Screw Threads 101

[widgitmaster]

Screw Threads 101

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.

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. Ballscrews are 80% - 90% efficient.

BSF
British Standard Fine. A thread form based upon the British Standard Whitworth form but with a finer thread (more threads per inch for a given diameter). This thread form was first introduced in 1908, the thread form is specified in BS 84: 1956.

BSW
British Standard Whitworth. A thread form developed by Sir Joseph Whitworth in 1841. The thread form has rounded roots and crests, the thread form is specified in BS 84: 1956. This thread form was superceded by the Unified thread in 1948 and then the metric thread form.

Classes 1A and 1B:
For work of rough commercial quality where loose fit for spin-on-assembly is desirable.

Classes 2A and 2B:
The recognized standard for normal production of the great bulk of commercial bolts, nuts and screws.

Classes 3A and 3B:
Used where a closed fit between mating parts for high quality work is required.

Class 4:
A theoretical rather than practical class, now obsolete.

Class 5:
For a wrench fit. Used principally for studs and their mating tapped holes. A force fit requiring the application of high torque for semi-permanent assembly.

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€(tm)s column strength.

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€(tm)s critical speed. High speed drives should be evaluated and driven below 80% of the calculated, theoretical critical speed.

Cut Thread:
Threads are cut or chased; the unthreaded portion of shank will be equal to major diameter of thread.

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

Diametral:
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.

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 Ballscrew systems.

Friction:
The resistance to motion of a screw and nut. Power screws exhibit sliding friction as the screw and nut surfaces pass each other. Ballscrews 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(r) Screws:
A power screw series characterized by the use of multiple start threads to produce faster linear motion than Acme screws. Hi - lead(r) screws have a minimum of 2 starts and a lead not greater than the diameter of the screw.

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(r) and Torqspline(r) 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.

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.

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.

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.

NPT:
"National Pipe Thread." A specification for tapered pipe threads from ANSI (American National Standards Institute). Actually taken from American National Standard Taper Pipe Threads. When listed as NPT-F, indicates female ends. American Standard taper pipe thread conforming to requirements of Federal Standard H28, part II, section VII.

NPT Vs. NPTF Taper Pipe Threads

The two most common taper pipe threads used in the United States are NPT and NPTF. Applications range from electrical conduits and hand railings to high-pressure pipe lines that carry gas or caustic fluids. NPT threads are for mechanical or low-pressure air or fluid applications and require the use of sealing compounds like Teflon tape, to provide the seal. When the application is more critical, and the sealing compound may fail due to high heat or pressure, NPTF Dryseal threads are used. This mechanical seal is produced by the mating and slight crushing of the threads when a wrench is applied to tighten the fittings.

Visually, both threads appear to be identical. Both have a ¾” taper over one foot of length. Both have the same pitch diameter at the top of the hole of internal threads or end of the pipe on external threads, and both have the same thread lengths or depths. However, there is a subtle difference in the thread form that differentiates the two. The major and minor diameters of both threads differ slightly. With NPT threads, after a wrench is applied, slight spaces at the major and minor diameters may exist that would allow the assembly to leak and therefore a sealing compound is used to fill any gaps. On the other hand, NPTF threads are designed to ensure that sufficient crushing of the entire thread form will take place to produce a mechanical seal.

How does the difference in thread forms effect the tooling used to produce NPT and NPTF threads? Taps are available for both NPT and NPTF threads having the appropriate form to produce each type of thread. Since NPT threaded parts require sealing compounds, it is acceptable to use an NPTF tap for NPT applications. However, NPT taps cannot be used for NPTF applications, as it will likely produce a thread that will leak. The same is true of external threads. In most cases the tap drill is the same for both forms.

The most significant difference in the two threads is the inspection required. Since sealing compounds will be used for NPT threads, only a single plug with a step, known as an L1 plug (internal thread) or a single thin L1 ring (external) are required to check size. However, since the taper and the position of major and minor diameters are so critical to the sealing of NPTF threads, the additional threads in the assembly known as L2 and L3, and the major and minor diameters are inspected with either special plug or ring gages.

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 page 68 for more details about thread pitch. 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.

