SATRA STM 609 Safety Footwear Impact Test machine

Describing the importance of this SATRA test machine in assessing the effectiveness of protective shoes.

by Peter Allen

Many workers face potentially debilitating toe injuries if the safety footwear they are wearing is not fit-for-purpose. Safety footwear manufacturers also run a risk of litigation with hefty financial penalties if the toe caps they select do not live up to expectations. It is important to ensure that these components will actually provide the expected level of shielding.

Two vital aspects can be tested with the SATRA STM 609 Safety Footwear Impact Test machine: toe cap impact protection and metatarsal impact protection. The tests can be conducted in accordance with three sets of methods – European EN ISO 20344/5/6 (2004), Canadian Z195 (2009) and American ASTM F2413 (2011). Analysis can be carried out on the toe cap before shoemaking (for example, under EN12568:2010), or from a section cut from the complete footwear. Assessment of protection for the metatarsal bones of the foot is conducted only on complete footwear.

Protective toe caps provide ‘defended space’, which must be maintained under impact. Two types of deformation can occur following an impact: ‘elastic’ and ‘plastic’. Elastic deformation is the deflection of the toe cap which is recovered following the impact, whereas plastic deformation is the permanent change of shape of the material after the impact. Both types of deflection are common. Not only the condition of the toe cap after an impact should be assessed; checking the maximum amount of protection offered against impact intrusion into the toe space is also required.

Key impact tests

Impact tests, as required as part of legislative requirements for safety footwear, typically specify the following:

• An impact energy to which the specimen on test is to be subjected
• An impact anvil geometry and associated mass
• A location for where impacts are to strike the test specimens
• Requirements for anchoring the footwear or component
• Criteria for acceptance after the test.

These represent some key requirements which an impact test machine capable of conducting these tests needs to accommodate.

To determine the maximum ingress into the toe space during the test, a cylinder of modelling clay is placed inside the toe cap prior to the test. This material has negligible spring-back when compressed, so it will retain the depth to which it was compressed. After the test, the height of the compressed cylinder is measured and checked against minimum heights specified in the relevant standards. For assessing the impact in the case of the metatarsal test, a moulded wax impression is fitted into the footwear and measurements are taken from this moulding before and after the test.

The SATRA STM 609 test machine imparts the specified impact energy by allowing an anvil of accurate mass to fall from a specified height. The anvil in the SATRA STM 609 test machine is guided in its fall by double-wheeled, low friction bearing units located between two vertical guides. An electric motor drive incorporated in the machine is used to hoist the anvil to the correct height. The anvil is then released via the control system and allowed to free-fall onto the target sample. Different anvils can be readily fitted to conform to the requirements of the particular test required.

At the base of the machine, an adjustable mechanism catches the anvil on rebound from the initial impact, and so does not impart a subsequent impact on the specimen. A speed sensor is incorporated immediately above the impact point. The machine’s computing system utilises the recorded speed and the known mass of the anvil to calculate the impact energy imparted (to confirm that the specified impact energy has been applied). This displays on the operating screen, along with other relevant information including the drop height. The display screen is also used to enter data and initiate the test. During set-up, the anvil is lowered to a position just above the specimen, which can then be accurately positioned and clamped. A jog function is used to position the anvil in contact with the specimen. The machine then raises the anvil to the required height (depending on the particular test), before releasing the anvil, which impacts the test specimen. The machine incorporates safety interlocked glazed security shields to provide protection from the falling anvil or any flying debris resulting from a test.

STM 609 can hold sections of complete footwear, individual components or toe sections cut from footwear. Clamps allow the specimens to be locked in the correct position.

SATRA test machines are designed and manufactured by SATRA, and in regular use in SATRA’s own laboratories. Test performance can therefore be monitored over extended periods, user experience being fed back into the design.

The SATRA STM 609 test machine can be used for compliance testing and to verify component or footwear assembly performance for product development. It can also be used to assist with the quality auditing of production items.

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

Publishing Data

This article was originally published on page 36 of the December 2011 issue of SATRA Bulletin.

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