When dealing with threaded fasteners like bolts and screws, two fundamental concepts define the thread geometry and function: Pitch and Lead. While often confused, particularly in single-start threads, they describe different aspects of the thread structure.
1. What is Screw Pitch?
Screw pitch is the fundamental measurement of thread density.
- Definition: The pitch of a screw is the axial distance between a point on one thread and the corresponding point on the very next thread, measured parallel to the thread axis.
- Measurement: It is typically expressed in millimeters (mm) for metric threads (e.g., 1.5 mm) or as Threads Per Inch (TPI) for imperial threads (e.g., 20 TPI).
- Significance: Thread pitch meaning directly relates to how fine or coarse the thread is. A smaller pitch means more threads per unit of length (finer thread), resulting in a tighter, stronger, but slower engagement. What is pitch in bolt? It is the primary specification for metric bolts (e.g., M8 * 1.25, where 1.25 is the pitch).
2. What is Lead (Thread Lead)?
Lead defines the axial distance the screw advances in one complete revolution (360°).What is the lead of a screw? It is the functional measure of travel.
- Definition: The lead of a thread is the distance the screw moves along its axis when the screw (or nut) is rotated exactly one full turn.
- Formula: Lead= Pitch*Number of Starts
- Significance: Screw lead is crucial for power transmission screws, like the lead screw pitch or pitch of ball screw, as it determines the speed of linear motion.
3. Pitch vs Lead: The Difference (Lead vs. Pitch)
The distinction between pitch vs. lead lies in the concept of starts. A “start” refers to the number of individual, interlacing helical threads running along the shaft.
| Feature | Pitch | Lead |
| Definition | Distance between adjacent threads. | Distance the screw advances in one rotation. |
| Formulaic Relationship | Pitch= Lead/Starts | Lead= Pitch*Starts |
| Single-Start Thread | Pitch= Lead | Pitch lead are equal. This is true for nearly all standard fasteners. |
| Multi-Start Thread | Pitch< Lead | Lead is greater than the pitch. Used to achieve fast travel without weakening the thread profile. |
| Primary Use | Specifying standard fasteners; thread strength. | Defining linear travel speed (lead screw pitch). |
For standard machine screws and bolts, the number of starts is always one, so what is a screw pitch is the same as what is the lead of a thread.
4. Importance in Power Transmission
The difference between pitch lead screw specifications is most critical in motion control:
- Ball Screw Pitch: In a ball screw, the pitch and lead are carefully selected to balance speed, load capacity, and precision.10 A smaller pitch provides more precision and load capacity, while a larger lead provides greater speed.
- Multi-Start Threads: Mechanisms that require fast linear motion (e.g., a pen cap or a quick-release clamp) use multi-start threads, where the lead is 2 x3x, or more times the pitch. This allows rapid engagement without relying on a coarse thread that would reduce shear strength.
5.Final Thoughts
Understanding the pitch screw, the lead of the screw, and the relationship between lead vs pitch is essential for engineering design, fastener selection, and mechanical performance. Whether you’re specifying a bolt, selecting a lead screw, or optimizing a ball screw pitch for precision machinery, mastering these concepts ensures reliable and efficient operation.