Property Class:
A designation system which defines the strength of a bolt or nut. For metric fasteners, property classes are designated by numbers where increasing numbers generally represent increasing tensile strengths. The designation symbol for bolts consists of two parts:
1. The first numeral of a two digit symbol or the first two numerals of a three digit symbol approximates 1/100 of the minimum tensile strength in MPa.
2. The last numeral approximates 1/10 of the ratio expressed as a percentage between minimum yield stress and minimum tensile stress.
Hence a fastener with a property class of 8.8 has a minimum tensile strength of 800 MPa and a yield stress of 0.8x800=640 MPa.
The designation system for metric nuts is a single or double digit symbol. The numerals approximate 1/100 of the minimum tensile strength in MPa. For example a nut of property class 8 has a minimum tensile strength of 800 MPa. A bolt or screw of a particular property class should be assembled with the equivalent or higher property class of nut to ensure that thread stripping does not occur.

Righthand Thread:
A screw thread that is screwed in by rotating clockwise. The majority of screw threads are right handed.

Rolled Thread:
A thread formed by plastically deforming a blank rather than by cutting. The majority of standard fasteners have their threads formed by rolling. Most threads are rolled before any heat treatment operation. Significant improvements in fatigue life can be achieved by rolling the thread after heat treatment, this improvement is due to compressive stresses being induced in the roots of the thread. However, because of the increased hardness of the bolt blank, the die life can be significantly reduced. Rolling the thread also generally improves the surface finish which can have a beneficial effect on fatigue life.

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".

Self-Locking:
A screw and nut system which 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.

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.

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

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.

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€(tm)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.

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

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.

UNC:
Unified National Coarse Thread. A standard kind of coarse straight thread used on fittings, nuts, and bolts. Not for pipes.

UNF:
Unified National Fine Thread. A standard kind of fine straight thread used on fittings, buts, and bolts. Not for pipes.
In November 1948 the Unified thread was agreed upon by the UK, the US and Canada to be used as the single standard for all countries using inch units. In 1965 the British Standards Institution issued a policy statement requesting that organisations should regard the BSW, BSF and BA threads as obsolescent. The first choice replacement for future designs was to be the ISO metric thread with the ISO inch (Unified) thread being the second choice.

Metric threads are designated by the letter M followed by the nominal major diameter of the thread and the pitch in millimeters. For example M10 x 1.0 indicates that the major diameter of the thread is 10mm and the pitch is 1.0mm. The absence of a pitch value indicates that a coarse thread is specified. For example stating that a thread is M10 indicates a coarse thread series is specified of diameter 10mm (giving the thread a pitch of 1.5mm).

The thread form for Unified and Metric threads are identical. If

p = pitch of the thread
d = depth of the thread
r = radius at the top and bottom of the threads

then:

d = 0.54127 p
r = 0.14434 p

Whitworth Thread:
A screw thread, also known as the British Standard Whitworth (B.S.W.), used principally in Great Britain.

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.

[chronon1]

damn , that was a lot ,, i feel like i'm back in school , at state school where i began studying in mechanical engineering technology, internal combustion engines ! (but then switched to electrical after 1 sem)

[NC Cams]

To figure out stretch of a screw/bolt, the old

e = (P*L)/(A*E) formula may be handy where

e = deflection, inch
P = load, lbs
L = Length under tension
A= cross sectional area, sq in
E = material modulus, as appropriate.

As is the case with most math, you can solve for any missing variable (take P for example) if you have the others and move things around as needed.

[widgitmaster]

Tap & Screw Thread Nomenclature

Allowance: The minimum clearance or maximum interference which is intended between mating parts.

Angle of thread: The angle included between the flanks of a thread measured in an axial plane.

Back of taper: A slight taper on threaded portion of the tap making the pitch diameter near the shank smaller than that at the chamfer.

Basic: The theoretical or nominal standard size from which all variations are made.

Chamfer: The tapered and relieved cutting teeth at the front end of the threaded section. Common types of chamfer are: Taper, 8 to 10 threads long; Plug, 3 to 5 threads and Bottoming, 1.5 threads.

Crest: The top surface joining the two sides or flanks of a thread.

Cutting face: The leading side of the land.

Flute: The longitudinal channels formed on a tap to create cutting edges on the thread profile.

Heel: The following side of the land.

Height of thread: In profile, distance between crest and bottom section of thread measured normal to the axis.

Hook face: A concave cutting face of the land. This may be varied for different materials and conditions.

Interrupted thread: Alternate teeth are removed in the thread helix on a tap having an odd number of flutes.

Land: One of the threaded sections between the flutes of a tap. Lead of thread: The distance a screw thread advances axially in one turn.

Major diameter: The largest diameter of the screw or nut on a straight screw head.

