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Ball Screw Assembly Terms and Definitions

Return list Source:WILL Browse:- Release date:2016-03-26 14:27:00【Big In Small

Backdriving: This is the ability of the ball screw to reverse by applying a thrust load to the ball nut to drive the ball screw and develop torque, or applying a thrust load to the ball screw to drive the ball nut. The higher the relationship of the lead of the ball screw to its diameter the less effort is required to obtain a smooth, positive backdriving effect. The lead of the ball screw should ideally be equal to the diameter of the ball screw. Any ball screw with a lead less than one third of its diameter is not recommended on a backdriving application.

Backlash (Axial Lash): The axial free movement between the ball nut and ball screw. It determines the mount of lost motion between the ball nut and ball screw on a horizontal application. Backlash on standard ball nuts range from .004 to .018, depending on ball screw size.

Ball Screw Assembly Terms and Definitions

Ball Circle Diameter (BCD): The diameter of the cylinder generated by the center of the bearing balls when in contact with the ball screw and ball nut.

Ball Screw Life (Life Expectancy): Is expressed as total revolutions or inches of travel a ball screw assembly will operate under a rated load in a clean environment with proper lubrication. About 90% of all ball screws assemblies operated at rated loads will meet or exceed a million revolutions of life before evidence of fatigue appears. Although 10% may not reach a million revolutions, 50% could exceed 5 million revolutions.

Circuit: The closed path of recirculating balls in the ball nut.

Column (Compression) Load: A load that tends to buckle or compress the screw shaft.

Column Strength: The maximum compressive load that can be applied to a ball screw assembly without resulting in failure due to elastic instability. Column strength is a function of the ball screw diameter, unsupported length of the balls screw and the rigidity of the end supports.

Critical Column Speed: The maximum speed at which a ball screw or ball nut can rotate without producing destructive resonant vibrations. The critical speed is a function of the ball screw diameter, the unsupported length of ball screw and rigidity of end supports and RPM.

Cycle: A complete forward and return stroke.

Dynamic Load (Operating) Rating: The maximum thrust load under which a ball screw assembly will achieve a minimum of 1,000,000 revolutions of travel life.

Efficiency: The ratio of work output divided by work input. Generally this is greater then 90% for ball screw assemblies.

End Fixity: This refers to how ball screw assembly ends are supported. The end fixity is determined by system stiffness calculation, column load and critical speed. The three basic end support variations are free (no support), simple (single point support) and fixed (spaced support points). The Heli-Tek ball screw assembly selection wizard can help determine your optimal end fixity.

Finish on Ball Screw Assemblies: Ball screw assemblies are supplied with a polished or ground finish. Other finishes and coatings are available upon request.

Lead: The linear distance the ball nut or screw will travel in one revolution.

Lead Accuracy: The maximum variation of linear displacement measured in inches per foot. Heli-Tek grade 7 accuracy is .0020 inches per foot (52um/300mm), grade 5 accuracy is .0010 inches per foot (23um/300mm) and grade 3 accuracy is .0005 inches per foot (12um/300mm).

Lubrication: Ball screws assemblies should not be operated without proper lubricant. A spindle lube of 10 weight oil is recommended. For ball screw assemblies with infrequent cycles or lubrication maintenance, light lithium grease is recommended.

Machined End Configuration: Ball screws can be configured with or without end journals. Heli-Tek provides optional standard end machining to accommodate all of our end support variations upon request. Ball screws can also be machined to customer specifications if a drawing is submitted.

Major Diameter: The outside diameter of the ball screw thread.

Matched Leads (Synchronous Screws): Screws synchronization is achieved by selecting a ball screw with closely matched lead errors. These ball screws can be driven in tandem by a single motor source to eliminate binding. To order synchronous ball screws ask for matched leads when ordering.

Material/Hardness: Most ball screws and ball nuts are made of alloy or carbon steel. They are hardened to Rc 56 minimum.

Minor Diameter: The root diameter of the ball screw thread, measured at the bottom of the thread.

Multiple Circuit Ball Nut: A ball nut with two or more closed paths, each containing recirculating balls. Used to carry heavier loads that can be handled by a single circuit ball nut.

Off-Center Load: A Load that tends to cock the ball nut on the ball screw. This type of loading will reduce ball screw life.

Pitch: The axial distance between succeeding ball grooves; the same as the lead for a single start thread.

Preload: The use of two groups of balls, loaded opposite directions to eliminate backlash in the ball screw assembly. Preloading increases screw stiffness and provides for accurate positioning with very little increase in applied torque.

Retrofitting Ball Screw: Heli-Tek has been able to engineer ball screw assemblies for applications that do not currently utilize ball screws along with reverse engineering currently integrated ball screws for greater improvement in life and performance.

Screw Straightness: .010 inches per foot TIR, not to exceed .025 inches TIR over entire length of screw.

Selective Fit:Ball nuts can be custom fitted with oversized balls to reduce backlash to .003 inch.

Side Load: A load that is applied perpendicular to the ball screw. This type will also reduce the life of a ball screw assembly.

Single Circuit Ball Nut: A ball nut with only one closed path for recirculating balls.

Static Load: The maximum load including shock load that can be applied to a stationary ball screw assembly before there is permanent deformation of the ball track in the ball nut or ball screw.

Stroke: The linear distance in either direction that a ball screw or ball nut travels in moving the load.

Temperature: Ball screw assemblies operate with normal efficiency between temperatures of -65 degrees F. to 300 degrees F. with suitable lubrications.

Tension Load: A load that tends to stretch the ball screw. A greater selection of ball screw sizes are available when tension loading is present because there are no column load limitations.

Thrust Load: A load parallel to and concentric with the center line of the ball screw, which acts continuously in the same direction. Thrust loading is the recommended method of attaching the load to the ball screw.

Wipers: Wipers are used with ball nuts to prevent foreign materials from entering inside.