Technical field

The invention relates to a device for determining the coefficient of dynamic friction of flexible planar and/or linear structures.

The invention relates to a method for determining the coefficient of dynamic friction of flexible planar and/or linear structures.

Background art

The coefficient of dynamic friction is a physical quantity describing the dynamic friction of different materials under contact stress during their mutual movement. This coefficient is used, for example, in the evaluation of various textile materials (linear or planar), e.g., in determining or verifying their applicability for different applications (production of parachutes, safety belts, tyres, conveyor belts, etc.) or their processability by different technologies (by which these materials have not yet been processed or have been processed under different conditions - e.g., high-speed spinning technology), etc.

Patent CZ 301502 discloses a method of determining the static and dynamic coefficient of friction of a longitudinal structure and a measuring device for performing the method. The device comprises at least one conductor with a curved-cylindrical surface, e.g., a knitting hook, intended for being wrapped by the monitored longitudinal structure, a sensor of tensile force connected to a recording and/or evaluation device and a device for inducing a reciprocal uniform movement of the monitored longitudinal structure. To determine the coefficient of friction, the longitudinal structure to be monitored is wrapped around the cylindrical surface of at least one conductor, whereby at one end it is connected to the sensor of tensile force and at the other end, a suitable weight is suspended to create preload. Thereafter, the longitudinal structure moves uniformly reciprocally in both directions of its length by the action of the device for reciprocating uniform movement, the magnitude of the tensile force acting therein being measured by the sensor of tensile force. This procedure determines a pair of tensile forces, one of which is greater by the effect of friction between the longitudinal structure and the conductor(s) and the other is less by the effect of friction between the longitudinal structure and the conductor(s), whereupon from the measured values of tensile forces and the magnitude of the wrapping angle of the longitudinal structure around each of the conductors, the resulting magnitude of the coefficient of friction between the longitudinal structure and the conductor(s) is calculated for each of the tensile forces on the basis of Euler's formula.

The disadvantage of this measuring device and method for determining the static and dynamic coefficient of friction is the fact that they are not suitable for determining the coefficient of dynamic friction during continuous friction with long duration and/ or at a high speed of the movement of the longitudinal structure, because the testing time and the speed of the longitudinal structure are limited by the length of the monitored longitudinal structure and by the fact that this structure performs a reciprocating movement, during which it stops at dead ends, where the direction of its movement changes. Another disadvantage of the device is the fact that it does not allow to simply test the mutual dynamic friction of two longitudinal structures - different or identical.

The objective of the invention is a device for determining the coefficient of dynamic friction during friction of flexible planar and/or linear structures and a method for determining the coefficient of dynamic friction which would eliminate the disadvantages of the background art.

Principle of the invention

The objective of the invention is achieved by a device for determining the coefficient of dynamic friction of flexible planar and/or linear structures, which comprises a sensor of tensile force acting in a first flexible planar or linear structure which is connected to a computing unit provided with means for determining the coefficient of dynamic friction of the surfaces of two flexible planar or linear structures, and further comprises a tensioning means for tensioning the first flexible planar or linear structure by tension force. A friction drum coupled to a drive for rotational movement about a longitudinal axis is mounted transversely rotatably between the sensor of tensile force and the tensioning means, the friction drum being provided with means for arranging a second flexible planar or linear structure on its surface. This device, in contrast to the devices known from the background art, can be used to determine the coefficient of dynamic friction of the surfaces of two flexible planar and/or linear structures, wherein the length of the continuous operation of the device is essentially arbitrary and limited only by the resistance of the tested materials.

In a preferred embodiment of the device according to the invention, the tensioning means is coupled to a sensor of tension force which is connected to the computing unit that makes it possible to correct the effect of possible vibrations of the friction drum on the tensile force measured by the sensor and to automatically detect rupture of the first, rubbed, flexible planar or linear structure.

In a preferred embodiment of the device according to the invention, the friction drum is hollow, the means for arranging the second flexible planar or linear structure on its surface being accommodated in its inner space. At the same time, a longitudinal slot is formed in the shell of the hollow friction drum for introducing the second flexible planar or linear structure to these means.

The actual speed of the friction drum during the determination of the coefficient of dynamic friction is influenced by the resistance of dynamic friction and is therefore lower than the pre-set idle speed. In a preferred embodiment, therefore, the friction drum is associated with a speed sensor connected to the computing unit provided with means for detecting the speed of movement of the surface of the second, frictional, flexible planar or linear structure over the surface of the first, rubbed, flexible planar or linear structure, so that the actual speed of the dynamic friction of the surfaces of the two flexible planar or linear structures can be monitored, analysed and, if necessary, controlled.

