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Exploring Rotary Ball Splines

July 19, 2012
Angular-contact ball bearings or crossedroller bearings? A few factors help determine the pros and cons of each type

Authored by:
Naoki Yamaguchi
Assistant Technical Manager
NB Corp. of America
Hanover Park, Ill.
Edited by Robert Repas

The first step in specifying a rotary ball spline is to lay out the parameters of stroke length, velocity, applied load, mounting space, duty cycle, required life, dimensions, installation direction, environment, and accuracy. You’re now ready to factor in the pros and cons between angular-contact ball bearings and crossed-roller bearings.

In addition to the ball spline’s forte in transferring torque, it adds a nut that rotates on the ball spline shaft. This makes the unit capable of simultaneous linear and rotary motion with submillimeter accuracy.

A ball spline has three basic components: a grooved shaft, a spline nut, and ball bearings. Unlike ball bushings, the ball-spline nut rides on a grooved shaft that prevents the nut from rotating. This lets torque transfer through the shaft to a nut or vise versa. Typical applications see the shaft or nut mounted to a fixed structure.

With a fixed shaft, the ends of the shaft can be turned down to accommodate radial-bearing mounts that support rotation. However, when the nuts are fixed, the radial bearing must mount on the outer diameter of the nut, which can make the overall assembly bulky. In addition, the nut is not as easily modified to accommodate various bearing sizes.

This is where the rotary ball spline becomes efficient and economical. Because rotary ball splines already have a radial-support bearing built into the ball-spline nut, a user needn’t find, source, and integrate one. Moreover, the overall dimension of the rotary ball-spline nut is more compact than a standard ball-spline nut with an added radial support bearing.

Importance of preload

A forced rotation of either the shaft or spline nut (not the rotary nut) makes both nut and shaft rotate together. The ball bearings of the nut are secured by the grooves. An angular backlash happens when the space between the spline nut and shaft provides a bit of wiggle room. As the shaft starts to rotate, the nut doesn’t immediately follow or it moves just a short distance that doesn’t match the pitch angle of the shaft vis-a-vis its rotation. This is angular backlash.

Placing a preload or force on the ball bearings removes that wiggle room, forcing the nut to respond to the slightest rotation of the shaft. However, higher preloads also place more force against the balls, pressing them tighter into the grooves producing more rolling friction. Selecting the appropriate preload grade for the application is critical to maximize the ball-spline product life, rigidity, accuracy, and maintain smooth movement. Manufacturers typically offer standard preload values or will provide a custom preload amount.

Preload also raises the rigidity of the spline by reducing assembly deformation under load. Because initial deformation is much greater on steel, predeforming the components by inserting larger diameter balls can reduce the amount of deformation when the spline assembly is loaded for an application. As a result the assembly becomes more rigid with a higher accuracy.

It’s the number of grooves on the spline shaft along with the number of points of ball contact in the grooves that dictate torque ratings. Ball-spline shafts with four grooves have higher torque ratings than those with three grooves. Likewise, Gothic-arch-shaped grooves with a four-point contact design provide higher ratings than circular archshaped grooves, which have two points of contact.

The Gothic arch eliminates any clearance that could lead to deflection and is, therefore, best for applications that need maximum precision. Four-point contact also augments load capacity and rigidity for greater load and torque. A four-groove spline with a four-point ball contact produces a total of 16 contacts per spline shaft whereas splines with three grooves and two contacts per ball only provide six points of contact. Though different ball splines might be exactly the same size, this shows why they can have quite different torque ratings.

What affects accuracy?

The symmetry of the spline shaft affects the shaft’s maximum rotational speed and stability through the amount of vibrations produced by the shaft as it rotates. Spline shafts vary as to whether they are precision ground, ground, or drawn steel bar. They also vary as to the grade of the base material. Manufacturers rank shafts by characteristics such as the tolerance of ground shafts, perpendicularity to the end face, concentricity of the part-mounting section in relation to the support section, as well as the material grade.

