The Power of a Ball Spline: Design, Function, Applications

The ubiquitous ball spline. Basically, it is a type of mechanical element used in a variety of industrial applications where precise movement and alignment is required, like machine tools, robot arms and linear actuators.

To better understand ball splines, we need to know what a spline is and its function.

A spline is a mechanical element used to transmit 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.

A Close Look at the Components

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.

The advantage of ball splines over other splines is the high degree of accuracy and rigidity that can be maintained, as the cylindrical rollers distribute the load evenly across the tracks, allowing for a slight misalignment between components without compromising precision.

Types of Splines

While there are many types of splines, the most common are involute and straight sided.

Straight-sided splines have teeth parallel to the axis of linear motion or rotation, whereas 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.

The rollers rotate around their own axis while also rolling between the tracks when linear motion or torque is applied, which allows for a smooth and precise movement no matter the speed or varying misalignments and loads.

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

Common Ball Spline Applications

Industrial applications requiring precise motion control and alignment commonly require ball splines. These include linear actuators, robot arms, medical equipment, machine tools and aerospace and defense.

Ball splines have contributed to advancing modern engineering and technology, playing a vital role as components 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.”¹ These can be used in applications that include transport equipment and robotics.

Ball splines are used in applications to sustain heavy loads to allow torque to transfer smoothly. NB Corp. ball splines, like many manufacturers’ splines, including various shaft and nut configurations, depending on the requirements.

NB Corp. ball splines are comprised of a shaft and nut assembly, with shafts that feature raceway grooves. Like many common ball splines, the nut assemblies consist of an outer cylinder, side rings, retainer and ball elements.

Configuring the ball splines in this manner offers such advantages as a reliable, smooth motion with specially designed shafts and spline nuts. The design offers precise high torque transmission capabilities, providing an effective contact angle between the ball elements and shaft. In addition, higher rigidity and positioning accuracy can be achieved with configured adjustable preloads, so that high-speed motion performance can be realized in a compact outer cylinder design.

The wide variety of requirements for specific ball splines requires variations in design.

For example, NB Corp. manufactures different ball spline nuts like the SSP/SSPS cylindrical-type spline nuts with a key groove and special key; SSPM cylindrical-type spline nuts without a key groove, with lock plates; and SSP-AM/SSPS-AM special compact cylindrical-type spline nuts with a countersink and special key, featuring flanged spline nuts and flanged nuts with two side cut flange, of which all can be customized by machining spline shafts, and all featuring standard size seals to protect against dust.

Ballscrew Splines

Two types of ballscrew splines from NB Corp. are the SPBR and the SPBF, which are typical for applications in assembly machines, machine loaders and SCARA robots.²

The SPBR type has a rotary ballscrew nut and rotary ball spline nut, with the rotary ballscrew nut an integration of ballscrew nut and angular contact bearings; the rotary ball spline nut is an integration of ball spline nut and angular contact bearings.

The SPBF type has a rotary ballscrew nut and a ball spline nut. While slightly different, each provide positioning on a single axis for linear, combined spiral motion and rotary motion. Being reliable and compact, the consistency of ballscrew splines have made them an essential tool of the robotics industry.

Small rotary ball splines like the SPR, SPB-KP are sized from 6 to 25 mm in diameter, with the smaller diameters being small in length. These smaller ball splines are in demand for such items as artificial limbs or the DeVinci surgical suite.

A number of companies specialize in small diameter ball splines for this market, with six models offering spline shaft diameters in the four-12 mm range. The C-Lube Linear Ball Spline MAG guides provide continuous linear motion of an external cylinder as it slides along a spline shaft, and is specifically designed for maintenance-free operation.³

With a built-in lubricating element, it can require no maintenance for 20,000 km, as it utilizes capillary action. The minimal requirement for lubrication makes this ball spline environmentally friendly.

What is of interest to engineers is that the external cylinders and spline shafts can be mixed and matched easily to meet specific applications, thanks to the designed interchangeability specifications.

The demands presented by fluctuating and complex loads are accommodated by a four-point contact and a double-row raceway design, which provides accuracy and rigidity.

Conclusion

From robotic arms to machine tools to linear actuators, ball splines are an integral component that ensures the smooth transmission of power. Understanding the relationship of torque and the rotating components are at the heart of choosing and maintaining a quality ball spline.