Load Capacity According to ANSI Standard
According to the ANSI standard, dynamic load capacity of a ball screw is the load under which it will reach a life of 1 million inches of travel. If the lead is smaller than 1 inch, then the load capacity per ANSI definition is smaller than the load capacity of the same ball screw expressed according to ISO / DIN standard. This is because the ANSI load capacity defines a load for which the resulting life in this case (lead less than 1 inch) is greater than 1 million revolutions.
The opposite holds true for a ball screw with lead larger than 1 inch. For such a case, the ANSI load capacity will be higher than the ISO load capacity, although the ball screw itself is identical!
To compare load capacity of a ball screw per ANSI standard to one per ISO / DIN standard, a conversion must be made. This equation will also convert lbf into Newtons:
Ca DDIN / ISO load capacity [N]
Pi Load capacity per ANSI [lbf]
P Lead [mm]
Preload is usually defined as a percentage of dynamic load capacity. Nuts with 4-point contact normally have a preload of 5 - 8% of dynamic load capacity, while for nuts with 2-point contact it is 8 - 10%. Keep in mind this refers to ISO/ DIN load capacity only. If the ANSI standard is used, then the percentages may change!