Ball Screw Definitions

Angular-Contact Thrust Ball Bearing– a special bearing that can withstand large thrust capacities as well as an axial load. They are used with fixed screw ends.

Arrestor Nut– a nut that is attached to the Ball Nut and is locks into the Ball Screw raceway to prevent damage from a failed Ball Nut in a non-horizontal application.

Axial Lube Port– a port on the ball nut that allows re-lubrication of the assembly.

Backlash– The free movement of a ball nut on a stationary ball screw caused by small gaps between the ball bearings and the ball nut or the ball bearings and the ball screw. A ball screw can be ordered preloaded, which means there is minimal backlash. 

Ball Diameter– the diameter of the ball bearings that ride between the ball nut and the ball screw. 

Ball Nut– a nut that contains ball bearings which contact a helical raceway on a screw. A ball nut also contains an interior ball pickup and recirculation system.

Ball recirculation system– a system which picks up the ball bearings at the rear of the nut and redirects them to the front for constant recirculation.

Ball Screw Assembly– an assembly consisting of a ball nut and a ball screw shaft. Ball screws convert rotational motion from a motor shaft or a hand crank, to linear motion.

Ball Screw– cylindrical shaft that has a helical raceway for the ball bearings in the ball nut to travel.

Ball Track Turns– sometimes called “Number of Circulations” is the number of times a ball bearing will travel around the ball screw before being recirculated.

Bearing– installed in the pillow block to provide low friction support for the screw to rotate.

Buckling– when a force perpendicular to the ball screw causes a deformation.

Characteristic Speed– the value of the nominal diameter multiplied by the rpm of the screw shaft.

Deep Groove Ball Bearing– low friction bearing used on floating screw ends.

Double Ball Nut– two single ball nuts that are tensioned against each other to reduce backlash.

End Seals– closure elements attached to the ball nut body and in sliding or non-sliding contact with the ball screw shaft such that foreign objects can’t enter the ball nut. End seals also assist in retaining lubricants.

Front Lube Unit– a unit mounted to the ball nut to provide continuous lubrication, increasing the life of the assembly. With an addition of the front lube unit, you will not have to re-lubricate the assembly for up to five years or 300 million revolutions

Lead– the distance, in mm, traveled by the nut when the ball screw makes one full rotation.

Maximum Linear Speed– the theoretical maximum speed of the ball nut in m/min.

Load Rating Dynamic– a value of the axial force of constant magnitude and direction in which 90% of ball screw assemblies can have a working life of over one million revolutions.

Load Rating Static– a value of the axial force that will permanently deform a ball bearing. The amount of deformation required is 0.0001 multiplied by the diameter of the ball bearing.

Nominal Diameter– the diameter of the Ball Screw at its widest measurement which includes the threads,

Nut Housing– a bracket joined to the ball nut to create a large surface for components to mount to. Nut housings can be made from aluminum or from steel. 

Pillow Block Unit– also called a vertical bearing unit is a support block at the screw end that acts as a support for the end of the ball screw and contains bearings to allow near frictionless rotation of the ball screw shaft. Pillow blocks can be made from aluminum or from steel. 

Pitch– the distance between two threads on the ball screw.

Positioning Accuracy– the difference in displacement of the actual ball nut travel and the calculation of the ball nut travel by multiplying the lead of the ball nut by the number of rotations made by the ball screw shaft.

Preload– preloading a ball screw assembly reduces backlash. Preload classes are C00, C0-C5. Some ball nuts have adjustable preload.

Retaining Ring– used to keep the bearing in the same place on the ball screw.

Rigidity– when a ball screw is more rigid, elastic deformation has less of an effect on it under a load.

Ring Nut– a nut that is screwed into the pillow block unit to secure the bearings and shaft for a fixed end screw.

Screw End Forms– forms are the identifying the machining for different shapes of ball screw ends. The left form end and right form end of a screw do not have to be the same. There are options to have keyways or wrench flats machined as wells as centering hole, a hex socket or a female thread. 

Tolerance Grade– the tolerance grade of a ball screw is the permissible target deviation (in um) of a ball nut given a 300mm travel. Tolerance grades are T3, T5, T7 and T9.

Common Abbreviations

BASA      Ball Screw Assemblies

Ltot          Overall Length of a Ball Screw Shaft

D0           Nominal Diameter of Ball Screw Shaft

P             Lead of Ball Screw Shaft

DW          Ball Diameter

i              Number of Ball Track Turns in Ball Nut

C             Dynamic Load Rating in Newtons

C0           Static Load Rating in Newtons

Vmax       Maximum Linear Speed (m/min) for Ball Nut

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