21-06-2012, 02:11 PM
Device for Measuring Sliding Friction on Highloft Nonwovens
[attachment=24991]
Abstract
When measuring the sliding friction on highly compliant materials such as fabric batting and foam rubber, a substantial portion of the apparent friction is due to the deformation of the substrate. A new friction instrument consisting of a sled within a sled has been developed that eliminates the contribution of this deformation and provides the true sliding friction, as well as the force required to deform the substrate. The friction coefficient as measured using a conventional steel sled sliding on high loft polyester batts increased as the number of polyester batts increased. Using the new, guarded friction sled, the friction coefficient was independent of the number of supporting batts, thus separating the deformation forces from the sliding forces.
Introduction
Friction is a critical property for many applications. In some applications, high friction is desired, while in others low friction is preferred. It is not surprising then that understanding and measuring friction have been investigated for more than 2500 years1. Amonton's law was originally published by Da Vinci and later rediscovered by Amonton2. Amonton's law states that the friction depends only on the force normal to the contact plane. From this, the friction coefficient can be defined as:
Description of Frictometer
a picture of the guarded friction meter. It is made from a single piece of steel, measuring 12 cm x 15 cm x 2.5 cm. The outer, conventional sled, S, is pulled by a tension load transducer (Omegadyne LCFD-50, Sunbury, OH), Fc, via cord C. A semicircular section is cut out of the center of this sled to form the guarded sled, G, which is pulled by a compression load transducer (Omegadyne LC8100-250-25), Fg, via a screw that passes through Fg, through a washer, and through a slot, which allows G to move vertically relative to S. Appropriate spacing between S and G is maintained by an additional washer. Both S and G are made from the same material and are the same thickness so that the pressure is the same under both sleds. Additional weights can be added to alter the loads and the pressures. The shape of the inner sled can be chosen as appropriate for the desired test conditions. In our instrument, it is semicircular to avoid snagging on a fibrous substrate. The front edges of both sleds are rounded slightly to avoid digging into soft substrates. A motor (1 rpm, Merkle Korff Industries, Des Plaines, IL) attached to a 10:1 reduction gearbox (Gam Gear, Chicago, IL) pulls the compound sled via a cord at a rate of 5.30 mm/min. The load transducers are read using an A/D card (CIO-DAS801 from Omega Engineering, Inc., Stamford, CT) and the data are collected using LabView (National Instruments, Austin, TX).
Test Procedures
During a test, the sample is placed on an aluminum block and the back edge is clamped in place. The compound sled is placed on a flat surface and the unloaded readings, Runloaded, for the transducers are read. Next, the load cells are calibrated by suspending the compound sled via the towing cord and reading the load cells again to obtain the loaded reading, Rloaded. The calibration factor is simply:
Conclusions
A new, guarded friction sled has been developed for measuring friction of highly compliant materials. On materials that do not readily deform during the measurement, the conventional sled and the guarded sled provide the same friction coefficient within a few percent. The measured friction coefficients obtained by both sleds vary with the materials being tested. However, on a highly compliant material, such as a polyester batt, the sliding friction between the sled and the substrate measured can be vastly different for the conventional sled and the new guarded sled. Friction measurements made using the guarded sled are independent of the substrate deformation, while the surrounding conventional sled measures an apparent friction coefficient that contains contributions due to both sliding friction and substrate deformation. This difference is especially important in compound fabrics such as fabric-and-foam laminates, or when using high loft nonwovens, even when they are used as a backing material.
[attachment=24991]
Abstract
When measuring the sliding friction on highly compliant materials such as fabric batting and foam rubber, a substantial portion of the apparent friction is due to the deformation of the substrate. A new friction instrument consisting of a sled within a sled has been developed that eliminates the contribution of this deformation and provides the true sliding friction, as well as the force required to deform the substrate. The friction coefficient as measured using a conventional steel sled sliding on high loft polyester batts increased as the number of polyester batts increased. Using the new, guarded friction sled, the friction coefficient was independent of the number of supporting batts, thus separating the deformation forces from the sliding forces.
Introduction
Friction is a critical property for many applications. In some applications, high friction is desired, while in others low friction is preferred. It is not surprising then that understanding and measuring friction have been investigated for more than 2500 years1. Amonton's law was originally published by Da Vinci and later rediscovered by Amonton2. Amonton's law states that the friction depends only on the force normal to the contact plane. From this, the friction coefficient can be defined as:
Description of Frictometer
a picture of the guarded friction meter. It is made from a single piece of steel, measuring 12 cm x 15 cm x 2.5 cm. The outer, conventional sled, S, is pulled by a tension load transducer (Omegadyne LCFD-50, Sunbury, OH), Fc, via cord C. A semicircular section is cut out of the center of this sled to form the guarded sled, G, which is pulled by a compression load transducer (Omegadyne LC8100-250-25), Fg, via a screw that passes through Fg, through a washer, and through a slot, which allows G to move vertically relative to S. Appropriate spacing between S and G is maintained by an additional washer. Both S and G are made from the same material and are the same thickness so that the pressure is the same under both sleds. Additional weights can be added to alter the loads and the pressures. The shape of the inner sled can be chosen as appropriate for the desired test conditions. In our instrument, it is semicircular to avoid snagging on a fibrous substrate. The front edges of both sleds are rounded slightly to avoid digging into soft substrates. A motor (1 rpm, Merkle Korff Industries, Des Plaines, IL) attached to a 10:1 reduction gearbox (Gam Gear, Chicago, IL) pulls the compound sled via a cord at a rate of 5.30 mm/min. The load transducers are read using an A/D card (CIO-DAS801 from Omega Engineering, Inc., Stamford, CT) and the data are collected using LabView (National Instruments, Austin, TX).
Test Procedures
During a test, the sample is placed on an aluminum block and the back edge is clamped in place. The compound sled is placed on a flat surface and the unloaded readings, Runloaded, for the transducers are read. Next, the load cells are calibrated by suspending the compound sled via the towing cord and reading the load cells again to obtain the loaded reading, Rloaded. The calibration factor is simply:
Conclusions
A new, guarded friction sled has been developed for measuring friction of highly compliant materials. On materials that do not readily deform during the measurement, the conventional sled and the guarded sled provide the same friction coefficient within a few percent. The measured friction coefficients obtained by both sleds vary with the materials being tested. However, on a highly compliant material, such as a polyester batt, the sliding friction between the sled and the substrate measured can be vastly different for the conventional sled and the new guarded sled. Friction measurements made using the guarded sled are independent of the substrate deformation, while the surrounding conventional sled measures an apparent friction coefficient that contains contributions due to both sliding friction and substrate deformation. This difference is especially important in compound fabrics such as fabric-and-foam laminates, or when using high loft nonwovens, even when they are used as a backing material.