematic difference between pneumatic, semi-pneumatic (cushion) and solid tires as regarda coefficient of friction; the details of individual design and material are the deciding factors; this is in agreement with the results of Bredtscheiner (Verkehrstechnik, 1922; see Schaar, “Die Beanspruchung der Strassen durch die Kraftfahrzeuge,” Zementverlag, 1925). (v) There is no simple relationship between the coefficient of friction and the compressibility or area of contact of the tire. (vi) The static friction perpendicular to the direction of travel is greater than in this direction. (vii) The coefficient of friction depends on the type of road surface, its de-formability, and especially on the presence or absence of dust, mud, or water. (viii) Rubber tires have a much higher coefficient of friction than iron tires, especially on dry hard surfaces. (ix) The static friction is 10 to 20 per cent higher than the dynamic friction.

A method for determining the high-temperature coefficient of friction

Plasticheskie massy ◽

10.35164/0554-2901-2021-5-6-4-5 ◽

2021 ◽

pp. 4-5

Author(s):

V. V. Kovriga ◽

A. S. Vasil'eva ◽

A. I. Malikov

Keyword(s):

High Temperature ◽

Temperature Coefficient ◽

High Temperatures ◽

Coefficient Of Friction ◽

Static Friction ◽

Kinetic Friction ◽

Thermal Chamber ◽

A method for estimating the coefficient of friction at high temperatures up to 220°C in the thermal chamber of a bursting machine has been developed. It is shown that the coefficient of kinetic friction with a change in temperature from 25°C to 220°C varies from 0.04 to 0.1. In the developed method, the coefficient of static friction and the coefficient of kinetic friction are determined. The coefficient of static friction at a temperature of 25°C to 220°C varies from 0.06 to 0.13.

Effect of Stick - Slip Phenomena between Human Skin and UHMW Polyethylene

Pertanika Journal of Science and Technology ◽

10.47836/pjst.29.3.06 ◽

2021 ◽

Vol 29 (3) ◽

Author(s):

Emad Kamil Hussein ◽

Kussay Ahmed Subhi ◽

Tayser Sumer Gaaz

Keyword(s):

Friction Coefficient ◽

Human Skin ◽

Coefficient Of Friction ◽

Normal Load ◽

Static Friction ◽

Time Interval ◽

Dynamic Friction ◽

Sliding Velocity ◽

Stick Slip ◽

The present paper investigates experimentally effect of applied load and different velocity on the coefficient of friction between two interacting surfaces (human skin and Ultra-high-molecular-weight polyethylene (UHMW- polyethylene) at static and dynamic friction. It is possible to conclude specific point based on the above practical part and frictional analysis of this investigation as the most important mechanical phenomenon was creep has been observed a stick time interval where the static friction force is significantly increased during this stroke. The analytical model for stick-slip of skin and UHMWPE is proposed. The difference between static and kinetic friction defines the amplitude of stick-slip phenomena. The contact pressure, the sliding velocity, and rigidity of system determine the stability conditions of the movement between skin and UHMWPE. Experiments were carried out by developing a device (friction measurement). Variations of friction coefficient during the time at different normal load 4.6 and 9.2 N and low sliding velocity 4, 5, 6 and 7 mm/min were experimentally investigated. The results showed that the friction coefficient varied with the normal load and low sliding velocity. At static friction, the coefficient of friction decreased when the time increases, whereas, at dynamic friction, the coefficient of friction decreased when the time increased at normal load 4.6 and 9.2 N.

A STUDY OF STATIC FRICTION IN RECIPROCATING MOVEMENT OF SLIDING PAIR STEEL-EXPANDED GRAPHITE

Tribologia ◽

10.5604/01.3001.0010.6913 ◽

2016 ◽

Vol 270 (6) ◽

pp. 131-138 ◽

Cited By ~ 1

Author(s):

Aleksandra REWOLIŃSKA ◽

Piotr KOWALEWSKI ◽

Karolina PERZ ◽

Marta PACZKOWSKA

Keyword(s):

Coefficient Of Friction ◽

Sliding Friction ◽

Applied Pressure ◽

Expanded Graphite ◽

Static Friction ◽

Kinetic Friction ◽

Reciprocating Motion ◽

Distinctive Character ◽

Reciprocating Movement

The paper presents the results of coefficient of static and kinetic friction depending on the load. During the study, the sample in the form of a pin with expanded graphite, mounted in a holder, was forcibly pressed the Fn to the steel countersample. The device on which the tests were carried out research allows sliding friction in reciprocating motion. It has been found that there is a noticeable difference between the coefficient of static friction and kinetic for both fixed and different pressures. In the field of applied pressure, there were no significant their impact on the coefficient of friction; applied force was not sufficiently high which may have contributed to this state. The study had a distinctive character.

