Ball Screw Life Expectancy

Definitions

The dynamic load ratings shown on the product specification pages indicate the load that can be carried for 1 million revolutions based on 90% reliability. The charts on pages 11 relate life to load. In applications where the load and rotational speed is relatively constant over the entire stroke, use the highest load in selecting a ball screw to provide a factor of extra life. For applications where the loads and/or rotational speed vary significantly, an equivalent load can be calculated using the following formula:

WHERE:

  • Fm = equivalent axial load (N)
  • Fj = each increment of axial load (N)
  • qj = percent of stroke at load Fj
  • nj = rotational speed at load Fj (rpm)
  • nm = equivalent rotational speed (rpm) and is given by the following equation:

Life (revs)

The life required in revolutions is determined by multiplying the total stroke in millimeters by the total number of strokes required for the designed life of the equipment and then dividing by the lead of the ball nut. Ball nut life is greatly influenced by the operating condition, including speed and vibration the assembly may see. A fatigue factor must be considered when calculating life. To calculate the life for a ball nut use the following formula:

Operation Condition fw (Fatigue Factor)
No External Vibration 1.0 - 1.2
Indirect Vibration 1.2 - 1.5
Direct Vibration OR High Cyclical Impact 1.5 - 2.5
Direct Vibration AND High Cyclical Impact 2.5 - 3.5

Indirect Vibration - Any vibration associated near the screw mounting which influences the stability of the assembly.

Direct Vibration - Any vibration directly linked to the screw assembly which influences the stability of the assembly.

High Cyclical Impact - Any repetitive impact or high deceleration of the ball screw assembly.

Life (adjust to reliability)

If operation reliability higher than 90% is required, then the theoretical life must be corrected by using a reliability factor (far) according to the table.

Reliability (%) far
90 1
95 0.62
96 0.53
97 0.44
98 0.33
99 0.21

Life (hours)

If total time is needed, the following equation can be used to find the life measured in hours:

EXAMPLE:

Given that a MBN12×2R-3FW ball nut will be used with 98% reliability with smooth operation under the following conditions:

  • F1 = 250 N ; q1 = 45% ; n1 = 150 rpm
  • F2 = 1525 N ; q2 = 55% ; n2 = 350 rpm

The life of the ball nut will be as follows:

This will bring nm to a value of 260.

Which results in a Fm value of 1412 N.

Taking the 12×2 (MBN12×2R-3FW) where Ca = 2811 N

Adjusting for 98% Reliability:

The variable Lh then equals 166.9 hours

Figure 1