What is Preloading and why it is done?
Preload is the elimination of internal clearance between the rail and carriage or between the ball screw and ball nut. This makes the system more rigid by creating a load on the linear guide or ball screw, which reduces deflection when an external force is applied.
Preloading is used to increase the accuracy and rigidity of a ball screw system. It makes the system more rigid by creating a load between the ball screw nut and shaft, which reduces structural displacement when under an external force.
|Increases Rigidity||Reduces Backlash||Ensures desirable wear characteristics|
Preloading in Ballscrews:
There are multiple ways to preload a ballscrew, with the most common being the use of oversized balls in the ball nut or by the use of a double nut system.
The first method is the most common and most cost-effective method. Using the oversized balls reduces the clearance between the screw threads and the nut raceways, thus eliminating the backlash. The below video will guide you on what this type of preloading looks like.
The second method is to tension two single nuts against each other using a spacer or spring between them. This method is also referred to as a double-nut system and is found in applications where rigidity is critical and vibrations or shock loads are present, such as machine tools. Double nut systems provide the most rigidity but are the most technically challenging to install. They also require nearly twice the length of a single nut, taking up valuable travel length in space-constrained applications.
Preloading in LM Guideways:
Linear guide preload eliminates the clearance between the bearing block and the rail, which increases rigidity and reduces deflection when external loads are applied.
For LM Guides the only method for preloading is by using the balls/rollers whose diameter is slightly more than the raceways of Rail and Block. For applications where the positional accuracy is important, such as assembly and dispensing, a linear guide with light preload is generally recommended. A higher preload doesn’t necessarily provide better positioning accuracy unless deflection becomes a factor. Machine tools, for example, often have to move heavy loads that are positioned a long distance from the bearing blocks. In such case heavy preloads are recommended to ensure proper stiffness.
Below is the preload table for our LM Guideways:
More is not always Better!
More preload is not always better for the Linear Guideway. More preload will require more force to move the bearing block, which in case not only increases the generation of heat within the block but also increases the wearing of the block, thus reducing the overall life.
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