What are methods for inducing preload in a double ballnut？

A double ballnut is essentially two single ballnuts that are tensioned against each other to induce preload.The three common methods of inducing preload in a double ballnut are：

●With a spacer ring between the ball nut halves

When using spacer rings in a double ballnut assembly,the amount of preload is determined by the thickness of the spacer strip.If the spacer is not perfectly flat,or if the two nut halves are not perfectly perpendicular to the spacer,this method of preload will result in inconsistent contact angles of the balls.However,when machined to high tolerances,this design has consistent frictional torque and produces extremely smooth motion.

 

Preload can be imparted by a spacer or a spring between two ballnut halves.

●With a spring between the ball nut halves

The use of a spring between the two halves of the nut eliminates concerns about spacing flatness,but the maximum thrust that can be applied is limited by the spring.If the stiffness of the spring is overcome by the thrust,the spring will collapse and lose preload.However,with this method,wear has no effect on preload because the spring will adjust to the given conditions.

●By creating an offset in the lead

The lead offset method is structurally different from the shim and spring design,but provides the same end result.In this method,the lead offset is created during the manufacturing process.The offset is located approximately halfway up the nut body and creates preload through two points of contact of the ball.Lead offset ball nuts typically have fewer active ball grooves,which can reduce load capacity and thus dynamic life.However,the single housing allows for a more compact lead offset design than a design containing two separate ballnuts.Although only one nut body is used,some manufacturers refer to this nut type as a "double nut" because it creates two points of contact between the ball and raceway.For screws with multiple starting points,a similar effect can be achieved by moving the lead between threaded starting points rather than within a single threaded starting point.

 

A single nut with a lead offset is often called a "double ballnut" because it results in two-point contact of the ball bearing.

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