KEVOS® MAKEOS · Threads
General Purpose Acme Thread Limits (Classes 2G, 3G, 4G)
What are the major, pitch and minor diameter limits of a general purpose Acme screw and nut in class 2G, 3G or 4G, with the allowance for a long engagement or a multi-start thread, and what are the stress and shear areas of the screw?
Limiting diameters of general purpose Acme screw threads by the ASME/ANSI B1.5 formulas the handbook gives: pitch diameter allowances 0.008 √D (2G), 0.006 √D (3G) and 0.004 √D (4G) at the mean of the nominal size range, pitch diameter tolerances 0.006 √D + 0.030 √P (2G), 0.0028 √D + 0.014 √P (3G) and 0.002 √D + 0.010 √P (4G), the major and minor diameter allowances and tolerances, a 10 % allowance increase for each inch of engagement beyond two diameters, the internal allowance for multi-start threads, the crest and root flats, and the stress and shear areas.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Nominal (basic major) diameter D | in | Nominal (basic major) diameter of the Acme thread in inches. |
| Threads per inch | TPI | Threads per inch; the pitch is 1 ÷ n. |
| Class of thread | choice | General purpose class; 2G is preferred, 3G and 4G give less backlash. |
| Number of starts | starts | Number of thread starts; the lead is starts × pitch. |
| Length of engagement | in | Length of engagement of the screw in the nut. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Nominal (basic major) diameter D
- What it is
- Basic major diameter, which is also the nominal size of the thread.
- Units and format
- Enter a number in in: inches. The form accepts 0.25 in to 5 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the basic diameters and the √D allowances and tolerances.
- Get it right
- Use the nominal size from the thread designation, e.g. 1.000 for 1.000-5 ACME-2G.
What this means: Threads per inch
- What it is
- Number of thread pitches in one inch along the axis.
- Units and format
- Enter a number in TPI: threads per inch. The form accepts 1 TPI to 20 TPI. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 TPI as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the pitch, thread height and the √P part of the tolerance.
- Get it right
- Count the crests over one inch or read it from the designation; the pitch is 1 ÷ n.
What this means: Class of thread
- What it is
- Tolerance class of the general purpose Acme thread, which sets the allowance and tolerances.
- Units and format
- Choose one of 3 options from the list: 2G (preferred); 3G; 4G.
- Example
- The form opens with “2G (preferred)” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Closer classes have smaller allowances and pitch diameter tolerances.
- Get it right
- Class 2 is the usual fit; classes 3 and 4 give less backlash and are harder to make.
What this means: Number of starts
- What it is
- How many separate helical threads are cut side by side on the screw.
- Units and format
- Enter a number in starts: a number of thread starts. The form accepts 1 starts to 12 starts. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 starts as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Two, three and four or more starts add 50, 75 and 100 % of the allowance to the nut.
- Get it right
- Count the thread ends at the end of the screw; the lead is starts × pitch.
What this means: Length of engagement
- What it is
- Axial length over which the screw and nut threads are engaged.
- Units and format
- Enter a number in in: inches. The form accepts 0.000001 in to 100 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Beyond twice the diameter, each inch or part adds 10 % to the allowance.
- Get it right
- Use the nut length, or the depth of the shorter of the two threads.
Method and formulas
- P = 1 ÷ n; E = D − 0.5P; K = D − P.
- Allowance on the external pitch diameter = 0.008, 0.006 or 0.004 × √(mean of the nominal size range) for 2G, 3G or 4G, raised 10 % for each inch or part beyond an engagement of 2D.
- Pitch diameter tolerance = (0.006, 0.0028 or 0.002) √D + (0.030, 0.014 or 0.010) √P, with D the next tabulated nominal size at or above the diameter.
- External: major max D, min D − 0.05P (at least 0.005); pitch max E − allowance, min max − tolerance; minor max K − 0.020 (10 TPI and coarser) or 0.010, min max − 1.5 × tolerance.
- Internal: major min D + 0.020 or 0.010, max min + 0.020 or 0.010; pitch min E (plus 50, 75 or 100 % of the allowance for 2, 3 or 4+ starts), max min + tolerance; minor min K, max K + 0.05P (at least 0.005), using the coarser recommended pitch when P falls between two.
- Stress area = π ((d2 min + d1 min) ÷ 4)²; shear area per inch = π D1 max [0.5 + n tan 14.5° (d2 min − D1 max)].
- Example: a 1-5 ACME-2G screw has a pitch diameter of 0.892 to 0.87258 in.
Limitations
- The tolerances apply to single-start threads; multi-start internal threads take the extra allowance shown, and threads with more than four starts take 100 %.
- The pitch diameter allowance uses the mean of the nominal size range as the tables do, so it can differ slightly from 0.008 √D at the actual diameter.
- Gauges and inspection of Acme threads follow ASME/ANSI B1.5.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set the turning and inspection sizes of an Acme lead screw
- Find the limits of a non-standard Acme diameter and pitch
- Check the clearances of a multi-start Acme nut
Who it is for
- Machinists
- Quality inspectors
- Toolmakers
What you get
- Basic pitch and minor diameters, lead and lead angle
- Pitch diameter allowance and tolerance
- External and internal major, pitch and minor diameter limits
- Crest and root flats for the threading tool
- Stress area and shear area per inch of engagement
Interpreting the result
- Tolerances on the screw are minus from the maximum sizes; on the nut they are plus from the minimum sizes.
- The thread thickness tolerance is 0.259 times the pitch diameter tolerance.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1921–1929. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Useful alongside · Library collection
Power screws and linear motion: leadscrews, ball screws, linear guides and cams
In Machine Elements and Mechanical Design
What it covers: Turning rotation into accurate straight-line motion is central to machines and automation. How to size leadscrews and ball screws, choose guides and actuators, and design cam motion.
KEVOS® tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the General Purpose Acme Thread Limits (Classes 2G, 3G, 4G) calculator