One of the oldest screening devices is the trommel or revolving screen, which is a cylindrical screen typically rotating at between 35 and 45% critical speed. Trommels are installed on a small angle to the horizontal or use a series of internal baffles to transport material along the cylinder. Trommels can be made to deliver several sized products by using trommel screens in series from finest to coarsest such as the one shown; or using concentric trommels with the coarsest mesh being innermost. Trommels can handle material from55 mmdown to6 mm, and even smaller sizes can be handled under wet screening conditions. Although trommels are cheaper, vibration-free, and mechanically robust; they typically have lower capacities than vibrating screens since only part of the screen surface is in use at any one time, and they can be more prone to blinding.