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.
|
| Lead Screw Representation |
Calculator:
Note: 1 Single or multiple start Acme thread.
|
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 |
 |
|
Torque required to lower the load1,2,3 |
TL |
 |
|
Frictional torque of
the thrust collar |
Tc |
 |
|
Efficiency during lifting the load 3 |
e |
 |
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:
