Place a recirculating-ball bushing on a shaft and what do you get? Frictionless movement of the shaft. But, create grooves along the axial length of the shaft that correspond to the radius of the bushing's ball elements and you have frictionless linear movement coupled with two important characteristics - anti-rotation torque transfer and higher load capabilities.

That's a ball spline. It combines a linear bushing (nut) that can now handle greater moment loads, and a shaft, which, unlike a rail, can be rotated when needed. It is for high-speed motion and high-speed rotation.

There is the right ball spline for a range of automated operations: robotics, inspection, spinning, loading, coating, wire winding, grinding, indexing, die setting, transferring, conveyance, molding, drafting, measuring, optical measuring, welding, riveting, printing, book-binding, packaging, filling, and pressing.

So why isn't identifying the right ball spline for an application straightforward and easy? Well, it can be when semantic differences and ball splines are thought of in terms of how their configurations affect their functions, and their functionality is compared to application requirements.

Six load and accuracy factors

The ball spline bushing (generally referred to as a nut) has a load capacity (including moment load) that can be increased by manipulating any of six factors. Four of the factors relate to the area of ball contact: number of grooves in the shaft, shape of the grooves, length of the nut and of its raceways, and closeness of tolerances. Five and six are shaft rigidity and mounting systems.

Number of grooves in the shaft

Compared to a slide bush's shaft, a grooved spline shaft gives greater contact area so load capacity and life is greater than a like-sized slide-bush-and-shaft combination. The basic dynamic load ratings of a ball spline are typically 5~12 times that of a slide bush of a similar size.

The number of grooves in a spline shaft can number from two to six. However, in some instances the six-groove system fills so much of the space on the shaft that there is no room next to the nut's active ball paths for ball recirculation. Therefore, the nut has to protrude from the shaft for the balls to recirculate above and away from the shaft. Also, because the ball elements will fall out of the nut if the nut and shaft are separated, this particular type of six-groove-shaft ball-spline system must be handled with far greater care.

In the most popular four-groove configuration, the nut can have side-by-side active and recirculating paths, making this a much more compact system. Plus, all ball tracks are in contact with raceways, whereas only half are in contact in any one direction on some of the six-groove shaft systems. So, if the load doesn't require six raceways and four will do, space can be saved.

Shape of the grooves

The four-point contact design is called a gothic arch because of its shape. The gothic arch eliminates any clearance that could lead to deflection and is, therefore, best for applications requiring maximum precision. The four-point-contact increases the load capacity and rigidity so that it can handle a greater moment load. Typically, larger spline sizes utilize the gothic arch four-point contact grooves.

Of the groove designs on the market, the standard choice is between balls that make contact with the raceway grooves at two points or at four points. A slightly elliptical groove design allows the balls to make contact at two opposing points but allows a bit of clearance on the balls' sides that are perpendicular to the contact points. A change of shaft rotation direction may cause backlash of this circular arc type nut. Because there are larger contact area differences on a gothic arc, the inner part of the balls must rotate faster than the outer, which creates slippage and results in greater friction. For this reason, circular arc grooves are used for smaller, more friction-sensitive ball splines.

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