Replace it every 60 hours, otherwise the electric rotating tool will overheat.Carbon brush, suitable for For DREMEL 4000 90930-04 2615298790 595 395 300 6000 300 215 217 100.installation method:Step 1: Use a wrench to open the carbon brush cover.Step 2: Take out the carbon brush.Step 3: Replace the carbon brush with a new one.Step 4: Use a wrench to lock the lid firmly.Note: These brushes should be replaced every 60 hours.Colour:Photo ColorMaterial:carbon + metalPackage Contents:6 * Carbon BrushesComputer display is different, there may be color differenceOnly the above package content, other products are not included.Note: Light shooting and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.