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Preloading a nut using 2-point contact is accomplished by using a "double nut" configuration. Half the nut resists load in one direction. The other half in the other direction. This results in:
More efficient operation with the balls rolling on two points (no skidding).
Preloading lasts longer than 4-point contact.
Preloaded nuts can be interchangeable. Preloaded nuts that utilize adjustable double nuts can be purchased individually and do not have to be matched to the screw at the factory.
Ability to preload on lower-precision screws. On lower-precision screws, the double nut assembly utilizes a spring
between the two nut halves. This allows it some flexibility to run on a
lower precision screw. However, the spring force is the limit of load with no
A longer nut for the same load capacity. The nut must have double the
number of ball circuits to have same load capacity as an equivalent non
preloaded nut or 4-point contact preload nut.
There are different preloaded ball nut designs, but the most designs can be grouped into three categories:
Oversize ball (4 point contact)
Double nut (2 point contact)
Skip lead (2 point contact)
Each solution has benefits and challenges:
Oversized ball preload uses selective ball
bearings to remove the axial free play and create a preload. This type of preload maintains
full load capacity and the smallest footprint. But it requires a mating
screw with high lead accuracy and typically only good up
to a 5% preload.
Double nut preload utilizes two nuts that are combined and configured to remove
axial free play. This style can be
loaded with compensating springs, and can be adjusted to compensate for
wear and load capacity. The downside is cost and its size relative to the oversized ball nuts.
Skip lead preload is typical in a whirled or ground screw assembly. It results in a stiff nut with a higher level of preload (approximately 10%). This is a single nut solution but reduces the dynamic capacity of the system as only half the ball bearings are loaded at one time.
An alternative to preloading is to select the
ball bearings size to reduce the lash to a minimum without creating a preload. Similar to what is done for an oversized ball preload. Advantages of minimum lash are less drag on the system and increased smoothness.