The temperature of the rubbed flexible planar or linear structure, especially during long-term continuous testing, can temporarily or permanently affect the structure of the material and thus the magnitude of the dynamic friction coefficient. In a preferred embodiment, the friction drum is associated with a pyrometer for determining the temperature of the first, rubbed, flexible planar or linear structure. The pyrometer is preferably oriented perpendicularly or substantially perpendicularly to the surface of the friction drum, or it is oriented to the surface of the friction drum at an angle at which it has been calibrated for determining the temperature of the first flexible planar or linear structure. The pyrometer is connected to the computing unit provided with means for determining the temperature of the surface of the first flexible planar or linear structure at the place of its wrapping around the friction drum.

When determining the coefficient of dynamic friction of various combinations of surfaces and materials of flexible planar and/or linear structures, it is advantageous if it is possible to adjust the magnitude of the tensile force, thus achieving greater reliability of measurement by the sensor of tensile force, slower wear of rubbed materials, etc. In a preferred variant of embodiment, therefore, the sensor of tensile force is coupled to a clamping element for clamping the first flexible planar or linear structure with the possibility of movement, e.g., by being coupled to a positioning element for setting the angle of the wrapping of the friction drum by the first flexible planar or linear structure, etc.

Furthermore, the objective of the invention is also achieved by a method for determining the coefficient of dynamic friction of flexible planar and/or linear structures, in which the first flexible planar or linear structure, which is at one end connected to the sensor of tensile force and at the other end it is loaded by tension force, presses against the surface of the second planar or linear structure arranged on the surface of the rotating friction drum, wherein the sensor of tensile force measures the tensile force acting on this end of the first flexible planar or linear structure and from the known tension force and the measured tensile force, the coefficient of dynamic friction is determined according to the formula where f is the coefficient of dynamic friction of the surfaces of the given flexible planar and/or linear structures, F _t is the tensile force acting in the first flexible planar or linear structure, F _n is the tension force, by which the first flexible planar or linear structure is preloaded by the tensioning means, and a is the angle of the wrapping of the friction drum by the first flexible planar or linear structure in radians. Thus, in contrast to the methods known from the background art, it is possible to determine the coefficient of dynamic friction during uninterrupted, essentially long-lasting friction of surfaces of two flexible planar or linear structures and/or during friction of surfaces of two flexible planar or linear structures at high speeds.

Description of the drawings

In the accompanying drawings, Fig. 1 is a schematic cross-sectional view of a first exemplary variant of the device for determining the coefficient of dynamic friction of flexible planar and/or linear structures according to the invention, and Fig. 2 is a cross-sectional view of a second exemplary variant of the device.

Examples of embodiment

The device for determining the coefficient f of dynamic friction of flexible planar and/or linear structures 1 a, 1 b and the method of determining the coefficient f of dynamic friction according to the invention will be explained with reference to two exemplary variants of this device shown in Figs. 1 and 2 and their function.

The device for determining the coefficient of dynamic friction f of flexible planar and/or linear structures la, b, comprises a sensor 2 of tensile force Ft acting in a first flexible planar or linear structure 1 a. The sensor 2 of tensile force Ft is provided with a suitable fastening means 32 for fastening the first flexible planar or linear structure 1 a, such as a spring clamp, a hook, or another suitable fastening means for fixing the linear or planar structure. The device for determining the dynamic friction coefficient f further comprises a tensioning means 4 for loading the first flexible planar or linear structure 1 a with a predetermined tension force Fn, the tensioning means 4 being provided with a fastening means 34 for fixing the first flexible planar or linear structure 1_a.

