Roots vacuum pumps, or mechanical booster pumps, are dry vacuum pumps that are used together with backing pumps in a wide range of applications where large pumping speeds are required. Vacuum boosters operate completely contact-free and without sealing fluids such as oil or water in the working chamber, although with most models the two gearwheels of the synchromesh gearing and the bearings for the impeller shafts are lubricated with oil. Some roots vacuum / booster pumps are equipped with a by-pass valve.