Testing the scuffing resistance of leathers

Explaining the value of this assessment using the Lisson Tretrad method.

by David Smith

There are many applications where scuff resistance is an important quality for a product. To complement the material tests already available, SATRA has developed the SATRA TM448:2017 – ‘Scuffing resistance of leathers – Lisson Tretrad method’ test as a way to assess scuff resistance under arduous conditions using a Lisson Tretrad machine. Typical applications include children’s shoes and footwear used in sports applications, where a high degree of scuffing may occur.

Lisson Tretrad

The Lisson Tretrad machine is used to assess the propensity of a material to wear under a realistic scuffing motion, created by the movement of a large weighted rotating assembly called a ‘Tretrad’. At the heart of the machine are two Tretrad wheel assemblies, one on either side of the test machine, each with four equally spaced ‘feet’. Test specimens are mounted on the Tretrad feet, and these specimens travel forwards and backwards over a length of abradant – for example, synthetic grass. On completion of the test, each specimen is examined to determine the degree of scuffing that has occurred and the mass loss is recorded.

Each foot platform has a radius of curvature 112.5 ±1mm, a circumferential length of 100 ±1mm, and a width of 55 ±5mm, and each of the feet exerts a force of 150 ±2N onto the test surface when stationary. The machine has a pair of flat plates, one either side of the machine, across which the Tretrad foot assemblies travel. The test surface is defined by the width of the Tretrad feet and the length of the track over which the Tretrad ‘feet walk’. The length of the track from the front edge of the base plate to the furthest point of travel is 800 ±20mm. 

One cycle on the test machine consists of one forward and one backward travel of the Tretrad assembly, at a linear speed of 0.28 ±0.2m/s. When the assembly reaches the furthest point of travel it pauses, and the Tretrad assembly remains stationary for approximately one second. During this period of stoppage, and while the feet are not in contact with the test track, the Tretrad assembly rotates freely to ensure that on subsequent test cycles the Tretrad feet contact with different areas of the material on the track bed, leading to an even wear pattern on the test specimens.

The Tretrad assembly rotates at a speed 20 ±1 per cent greater than that of the linear speed to create a scuffing action. The rotating feet of the Lisson Tretrad are designed to spin faster than the movement of the assembly allows, thereby scuffing and pulling at the bed surface as they pass. This action is similar to the scuffing or dragging action of a person dragging the forepart of his or her footwear backwards with each step. Additionally, the mass of the Tretrad assembly means that each ‘step’ has a reasonable degree of force behind it, creating a small impact similar to that created by a foot landing. The Lisson Tretrad provides scuffing abrasion and wear by subjecting specimens to the stomping and scuffing action of these metal feet wrapped in upper material as they pass over it.

Advantages of this method

The Lisson Tretrad method has advantages over some conventional materials tests that evaluate only a small piece of material or a single specimen. Fairly sizeable pieces of material under test are used, and with two Tretrad assemblies on the machine, each with four ‘feet’, four pieces of the same material can be tested as four specimens at the same time. If both tracks are used to test the same material, eight specimens can be tested simultaneously.

Rather than simply subjecting the specimen to a rotational or geometric abrasive action, the test materials are subjected to abrasion similar to that created during a walking action. This scuffing abrasion can also be used to represent abrasion and wear likely to be encountered during interaction between footwear and a surface.

Click here for comprehensive information on SATRA's range of test equipment.

Publishing Data

This article was originally published on page 38 of the September 2017 issue of SATRA Bulletin.

Other articles from this issue »

---