Steinmeyer ball nuts are ideal for rotating nut designs because they are inherently mass balanced due to the symmetrical orientation of the ball deflectors. Driven (rotating) nuts and stationary shafts offer a number of advantages over conventional layouts (and some disadvantages):
- Resonance of the shaft is not as critical. Higher speeds may be reached when the shaft is not rotating.
- Pre-tensioning of the shaft is simplified, since the necessary forces do not need to be transmitted through bearings. Heating is reduced significantly.
- Stationary shafts allow easy internal cooling of the screw.
- Axial and torsional stiffness is higher, when axial loads and moments can be transmitted into the surrounding structure at both ends of the shaft rather than just one. Particularly long shafts with high lead/diameter ratio exhibit a clear increase in total rigidity compared to conventional layouts with rotating shafts.
There are however a few difficulties that are associated with the rotation of the nut:
- Lubrication of the nut is complicated since rotating unions are necessary to feed lubricant directly into the nut, and centrifugal forces push all lubricant away from the balls and ball tracks.
- Thrust bearings to be installed on the nut are relatively large in diameter and may restrict speeds.
- Direct drive requires a hollow shaft motor, which may be critical due to heat emission of the motor.
To solve these difficulties, Steinmeyer makes several proven solutions available. For example it is possible to install the bearing directly on the nut body by using our customized nut design which incorporates: the seat for the bearing; the locknut thread to hold it; and an interface for a pulley - which makes any extra parts obsolete and allows for a compact design. Even for the lubrication problem, we have a proven solution - see column on the right side of this page.
Rotating unions, used to feed oil into a rotating nut, can be quite troublesome because their leak rate can (with time) be higher than the actual oil flow. This means the oil can be thrown out of the nut before it reaches the balls, resulting in complete lubrication breakdown.
A simple and effective alternate solution is to drill a hole through the center of the shaft from one end and cross-drill another hole at a position where the nut passes over regularly - for example a tool change position or similar. The lube pump, which needs to be connected to the NC control, will then shoot a small quantity of oil into the nut when it is positioned over this cross hole. With proper lubricant and wiper selection, this technique should be sufficient until the nut returns to this position again.
This feature can even be combined, if necessary, with a coolant channel through the shaft.