A friction drum 5 with a horizontally oriented longitudinal axis 51 is mounted transversely rotatably in an unillustrated frame of the device between the sensor 2 of tensile force Fj and the tensioning means 4. The friction drum 5 is coupled to a drive 6 for rotatable movement about its longitudinal axis 51, which is coupled to a control unit 7. The drive 6 of the friction drum 5 preferably has adjustable speeds - for example, it is formed by an electric motor equipped with a frequency converter, etc. The friction drum 5 is provided with means for temporarily arranging a second flexible planar or linear structure 1 b on its outer surface. In the preferred variant shown in Figs. 1 and 2, the friction drum 5 is hollow and the means for arranging the second flexible planar or linear structure 1 b are accommodated in its inner space. Such means include, for example, at least two longitudinal swivel pins 52 provided with unillustrated spring clamps and/or slots for fastening the second flexible planar or linear structure 1 b. The swivel pins 52 are arranged in the inner space of the friction drum 5 parallel with its longitudinal axis 5T In this case, in addition, a slot 531 is formed in the shell 53 of the friction drum 5 for introducing the second flexible planar or linear structure 1 b to the means for arranging it which are accommodated in the inner space of the friction drum 5. In other variants, other means can be used for mounting the second flexible planar or linear structure b on the surface of the friction drum 5, it is also possible to use other means which will not interfere with its structure. These means can be arranged in the inner space of the friction drum 5 (e.g., at least one clamp provided with a tensioning arm), or on the surface of the friction drum 5 (e.g., a Velcro fastener, glue layer, etc.). The friction drum 5 is preferably associated with at least one known speed sensor 56, e.g., electromagnetic inductive sensor for sensing a metallic element of the friction drum 5, e.g., of a metallic longitudinal swivel pin 52, a laser sensor for sensing an unillustrated reference mark located on the surface of the shell 53 or of the face of the friction drum 5, etc. In addition, the friction drum 5 is preferably associated with a pyrometer 8 oriented, for example, perpendicularly or substantially perpendicularly to the shell 53 of the friction drum 5, and located at the place 1 1 of the wrapping of the friction drum 5 by the first planar or linear flexible planar or linear structure 1_a.

The sensor 2 of tensile force Ft and optionally also the speed sensor 56 and/or the pyrometer 8 is/are connected to a computing unit 9 of the device for determining the coefficient f of dynamic friction, which is provided with means for determining the coefficient f of dynamic friction and, optionally, also with means for determining other parameters of at least one of the flexible planar or linear structures 1_a, 1_b, of rotation speed of the friction drum 5, etc. The computing unit 9 is preferably connected to a control unit 7 of a drive 6 of the friction drum 5 and is provided with means for controlling the control unit 7.

In the variant of the device shown in Fig. 1 , the tensioning means 4 for loading the first flexible planar or linear structure 1 a is formed by a tensioning weight 41 provided with a fastening means 34 for fastening it to the first flexible planar or linear structure 1_a. The tension force Fn acting on the first flexible planar or linear structure 1 a in the rest state of the device then corresponds to the gravitational force F_ _g acting on the tensioning weight 41 .

In the variant of the device shown in Fig. 2, the tensioning means 4 is formed by a device for regulating the force, e.g., based on a spring and/or a lever, or a lever equipped with a sliding and/or interchangeable weight, which allows a more variable adjustment of the tension force Fn, or its change during the determination of the coefficient f of dynamic friction. In the illustrated variant of embodiment, the tensioning means 4 is oriented vertically upwards, but in variants not shown, it can be oriented substantially arbitrarily differently. The advantage of the upward orientation of the tensioning means 4 is the reduction of the build-up area of the device. In this variant, the device for determining the coefficient f of dynamic friction is provided with a sensor 2a of tension force Fn in the first flexible planar or linear structure 1 a when tensioned by the tensioning means 4. The sensor 2a of tension force Fn is arranged between the tensioning means 4 and the fastening means 34 for fixing the first flexible planar or linear structure 1 a and is connected to the computing unit 9. The tension force Fn measured by the sensor 2a of tension force Fn corresponds to the force set on the tensioning means 4. The tensioning means 4 is, for example, connected to the computing unit 9.

The advantage of the variant shown in Fig. 2 is the fact that a combination of the sensor 2 of tensile force Ft and the sensor 2a of tension force Fn allows to correct distortion of the tensile force Fj measured by the sensor 2 of tensile force Ft caused by vibrations which are transmitted to the first flexible planar or linear structure 1 a from the rotating friction drum 5. The currently measured tension force Fna is in the computing unit 9 compared to the value of the tension force Fn set by the tensioning means 4 and then, during the measurement, the computing unit 9 performs correction of the effect of the vibrations of the rotating friction drum 5.

The sensor 2 of tension force Fn is mounted in the device frame, for example, by means of a positioning element 1_0, which allows the sensor 2 and with it also the clamping point of the first flexible planar or linear structure 1a to be placed in any position relative to the friction drum 5 and thus change, e.g., the angle a of the wrapping of the friction drum 5 by the first flexible planar or linear structure 1 a. The positioning element 10 is designed, for example, as a horizontally oriented sliding and/or swivel pin with a locking screw, etc. The angle a of the wrapping of the friction drum 5 by the first flexible planar or linear structure 1 a is preferably adjustable in the range of 10 ° to 140 °, more preferably in the range of 22 ° to 105 °, or otherwise, as required.

