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: General Purpose (G), Centralizing (C), and Stub Acme. The General Purpose and Centralizing thread form have a nominal depth of thread of 0.50 x pitch and have a 29° included thread angle resulting in a trapezoidal tooth shape (some sizes have 40°). Metric trapezoidal thread forms have a 30° included thread angle.

When compared to general-purpose thread forms, centralizing threads are manufactured with tighter tolerances and limited clearance on the major diameter. When an acme nut is side loaded, a “G” class acme nut will “wedge” when both of 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. See “Load Definition” section for further information. (SEE FIG. 1) Stub Acme threads follow the same basic design, but have a thread depth less than one half the pitch.

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 tooth thickness is equal to the space between teeth.

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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.

LEAD = PITCH x STARTS

NOTE: Nook Industries acme screw designations reference major diameter and effective turns per inch. For example: 1/4" – 4 RH requires four turns for one inch of travel. A 1/4" – 4 RH has two starts and a 0.125" pitch. 0.125" pitch X two starts = 0.250" lead.

SCREW STARTS
The number of independent threads on the screw shaft; example one, two or four. (SEE FIG. 2)

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 x .500 inches per revolution = 12.000 inches of travel) With a Lead accuracy of 0.004 inch per foot, actual travel could be from 11.996 to 12.004 inches.

Refer to the listings in the design guide for the lead accuracy of a particular screw.

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STRAIGHTNESS
Although PowerAc™ Acme 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 from Nook Industries, the following straightness tolerances can be expected:

PowerAc™ Rolled and Milled Acme 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.

PowerAc™ Ground Acme Screws are straight within 0.001 inch/foot when shipped from the factory. If tighter straightness tolerances are required, contact Nook Industries customer service.

LIFE
PowerAc™ Acme 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 PowerAc™ Acme Screws.

EFFICIENCY
Efficiency of PowerAc™ Acme Screw assemblies range from 15% to 85%. These efficiencies are dependent upon nut material, lubrication, lead and thread form. The efficiencies for each assembly are listed with the screw and nut data.

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BACKDRIVING
Normally, acme 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 acme 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.

BACKLASH
Backlash (lash) is the relative axial movement between a screw and nut without rotation of the screw or nut. Backlash information for PowerAc™ Acme Screws and Nuts is listed with the screw data.

Lash will always increase with use. Nook Industries has developed several unique ways to reduce or remove the lash between the screw and nut. The PowerAc™ TAB™ Torsional Anti-Backlash Nuts are used in applications up to 5/8 inch.

For screw diameters over 5/8 inch, PowerAc™ No-Lash™ Flanges are available. The PowerAc™ No-Lash™ Flange is identical to a standard flange and includes slotted mounting holes. The backlash can be removed by using a second nut with a PowerAc™ No-Lash™ Flange in combination with a standard nut and flange. By rotating the slotted PowerAc™ No-Lash™ Flange and nut relative to each other, the thread in the second nut advances until the lash is reduced.

As the nuts wear and backlash increases, loosen the mounting bolts and readjust the PowerAc™ No-Lash™ Flange and nut until the lash is minimized. (SEE FIG. 3) For a complete PowerAc™ No-Lash™ Flange assembly order 2 standard nuts, 1 standard flange and 1 No-Lash™ Flange. For example a 3/4"-2 assembly requires the following:

2 – 20072 Standard Nuts
1 – 70262 Standard Flange
1 – 73262 No-Lash™ Flange

CAUTION - When the uncompensated lash is equal to or greater than 1/4 times the pitch, the assembly should be replaced.

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DYNAMIC LOAD
The maximum recommended thrust load which can be applied to the PowerAc™ Acme screw and nut assembly while in motion.

PV LOAD
Any material which carries a sliding load is limited by heat buildup. The factors that affect heat generation rate in an application are the pressure on the nut in pounds per square inch and the surface velocity in feet per minute. The product of these factors provides a measure of the severity of an application.

