The dynamic load capacity of a ball screw is typically defined by the DIN ISO 3408-5 standard, or in some cases, by the JIS B1192-1997 standard, both of which use 1 million revolutions as the basis for load capacity. However, some manufacturers determine ball screw dynamic load capacity according to the ANSI ASME B5.48-1977 standard (reconfirmed in 2013), which uses 1 million inches of travel as the basis.

The ANSI ASME B5.48 standard defines the basic (dynamic) load rating according to the number of balls that are carrying the load, the ball diameter, and the screw lead.

P_i = basic (dynamic) load rating (lbf) for 1 million inches of travel, per ANSI ASME

Z = number of load carrying balls per turn

d_i = ball diameter (in)

n = number of ball turns under unidirectional load

L_i = lead (in/rev)

The ANSI standard also provides the basic load rating in metric units, with the travel basis defined as 25,400 m (= 1 million inches).

P_m = basic (dynamic) load rating (N) for 25,400 m of travel, per ANSI ASME*

d_m = ball diameter (mm)

L_m = lead (mm/rev)

*Note that dynamic load capacity is typically denoted “C” in the DIN ISO standard, or “C_a” in the JIS standard.

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Since the life basis is different between the DIN ISO standard (1 million revolutions) and the ANSI ASME standard (1 million inches), the load ratings of the two standards aren’t directly comparable. However, the following equation allows users to compare the dynamic load capacity of a ball screw rated according to the DIN ISO standard with the dynamic load capacity of a ball screw rated according to the ANSI ASME standard:

C = basic dynamic load rating (N) for 1 million revolutions, per DIN ISO

P_i = basic (dynamic) load rating (lbf) for 1 million inches of travel, per ANSI ASME

L_m = lead (mm/rev)

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Because the DIN ISO (and JIS) ratings are based on revolutions, the actual linear travel that a ball screw can achieve depends on the screw’s lead. A screw with a lead of 10 mm that achieves a life of 1 million revolutions will have a shorter travel life in terms of linear distance (0.010 m/rev * 1,000,000 rev = 10,000 m) than a screw with a lead of 20 mm that achieves 1 million revolutions (0.020 m/rev * 1,000,000 rev = 20,000 m).

Conversely, the ANSI ASME load capacity rating is based on linear travel distance, regardless of the lead of the screw. So a screw with a lead of 0.25 inch that achieves a life of 1 million inches of travel will have made more revolutions (1,000,000 in ÷ 0.25 in/rev = 4,000,000 rev) than a screw with a lead of 0.50 inch that achieves 1 million inches of travel (1,000,000 in ÷ 0.50 in/rev = 2,000,000 rev) .

With this in mind, and demonstrated mathematically by the conversion equation above, you can see that if the lead of the screw is smaller than 1 inch (L_m < 25.4 mm), the load capacity per DIN ISO will be greater than the load capacity per ANSI ASME. Conversely, if the screw lead is larger than 1 inch (L_m > 25.4 mm), the load capacity per DIN ISO will be less than the load capacity per ANSI ASME.