Inch or metric ball screws: not just a question of units

Because ball screws are available in both inch and metric dimensions, designers sometimes begin the specification process by selecting a product family based on the unit of measure. This decision may prematurely exclude the best product for the application and lead to significant losses in time, labour and money. This article explains how sizing and selection questions centred on performance - instead of monikers - can lead to more efficient linear motion designs.

The lead of a ball screw is the linear travel that a ball nut will travel upon a single 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. 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. Similarly, a ball screw with a 25.000mm lead would mean that one revolution of the screw translates the load 25.000mm. 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. 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.

Inch series ball nuts and metric series ball nuts have traditional differences in performance and configuration that are much more important than the lead. Both are excellent products for the applications they were designed to solve. Inch series assemblies were designed as the workhorses 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 should not be 'inch or metric?' but 'transport or precision?'

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). Applications that require simply translating a load from one point to another use the lower accuracy classification known as 'transport grade' (typically +/-23um per 300mm or higher). Applications 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 +/-23um per 300mm or lower). The accuracy designation does not just provide a numerical deviation but also defines the overall accumulation of positional error. Transport grade error is cumulative and linear, whereas precision grade error is controlled and constrained.

Ball screws were invented and popularised in the USA for the automotive industry. 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. The metric series of ball screws was initially developed for machine tool applications that required a product that was accurate, repeatable and stiff. 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 (ie an inch series assembly with precision accuracy or a metric series assembly with transport accuracy). Thomson continues to supply and support both series of products, as the company has embraced the differences and understand that each product solves different problems.

Why select transport-grade ball screws?

When design engineers are selecting ball screws, the performance and price gap between a transport-grade and a precision-grade ball screw assembly is marginal, making the final product selection even more challenging. The transport-grade option can still offer some capability advantages when selected for the proper application.

Besides lead accuracy, transport-grade assemblies typically use external ball return systems to recirculate the ball bearings, making it easy to visually identify this product. External return systems tend to be noisier, less smooth and consume more space than internal recirculation systems. However, they are less expensive to manufacturer and enable the use of more ball bearings, increasing their load-bearing capacity or life expectancy. For those applications in which cost, load bearing or life expectancy are critical factors, designers should seek out manufacturers who specialise in transport-grade products.

Why select precision-grade ball screws?

When accuracy and repeatability are critical to the performance of a system, it is important to select the correct product to yield the desired results. Precision-grade assemblies tend to be quieter, smoother and have a smaller installed profile than transport-grade assemblies. Therefore, this product is suitable for accuracy-critical applications or where customers demand a higher level of operating performance. High-accuracy screws with a preloaded ball nut can eliminate the need for a positional feedback system, which can reduce the overall system cost.

How measurement scales define product lifecycles

Another point of differentiation between inch and metric series products is in how each scale describes the dynamic capacity - or life - of the screw. Inch series products describe life in terms of millions of inches of linear travel, whereas the metric series defines life in terms of millions of revolutions. An inch series ball screw having a 2000N dynamic capacity, for example, means that it would carry a 2000N load for 1 million inches before it reaches its estimated L10 end of life. Likewise, a metric product with a 2000N dynamic capacity means that it would carry a 2000N load for one million revolutions to reach the L10 life expectancy. Because the metric series has the screw lead in the equation, the dynamic capacity cannot be directly compared and used as a point of differentiation unless converted to similar units.

What to specify

The most important question to ask yourself when specifying ball screw accuracy is whether you need a transport or precision-grade option. Consideration of the differences between the product series are secondary considerations only and may then be used to sway the final product selection.

Both inch and metric series ball screws are excellent products if used in the correct application. As with any business decision, it is important to analyse the trade-off of performance and cost when making the final product selection. Select a transport-grade ball screw for applications that require a transport-grade screw. Select a precision-grade ball screw for applications that require a precision-grade screw. Do not let product monikers sway the decision when only performance parameters are important. Start the ball screw selection process by creating a performance specification based on important parameters such as load, speed and life. The correct product may then be an inch or metric series ball screw.

Go to for more information about inch and metric ball screws.

Thomson Industries Inc

1500 Mittel Blvd
Wood Dale
IL 60191

+1 630 694 3334

More technical articles
20 hours ago
Nabtesco delivers solutions for every drive application
To continue meeting increasing demand, Nabtesco is substantially expanding its production capacities with the construction of a new factory in Hamamatsu, 250 km southwest of the its headquarters in Tokyo – which will double the annual production capacity to two million precision gearboxes by 2026.
1 day ago
Festo showing ‘Progress in Motion’ at PPMA 2022
The Festo stand at PPMA 2022 will focus on key applications for the process and packaging sector. These include innovative process valve controls, servo-controlled flow wrapping and a new solution for controlling web tension with dancing rollers.
1 day ago
IFM Electronic makes IIoT easy at PPMA 2022
At this year’s PPMA Show, IFM Electronic is focusing on its moneo IIoT platform. This has been developed specifically to provide a convenient solution to the challenge of converting the huge amount of data produced by modern plant and machines into useful information that can be used to reduce downtime, optimise efficiency and cut costs.
1 day ago
Bürkert extends capabilities with Fine Controls partnership
The key distribution partner agreement between Bürkert and Fine Controls brings OEMs or end users in process control a distributor who can supply all the parts necessary to build a complete process system, as well as offering design and specification support.
5 days ago
Individual service for signal integrity
The demands on connection technology performance are constantly increasing. By providing customised support, Phoenix Contact helps customers select connection components to ensure the highest possible signal integrity.
5 days ago
Aveva MES rises in the leader ranks of the latest Gartner report
Aveva’s Manufacturing Execution Systems (MES) offering, which is deployed in the UK & Ireland by AVEVA Select partners SolutionsPT, has been recognised as a global leader in industrial software in the latest 2022 Gartner Magic Quadrant for Manufacturing Execution Systems.
6 days ago
Harwin and MSA Components sign German distribution deal
Harwin is bringing greater strength to its German supply chain through a new strategic partnership with MSA Components. The agreement allows MSA to offer all the latest products from Harwin’s extensive portfolio.
6 days ago
Siemens selected by Microsoft for RAMP Programme
Siemens Digital Industries Software has been selected to participate in the Rapid Assured Microelectronics Prototypes (RAMP) Phase II initiative.
6 days ago
Kontron receives VDC Research’s Gold Award for vendor satisfaction
Kontron has been recognised for outstanding customer satisfaction. The company has received the Gold Vendor Satisfaction Award for IoT and Embedded Hardware technology from VDC Research.
6 days ago
Mouser wins Vishay Distributor of the Year Award
Mouser Electronics has received the 2021 High Service Distributor of the Year award from Vishay Intertechnology, one of the world’s largest manufacturers of discrete semiconductors and passive electronic components.

Login / Sign up