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Flex testing in low temperatures

Outlining a range of SATRA test machines designed to evaluate flex resistance of components and materials at sub-zero temperatures.

Image © Osman Rara Unsplash

Flexing tests are sometimes required to be carried out at low temperatures. This determines the suitability of materials to flex at temperatures below 0°C. SATRA provides a range of these machines fitted with a low temperature cabinet specifically designed to suit the test.

Maximum operator safety is ensured by having the electrical control and drive systems outside the freezer area, where moisture cannot impair electrical safety. The control system incorporates a digital temperature controller which can be set to achieve the desired temperature down to -20°C subject to ambient temperature, plus a counter to indicate the number of flexes carried out. The counter can be pre-set to stop the drive mechanism when completed.

SATRA STM 101F upper material flexing machine

SATRA STM 101F is a 16-station version of the SATRA STM 601 vamp flexing machine. The stations are located inside the freezer area to allow simple loading of test specimens and easy/frequent inspections without having to stop the machine. This equipment is fitted with a counter which allows the total number of flexes to be pre-set.

This machine assesses the tendency of all types of footwear upper materials, clothing leathers and even upholstery coverings to crack or break as a result of flexing in wear. It is also suitable for testing plastic-coated fabrics, poromerics and woven fabrics and, in addition, can be used to test seam construction during flexing. The crease that is formed is an accurate representation of those created in normal wear.


The SATRA STM 141F Ross flexing machine

SATRA STM 141F Ross flexing machine

This low-temperature version of the SATRA STM 141 Ross flexing machine is used to assess the effect of flexing in sole unit samples under low temperature conditions. It has 12 stations to hold strip test pieces, but can also accommodate whole soles by adjusting the mechanism. While the STM 141/141F normally runs at 60 flexes per minute, a motor to produce 100 flexes per minute can be fitted for tests to ASTM standards.

SATRA STM 477F Bally vamp flexer

The 12-station SATRA STM 477F machine is a special version of the Bally vamp flexer. STM 477F equipment is used to conduct an alternative low temperature assessment to the SATRA vamp flex test, frequently referred to as the ‘Bally flex test’. Mentioned in some international standards, it is more severe than the vamp flex test and failure is usually indicated more quickly.


Controls on the SATRA STM 477F Bally vamp flexer

SATRA STM 612F SATRA/Bata belt flexing machine

The STM 612F SATRA/Bata belt flexing machine provides one of the best indications of the risk of cracking of shoe soles due to flexing in wear. The test is conducted by fixing the sole unit to an endless belt which passes around two pulleys of differing diameters. The larger main drive pulley provides the rotation while the smaller of the two pulleys determines the severity of the flexing action. The equipment is supplied complete with three small pulleys of varying diameters to increase or decrease the amount of flexing.

The endless belt can usually accommodate six sole units, subject to the style and size. Sole units are bonded to the belt using a strong adhesive. When very stiff sole units are tested, it may be necessary to stitch the toe of the sole unit to the belt. The belt is made of canvas and its construction allows the number of complete cycles to be counted which is equal to the number of flexes of the test sample. The flexing area is provided with a full guard interlocked with the machine controls to ensure the guard cannot be opened until the belt has stopped.

The control cabinet located at the side of the machine houses the motor drive controls, as well as a counter which can be pre-set to stop the machine when a predetermined number of flexes has been reached. This allows the examination of the sole units at intervals throughout the test duration. The standard machine is supplied as a bench top machine, although a separate stand is available as an optional extra.

The SATRA STM 612F low temperature variant of the STM 612 machine has been designed to allow flexing to be carried out at sub-zero temperatures, thus enabling the increased risk of cracking due to cold weather. The complete flexing

mechanism is built into an insulated chamber where the temperature can be reduced to a pre-set level. STM 612F is supplied complete with a stand that houses the refrigeration unit.

SATRA STM 465F whole sole flexing machine

This version of the SATRA STM 645 machine is used to determine the resistance of materials to cut growth during repeated flexing at low temperatures, and it is especially applicable to outsoles of protective footwear. The machine has three workstations presented horizontally for operator ease. Loading, unloading and measurements are also much easier to carry out with the workstations presented in this manner. Samples are set at 140 flexes per minute and the number is recorded on a counter which has the built-in facility to pre-set the number of flexes required.

A SATRA STD 465J jig can be used to locate an STD 465C chisel and support the test while the initial chisel cuts are being made in the soling. All moving parts which require user access are protected by a fully-interlocked safety guard, thus ensuring that the machine conforms to the latest safety regulations. A manual device which is used to measure the stiffness of an outsole may be required to see if the test piece must be flexed in accordance with EN ISO 20344.


The SATRA STM 465F whole sole flexing machine

SATRA STM 184F whole shoe flexing machine

The action of this low-temperature version of the SATRA STM 184 machine simulates the flexing of footwear during wear. The angle of flex is adjustable up to 50 degrees. The machine may be used to predict the failure of footwear in wear, and it is particularly useful in assessing problems associated with bottom construction.

Footwear is clamped to the machine at the toe end using the appropriate toe clamp from one of five pairs supplied. The heel end is secured to the flexing bar. A predetermining counter is fitted and once the appropriate number of flexes has been registered, it will stop the machine so that the shoe can be inspected.

The moving parts of the machine are covered by a hinged guard which is fitted with the latest safety device in order to prevent access while the machine is still in use. The machine is robust and non-corrosive materials are used where applicable in its construction.

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Publishing Data

This article was originally published on page 28 of the November 2022 issue of SATRA Bulletin.

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