Mastering Ball Screws Pt. 4: Increase in Ball Screw Speeds

The need for speed! Increasing speed will increase accuracy and there are a variety of challenges to overcome to get that optimization.  To review, a ball screw is a mechanical element that converts motor’s rotary motion into linear motion.  Therefore, there are two requirements for high speed optimization of ball screws.

  • Increase the linear motion distance per one rotation→ Use of large leads
  • High rotational speeds→ Increase RPM (simultaneously attaining high accuracy)

Since the accuracy increase is also needed at the same time as the ball screw speed increase is needed, (1) and (2) must be satisfied simultaneously.

Issues of Increasing Rotational Speeds

Ball screw speed increasing is associated with the following high level difficulties.

  1. Orbital speed of the steel balls: Speed limitations based on dmn value — dm: Ball diameter, n: Rotational speed
  2. Critical speed of a ball screw [formula (1)]→ the speed at which the screw starts to encounter bending vibrations
  3. Thermal deformation increase by heat rise
  4. Increased vibrations and noises
  5. Load (inertia, etc.) increase due to high speed motion and high acceleration/deceleration.

Formula 1Formula 1

Nc = Critical Speed

K = Factor Based on End Support Bearings

dn = Root Diameter of Ball Screw

Lcr = Unsupported Length of Ball Screw

Explanation:

  1. As the steel ball’s orbital speed (dmn value) increases, damages to the ball recirculation circuit components, such as tangs of return tubes, due to repeated shocks will become no negligible.
    Figure 1
  2. When the ball screw is rotated at high speeds, it will reach a critical speed where the natural frequency of the long narrow shaft suspended at both ends will bring a resonance vibration phenomenon at a certain speed.
  3. Ball screw temperature will rise as the speed increases due to the friction torque increased between the steel balls and the shaft. This temperature rise causes heat expansion and the accuracy will be degraded.
  4. Ball screw nuts have complex recirculation paths with internal discontinuities (not smooth). As the ball speed increases, associated vibrations and noises will also increase.
  5. Since the ball screw imparts high speed and high acceleration motion on objects, the loads on the ball screw increases.

Increasing ball screw speed has many limiting factors to take into consideration.  Proper calculation of the critical speed is recommended for less vibration.  This completes the Mastering Ball Screws series.  Thank you for tuning in!

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