LEADSCREW (POWER SCREW) TORQUE and EFFICIENCY CALCULATIONS for ACME THREADS

A Lead screw (or Power screw) is a device which converts rotational motion to a linear motion and transmits power. Square, Acme and Buttress thread forms are generally used as lead screws. Common applications of power screws are lead screw of lathes, screw presses, screw type of jacks and micrometer screw.

There is a large contact area between male and female threads in lead screws and this results large frictional loss during the operation. Due to these frictional losses, lead screws are not very efficient but generally self-locking. In self locking lead screws, the load can’t lower itself without an external effort. This feature is generally used to hold loads.

Ball screws are efficient alternatives to lead screws. In ball screws, ball bearings are recirculating between the screw rod and nut. This results a significantly less friction than the power screws. However,  brakes are needed to stop ball screws from overhauling.

Lead screw calculator for Acme thread was developed to calculate parameters of a lead screw with general purpose and stub Acme thread form. Calculated parameters by the calculator are required torque to raise and lower the load, efficiency and self-locking feature of lead screw. The formulas and parameters used in the calculator are given in the " List of Equations " section of this page.

The basic profile for Unified inch screw threads
Lead Screw Representation



Calculator:

INPUT PARAMETERS
Parameter Symbol Value Unit
Acme Thread Type -- ---
Nominal diameter (inches) - threads per inch --
Number of Start 1
Load F lbf
Coefficient of Thread Friction2 f ---
Coefficient of Collar Friction2 fc
Mean Collar Diameter dc in

Note: 1 Single or multiple start Acme thread.
Note: 2 See Friction Coefficients in Lead Screws

 


RESULTS
Parameter Symbol Value Unit
Nominal Diameter D --- in
Threads per inch n --- ---
Basic Pitch Diameter D2 --- in
TORQUE AND EFFICIENCY CALCULATIONS
Torque to Raise the Load TR ---
Torque to Lower the Load0 TL ---
Efficiency during Lifting e --- ---
Self Locking -- ---

Note: 0 If TL is negative, the lead screw is not self-locking.

Definitions:

Lead: The distance that screw (or nut) advances in one revolution.

Major Diameter: The maximum diameter of a thread which is diameter of the crest of a male thread or root of a female thread.

Minor Diameter: The minimum diameter of a thread which is diameter of the root of a male thread or crest of a female thread.

Multi Start Acme Thread: Provides fast relative traversing motion with lower mechanical advantage.

Stub Acme Thread: The thread height is less than general purpose and centralizing acme threads. The thread height is 0.3 pitches for Stub Acme thread. In general purpose form this is 0.5 pitches. Stub Acme thread form is provided for unusual applications where a coarse-pitch thread of shallow depth is required due to mechanical or metallurgical reasons.

Thread class: A classification system to classify the threads for interchangeability and manufacturability.

Threads per inch: Number of full thread per an inch length.


List of Equations:



Parameter/Condition Symbol Equation
Torque required to raise the load 1,2,3 TR Equation for torque required to raise the load
Torque required to lower the load1,2,3 TL Equation for torque required to lower the load
Frictional torque of the thrust collar Tc Equation for frictional torque of thrust collar
Efficiency during lifting the load 3 e Equation for efficiency during lifting the load

Note 1: dm is mean diameter and basic pitch diameter (D2) is used as mean diameter in the calculations.
Note 2: 2α is thread angle and 29 deg for Acme thread.
Note 3: l is the lead distance.

Supplements:



Link Usage
Friction Coefficients in Lead Screws Typical friction coefficients of thread and collar in lead screws
Acme General Purpose Screw Threaded Rod and Nut Sizes & Tolerances Calculator Dimensional calculations of general purpose single and multiple start Acme threads according to ASME/ANSI B1.5-1997 standard.
Stub Acme Screw Threaded Rod and Nut Sizes & Tolerances Calculator Dimensional calculations of single start Stub Acme threads according to ASME/ANSI B1.8-1988 standard

Reference: