What happens if I exceed the ballscrew critical speed?

Operating a ballscrew above its critical speed causes the shaft to resonate and whip violently, leading to excessive vibration, loud noise, rapid bearing wear, and potentially permanent damage. The 80% safety factor accounts for real-world imperfections.

Ballscrew Critical Speed Calculator

Q: Which ballscrew support condition should I choose?

Fixed-Supported is the most common: one angular contact bearing pair (fixed) and one deep-groove bearing (supported). Fixed-Fixed uses angular contact bearings at both ends for higher critical speed. Fixed-Free is for single-bearing actuators.

Q: How do I find the root diameter of my ballscrew?

The root diameter is the minor diameter at the bottom of the thread grooves. For a 1605 ballscrew it is typically 14.2 mm. Check your manufacturer's datasheet for the exact value, often labeled dr or shaft root diameter.

Calculate the maximum safe RPM for your ballscrew based on shaft diameter, length, and support conditions.

Feeds & Speeds Steps/mm Critical Speed Force RPM / SFM

Support Condition

Shaft Root Diameter

Root diameter (dr) in mm

The minor (root) diameter of the screw shaft, not the nominal ball diameter. Common sizes: 12, 16, 20, 25, 32 mm.

Unsupported Length

Distance between supports (L) in mm

The span between bearing supports. For Fixed-Free, measure from the bearing to the free end.

Calculated Parameters

Critical Speed (Nc)

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RPM

Safe Max (80%)

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RPM

Status

Enter values

Speed Zone ---

0 RPM 1000 3000 5000+

Formula: Nc = K × (dr / L²) × 10&sup7;

Worked Example

A typical CNC router uses a 1605 ballscrew (16 mm nominal diameter, 5 mm lead) with Fixed-Supported mounting and an unsupported length of 600 mm.

K = 3.927 (Fixed-Supported)

dr = 14.2 mm (root diameter for a 1605 screw)

L = 600 mm

Nc = 3.927 × (14.2 / 600²) × 10&sup7;

Nc = 3.927 × (14.2 / 360000) × 10&sup7;

Nc = 3.927 × 3.944 × 10&supmin;&sup5; × 10&sup7;

Nc = 3.927 × 394.4

Nc ≈ 1,549 RPM

Safe Max (80%) ≈ 1,239 RPM

At a 5 mm lead and 1,239 RPM safe max, the maximum linear speed is about 6,195 mm/min (244 in/min). For faster rapids, consider a shorter span, larger shaft diameter, or Fixed-Fixed mounting.

Frequently Asked Questions

What happens if I exceed the critical speed?

Operating a ballscrew above its critical speed causes the shaft to resonate and whip violently. This leads to excessive vibration, loud noise, rapid bearing wear, and potentially permanent damage to the screw and nut assembly. The 80% safety factor accounts for real-world imperfections such as slight runout, uneven loading, and temperature expansion that lower the theoretical limit.

Which support condition should I choose?

Fixed-Free applies when only one end has a bearing block and the other end is unsupported. This is rare on CNC machines but common in lead screw actuators. Fixed-Supported is the most common setup: one end has an angular contact bearing pair (fixed), and the other has a single deep-groove bearing (supported). Fixed-Fixed uses angular contact bearings at both ends with preload, giving the highest critical speed but requiring more precise alignment.

How do I find the root diameter of my ballscrew?

The root diameter is the minor diameter of the screw shaft (the smallest cross-section at the bottom of the thread grooves), not the nominal ball circle diameter. For a 1605 ballscrew (16 mm nominal, 5 mm lead), the root diameter is typically around 14.2 mm. For a 2005 screw it is roughly 18 mm, and for a 2510 screw around 22.5 mm. Check your manufacturer's datasheet for the exact value, often labeled as "d_r" or "shaft root diameter."