Thread spacing and nut travel per revolution agree for some screws, but they are not universally the same quantity.
Identify the specified distance
Thomson distinguishes screw pitch from lead. Lead is linear travel per revolution; for a multistart screw it equals pitch multiplied by the number of starts. Primary reference: Thomson screw pitch and lead.
A drive specification should state which quantity it uses. Threads per inch, pitch in millimetres and lead in millimetres per revolution should not be copied into one generic pitch field. Keep the designation and units intact.
A hypothetical multistart calculation
Imagine a fictional screw with 2 mm pitch and three starts. Its lead is 2 × 3 = 6 mm per revolution. At an assumed 100 revolutions per minute, nominal travel speed is 600 mm/min under the simplified geometry model. Using pitch as travel would give 200 mm/min and describe a different result.
Add start count, pitch, lead and rotational-speed basis to the worksheet. These inputs describe no HM axis, actual screw or positional performance. The calculation gives nominal travel, not an accuracy, load or life qualification.
Connect geometry with the complete drive requirement
After resolving travel, ask about the load, mounting, speed and other requirements of the identified assembly. A lead value does not prove the screw is suitable for the duty or that the connected drive reaches its required position.
For replacement or integration, retain the exact screw and nut identities with the motion conversion used by the controller. An apparently similar thread can produce a different distance per turn. The accepted record should make that conversion reproducible while keeping observed motion evidence and permitted component ratings as separate assessments.
Customer Questions
When do pitch and lead match?
For a single-start screw under the stated definitions; multistart lead includes the start count.
Does nominal lead establish positioning accuracy?
No. It defines travel geometry, not the complete positioning performance.
What should a controller motion conversion retain?
The identified screw lead, units and relevant drive relationship.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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