When calculating the life of ball screw, the load will depend on the vibration load or impact load, so it is necessary to modify the theoretical load. This correction factor is called the load factor. Therefore, as the actual load, the load (FA) in the life calculation is the product of the load (FE) calculated theoretically and the load factor FW.
Fa = Fe・fw
|Value of load factor (FW)|
|Degree of load||fw|
|No impact, smooth operation||1 ～ 1.2|
|Normal operation||1.2 ～ 1.5|
|Operation with shock and vibration||1.5 ～ 3.0|
When the ball screw rotates, it needs to consider the basic rated load or service life, but when it is stationary or rotating at low speed below 10rpm, it must consider the basic rated static load. The basic rated static load is the static load when the permanent deformation between the raceway surface and the contact surface of the steel ball reaches 0.0001 of the diameter of the rolling element. The rated static load must also be considered when the static load or impact load is large.
Generally, the safety factor of the rated static load of the ball screw in rotation is 1.
|The value of safety factor (FS)|
|Common load||1 ～ 2|
|Impact load||2 ～ 3|
The surface hardness of groove of ordinary heat treatment ball screw is HRC58-62. For the low load and low speed, the ball screw without heat treatment is sometimes used. When the groove hardness is less than hrc58, the corresponding basic dynamic load and rated static load will also be reduced relatively. We call this correction coefficient hardness coefficient. That is to say, when the groove hardness is below hrc58, the basic rated dynamic load and the rated static load shall be multiplied by the hardness coefficient FFH.
Ca’ = fH ・ Ca
C0a’ = fH’ ・ C0a
However, Ca and c0a are the basic dynamic and static loads when the groove hardness is HRC58-62