Metric vs Imperial Ball Screws: Key Differences

The main difference between metric and imperial ball screws is the dimensional system used to specify the screw diameter, lead, nut geometry and machined shaft ends. Metric ball screws use millimetres, while imperial-or inch-series-ball screws use inches and fractional-inch dimensions.

Neither system is inherently more accurate, efficient or durable. Performance depends on the ball screw's accuracy grade, manufacturing process, preload, material, load capacity, installation and lubrication.

Metric and Imperial Ball Screws at a Glance

How Metric Ball Screw Sizes Are Identified

Metric ball screw model numbers commonly include the nominal screw diameter and lead in millimetres. For example:

2510 ball screw = approximately 25 mm nominal screw diameter and 10 mm lead

Lead is the linear distance travelled by the nut during one revolution of the screw. A 10 mm lead therefore produces 10 mm of theoretical travel per revolution.

The number 2510 does not indicate 25 inches and 10 inches. A 25-inch-diameter ball screw would be an extremely large, special component and has no connection with the common 2510 model designation.

Model codes are not completely universal. The same diameter and lead can be combined with different nut types, ball circuits, preload levels and accuracy grades. The supplier's drawing should always be checked.

How Imperial Ball Screw Sizes Are Specified

Imperial ball screws specify diameter and lead in inches. The diameter may be written as a decimal or fraction, such as 0.750 in, 1.000 in or 1-1/4 in.

Lead may be expressed in inches of travel per revolution. Some drawings or catalogues may use other inch-based thread descriptions, so it is important to confirm whether the stated value means lead, pitch or revolutions per inch.

Lead and pitch are not always identical. Lead is the total linear travel per revolution, while pitch is the axial distance between adjacent thread grooves. In a multi-start screw, the lead equals the pitch multiplied by the number of starts.

Unit Conversion Does Not Create an Exact Replacement

The basic conversion is:

1 inch = 25.4 mm

However, converting the units does not mean that two nominal sizes are mechanically identical.

For example:

• 25 mm = approximately 0.9843 in, not exactly 1 in
• 10 mm = approximately 0.3937 in
• 0.400 in = 10.16 mm
• 0.200 in = 5.08 mm, not exactly 5 mm

A one-inch screw is therefore about 0.4 mm larger than a 25 mm screw. This difference is too large to ignore when matching a ball nut, support bearing or machined housing.

Small Lead Differences Accumulate With Travel

A metric lead and a nearby imperial lead may look interchangeable, but their travel difference accumulates with every revolution.

Consider a 5 mm lead ball screw and a 0.200 in lead ball screw:

• Metric lead: 5.00 mm per revolution
• Imperial lead: 0.200 in = 5.08 mm per revolution
• Difference per revolution: 0.08 mm
• Difference after 100 revolutions: 8 mm

If the controller is programmed for 5 mm per revolution but the installed screw moves 5.08 mm, the commanded and actual positions will diverge significantly. The controller's motion parameters must be updated when the lead changes.

Metric Does Not Mean More Accurate

The measurement system does not determine the accuracy of a ball screw. A precision-manufactured imperial ball screw can be more accurate than a general-purpose metric ball screw.

Accuracy depends on factors such as:

• Rolled or ground manufacturing process
• Specified lead-accuracy grade
• Cumulative lead error over the effective travel
• Travel variation within a defined interval
• Axial clearance or preload
• Support-bearing and mounting accuracy
• Temperature and thermal compensation

Metric ball screws may be specified using grades such as C3, C5, C7 or C10, depending on the applicable standard and manufacturer. An imperial catalogue may state permissible lead error in inches per foot or over another reference length.

Do not compare the grade name alone. Convert both specifications to the same reference length and unit before comparing permissible error.

Load Capacity Is Not Determined by the Unit System

A metric ball screw is not automatically stronger or weaker than an imperial ball screw. Load capacity depends on screw diameter, ball diameter, raceway geometry, number of loaded circuits, nut structure, material and heat treatment.

When comparing possible replacements, check both the basic dynamic load rating and the basic static load rating using the same unit system. Also review axial rigidity, allowable speed and calculated service life.

Be careful when converting load values. Kilograms describe mass, while newtons, kilonewtons and pound-force describe force. The units should not be treated as directly interchangeable without the correct conversion.

Nut Dimensions Must Be Checked

Even when the screw diameter and lead appear close, metric and imperial nuts may differ in:

• Nut body diameter
• Overall nut length
• Flange diameter and thickness
• Mounting-hole diameter
• Bolt-circle diameter
• Flange orientation
• Lubrication-port thread
• Pilot diameter and mounting shoulder

A replacement nut must fit the existing nut housing and mounting bolts. Adapters may be designed in some retrofit projects, but there is no universal conversion kit that makes every metric and imperial ball screw interchangeable.

End Machining and Support Units Are Different

Ball screw ends are machined to match support bearings, locknuts, couplings and motor connections. A metric screw may use metric bearing-seat diameters and threads, while an imperial assembly may use inch dimensions and inch threads.

Before replacing one system with the other, confirm:

• Fixed-side and supported-side bearing-seat diameters
• Bearing-seat lengths and shoulder positions
• Locknut thread size and direction
• Coupling bore diameter
• Support-unit mounting dimensions
• Overall shaft length and effective travel

View DLY ball screw support units when checking the bearing and mounting interface.

Which Type Should You Choose?

For a new machine, select the dimensional system that matches the rest of the equipment, drawing standards, bearings, fasteners and local spare-parts supply.

For a replacement project, the safest choice is usually to maintain the original dimensional system unless the complete assembly can be redesigned and verified.

Choose based on:

• Existing machine dimensions
• Required diameter and lead
• Nut mounting interface
• End-machining drawing
• Support-bearing dimensions
• Load and speed requirements
• Accuracy and preload requirements
• Controller settings
• Availability of replacement components

Information Needed for a Replacement

A model number alone may not provide enough information, especially when replacing an imperial ball screw with a custom metric assembly. Provide:

• Complete ball screw model number
• Screw diameter and lead
• Overall length and effective travel
• Nut dimensions and mounting-hole pattern
• Detailed drawings of both machined ends
• Accuracy grade or permissible lead error
• Preload or axial-clearance requirement
• Load, speed and mounting direction

If no drawing is available, clear photographs and measurements of the original screw, nut, bearing seats and threads can support an initial review. Critical dimensions should still be confirmed before production.

To understand the internal motion principle, read how a recirculating ball screw works .

Conclusion

Metric and imperial ball screws use different dimensional conventions, but neither system has an inherent advantage in accuracy, efficiency or load capacity.

The practical differences are the screw diameter, lead, nut mounting dimensions, machined shaft ends, bearing interfaces and the units used to specify accuracy. A nearby converted size is not necessarily an interchangeable replacement.

View DLY ball screw products for metric models, available accuracy grades and custom end-machining options.

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