Frictional Vibrations

Journal of Applied Mechanics ◽

10.1115/1.4011044 ◽

1955 ◽

Vol 22 (2) ◽

pp. 207-214

Author(s):

David Sinclair

Keyword(s):

Coefficient Of Friction ◽

Equations Of Motion ◽

Static Friction ◽

Uniform Motion ◽

Friction Characteristic ◽

Stick Slip ◽

Brake Lining ◽

Solid Bodies ◽

Frictional Vibrations

Abstract Frictional vibrations, such as stick-slip motion and automobile-brake squeal, which occur when two solid bodies are rubbed together, are analyzed mathematically and observed experimentally. The conditions studied are slow uniform motion and relatively rapid simple harmonic motion of brake lining over a cast-iron base. The equations of motion show and the observations confirm that frictional vibrations are caused primarily by an inverse variation of coefficient of friction with sliding velocity, but their form and occurrence are greatly dependent upon the dynamical constants of the mechanical system. With a constant coefficient of friction, the vibration initiated whenever sliding begins is rapidly damped out, not by the friction but by the “natural” damping of all mechanical systems. The coefficient of friction of most brake linings and other organic materials was essentially invariant with velocity, except that the static coefficient was usually greater than the sliding coefficient. Most such materials usually showed a small decrease in coefficient with increasing temperature. The persistent vibrations resulting from the excess static friction were reduced or eliminated by treating the rubbing surfaces with polar organic compounds which produced a rising friction characteristic.

A Theory of Dynamic Rubber Friction

Rubber Chemistry and Technology ◽

10.5254/1.3544844 ◽

1966 ◽

Vol 39 (2) ◽

pp. 320-327 ◽

Cited By ~ 10

Author(s):

A. Schallamach

Keyword(s):

Coefficient Of Friction ◽

Time Lag ◽

Quantitative Agreement ◽

Dynamic Friction ◽

Average Life ◽

Thermally Activated ◽

Molecular Adhesion ◽

Rubber Friction ◽

Rate Processes

Abstract Assuming dynamic friction to arise from the shearing and subsequent breaking of distinct bonds between the rubbing members, a general equation is derived for the frictional force which involves the number and average life of the bonds as well as the average time lag between breaking and re-making of a bond at a given site. In the case of friction between rubber and smooth, hard surfaces, the bonds are attributed to local molecular adhesion between rubber and track, both formation and breaking of the bonds being thermally activated rate processes. A theory developed on this basis reproduces the experimental results obtained by Grosch in that the coefficient of friction as function of the velocity has a pronounced maximum. The height of the maximum and the velocity at which it occurs are in semi-quantitative agreement with Grosch's findings.

THE INFLUENCE OF A CONSTANT STATE OF DEFORMATION ON THE FRICTION COEFFICIENT IN SELECTED THERMOPLASTICS (POLYMER–STEEL PAIR)

Tribologia ◽

10.5604/01.3001.0010.5992 ◽

2017 ◽

pp. 39-45 ◽

Cited By ~ 2

Author(s):

Maciej KUJAWA ◽

Wojciech WIELEBA

Keyword(s):

Room Temperature ◽

Tensile Deformation ◽

Small Deformation ◽

Static Friction ◽

Kinetic Friction ◽

High Deformation ◽

Polymer Properties ◽

Constant State ◽

Friction Polymer

The effect of tensile deformation on polymer structures and their mechanical properties is described in various papers. However, the majority of articles are focused on high deformation (a few hundred percentiles) at increased temperature. It causes changes in orientation and the crystallinity ratio. The authors of this paper asses the influence of strain (max. 50%) on hardness and the coefficient of friction (polymer–steel A1 couple) for selected polymers. The deformation was conducted at room temperature and maintained during tests. There was a significant reduction (up to 50%) of hardness after deformation, in the case of all examined polymers. In the case of PE-HD, the coefficient of kinetic friction almost doubled its value (89% increase). The reduction of the coefficient of static friction for sliding pairs that include PTFE and PA6 was about 26% (in comparison with non-deformed polymer). For all investigated polymers, hardness increased over time (up to 40% after 24 hours). Coefficients of static and kinetic friction decreased in 24 hours (up to 29% coefficient of static friction and 19% coefficient of kinetic friction). The research shows that a small deformation causes changes in polymer properties. Moreover, these changes appear at room temperature directly after deformation.

Measurement of Friction between Rubber-Like Polymers and Steel

Rubber Chemistry and Technology ◽

10.5254/1.3539910 ◽

1962 ◽

Vol 35 (2) ◽

pp. 379-387 ◽

Cited By ~ 1

Author(s):

D. I. James

Keyword(s):

Polymeric Material ◽

Coefficient Of Friction ◽

Frictional Force ◽

Steel Plate ◽

Dioctyl Phthalate ◽

Direct Reading ◽

Automatic Operation ◽

Drive Mechanism ◽

Flat Sheet ◽

Abstract A machine for measuring the coefficient of friction between a flat sheet of polymeric material and a ground steel plate is described in detail. An inverted lathe cross slide forms the basis of the drive mechanism. Frictional force is balanced against the tension developed in a spring (proof ring), the extension of which measured with a commercial displacement pickup, gives a direct reading of coefficient of friction. A few results are given for a PVC sample plasticized with 40% dioctyl phthalate. A circuit for automatic operation and recording is also described.

The Calculation of Roll Pressure in Hot and Cold Flat Rolling

Proceedings of the Institution of Mechanical Engineers ◽

10.1243/pime_proc_1943_150_025_02 ◽

1943 ◽

Vol 150 (1) ◽

pp. 140-167 ◽

Cited By ~ 352

Author(s):

E. Orowan

Keyword(s):

Pressure Distribution ◽

Yield Stress ◽

Coefficient Of Friction ◽

Static Friction ◽

Rolled Stock ◽

Theoretical Treatment ◽

Frictional Drag ◽

Roll Pressure ◽

Plate Rolling ◽

A numerical or graphical method is given for computing, in strip or plate rolling, the distribution of roll pressure over the arc of contact and the quantities derived from this (e.g. the vertical roll force, the torque, and the power consumption). The method avoids all mathematical approximations previously used in the theoretical treatment of rolling, and permits any given variation of the yield stress and of the coefficient of friction along the arc of contact to be taken into account. It can be used, therefore, in both hot and cold rolling, provided that the basic physical quantities (yield stress and coefficient of friction) are known. The usual assumption that the deformation could be regarded as a locally homogeneous compression has not been made, and the inhomogeneity of stress distribution has been taken into account approximately by using results derived by Prandtl and Nádai from the Hencky treatment of two-dimensional plastic deformation. It is found that the discrepancy between the roll pressure distribution curves calculated from the Kármán theory and those measured by Siebel and Lueg is due to the assumption in the theory that the frictional drag between the rolls and the rolled stock is equal to the product of the roll pressure and the coefficient of friction. If frictional effects are dominant, as in hot rolling, this product may easily exceed the yield stress in shear which is the natural upper limit to the frictional drag, and then static friction, instead of slipping, occurs. This has been taken into account in the present method, and the calculated curves of roll pressure distribution show good agreement with the curves measured by Siebel and Lueg.

Study of Self-Locking Mechanism of Belt Friction

ASME/STLE 2007 International Joint Tribology Conference, Parts A and B ◽

10.1115/ijtc2007-44058 ◽

2007 ◽

Author(s):

Keiji Imado

Keyword(s):

Contact Angle ◽

Coefficient Of Friction ◽

Exponential Function ◽

Frictional Force ◽

Cylindrical Surface ◽

Loading Condition ◽

The Self ◽

Locking Mechanism ◽

Severe Loading

It is well know that the belt friction is expressed in an exponential function of a product μ and θ, the coefficient of friction and the angle of contact between the flexible belt and the cylindrical surface respectively. So the frictional force increases greatly with an increment of contact angle θ. Using this property, many kinds of buckles were developed to fasten belt. But the locking condition of belt is not obtained from the equation unless θ is of infinity. Their locking conditions were not clarified theoretically. In practice, the product of μθ is usually less than θ, so that the exponent of the product μθ is not so large. Then some slippage may occur in case of severe loading condition. This study is focusing on a self-locking mechanism of a simple buckle developed for flat belt. The belt in the buckle is partially wound again over the belt. According to the equation derived, the fraction of the tight side belt tension to the loose side belt tension is significantly affected by the angle of double-layered segment. With an increment of angle of doublelayered segment, the fraction increases to infinity, which means the occurrence of belt locking. The locking condition is determined by the geometry of the buckle and the coefficient of frictions. The frictional force is automatically generated by the tension of belt so that the self-locking mechanism is realized in the buckle. The equation derived was confirmed by the experiments.