Page 80 of catalogue Drive Units with Ball Screw Drives of Bosch Rexroth AG

80

Bosch Rexroth AG

Drive Units with Ball Screw Drives

R310EN 3304 (2007.05)

Calculations and Examples

Sizing of the FAR Drive Unit
Basic benets of systems with driven Nuts
Mass moment of inertia In long Screws, only the nut has to be set into rotation during the acceleration phase, and not the screw. The mass moment of inertia is therefore not a determining factor. The ball nut’s mass moment of inertia is relatively small and no longer depends on the specied stroke.

Dynamics

Complex end bearing designs, e.g. xed bearings with Angular contact thrust ball bearings at both ends, are no longer required to achieve high dynamics.

Stretching

Since screw is stationary, it can be stretched with relatively little effort. – Higher permissible axial load (buckling load); not limited by end bearings – Compensation of thermal effects – Higher total rigidity

Liquid cooling

Improved cooling can be easily accomplished by selecting a hollow screw. Since the screw is stationary, it can be cooled with relatively little design effort. With a controlled cooling system, length variations due to temperature uctuations can be virtually eliminated.

Design and manufacturing tolerances

The use of nuts with high parallelism and radial runout accuracy reduces whipping of the screw to a minimum. All functional elements come from a single source, so there is no need for customer-built designs.

Critical speed

nk fnk

d2 ln

7 –1 2 10 (min )
. –1

nkperm. = 0.8 nk (min )

nk = critical speed nkperm. = permissible operating speed fnk = corrector value determined by bearing d2 = core diameter, see Dimension Tables ln = critical screw length for preloaded nut systems

(min–1) (min–1)

(mm) (mm)