Trochoid pumps belong to the class of so called rotary piston pumps, which (see overview of Table 2.1) in turn belong to the group of rotary pumps. With rotary piston pumps, the piston’s center of gravity runs along a circular path about the rotational axis (hence rotary piston machines). A rotary piston pump can – in contrast to the rotary plunger pump – be completely
balanced dynamically. This offers the advantage that larger pumps can operate without vibration so that they can be installed without needing foundations. Moreover, such pumps may be operated at higher speed, compared to rotary plunger pumps (see below). The volume of the pumping chamber with respect to the volume of the entire pump – the so called specific volume – is, in the case of trochoid pumps, approximately twice of that of rotary plunger pumps. Larger rotary plunger pumps run at speeds of 500 rpm. The trochoid pump may run at 1000 rpm and this applies also to larger designs. It is thus about four times smaller compared to a rotary plunger pump having the same pumping speed and runs without producing any vibrations. Unfortunately the advantages in the area of engineering are combined with great disadvantages in the area of manufacturing, so that today LEYBOLD does not produce trochoid pumps any more. Operation of such a pump is shown in the sectional diagram of Fig. 2.12.
Table 2.1 Classification of vacuum pumps
Fig. 2.12 Cross section of trochoid pump