Precision Machine Design: Difference between revisions
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How will the ballnut connect with the moving element. If the ballnut is "hanging off" too much for a given load-time curve during normal operation, then the ballnut will heavily deflect; also, the ballnut and ballscrew will suffer uneven wear, especially with shock forces. | How will the ballnut connect with the moving element. If the ballnut is "hanging off" too much for a given load-time curve during normal operation, then the ballnut will heavily deflect; also, the ballnut and ballscrew will suffer uneven wear, especially with shock forces. | ||
How the ballscrew will connect to the rotating element. Conventionally, the rotating element to be interfaced is a shaft, aka round bar; rod. Conventionally, the ballscrew has a precision round geometry on both ends. | How the ballscrew will connect to the rotating element. Conventionally, the rotating element to be interfaced is a shaft, aka round bar; rod. Conventionally, the ballscrew has a precision round geometry on both ends. | ||
The shaft coupling is either flexible (it can accommodate misaligned shafts) or rigid (does not bend at all; stronger than flexible but will cause uneven wear for misaligned shafts). The coupling mechanism is either set screw pressing into the shaft, clamp (where a screw on the wall of the shaft coupling is tightened. Pin, key, splined are alternatives associated with other high strength applications. | |||
Revision as of 05:47, 15 October 2012
Rotary to Linear Motion Conversion
Ballscrews and ballnuts are the conventional technologies.
Watch out for backlash where the ballscrew screws into the ballnut. Even if the ballscrew rotates, the nut will not move until the ballthread tolerance is cleared. Anti-backlash/zero-backlash technology is the solution.
Watch out for thermal expansion/contraction. Ballscrews will expand and contract in size if the temperature rises or falls. This will affect linear precision as well as the fit between the ballscrew and the ballnut. Thermal compensation hardware is the solution.
Watch out for kinetic tension. Ballscrews that experience great axial forces will deflect whatever is supporting the ballscrew at either end. This is why the ballscrew mounts at each end of the ballscrew must be firmly mounted. Also, pre-loading tension into the mount-ballscrew-mount assembly will increase the rigidity of the system because the force-deflection curve of the assembly is non-linear in a good way (more deflection with low forces than later on with higher forces).
How will the ballscrew be set up to handle axial forces from preloading and normal operation? Thrust bearings at both ends of the ballscrew between each end of the ballscrew thread and the mounting block.
How will the ballnut connect with the moving element. If the ballnut is "hanging off" too much for a given load-time curve during normal operation, then the ballnut will heavily deflect; also, the ballnut and ballscrew will suffer uneven wear, especially with shock forces.
How the ballscrew will connect to the rotating element. Conventionally, the rotating element to be interfaced is a shaft, aka round bar; rod. Conventionally, the ballscrew has a precision round geometry on both ends.
The shaft coupling is either flexible (it can accommodate misaligned shafts) or rigid (does not bend at all; stronger than flexible but will cause uneven wear for misaligned shafts). The coupling mechanism is either set screw pressing into the shaft, clamp (where a screw on the wall of the shaft coupling is tightened. Pin, key, splined are alternatives associated with other high strength applications.