Thomson continues to supply and support both series of products as it has embraced the differences and understand that each product solves a unique application. an inch series assembly with precision accuracy or a metric series assembly with transport accuracy). The positional accuracy of each series is still the main defining point of differentiation between the products today, although it is certainly possible to produce a product that crosses or overlaps the value proposition (i.e. The metric series of ball screws was initially developed for machine tool applications that required a product that was accurate, repeatable and stiff. Therefore, the product series has maintained the key features critical to this application: low cost, high performance (efficiency, load capacity and life expectancy), and a positional accuracy that is not critical. Transport grade error is cumulative and linear, whereas precision grade error is controlled and constrained.īall screws were invented and popularised in the US for the automotive industry. The accuracy designation doesn’t just provide a numerical deviation but also defines the overall accumulation of positional error. Applications that require simply translating a load from one point to another use the lower accuracy classification known as “transport grade” (typically ☒3µm/300mm or higher).Īpplications requiring knowledge of the absolute position of that load at every point in the stroke without external feedback use the higher “precision grade” accuracy (typically ☒3µm/300mm or lower). Positional accuracy is a much more valuable metric, and there are two classifications (Transport or Precision) with multiple grades (P0, P3, P5, T5 and T7 to name a few). Inch series assemblies were designed as the work horses for low-cost, transport-grade applications: “I need to move a large load from Point A to Point B efficiently and cost effectively.” Metric series assemblies have evolved as the precision solution: “I need to move a large load efficiently while maintaining precise location throughout the stroke.” Therefore, the question shouldn’t be “inch or metric?” but “transport or precision?” Both are excellent products for the applications they were designed to solve. Inch series ball nuts and metric series ball nuts have traditional differences in performance and configuration that are much more important than the lead. Everything else being equal, the unit of measurement is little more than a parameter menu option on a motion configuration screen and, therefore, does little to actually differentiate the product. From a functional standpoint, regardless of whether the lead is specified in inches or millimetres, both screws translate the load in exactly the same way. Similarly, a ball screw with a 25.000mm lead would mean that one revolution of the screw translates the load 25.000mm. A 1.000 inch lead on an inch-series ball screw means that the load, as attached to a nut, will travel 1.000 inch with each revolution of the screw. The unit of measure used to define this motion is then typically assigned to the ball nut as the product series. The lead of a ball screw is the linear travel that a ball nut will travel upon a single revolution of the screw. This article explains how sizing and selection questions centred on performance – instead of monikers – can lead to more efficient linear motion designs. This decision may prematurely exclude the ideal product for the application and lead to significant losses in time, labour and expense. Inch series ball nuts provide dependable accuracy and repeatability, and are ideal for assemblies designed for inexpensive, transport-grade applications
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