Seems like everyone’s pushing the limit on ball screw speed lately. Steinmeyer introduced our UltraSpeed ball screw series several years ago, boasting of Dn values to 160,000. First, what’ s Dn value and what does it signify? Very simply, answering the second question first – the Dn value actually only refers to the speed rating of the ball nut. The speed rating of a ball screw assembly is dependent on the shaft diameter, the shaft length, and the type of bearing supports. And a ball screw shaft is always subject to maximum rotational speed defined by its critical speed – unless the system employs a stationary shaft with rotating nut. The Dn value for a ball nut is derived by multiplying the nominal diameter of the shaft in mm and the maximum speed the nut will sustain, in revolutions per minute (rpm). For example, a ball nut used with a shaft diameter of 25 mm and having a Dn value of 60,000 can operate at a rotational speed up to 2400 rpm. Again, this does not mean the entire ball screw can operate at that speed – this depends on the shaft length and the type of support bearings. Typically ball nuts with tubes are limited to Dn values of 50,000 to 70,000. This is due to the construction, that is the presence of tubes and additional guiding pins which significantly limit the maximum operating speed. With internal ball deflectors, the Dn value approaches 120,000 – but not necessarily from all suppliers so better check. And as mentioned Steinmeyer introduced a ball screw series several years ago with Dn value of 160,000. This means that a 40 mm diameter Steinmeyer ball screw with a ball nut having a Dn value of 160,000 is capable of operating at speeds to 4,000 rpm. Now very soon after we introduced this new series, one or more competitors reacted and introduced products with even higher Dn values, in excess of 200,000. But so what? Try finding a motor with sufficient torque to operate (reliably) beyond 4,000 rpm – you’ll have a hard time. And even if you do find one, what about the heating that results from operating at such speeds? You’ll find the thermal rise is well beyond what is considered safe. If you think a ball screw is capable of operating at 5,000 rpm and beyond, good luck with that. Our, and some of our customers’, tests have shown otherwise. So caveat emptor – make sure you understand the ramifications of operating a ball screw at high speed and design accordingly. Because to a large extent, the Dn argument is little more than “mine’s bigger than yours”.