What’s the Difference Between a Ball Spline and a Bearing?

Ball splines and bearings: each play a role in mechanical function, and each are unique.

All types of machinery are provided with supports for rotating shafts, and that supporting device is a bearing—the machine element that constrains relative motion, used to reduce the friction between moving parts.

A ball spline is a type of mechanical element used in a variety of industrial applications where precise movement and alignment is required—transmitting linear motion or torque between two linear or rotating components. It is a type of mechanical coupling that allows for a slight misalignment between connected components as it transmits power.

Ball splines and bearings each have their specific uses, advantages and strengths.

Bearings

A bearing is used to guide, support and restrain moving the element, carrying the load as a stationary member and should not be confused with a bushing¹, which is a round-shaped cylindrical component, its primary use being static for support, or dynamic for motion.

The bearing supports the part of the shaft known as the journal, which is a moving member. Classified according to the type of operation they do, bearings are identified by: the motions allowed; or to the directions of the forces applied or loads to the parts.

Shafting in workshops, engine crankshafts, automobile axles, machine tool spindles on lathes, drilling, milling machines are all common applications of bearings.

There are many types of bearings², among them:

Ball bearings
Bush bearings
Thrust bearings
Foot step bearings
Rolling contact or anti-friction bearings
Roller bearings
Cylindrical roller bearings
Tapered roller bearings
Needle roller bearings
Plain bearings
Linear bearings
Fluid bearings
Magnetic bearings
Jewel bearings
Flexure bearings

Common Types

Located between two annular metal pieces, known as races, the most common type are ball bearings, which have a set of balls. These bearings have two races, with the outer race fixed and the inner race free to move. While providing little friction while in operation, it has limited bearing capacity due to small contact area between the race and the balls.

A roller bearing³ provides low-friction movement for a bearing block or bushing in a cylindrical unit, while a ball bearing does this as a spherical—the only real difference being the contact surface between the bearing and the rail.

For ball bearings the contact surface is just a single point, assuming there is no deformation, which creates an inherent strength limit. Roller bearings have an entire line of contact, greatly increasing the rigidity, maximum load capacity and stability of the system.

Deep-groove ball bearings are the most often used type of bearing. They are utilized in many uses because of their simple form, which also makes them easy to maintain and less sensitive to operating circumstances.

An angular contact ball bearing has a contact angle. This indicates that forces are moved at a specific angle from one racetrack to the next.

With self-aligning ball bearings, the inner ring raceway of the inner ring contains two rows of balls with a cage guiding the balls, but the outer ring swivels within the inner ring companion. This allows for a certain amount of self-alignment within the application.

In thrust ball bearing, the bearing pressure will be axial. The axis of the shaft may be vertical or horizontal. If the axis of the shaft is vertical, the thrust bearing is known as the footstep bearing. If the axis of the shaft is horizontal, the thrust bearing is known as a roller bearing.

Ball Splines

A typical ball spline consists of a set of cylindrical rollers, an outer sleeve and an inner shaft.

The outer sleeve is a cylindrical tube machined with a set of tracks or grooves on the inner surface. Similarly, the inner shaft is a cylindrical rod machined with a set of matching grooves or tracks, but on the outer surface.

Made of hardened steel or ceramic, the rollers are held in place between the tracks on the outer and inner surfaces. Depending on the application, speed and load and requirements, the size and number of rollers can vary.

Unlike slide bushings, ball splines have the nut sliding along a grooved shaft instead of a standard shaft or rail, as the grooved shaft rotates, allowing the ball splines to transmit or resist torque while delivering linear motion.⁴

Typically, available with either gothic or circular grooves, the circular grooves feature slightly elliptical cross sections, and two points of contact provided for the balls in these grooves—one on the nut and one on the shaft.

Gothic grooves are narrower at the ends, providing four points of contact which can handle higher loads, but with increased friction. The points of contact consist of two on the nut and two on the shaft and two on the nut.

Ball spline shafts have 2-6 grooves running along their length, with six-groove shafts having the highest load capacity—but requiring a large nut to accommodate the separate tracks. The most common are the four-groove ball splines, unless the shaft diameter goes below a certain limit, then two-groove setups are commonplace.

Types of Splines

There are many types of splines, but the most common are straight sided and involute.

Straight-sided splines have teeth parallel to the axis of linear motion or rotation. The teeth on involute splines are angled, allowing for more gradual engagement between components.

A ball spline—or what is sometimes called a rotary ball spline or roller spline—incorporates cylindrical rollers as the transmission elements, the rollers held in place between two tracks, which are machined onto the outer and inner surfaces of the spline.

Ball splines all share the same basic principle no matter the design or configuration, using cylindrical rollers as the transmission elements.

Common Applications

Industrial applications requiring precise alignment and motion control require ball splines, which include medical equipment, linear actuators, aerospace, robot arms, machine tools and defense, playing a vital role in a vast array of industrial and technological systems.

For example, Nippon Bearing (NB Corp.) SSP/SSPM ball splines “are configured as a linear motion mechanism utilizing the recirculating motion of ball elements that can sustain loads and transfer torque either simultaneously or independently.”⁵ These can be used in applications that include transport equipment and robotics, sustaining heavy loads to allow torque to transfer smoothly

Conclusion

Bearings and ball splines each are crucial to the smooth delivery of power. What is important to remember is that bearings provide the support for rotating shafts, reducing the friction between moving parts, and ball splines transmit the torque between the two linear or rotating components.

Each are crucial and complementary to the transmission of power.