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#272 Ball Screws -14: Production Method

Category : Linear Motion Components
January20, 2017

Ball screws are roughly divided into "grinding ball screws" and "rolled ball screws". There is not much difference in the components constituting both types of ball screws. The major differences in production methods for each are (1) how the screw shaft is formed, and (2) built-in capability and its inspection according to the capabilities of ball screws. See the description below.

(1) Screw shaft

Simplified process of screw shaft production is as follows:

Material processing → Outline machining → Grooving (rolling or cutting) → Heat treatment → Straightening → Tempering → Groove finishing

  • In this production process, grinding screw shafts are manufactured differently from rolled screw shafts in terms of the following: grooving is made more precisely by grinding work; precise grinding is applied for other outer shapes (outer diameter and shaft end); lapping is applied to grooves in the final process.
  • In addition, subzero treatment may be performed on grinding screw shafts during heat treatment in order to improve abrasion resistance while minimizing aging deformation of shafts (change of shape after a long period).

During the screw grinding work, grinding fluid temperature is controlled to stay at the same temperature as that of the temperature-controlled working chamber to allow precise machining of long screw shafts.

Example of thread grinder

(2) Nut

Simplified process of nut production is as follows:

Material processing → Groove cutting → Heat treatment → Nut outline grinding → Groove grinding → Circulating unit assembly → Steel ball insertion → Inspection

  • The difference in the production of nuts for rolled ball screws and grinding ball screws is the additional production process applied for the latter: the groove grinding and outer shape grinding of nuts so that the outer shape of the nut can be used as an assembly reference plane.

(3) Ball screw assembly

  • The difference between the two in the assembly process is setting of "preloading" performance required for grinding ball screws. Since the rolled ball screws do not require this setting, the assembly is very simple.
  • Preload inspection is conducted for grinding ball screws after steel-ball assembly. If the required preload performance is not confirmed, it is necessary to reassemble the steel ball with different diameter and perform this inspection again.
  • After inspecting the finished goods, grease is applied, and then the product is packaged for shipping.
  • Inspection items: Screw shaft lead, effective diameter, groove shape, total run-out, outer dimensions of screw shafts and nuts (pre-load dynamic torque)