January 2017 Archives

A "spring" is used for various structures. Each spring has its purpose, but it plays more of a supporting role than a lead. In spite of this image, springs are one of the most important LCA components in ongoing technological advancements because they are closely related to making the product more compact and lightweight, improving products' reliability, high-speed performance, and operability.

(1)Origin of the word "spring"

(2)Functions of springs

The following characteristics of springs are utilized for mechanical components.

The following table summarizes the uses of springs according to operating conditions.

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:

• 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.

(2) Nut

Simplified process of nut production is as follows:

• 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)

This volume explains the life (in a broad range) of ball screws designed for ultra-precision positioning through case examples.

Abnormal life of preloaded ball screws

"Preloaded ball screws" are selected for uses requiring high precision and rigidity in order to prevent a backlash during direction changes. This volume explains how the preloaded torque is measured as well as the preload control using measurement and quality control data.

(1) How to measure dynamic drag torque

• Dynamic drag torque is the actual torque when a nut for a preloaded ball screw rotates without being affected by the external load. JIS B1192-7.6 defines the measurement method. (See [Fig.1] for details.)
• In an actual measurement, a load cell is used to measure the force required to stop a nut while the screw shaft is rotating. To calculate the torque, this force F is multiplied by the distance between the screw shaft center and the measuring point (arm length L).

(2) Actual measurement data and quality control

• For ultra-precision ball screws for positioning and high rigidity ball screws for machine tools, the dynamic drag torque is a required inspection item for quality control in the shipping preparation. [Fig.2] is dynamic drag torque data of ultra-precision & high rigidity ball screws for machine tools at factory shipment.
• For quality control at factory shipment, the dynamic drag torques in both directions are measured so that both of them meet the acceptance criteria of preloaded torque. Data in [Fig.2] shows the preloaded torque range between 20.6 and 37.2 N・cm.

• The graph of dynamic drag torque shown in [Fig.3] is the measurement data of the ball screw of [Fig.2] after using it for a long period. The data indicates that the preload has been reduced owing to wear and fatigue of the ball screw. The measurement values are in the range between 9.8 and 24.5 N・cm. Compared to the initial value at factory shipment, the preload has been reduced by about 30 to 40%.
• This state is referred to as "preload loss", which could cause a backlash that did not exist before, resulting in reduced accuracy or rigidity, as well as servo instability.