Minor diameter: The smallest diameter of the screw or nut on a straight screw head.

Neck: The reduced diameter; on some taps, between the threaded portion and the shank.

Pitch: The distance from a point on one thread to a corresponding point on the next thread, measured parallel to the axis.

Pitch diameter: On a straight screw thread, the diameter of an imaginary cylinder where the width of the thread and the width of the space between threads is equal.

Point diameter: The diameter at the leading end of the chamfered portion.

Radial: The straight face of a land, the plane of which passes through the axis of the tap.

Rake: The angle of the cutting face of the land in relation to an axial plane intersecting the cutting face at the major diameter.

Relief: The removal of metal behind the cutting edge to provide clearance between the part being threaded and a portion of the threaded land. Also, see back taper.

Chamfer Relief: The gradual decreasing land height from cutting edge to heel on the chamfered portion of the tap land to provide radial clearance for the cutting edge.

Con-Eccentric Relief: Radial relief in the thread form starting back of a concentric margin.

Eccentric Thread Relief: Radial relief in the thread form starting at the cutting edge and continuing to the heel.

Root: The bottom surface joining the flanks of two adjacent threads.

Side of flank of thread: The surface of the thread which connects the crest to the root.

Shank: The portion of the tap by which it is held and driven.

Spiral point: An oblique cutting edge ground into the lands to provide a shear cutting action on the first few threads.

Square: The squared end of the tap shank.

Thread: The helical formed tooth of the tap which produces the thread in a tapped hole.

Thread lead angle: The angle made by the helix of the thread at the pitch diameter; with a plane perpendicular to the axis.

Threads per inch: The number of threads in one inch of length.

Thread: SINGLE: A thread which is equal to pitch. DOUBLE: A thread in which lead is equal to twice the pitch. TRIPLE: A thread in which lead is equal to triple the pitch.

[tobyaxis]

Very in depth and practical for new members that want to learn. I can't tell you how many times I have told fellow co-workers that cranking down a cap screws too much only weakens it and destroys the threads. This is a good reference in this forum and it refreshed the mind on a few things that I haven't had to think about in a while. Not to sound picky but drawings would be good for members that don't have basic knowledge of fasteners. Other than that it's a perfect reference and should have a Quick Link in this Forum.

Nice work Eric! I always enjoy your Posts.

Would you like to write a reference post on Gears, Pulleys, and Sprockets now LOL

[smarbaga]

yes thank you very much.
this looks like cut and paist (a keeper )

[Mat-C]

"See page 68 for more details about thread pitch." ... are there going to be 67 more pages?

[spunky1974]

Now my eyes hurt from reading all that thanks the reference will be helpful....I got get some visine ouch...

[lgalla]

I have discounted Acme screws from the beginning.Now I feel sorry for acme.
Does anyone like Acme screws?
Are they better than R&P or ballscrews in some cases?I read somewhere ballscrews are ground with an Acme thread controled machine.
Does Acme require angular contact bearings?If you go "all out"Precision rod,angular bearings,bronze nut or preload plastic etc.Are you producing a viable linear device?If you go this route it is very pricy.Generally it looks like hobbyists use El-cheapo Acme components.
Friction.I understand acme has high friction.For a nube like me, so what,use a bigger stepper.I can imagine heat would slowdown the nut, requiring more torque from the motor.Not wanting to show my ignorance on CNC control I will ask anyways.If one axis heats up and slows down,does the controller know?Or is it possible now to lose steps?
Larry

[Mat-C]

I doubt heat would slow things down unless the expansion causes binding, but it might affect the lubricant. Main disadvantage of screws vs acme seems to be the lack of reverse braking, ie you can power a screw backwards, whereas acme will hold its position.

The heating and thus expansion of the screw will affect accuracy: for steel it's about 13 ppm/C. So 50 degrees C (~100F) throws you off by over half a millimetre per metre, and even just 5 degrees (~10F) loses you 50 microns (2 thou).

The controller usually won't "know" about thermal effects. You need to make the controller and software have sufficient margin to avoid lost steps (or monitor them with an encoder and "check" as you go). Thermal effects on screws can't really be measured (esp. as won't be uniform).

Best way to fix thermal issues like this is with a separate linear encoder to monitor absolute position.

[svenakela]

Great summary, I'll make this thread a sticky.

[widgitmaster]

Originally Posted by svenakela Great summary, I'll make this thread a sticky.

Thanks I think?
What's a "STICKY THREAD"
How do we use them?

Eric