Acme Screws

A screw thread series first developed in 1895 to replace square threads for use in positioning and power transmission. Acme screws are the most widely used power screws, popular in all industries for linear motion. See the Acme section for more information.

Backdrive

The action of converting thrust to torque, common to bearing screws and multiple start power screws. Technically, a screw will backdrive if its backdrive efficiency is greater than 0.

Backlash

The amount of free movement between a screw and nut. Axial backlash (or simply “lash”) is measured along the screw axis and radial or transverse backlash is measured transverse to the screw axis.

Ball Screw

A screw/nut drive system utilizing a series of bearing balls between the screw and nut to improve the screws mechanical efficiency and wear life. Ball screws are 80% – 90% efficient. See the Ball Screws section for more information.

Column Strength

Compressive loads on screws are limited by the screws column strength. The maximum compressive load that can be applied to a slender screw shaft without the screw shaft failing by elastic instability is the shaft’s column strength. See the Column Loading section in Application Engineering for more information.

Creep

The change in position of a screw and nut under load. Creep can occur because of classic mechanical creep in the screw under very high loads or because of vibration that changes the relative position of the screw and nut by backdriving. Screw systems that are theoretically self-locking may creep under vibration unless they are mechanically locked in place.

Critical Speed

Rotating screw shafts will develop natural frequencies of vibration depending upon their length, size and end mountings. The speed at which this vibration occurs is predictable and is called the shaft’s critical speed. High speed drives should be evaluated and driven below 80% of the calculated, theoretical critical speed. See the Critical Speed section in Application Engineering for more information.

Cycle

One complete working stroke of a screw and nut system, typically up and down or in and out, etc.

Diametral Pitch

A term relating to worm threads which indicates the pitch of the thread. Diametral Pitch is equal to p (3.1415926) divided by the actual pitch in inches. Popular but not standard diametral pitches for worm threads are 64, 48, 32, 24, 16, 12, 10, 8 and 6. Many other pitches are used depending upon the particular worm thread requirements.

Drive Torque Ratio

The drive torque ratio is the ratio of drive torque about the screw (or nut) axis per unit of axial thrust load on the nut (or screw). The drive torque ratio is used to size motors, belt drives and other power transmission components of a power screw drive system.

Duty Cycle (%)

The amount of time a screw or nut system is in motion expressed as a percentage of total time.

Efficiency (%)

The ratio of work output to work input expressed as a percentage. Efficiency varies greatly with different screw series from approximately 30% for Acme systems up to 90% for Ball screw systems.

Freewheeling

The ability of a screw to continue to rotate without producing linear motion at the ends of travel. See the Freewheeling Ball Screws for more information.

Friction

The resistance to motion of a screw and nut. Power screws exhibit sliding friction as the screw and nut surfaces pass each other. Ball screws exhibit rolling friction as the balls roll in the nut and screw thread grooves and as they rub against each other. Friction varies greatly depending upon lubrication, type of materials in contact, unit pressure, type of screw thread, lead angle, etc.

Hi – lead® Screws

A power screw series characterized by the use of multiple start threads to produce faster linear motion than Acme screws. Hi – lead® screws have a minimum of 2 starts and a lead not greater than the diameter of the screw. See the Hi – lead section for more information.

Key Torque

The amount of torque necessary to force a translating nut to move axially along its mating screw so called because nuts are often keyed inside their housings to prevent rotation. Key torque is proportional to the drive torque

Lead

The distance a screw thread advances in one revolution. The lead of a screw is equal to the pitch of the screw multiplied by the number of starts on the screw. For example, a .200 pitch screw with 5 starts would have a lead of 1.000 in./rev.

Lead Angle

The angle made by the helix of a screw thread with a plane perpendicular to the screw axis. Lead angles are not measurable, they are calculated based upon the lead of the thread and an arbitrary diameter – typically the pitch diameter.

Lead Error

The deviation from nominal lead resulting from manufacturing variations. Standard lead error tolerances for Ball Screws are .010 in. per foot. Lead errors for Acme, Hi – lead® and Torqspline® series leadscrews are .009 in. per foot with precision tolerances of .006 and .003 available in selected sizes. All tolerances apply plus and minus, however, thread rolled product errors are generally the same throughout the entire screw and can be easily compensated for with appropriate profiling software.

Linear Speed

See Speed below.

Major Diameter

On a screw thread, the major diameter is the diameter of a cylinder formed by the crests of the screw. On a nut thread, the major diameter is the diameter of a cylinder formed by the roots of the threads. It is common to specify threads beginning with their major diameter. See the Major Diameter entry in the Identifying Screw Threads section for more details.