TENSION LOAD
A load that tends to “stretch” the screw. (SEE FIG. 4)

COMPRESSION LOAD
A load that tends to “squeeze” the screw. (SEE FIG. 4)

THRUST LOAD
A load parallel to and concentric with the axis of the screw. (SEE FIG. 5)

OVERTURNING LOAD
A load that tends to rotate the nut around the longitudinal axis of the screw. (SEE FIG. 5)

SIDE LOAD
A load that is applied radially to the nut. (SEE FIG. 5)

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MOUNTING AND PINNING OF ACME NUT FLANGE
Flanges must be attached to acme nuts. The preferred method of locking a flange to a nut is a pin or set screw parallel to the screw which intersects the flange/nut mounting thread. Because of the dissimilarity of materials, the hole may need to be milled, not drilled.

Alternatively, the flange may be drilled and tapped radially for a set screw. After assembly of the flange to the nut, spot drill the nut threads through the flange and install a dog point set screw from the flange O.D. into the nut O.D. threads. Avoid getting metal chips in the nut when drilling. (SEE FIG. 6 and 7 for pin size)

Commercially available thread adhesives may be used for light load applications. Follow the manufacturers recommendations to ensure a satisfactory bond. Avoid getting the adhesive onto the screw thread.

LUBRICATION
Proper lubrication must be provided to achieve satisfactory service life. Nook PowerAc™ lubricant (E-100 spray lube or PAG-1 grease) is recommended for applications using PowerAc™ nuts.

NOTE: PowerAc™ PLAST/AC™ nuts are considered self-lubricating and may be operated without lubrication. To maximize the life of PLAST/AC™ nuts an initial lubrication is recommended.

Lubrication intervals are determined by the application. It is required that screw assemblies are lubricated often enough to maintain a film of lubricant on the screw. See the Acme Lubrication page for more details.

DRIVING TORQUE
Driving torque is the torque required to move a load and is calculated by multiplying the force needed by the “Torque to raise one lb.” value listed in the technical data section for each screw and nut size.

EXAMPLE: To lift a 1,000 lb. load using a 1"- 6 RH acme screw, 74 in. lbs of torque are required. 074in.-lb/lb. x 1000 lb. = 74 in.-lb.

TEMPERATURE
With proper lubrication, PowerAc™ Acme Screws with BRONZ/AC™ nuts operate efficiently between 15°F and 350°F, and PLAST/AC™ nuts between 15°F and 175°F. Consult the factory for low temperature applications.

END MACHINING
To obtain optimum performance of your acme screw assembly, it is recommended that the machining be performed at the Nook Industries factory. Screws may be purchased machined to your specifications or to standard end machining designs.

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EZZE-MOUNT™
Lead screws in operation generate an axial load and a radial load; therefore, end mounts must be designed to accommodate these loads. Nook Industries has designed precision end mounts to work specifically with lead screws. For a detailed description of these bearing supports, see the EZZE-MOUNT™ section.

An EZZE-MOUNT™ can be shipped pre-assembled to a PowerAc™ Acme Screw. Refer to the PowerAc™ Acme Screw Assemblies for a complete assembly.

OPTIONAL SURFACE COATINGS
PowerAc™ Acme Screws are available with optional corrosion resistant and/or lubricated finishes like Nickel, Teflon, or Hard Chrome; consult Nook Industries for detailed specifications.

BOOTS AND BELLOWS
For contaminated environments, use of a boot or metal cover to protect the acme screw assembly is recommended.

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.

LOAD
The loads that need to be considered are the static loads, dynamic loads, reaction forces and any external forces affecting the screw. See Load definitions section above for details.

SPEED
The travel rate (linear speed) is the rpm at which the screw or nut is rotating multiplied by the lead of the screw.

LENGTH
The unsupported length of the screw.

END FIXITY
End fixity refers to the method by which the ends of the screw are supported. The degree of end fixity is related to the amount of restraint of the ends of the screw. Examples of the three basic types of end fixity are: