PowerTorq Precision Ball Spline Engineering Glossary and Technical Data

PowerTorq™ Ball Splines are convenient and efficient devices that allow friction free linear motion while transmitting torque. Because of their reliability and high efficiency, they are utilized to replace conventional splines. In a ball spline assembly, recirculating bearing balls carry the load between the rotating member (inner race) and the rotating/translating member (outer race).

Ball Spline Terms

Active Circuits

The closed path that the bearing balls follow through the outer race is referred to as a circuit. The number of potential circuits varies with the diameter of the spline shaft. When a circuit is loaded with bearing balls, it is referred to as an "active circuit". PowerTorq™ Ball Splines may have three or six active circuits. (See Image 4 in Figure 1 Below).

Return Guides

The outer race component through which the bearing balls are recirculated is referred to as the return guide. PowerTorq™ outer races are available with stamped return guides or high performance solid return guides.

Ball Circle Diameter

The ball circle diameter is the diameter of the circle generated by the center of the bearing balls when in contact with the inner and outer race.

Land Diameter

The land diameter is the outside diameter of the inner race. This diameter is less than the ball circle diameter.

Root Diameter

The root diameter is the diameter of the inner race measured at the bottom of the groove. This is the diameter used for critical speed calculations.


Although PowerTorq™ Ball Splines are manufactured from straight, cylindrical material, internal stresses may cause the material to bend. When ordering random lengths or cut material without end machining, straightening is recommended. Handling or machining of splines can also cause the material to bend. Before, during and after machining, additional straightening may be required.

When ordering splines with machined ends from Nook Industries, the following straightness tolerances can be expected:

PowerTorq™ Ball Splines are straight within .010 in/ft when shipped from the factory, and do not exceed .030 inch in any 6 foot section. Twist is limited to .015 in/ft at the ball circle diameter.


A ball spline assembly uses rolling elements to carry a load similar to an anti-friction (ball) bearing. These elements do not wear when properly lubricated during normal use. Therefore, ball spline life is predictable and is determined by calculating the fatigue failure of the components.

Proper lubrication, regular maintenance, and operation within specified limits will allow PowerTorq™ Ball Splines to operate to the predicted life. See page 221 of our Precision Screw Assemblies Catalog for life calculations.


The use of rolling elements in a PowerTorq™ Ball Spline result in a low coefficient of friction.

Rotational Lash

Backlash or lash is the relative rotational movement of an outer race with no rotation of the inner race (or vice versa). Rotational backlash for the PowerTorq™ Ball Splines is limited to a range of .005" to .009" at the ball circle diameter.

Selective Fit

When less than standard lash is required and a preloaded outer race cannot be used, outer races can be custom-fit to a specific inner race with bearing balls selected to minimize rotational (angular) lash.

Load Definitions

Dynamic Torque Load

The torque load which, when applied to the ball spline assembly, will allow a minimum life of 1,000,000 inches of travel.

Static Torque Load

The maximum torque load (including shock)that can be applied to the spline assembly without damaging the assembly.

Overturning Load

A load that rotates the outer race around the longitudinal axis of the inner race. (See Image 5 in Figure 1 Below).

Side Load

A load that is applied radially to the outer race. (See Image 5 in Figure 1 Below).

CAUTION: Although a side load will not prevent the ball spline from operating, the outer race is not designed to operate with a side load, such as those generated from pulleys, drive belts or misalignment.


Preload is a load introduced between an outer race and screw assembly that eliminates radial movement. Preloaded assemblies provide zero backlash for excellent repeatability and increased system stiffness.

Optional Standard Keyways

Typically, outer races are mounted by machining a keyway into the outer race, inserting a key, and then sliding the outer race into a keyed bore. Standard machined keyways are available. See product pages 222-225 for details.

Transferring Outer Races From Shipping Arbor

Standard Races

PowerTorq™ Ball Spline outer races are shipped on arbors. Transferring the outer race from the arbor to the ball spline can be achieved by placing the arbor against the end of the spline and carefully sliding the outer race onto the inner race.

If the I.D. of the arbor is not able to slip over the O.D. of the end journal, apply tape to the journal to bring the O.D. up to the root diameter. The outer race can then be transferred across the taped journal onto the ball spline.

CAUTION: Removal of the arbor from the outer race will result in the loss of the bearing balls.

NOTE: The set screw is used for transportation only and needs to be completely removed after installation.

PowerTorq™ Material Specification

PowerTorq™ inner races are made of high quality 4150 alloy steel, induction hardened to Rc 56-60. PowerTorq™ outer races are made of hardened steel with ball tracks heat treated to Rc 56-60. PowerTorq™ ball spline inner and outer races are protected with a black oxide finish.


Proper and frequent lubrication must be provided to achieve predicted service life. A 90% reduction in the ball spline life should be anticipated when operating without lubricants.

Standard lubrication practices for antifriction bearings should be followed when lubricating ball splines. A light oil or grease (lithium-based) is suitable for most applications. Lubricants containing solid additives such as molydisulfide or graphite should not be used.

E-900 Ball Screw Lubricant is oil that has been developed specifically for ball screws and ball splines and is available as a spray or liquid.

Lubrication intervals are determined by the application. It is required that spline assemblies are lubricated often enough to maintain a film of lubricant on the inner race.


PowerTorq™ Ball Splines will operate between -65°F and 300°F with proper lubrication.

End Machining

To obtain optimum performance of your ball spline assembly, it is recommended that the machining be performed at the Nook Industries factory. Splines may be purchased machined to your specifications.

Annealed ends can be provided on precision ball splines to facilitate end machining of journals.

End Fixity

End fixity refers to the method by which the ends of the spline are supported. See the Ball Screw technical section for a further explanation of end fixity, page 78-79.

Critical Speed

The speed that excites the natural frequency of the spline inner race is referred to as the critical speed. Resonance at the natural frequency of the inner race will occur regardless of orientation (vertical, horizontal, etc.).

The critical speed will vary with the diameter, unsupported length, end fixity and rpm. Since critical speed can also be affected by shaft straightness and assembly alignment, it is recommended that the maximum speed be limited to 80% of the calculated valve. The formula used to calculate critical speed is found on page 220. The critical speed chart can also be used to quickly determine the minimum diameter.

E-900 Lubricant

E-900 Ball Screw Lubricant (See Image 6 in Figure 1 Below) protects against inter-ball friction, wear, corrosion, and oxidation. E-900 Ball Screw Lubricant may be used on PowerTorq™ Ball Splines. E-900 will provide a lasting film for wear protection and resistance to corrosion. With an operating range of -65° to +375°F, E-900 has low rolling friction characteristics and helps reduce inter-ball friction in ball spline assemblies.

For optimum results, the ball spline assembly should be in good repair and free of dirt and grease. Used regularly, E-900 will extend the life of ball spline assemblies. It should be applied generously on the entire length of the spline.

PART # NLU-1003 1 CAN weight of 1 lb.
PART # NLU-2003 1 CASE with 12 cans - total weight of 13 lb
PART # NLU-1004 1 BOTTLE weight of 32 oz.
PART # NLU-2004 1 CASE with 12 quarts - total weight of 25 lb 5 oz.
Figure 1