Minor Diameter

On a nut thread, the minor diameter is the diameter of a cylinder formed by the crests of the nut threads. On a screw thread, the minor diameter is the diameter of a cylinder formed by the roots of the threads. Formerly, the minor diameter was called the “root diameter” and it still commonly referred to as root diameter. See the Minor Diameter entry in the Identifying Screw Threads section for more details.

Module

A system of gear tooth proportions based upon the metric system and popular in Europe and Asia. The Module system is similar to the Diametral Pitch gear system which is the dominant system in the USA. The units for Module are often not given but assumed to be millimeters. A 1mm Module worm is equivalent to a 25.4 Diametral Pitch worm with a pitch of .1237 inch. To obtain the pitch of a worm in inches, multiply the Module of the worm by Pi and divide by 25.4 mm/inch.

Pitch

The pitch of a screw thread is the distance from one thread groove to the next measured axially. Pitch is often misinterpreted as threads per inch. A thread with a pitch of .200 is commonly but mistakenly referred to as a “5 pitch thread”. To be correct, it should be referred to as 5 threads per inch (the reciprocal of 5 = .200). See Screw Thread Pitch entry in the Identifying Screw Threads for more details. Thread pitch is equal to the thread lead divided by the number of starts.

Pitch Diameter

The pitch diameter of a screw thread is the diameter at which the thread thickness and the thread space are equal. See the Pitch Diameter entry in the Identifying Screw Threads section for more details.

Root Diameter

A term referring to the minor diameter of a screw thread or the major diameter of a nut thread. Root diameter has been replaced by the more accurate terms “major diameter” and “minor diameter”. See discussion of major and minor diameters in the Identifying Screw Threads section.

Self-Locking

A screw and nut system that will not convert thrust to torque is self-locking. Self-locking screws will not backdrive. Single lead Acme screws are self-locking. All other power screws may not be self-locking. See Creep for related information.

Speed

The rate of travel of a screw and nut system measured as linear speed typically in inches per minute (ipm). The linear speed and the rotational speed of the screw or nut are related by the lead of the screw system. By multiplying the rotation speed (RPM) by the screw lead (In./rev.) the linear speed is obtained. For example, a screw rotated at 100 RPM with a lead of 2 inches/rev. will have a linear speed of 200 inches per min. (200 ipm).

Start(s)

A term which describes the number of independent threads on a screw (see Identifying Screw Threads).

Stroke

The linear movement of a screw and nut drive system typically measured in inches. Synonymous with Travel below.

Surface Speed

The speed of the screw surface relative to the nut surface in a screw and nut drive system usually expressed in surface feet per minute. The surface speed is much higher than the linear speed as a very small linear move requires a large increment of surface movement of the nut surface relative to the screw surface. See Surface Travel below.

Surface Travel

The total distance traveled in a screw and nut drive system by the nut surface relative to the screw surface. Surface travel is always much greater than the nut linear travel and is equal to the linear travel multiplied by (Pi times the Major Diameter) and divided by the cosine of the lead angle. An Acme screw 1 inch diameter and 5 threads to the inch will have a surface travel of 15.74 inch per inch of linear travel.

T-Nose Nut

The slang term for “threaded nose nut”, a nut style whereby the end of the nut is threaded with Unified threads for fastening to a mounting flange or the customer’s mating internally threaded mounting.

Tap

Cutting tool used to produce internal power transmission screw threads. Power transmission screw threads such as Acme threads require special taps and much more power than fastener thread taps.

Threads per Inch

The reciprocal of the pitch of a thread is the number of threads per inch. Threads per inch is often confused with pitch. Users frequently mis-state the pitch of a thread as the number of threads per inch. A reference to a “5 pitch thread” usually refers to 5 threads per inch or an actual pitch of .200 in. See the information on thread pitch in the Useful Formulas section.

Thrust

Thrust is the amount of linear output force produced by a given input torque in a screw and nut drive system. Also, the amount of linear input force applied to a screw or nut necessary to produce a given output torque in a screw and nut drive system that is not self-locking.

Torqspline® Lead Screws

A power screw series characterized by the use of multiple start threads to produce very fast linear motion. Torqspline® Lead Screws have a minimum of 4 starts and a lead greater than the diameter of the screw. See the Torqspline® Lead Screw section.

Torque

The amount of rotational force applied to a screw or nut necessary to produce linear thrust in a screw and nut system. Also the amount of angular force produced by a linear thrust in a screw and nut system that is not self-locking.

Travel

The linear movement of a screw and nut drive system typically measured in inches. Synonymous with Stroke above.

Worm Threads

A screw thread used with a mating wormgear (sometimes called a wormwheel), helical gear, spur gear or rack to reduce speed, increase torque. Worms are generally used on shafts mounted at 90°. Worm threads are not standardized but are produced in popular pitch series such as Diametral Pitch, Circular Pitch and Metric Module Pitch. See Diametral Pitch (Table 37) for related information.

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