Acme Threaded Rod 101

Christopher Nook by Christopher Nook

tagged as:

Acme Screws, Acme Nuts

The acme thread form, established over 100 years ago, replaced square thread screws, which had straight-sided flanks and were difficult to manufacture.

Prerequisite for Acme Threaded Rod 101: Are Lead Screws Right For You?

Thread Types

The acme thread form, established over 100 years ago, replaced square thread screws, which had straight-sided flanks and were difficult to manufacture. 

There are Three Main Classes of Acme Thread Forms

    1. General Purpose (G)
    2. Centralizing (C)
    3. Stub Acme

The General Purpose and Centralizing thread forms have a nominal depth of thread of 0.50 × pitch and have a 29° included thread angle. Trapezoidal thread forms have a 30° included thread angle. Precision lead screw assemblies have a 40° angle. (Please refer to Figure 1)

Acme threaded rod forms

Compared to General Purpose Thread Forms

Centralizing threads are manufactured with tighter tolerances and reduced clearance on the major diameter. Stub acme threads follow the same basic design, but have a thread depth less than one half the pitch. 

If an acme nut is side loaded with a radial load, a “G” class will “wedge” when the nut thread flanks come in contact with the screw thread flanks. To prevent wedging, less clearance and tighter tolerances are allowed between the major diameter of the nut and the major diameter of the screw. 

CAUTION - Although a side load will not cause a centralizing thread to wedge, the nut is not designed to operate with a side load such as a pulley, drive belt, etc. 

Threaded Rod Terms

Land (Major) Diameter - The outside diameter of the screw.

Pitch Diameter - On an acme screw, this diameter is approximately halfway between the land diameter and the root diameter. It is the diameter at which the thread thickness is equal to the space between threads.

Root (Minor) Diameter - The diameter of the screw measured at the bottom of the thread.

Pitch - The axial distance between threads. Pitch is equal to the lead in a single start screw.

Lead - The axial distance the nut advances in one revolution of the screw. The lead is equal to the pitch times the number of starts.

PITCH x STARTS = LEAD

NOTE: Helix precision lead screw designations reference nominal diameter and lead. For example: 250×125 screw advances 0.125" in one revolution and requires eight turns for one inch of travel. A 250×125 screw has 2 starts and a 0.062” pitch.

0.062" PITCH x TWO STARTS = 0.125" LEAD

Lead Accuracy - Lead accuracy is the difference between the actual distance traveled versus the theoretical distance traveled based on lead. For example: A screw with a 0.5 inch lead and 0.004 inch per foot lead accuracy rotated 24 times theoretically moves the nut 12 inches. (24 Revolutions × .500 inches per revolution = 12.000 inches of travel) With a lead accuracy of .0003”/inch, actual travel could be from 11.996 to 12.004 inches. 

Screw Starts - The number of independent threads on the screw shaft; example one, two or four. (Please refer to Figure 2)

Acme threaded rod screw starts diagram

Matched Lead - When multiple screws are used to move a load with precise synchronicity, screws of similar lead accuracy can be factory selected and supplied as sets. Consult factory for matched lead set tolerances.

Straightness - Although precision lead screws are manufactured from straight, cylindrical material, internal stresses may cause the material to bend or yield. 

When ordering random lengths or cut material without end machining, straightening is recommended. Handling or machining of screws can also cause the material to bend or yield. Before, during and after machining, additional straightening is required. When ordering screws with machined ends the following straightness tolerances can be expected: 

Precision rolled and milled lead screws are straight within 0.010 inch/foot and will not exceed 0.030 inch in any 6-foot section, when shipped from the factory. Precision ground lead screws are straight within 0.001 inch/foot when shipped from the factory. If tighter straightness tolerances are required, contact Helix customer service.

Life - Precision lead screws are manufactured from high quality materials with excellent dynamic properties. Because of the variable effects of friction, lubrication and cleanliness, a specific life cannot be predicted. Proper lubrication, regular maintenance, and operation within specified limits will extend the life of lead screws. 

Backdriving - Normally, lead screws are used to convert rotary motion into linear motion. Backdriving is the result of the load pushing axially on the screw or nut to create rotary motion. Generally, a nut with efficiency greater than 50% will have a tendency to backdrive. If a selflocking assembly is required, select a nut with efficiency below 35%. 

CAUTION - Vibration can cause any lead screw assembly to creep or backdrive. When using lead screws, applications should be analyzed to determine the necessity of a brake, especially when the possibility of injury may occur.

Efficiency - Efficiency of precision lead screw assemblies range from 15% to 85%. These efficiencies are dependent upon acme threaded rod nut material, lubrication, lead and thread form. The efficiencies for each assembly are listed on the following pages. For more information on deciding which nut material to use download this FREE presentation:

Free PowerPoint How to Compare Acme Nuts Material

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