Beier variable-ratio gear

The Beier variable-ratio gear or Beier variator is a mechanical drive offering a continuously variable gear ratio between input and output.

The gear relies on the inter-meshing of a number of thin disks. By varying their separation, the effective radius of one disk varies, thus changing the overall gear ratio. This varying-radius principle is shared by a number of other variator mechanisms. An advantage of the Beier is that a large number of disks may be stacked on a common shaft, thus increasing its torque capacity for only a small increase in overall length.

The mechanism consists of a spring-loaded stack of thin disks on a central shaft. Around this are arranged other stacked disk packs on a number of planetary shafts, usually three. These shafts are mounted on swinging arms, so that they may be moved in and out together. The central disks are thin, with a thickened rim. The planet disks are tapered across their radius, at an included angle of around 3°. When the disks are intermeshed, they thus only contact at the rim of the central pack, no matter what the spacing of the shafts. These planetary disk packs have a gear meshing with a gear on the swing arms' fulcrum shafts, which in turn meshes with a central gear, used for the input drive. The central shaft is the output shaft and the gear is used as a reduction gear. When the disks are moved outwards, the central rim drives on the outside radius of the similar-sized tapered disks and the overall ratio is around 1:1. As the swing arms are moved inwards, the tapered disks are forced between the central rims against their spring-loading. The effective radius of the tapered disks is thus reduced and the gear ratio increases. The gear is made in a variety of ratio ranges, typically from 3:1 to 10:1. Overall efficiency is good, greater than 90%.

Unlike most other variators, but in common with the Hele-Shaw clutch, the Beier gear does not rely on friction between the disks, but rather on viscous drag through a thin oil film between them. This has little slip, less than 1%,¹ and the lack of friction reduces the losses through heat and thus permits a small compact mechanism to still handle a high power. Oil is often pumped through the drive, also having a cooling effect.

The Beier gearbox was an Austrian friction-type variable-ratio drive; a fully automatic version was developed for use with a bus engine.²

The Beier variable-ratio gear was incorporated in the 1951 design of the E.145 Napier Nomad II, which first ran in December 1952.³ The Nomad II was a compound two-stroke diesel aircraft engine in which exhaust gas from the diesel drove an axial turbine, which in turn drove a 12-stage axial compressor used for scavenging and supercharging. The Beier drive coupled the turbomachine shafting to the piston-engine crankshaft, allowing the crankshaft to drive the compressor at low power and allowing surplus turbine power to be returned to the crankshaft at higher power settings.³

In contemporary industrial use, the Beier variator is used as a mechanical adjustable-speed drive, commonly driven by an electric motor.⁴ The variator provides mechanical speed adjustment by changing the engagement between its driving and driven discs, and may be combined with a Cyclo reducer as a Beier-Cyclo variator for adjustable low-speed output.⁴