The difficulty lies in machining all of the shaft grooves in such a manner that they maintain high accuracy and linearity all along the length of the shaft. Nonground or drawn spline shafts are, naturally, of lower accuracy.

Generally, manufacturers present three accuracy ratings: Precision (meaning their highest accuracy); High (meaning their standard grade — usually a stock item); and Normal or Commercial (often a nonground shaft). However, one manufacturer’s top grade can be another’s standard grade. Comparing accuracy grades basically comes down to comparing values of shaft-diameter tolerance, straightness, perpendicularity, and concentricity.

Sometimes a lesser degree of accuracy is acceptable. If the primary concern is torque transfer, linear transfer, rotational motion, or length, then drawn, nonground spline shafts may be the best choice. Some drawn shafts can use the same nuts as ground splines, but load capacity is reduced because the nut is traveling in a nonground raceway groove. However, they are less expensive and can be as long as 5 meters, making them appropriate for material transfer and handling applications.

Crossed-roller bearings

Crossed-roller bearings work like ball bearings, except the bearings housed within the rotary nut are cylindrically shaped instead of ball shaped. The rollers crisscross each other at a 90° angle and move between “V-grooved” bearing ways. The grooves are ground into the spline nut outer diameter and the rotary flange attached to the nut.

Crossed-roller bearings offer a line of contact versus a ball bearing’s points of contact. This creates a broader contact surface that can carry a heavier load. It also provides more rigidity, less deformation and, thus, more accuracy, compared to the point contact of balls. This makes crossed-roller bearings one of the most accurate forms of mechanical motion.

As rotary-support bearings, both angular-contact ball bearings and crossed-roller bearings can support axial and radial loads. One can expect either or both types of loads depending upon the application. An angular-contact ball bearing uses two rows of balls whereas a crossed-roller bearing uses a single row. However, single-row bearings show some issues with speed and wear.

When comparing dimensions, one is immediately struck by the size difference when the rotary-support bearing rides on crossed-roller bearings opposed to one that rides on angular-contact ball bearings. On the spline comparishaft, the spline nuts are basically the same between the two bearing types with the spline shaft riding on ball bearings. But the unit with crossedroller elements in its rotary-support bearing is more compact. The reason it takes up so much less space is that the rotary nut and spline nut is one unit. The crossed rollers are directly attached to the spline nut’s outer cylinder. In addition, the crossed-roller rotary-support bearing is thinner.

The crossed-roller type is compact with a relatively high-load capacity in terms of the crossed-roller section size. It’s ideal when you need to keep something compact and to support a high load for an application that doesn’t need continuous rotation.

Because an angular-contact type is larger, the rotational section is larger and the overall spline dimensions are larger. This larger footprint reduces the compactness of the overall application.

The larger contact area of crossed rollers over ball bearings reduces elastic deformation. Because of this greater stiffness, crossed rollers provide consistently precise movement. There is a direct correlation between the contact area of the crossed rollers and load capacity. The roller-toraceway contact greatly boosts load capacity.

However, for applications that continuously rotate, crossed rollers wear out more quickly than ball bearings because more of their contact area is in use. For example, an angular- contact type is a better choice for driving the spindle driveshaft of a grinding machine, a conveyor belt, or a wire winder. But, crossed rollers are highly desirable when the attached gripper rotates back and forth, as when changing the angle of an item with only a quarter or half rotation.

In a rotational-speed comparison, angular contact is more advantageous than crossed roller. Comparing the two types at a standard 16-mm size (the diameter of the spline), the crossed-roller nut is capable of only 1,080 rpm versus the 4,000-rpm maximum rotational speed for an angular- contact nut.

As crossed-roller rotary-nut technology is new, most existing applications use the angular-contact bearing-supported ball spline. That makes changing to the newer technology expensive as it’s more compact. However, most angular-contact bearing-supported ball splines are interchangeable, size-wise. Though the outer dimensions of interchangeable rotary ball splines may be the same, there are design advantages to consider such as the number of grooves on the spline, the points of ball contact within the grooves, and shaft rigidity. The choice is now up to you.

© 2012 Penton Media, Inc.

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