Have you ever wondered, “what is preload”? We’re here to help! Applications that use linear bearings or ball screws often require precise positioning, which means that the motion components must be rigid. And while recirculating ball guides and screws are able to carry large loads, they inherently have clearance, or backlash, due to the size and fit of the balls between the raceways. To eliminate this backlash and increase rigidity, preload must be applied.
What is preload?
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.
Preload for linear guide systems
For a linear guide, preload is achieved by loading the guide block with balls whose diameter is slightly larger than the distance between the block and rail raceways (oversized balls). The diameter of the balls relative to the distance between raceways determines the amount of preload that is reached. Common preload amounts for linear guides are 2, 5, or 8 percent of the dynamic load capacity. For example, a linear guide with a dynamic load capacity of 25,000 N and 2 percent preload would have a preload force of 500 N.
Some manufacturers provide a preloaded system as a matched set – rail and carriage. In this case, if the carriage is used on a different guide rail, the specified preload cannot be guaranteed. However, for ease of servicing, specifying, and inventory, manufacturers are more commonly providing interchangeable carriages and rails, where the preload is specified and set in the carriage, allowing carriages to be used on different rails while maintaining the same preload. When specifying a preloaded linear guide, it’s important to be aware of which type – matched set or interchangeable – the manufacturer provides.
Deflection of a linear guide with no preload (Z0), light preload (Z1), and medium preload (Z3) Image courtesy NSK Ltd.
Preload for ball screw systems
There are multiple ways to achieve preload for a ball screw system, with the most common three being the use of oversized balls in the ball nut, an adjustable diameter ball nut, or a double-nut system.
The first approach is analogous to the method used to achieve preload in a linear guide system. Oversized balls are loaded into the ball nut, eliminating the clearance between the screw and nut raceways. This is a relatively low-cost option and can be achieved with a variety of nut styles, making it a common choice for applications requiring low preload.
For applications where the preload does not need to be set to a precise amount, or when it should be variable – to account for changing application conditions or wear – an adjustable nut is the best option. In this case, the nut is provided with a slot (think of the letter “c”). An adjusting screw allows the user to decrease the internal diameter of the nut, eliminating clearance between the raceways and increasing the preload. The adjustable nut method is inexpensive and allows flexibility, but since the preload cannot be set to an exact amount, it is most commonly found in applications where rigidity is important, but not extremely critical.
When high preload amounts, typically between 7 and 10 percent, are required in a ball screw system, the most common 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.
Application requirements determine the amount of preload necessary.
How to choose the correct preload
When it comes to choosing preload, more is not necessarily better. Preload increases the force (or torque) required to move the linear carriage (or ball nut), which can require a larger motor and related components, increasing cost and complexity. High preload also causes additional heat to be generated inside the carriage (or ball nut), which increases wear and reduces life. And finally, preload is a load applied to the bearing and must be accounted for when calculating bearing life, meaning that its effect is amplified by the cubed power in the bearing life equation.
Like other application forces, preload force affects bearing life.