Determination of the abrasion and friction properties on deep-drawn sheets
Special testing machine 20 kN for draw-bead tests
Sheet metal forming test methods provide characteristic values for metal processing. One sheet metal forming process is deep drawing. In the automotive industry, the demands on the properties of sheet metal are constantly increasing: these include, for example, high stability with simultaneous formability and the lightest possible weight.
This special testing machine can be used to determine the abrasion and sliding properties of deep-drawn sheet metal used in automotive construction. These sheets are used, for example, for bonnets, wings, doors or pillars. Car body parts like these are produced by car manufacturers through automated deep drawing. Because they are irregularly shaped sheet metal parts, the forming processes are complicated.
In order to control the flow of material so that the sheet metal does not crumple or even tear locally during forming, so-called draw beads are incorporated into the drawing edges of the forming tool. They act like brakes and prevent excessive local material flows into the drawing die. These draw beads are the neuralgic points in the drawing process. They only fulfil their function if optimum friction conditions prevail between them and the sheet. These friction conditions are set by lubricants or coatings on the body sheet during production. Galvanised sheet metal has inherent sliding properties that are used for pulling. The coefficient of friction is the measure of friction. It has to be determined in order to select the right lubricant or the right coating for a particular sheet. The test method for this is the Draw Bead Test (DBT), in which a sheet metal sample is drawn through a clamping device equipped with a drawing bead at a defined force and speed. The approximately man-high, spindle-mechanically driven machine can test sheet metal strips of 50 mm width, 400 mm length and up to 2 mm thickness. The hydraulic pressure axis is horizontal, the pull-off axis vertical. With contact forces of up to 50 kN, the sheet can be pulled through the clamping device at variable speed. Test speeds of up to 150 mm per second are achieved. For this reason, the safety enclosure prescribed for such high test speeds only permits tests with the safety door electronically locked.
By replacing the DBT clamping device with another one, without a pulling bead, in a few simple steps, friction force tests can also be carried out. These tests are carried out with particularly high contact pressure and maximum pull-through speed. The tests are used to test the durability of surface coatings - for example, the zinc coating already mentioned - as the forming must not cause any damage to the corrosion protection of the sheet metal.High-precision, high-speed thermocouples integrated into the clamping jaws record their temperature change during the test, which is also a measure of the frictional force. Another extra concerns the design of the clamping jaws. Special ball-bearing stops on the edges of the clamping jaws prevent sheet samples from deforming like sabres and running out of the holder during the test due to slightly uneven contact pressure across the jaw cross-section.