White Paper: Robotics, Automation & Control
Evaluating Rotary Ball Spline Technologies
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An essential question engineers ask before specifying a rotary ball spline is: Should the rotary spline nut be riding on angular contact ball bearings or crossed roller bearings?
Before evaluating the pros and cons of the two rotational nut technologies, the first step is determining the design application’s requirements for stroke length, velocity, applied load, mounting space, duty cycle, required life, dimensions. installation direction, environment and accuracy.
To best understand how to make this analysis, it’s helpful to review how the technologies work.
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Overview
This document from NB Corporation evaluates rotary ball spline technologies, focusing on the choice between angular contact bearing supported and crossed roller bearing supported rotary ball splines. Rotary ball splines enable simultaneous linear and rotational motion with high precision and efficient torque transfer, making them ideal for applications like automotive assembly, packaging machinery, and robotics.
A rotary ball spline consists of a grooved shaft, a spline nut, and ball bearings. Unlike ball bushings, the spline nut rides on the grooved shaft, preventing rotation of the nut relative to the shaft and enabling torque transfer. Rotary ball splines include a built-in radial support bearing integrated into the spline nut, enabling a compact and efficient design.
The two types of rotary spline nut bearing technologies examined are angular contact ball bearings (standard for continuous rotation applications) and crossed roller bearings (ideal for compactness and high load capacity but not continuous high-speed rotation). Angular contact bearing nuts are larger due to the bigger rotary section, increasing overall dimensions and footprint, which reduces compactness.
Crossed roller bearings use cylinder-shaped rollers arranged orthogonally, providing line contact rather than point contact (as with balls). This line contact offers greater rigidity, reduced elastic deformation, and higher accuracy due to greater contact area and load capacity. Crossed rollers are particularly suited for oscillating or intermittent rotation applications rather than continuous high-speed rotation because their extensive contact area leads to faster wear.
In terms of performance, angular contact bearing types support higher rotational speeds (up to 4,000 rpm for a 16 mm diameter spline) compared to crossed roller types (about 1,080 rpm). Crossed rollers tend to wear out faster in continuous rotation scenarios but excel in applications requiring compact size and high load capacity with intermittent rotation.
Additional factors affecting performance include the spline shaft’s number of grooves and ball contact points, which influence torque capacity and accuracy. Gothic arch grooves with four-point contact enhance precision and rigidity compared to circular grooves. Shaft grading and machining precision also impact stability and maximum rotational speed.
Retrofitting older systems tends to be more feasible with angular contact bearing supported ball splines due to size compatibility, whereas adopting crossed roller technology may require redesign due to differences in compact size and design features.
In summary, selecting between angular contact and crossed roller bearing rotary ball splines depends on application demands such as rotational speed, load, compactness, and duty cycle, with crossed rollers favoring compact, high load, intermittent motion and angular contact bearings favored for continuous high-speed rotation applications.