If necessary, the device for determining the coefficient f of dynamic friction at the guide point of the first flexible planar or linear structure 1 a is provided with (fixed or rotatable) guide element/elements 20. In the variant shown in Fig. 2, such a guide element 20 is arranged between the friction drum 5 and the tensioning means 4 and serves to change the guiding direction of the first flexible planar or linear structure 1a.

When determining the coefficient f of dynamic friction using the abovedescribed device according to the invention, the second flexible linear structure 1 b, such as a textile cord, a textile belt, a textile fiber bundle, a glass fiber bundle, a yarn, a thread, etc., or a flat structure, such as a strip of sheet metal, leather, plastic foil, sandpaper, etc., is arranged on the outer surface of the friction drum 5 preferably in such a manner that it covers at least a part of its surface around its entire circumference. Subsequently, the surface of the friction drum 5 with the second flexible planar or linear structure 1 b arranged at a defined angle a is wrapped by the first flexible planar or linear structure 1 a, which may or may not be the same as the second flexible planar or linear structure 1 b. At the same time, the first flexible planar or linear structure 1 b is fixed at one end in the fastening means 32 coupled to the sensor 2 of tensile force Fj and at the other end in the fastening means 34 coupled to the tensioning means 4. The tensioning means 4 is loaded with a predetermined tension force Fn. The friction drum 5 is then set into rotational movement about its longitudinal axis 51, preferably, e.g., at a constant speed of 50 to 5000 revolutions per minute in a direction away from the sensor 2 of tensile force Ft towards the tensioning means 4 (see arrow d in Figs. 1 and 2). In doing so, the surface of the first flexible planar or linear structure 1 a rubs the surface of the second flexible planar or linear structure 1 b arranged on the surface of the friction drum 5, as a result of which a frictional force Ff induced by the effect of this dynamic friction is induced in the first flexible planar or linear structure a, which acts in the same direction as the tension force Fn and which combines with the tension force Fn to form the resulting tensile force Ft measured by the sensor 2 of tensile force F^ The magnitude of the tensile force Ft measured/being measured by the sensor 2 of tensile force Ft is stored, or continuously stored, in the computing unit 9 and then the coefficient f of dynamic friction is determined, or continuously determined, by the computing unit 9 according to the following formula derived from the Euler relation: where f is the coefficient of dynamic friction of the surfaces of the given flexible planar and/or linear structures la, 1_b, Ft is the tensile force sensed by the sensor 2 of tensile force acting in the first flexible planar or linear structure la, Fn is the tension force by which the first flexible planar or linear structure 1 a is preloaded by the tensioning means 4 and a is the angle of the wrapping of the friction drum 5 by the first flexible planar or linear structure 1a in radians.

In case of need, the rotation speed of the friction drum 5 can be changed during the measurement. If the device for determining the coefficient f of dynamic friction is provided with the speed sensor 56 of the friction drum 5, it is possible, simultaneously with the determining of the coefficient f of dynamic friction, to determine the speed of the movement of the second flexible planar or linear structure 1 b on the surface of the first flexible planar or linear structure 1 a from the actual revolutions of the friction drum 5, or determine the speed of the movement, or adjust the speed to a required value during the measurement by means of controlling the drive 6 of the friction drum 5 by the control unit 7.

If the device for determining the coefficient f of dynamic friction is provided with the pyrometer 8, it is possible, during the determination of the coefficient f of dynamic friction, to measure the temperature or a change in the temperature of the first flexible planar or linear structure 1 a at the place of contact with the second flexible planar or linear structure 1 b and, if appropriate, to monitor the dependency of the coefficient f of dynamic friction on the temperature, or vice versa.

The device for determining the coefficient of dynamic friction allows to test the dynamic friction of the surfaces of the two flexible planar or linear structures 1 a, 1 b for essentially any time, limited only by the resistance of the rubbed materials to frictional wear, while it is possible to simulate the actual conditions of dynamic friction of these structures. List of references

1a First flexible planar or linear structure 1 b Second flexible planar or linear structure 11 Place of wrapping 2 Sensor of tensile force

2a Sensor of tension force

32 Fastening means

34 Fastening means 4 Tensioning means 41 Tension weights 5 The friction drum

51 Axis of the friction drum

52 Longitudinal swivel pin

53 Shell of the friction drum 531 Slot in the shell of the friction drum 56 Speed sensor 6 Drive of the friction drum

7 Control unit

8 Pyrometer

9 Computing unit

10 Positioning element

20 Guiding element

Fg Gravitational force of the tensioning means

F _n Tension force

Ft Tensile force

F _f Friction force a Wrapping angle d Direction of the drum rotation f Coefficient of dynamic friction