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Demystifying Ball Spline Specs

Wed, 07/11/2012 - 8:50am
Matt Proske

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Why isn’t identifying the right ball spline for an application straightforward and easy? Demystify the semantic differences in the literature and it will, at least, be easier. We’ll do this by thinking of ball splines in terms of how their various configurations affect their functions and then compare their functionality 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 relate to the area of ball contact, i.e., the number of grooves in the shaft, the shape of the grooves, the length of the nut and of its raceways and how close the tolerances are. Five and six are shaft rigidity and mounting systems.

Number Of Grooves In The Shaft
The number of grooves in a spline shaft can number from two to six. The more grooves, the greater the contact area. 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 the nut has to be much larger. So, if the load doesn’t require six raceways and four will do, space can be saved.

Shape Of The Grooves
The 4-point contact design is, because of its shape, called a gothic arch. The gothic arch eliminates any clearance that could lead to deflection and is, therefore, best suited for applications requiring maximum precision. The 4 point-contact increases the load capacity and rigidity so that it can handle a greater moment load.

A change of shaft rotation direction may cause backlash of the circular arc type nut, however, 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 and larger spline sizes usually utilize the gothic arch 4-point contact grooves.

Another shaft configuration is available which is designed for very high loads. It uses semi-triangular protrusions along the spline shaft. Its main drawback is that it makes it more difficult for the user to machine the ends of the shaft and it is less stiff. This type of spline shaft is highly susceptible to vibration since straightness can be somewhat difficult to maintain.

Length Of The Nut And Of Its Raceways
The longer the nut, the more active balls supporting the load. However, some raceways are designed to more efficiently take advantage of the nuts length and pack more balls into its active portion. The more active balls in the nut’s raceways, the more moment load the nut can support. To increase moment load capacity, multiple nuts can be used in tandem.

Nut Tolerances
When raceways are precisely ground (not drawn) they better conform to the shape of the balls in the nut and grooved spline shaft. This results in opposing raceways having the same angles of contact, which eliminates backlash.

The clearance between the balls and the grooved spline shaft is controlled by grinding the nut and grooved spline shaft raceways accurately and also by installing the proper ball size grade – using the nut’s bearing outer cylinder wall to adjust to the different sizes.

Each spline nut should be individually preloaded at the factory with larger ball grades that reduce the clearance between spline shaft groove and nut groove. Where less vibration and less fluctuation of torque are present, standard pre-load is sufficient to ensure smooth linear movement. For a load subjected to minor twisting, alternating moment load and vibration, a light pre-load is preferable.

Shaft Rigidity
Generally, manufacturers present 3 accuracy ratings comparable to High (meaning their highest precision), Medium (meaning their standard grade – usually a stock item) and Low (often a non-ground shaft).  However, one manufacturer’s top grade can be another’s standard grade. Comparing accuracy grades comes down to comparing measures of the ranking characteristics – shaft diameter tolerance, straightness, perpendicularity and concentricity.

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

Three Mounting Systems: Key, Flange and Keyless
The mounted housing has to turn with the nut. The standard mode of mounting a cylindrical nut is with a key. For this system, the cylindrical nut will have a keyway and separate key. A matching keyway must be bored into the housing or block that will be mounted on the cylinder nut. It is critical that the bore precisely fit the key to prevent vibration. In addition to the key slot in the middle of the nut, this system also requires a snap ring, a push plate or another fixing method to prevent the nut from slipping out of the housing.

The second mounting system is the flange nut. A flange nut is much simpler to install because it only requires a rough bore and mounting holes drilled and tapped to secure the flange to the housing. Though, to fit the nut into the housing, a hole has to be bored, this boring does not require the accuracy demanded by a keyed nut.

The third mounting system is the keyless cylinder nut. It is very suitable for small size compact applications. It is similar to a flange type nut. Instead of a fixed flange on the nut, there is a squared attachment that slots into the nut to prevent rotation. The slot can be made with holes for screws so that the nut will be fixed similarly to the way that the flange is fixed to housing. But it is much more compact than the standard flange.

Matt Proske is an application sales engineering manager for Sigma Plastic Services Inc., based in Schaumburg, IL.  A graduate of Kent State University in Manufacturing Engineering, Matt assists customers in the application of SigmaSoft, the company's advanced process simulation software for injection molders of thermoplastics, elastomers and TPE's. 

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