The weight of the unsupported leadscrew length refers to the mass of the freely cantilevered—that is, not additionally supported—section of the leadscrew in a leadscrew-nut drive. This refers to the portion of the leadscrew that “hangs freely” between two bearing points or between a bearing and the nut and is not supported or held by additional guide elements.
The weight of this freely cantilevered section is an important parameter in the design of ball and trapezoidal screw drives, as it has a direct influence on:
- Spindle Deflection
- Natural Frequencies and Vibration Behavior
- Critical speed
- Operational Stability at High Speeds
- Wear and Bearing Load
The greater the weight of the free section of the spindle, the greater the bending stresses and the lower the critical speed: The spindle is more prone to oscillations, vibrations, and potential instability.
How is weight determined?
The weight is calculated as follows:
- unsupported spindle length
- Spindle diameter (outer diameter)
- Material density
- Geometry (solid shaft or hollow shaft)
Simplified formula:
Use in Design and Calculations
The weight is required in particular for:
- Calculation of the Critical Speed (Whirling/Buckling)
- Calculation of the natural frequency dn
- Simulation of deflection under dead load
- Determination of the maximum
- Evaluation of Bearing and Nut Loads
In long, slender spindles—for example, those with long travel distances—this value becomes particularly important because the spindle’s own weight becomes the dominant factor influencing its dynamic behavior.
Practical Example
A 1.2 m long, unshielded 20 mm spindle exhibits significantly more deflection and has a much lower critical speed than a spindle of the same length that is supported by an intermediate bearing or a guide. The increased weight of this section acts as an additional load, causing the spindle to “sag” and facilitating vibrations.