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Swashing Errors In Ball Screws In Miniature Applications

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Swashing Errors In Ball Screws In Miniature Applications
Swashing Errors In Ball Screws In Miniature Applications

Video: Swashing Errors In Ball Screws In Miniature Applications

Отличия серверных жестких дисков от десктопных
Video: Introduction to Ball Screws 2023, February

By definition, a wobble error is an irregular pitch error that occurs at intervals in a thread. This principle is difficult to illustrate unless you unwind the thread from the spindle core (Figure 1).

Guided by ISO, DIN and JIS industry standards (Fig. 2), many designers define the lead accuracy of ball screws in terms of the accumulated error over 300 mm (V 300). However, it is often overlooked that the measurement or control of the pitch accuracy per revolution (V ) can be significantly more useful. In view of the wobble error - often referred to as "wobbling thread" due to the resulting unpredictable fluctuating movement of the ball screw drive - the pitch accuracy per revolution is a critical parameter, especially in miniature applications where the total stroke length is less than 300 mm.

Many neglect the mistake

Many designers incorrectly assume that the V error is negligible in relation to the total pitch accuracy V 300 and therefore ignore this error when specifying a ball screw drive. As shown in Figure 3, the pitch error per revolution for a common P5 spindle can be 8 µm or 1/3 of the permissible pitch error over 300 mm (23 µm).

Detecting and quantifying such errors can be difficult even for the manufacturer, since this requires expensive special analysis equipment that is not available everywhere. This is why many manufacturers rely on manual individual measurements. So you do without dynamic, continuous measurements that control and log 100 percent accuracy over the full stroke. A ball screw with, for example, 12 mm diameter and 2 mm pitch has 150 gears over 300 mm. This means that more than 600 measurements are necessary to accurately reproduce the properties of the spindle.

Picture gallery

Picture gallery with 5 pictures

V 300 refers to longer spindles

When inquiring about screw drives for precision devices with stroke lengths shorter than 300 mm, some suppliers usually manufacture a spindle with an accuracy of V 300related to a longer spindle and provide the customer with a shorter section that fits their requirements. A customer specifies e.g. B. a P5 spindle (23 µm / 300 mm) with 125 mm stroke, the manufacturer could start with a two meter long P5 spindle and cut a 250 mm section for processing. Basically, the customer gets what he ordered - but possibly disregarding the actual performance parameters that are critical to the positioning requirements of the application. As a result, expensive post-processing may be necessary, so that the end customer's requirements are only met with a delay by the OEM.

Take, for example, the case of a manufacturer of medical fluid pumps that specifies an accuracy of P5 and a stroke of 150 mm for a new medical device. Not familiar with the subtleties of the spindle characteristics, only a pitch accuracy of 23 µm / 300 mm was specified in the specification.

Although this pump manufacturer was unable to measure the spindle accuracy separately, it was able to dynamically capture the fluid output and compare this data with the spindle accuracy. In the end he had to reject the standard spindle, although it corresponded to the originally required accuracy. Specifying an accumulated error over 300 mm turned out to be irrelevant for this application. A special specification was therefore necessary to achieve the desired performance.

By comparing the data between the dynamic fluid output quantities and the dynamic spindle behavior, the spindle properties that had a negative effect on the pump performance could be determined. This data was used to revise the specification of the spindle and to eliminate the unwanted error.

Content of the article:

  • Page 1: Wobble errors in ball screws in miniature applications
  • Page 2: Consider the entire context
  • Page 3: Costs also play a role

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