How to Specify the Right Lead Screw Without Mismatching the Thread, Nut or Drive

Updated September 3, 2026

A trapezoidal threaded rod is easy to recognize and surprisingly easy to order incorrectly. Two rods can share the same nominal diameter yet differ in thread family, pitch, lead, number of starts, hand, fit, accuracy and compatible nut. Even a mechanically compatible screw and nut may be unsuitable once stroke, speed, support, backlash or load holding enters the design.

The useful unit of selection is therefore not the rod alone. It is the complete motion system: screw, nut, bearings, coupling, retention, supports, lubrication and the structure that keeps everything aligned.

This guide explains how to identify and specify that system. It does not provide universal load ratings or size recommendations. Loaded, long, fast, vertical and safety-critical mechanisms require verified manufacturer data and application-specific engineering checks.

What a trapezoidal threaded rod is—and what it does

A trapezoidal threaded rod is a cylindrical rod with an external thread whose profile is trapezoidal. Paired with a matching internal thread in a nut, it is commonly used as a lead screw or power screw to convert rotary motion into linear motion.

There are two basic arrangements:

• The screw rotates while the nut is prevented from rotating, causing the nut to travel along the screw.
• The nut rotates while the screw is prevented from rotating, causing the screw or its attached carriage to move axially.

The first arrangement is familiar in vises and many positioning axes. The second can solve different packaging or inertia problems. Neither is universally preferable.

Commercial vocabulary is less precise than engineering vocabulary. A supplier may call the same general object a threaded rod, threaded bar, spindle, lead screw or power screw. Those terms overlap, but the title alone does not establish the part’s thread standard, lead accuracy, straightness, duty rating or suitability for positioning.

Trapezoidal forms are associated with lathe lead screws, vises, jacks, machine tools, conveyors, valves and positioning mechanisms. These are typical applications, not evidence of the capacity of a particular rod. Diameter and an application label are not substitutes for product-specific load data.

A working drive normally includes:

• A mating nut
• Radial and thrust support
• A motor, handwheel or gearbox
• A coupling or other torque-transfer feature
• Collars, shoulders, locknuts or retaining rings
• Linear guides that resist carriage rotation and side load
• A lubrication and contamination-control plan

The thread engagement creates axial motion. It should not automatically be expected to locate the shaft radially, absorb all thrust, correct misalignment or replace proper linear guidance.

Retail categories make this distinction important. For example, McMaster-Carr’s trapezoidal-threaded-rod category includes filters for lead screws, nuts and collars as well as Acme, metric trapezoidal and unified-thread products. A displayed material, length or diameter may therefore describe an accessory rather than the rod itself.

Treat the product title as a starting point. The controlling information is the dimensioned drawing and documented specification for the complete screw-and-nut pair.

Metric trapezoidal, Acme and ordinary metric threads are not the same

“Trapezoidal” describes a broad profile family, not one universal thread. Metric trapezoidal and imperial Acme threads are related motion-thread forms, but their geometry and designation systems differ. Ordinary metric fastener threads are a separate V-thread family. Ball screws use another screw-and-nut geometry entirely.

System	Basic form	Typical designation	Main role	Compatibility rule
Metric trapezoidal	Commonly described as a 30-degree included profile; DIN 103 is a relevant geometry reference	Tr20×4, for example	Sliding lead or power screw	Match the metric trapezoidal standard, diameter, pitch, lead, starts, hand and fit
Imperial Acme	Standard Acme uses a 29-degree included profile and is associated with ASME/ANSI specifications	Inch diameter plus threads-per-inch or lead designation	Sliding lead or power screw	Use a matching Acme nut of the specified form and class
Ordinary ISO metric	60-degree V-profile	M10×1.5, for example	Primarily fastening	Do not pair with a trapezoidal nut
Ball screw	Matching helical grooves with recirculating balls	Manufacturer- and standard-specific	Low-friction linear motion	Use the specified ball nut and screw as a matched system

The 30-degree metric trapezoidal and 29-degree standard Acme descriptions are common commercial distinctions; the associated geometry and designation conventions are summarized in this overview of trapezoidal thread forms. Final procurement should still be checked against the governing standard and supplier drawing.

The one-degree angle difference is not the only issue. Dimensions, pitches, tolerances and designation conventions can also differ. Metric trapezoidal screws and Acme nuts should never be assumed interchangeable. Metric & Multistandard expressly warns against that substitution on its trapezoidal threaded-rod listing.

Ordinary metric threaded rod is different again. Its 60-degree profile is primarily intended for fastening. It can produce motion when turned through a nut, and that may be acceptable for an inexpensive, modest, noncritical mechanism. Pitch alone, however, cannot establish whether it will meet the required load, backlash, lead quality, repeatability or life.

Community experience can identify possibilities but cannot guarantee performance. An Engineering Stack Exchange discussion describes ordinary-threaded-rod mechanisms, one-direction positioning and opposed-nut preload. Those reports are anecdotal and configuration-specific; they do not convert commodity fastener rod into a specified precision lead screw.

Standards references also require careful reading. A seller may attach DIN 975 or DIN 976 to a fully threaded bar, but that reference alone does not establish DIN 103 motion-thread geometry, lead accuracy, straightness, fit or positioning quality. One Aspen Fasteners product listing, for example, describes an M30×6 trapezoidal bar under DIN 975 but supplies no dimensional tolerances, lead-accuracy specification or technical drawing in the cited page.

Compatibility rule: match all of the following, not merely nominal diameter:

1. Thread family and governing standard
2. Nominal diameter
3. Pitch
4. Lead
5. Number of starts
6. Right- or left-hand direction
7. Fit or clearance
8. Actual screw and nut profile

An 8 mm Acme-like screw, an M8 fastener rod and a Tr8 spindle are not interchangeable simply because each designation contains the number eight.

How to read diameter, pitch, lead, starts and thread direction

Thread labels become easier to compare once five terms are kept separate.

Thread terminology

• Nominal diameter: The named or reference diameter of the thread, not necessarily a complete measured outside-diameter specification.
• Pitch: The axial distance from one thread ridge to the corresponding point on the next ridge.
• Lead: The axial distance traveled in one complete revolution.
• Starts: The number of independent helical threads wrapped around the shaft.
• Hand: The direction in which the helix advances—right-hand or left-hand.

For a single-start screw:

Lead = pitch

For a multi-start screw:

Lead = pitch × number of starts

That relationship matters because two screws with the same pitch can produce very different travel per revolution.

Decoding standard-style designations

Under the common designation interpretation:

Tr60×9 identifies:

• A metric trapezoidal thread
• 60 mm nominal diameter
• 9 mm pitch
• A single start when no separate lead is stated
• 9 mm nominal travel per revolution

Tr60×18(P9)LH identifies:

• A metric trapezoidal thread
• 60 mm nominal diameter
• 18 mm lead
• 9 mm pitch
• Two starts, because 18 ÷ 9 = 2
• A left-hand thread

These examples follow the notation summarized in the linked trapezoidal-thread overview above. For purchasing, the supplier should confirm the interpretation on a dimensioned drawing rather than relying on shorthand alone.

Why “T8” is incomplete

Marketplace shorthand often compresses an 8 mm trapezoidal lead screw to “T8.” That does not define pitch, lead or starts. One commodity listing offers nominally 8 mm products in these combinations:

Marketplace description	Pitch	Lead	Implied starts
T8 variant	1 mm	1 mm	1
T8 variant	2 mm	2 mm	1
T8 variant	2 mm	4 mm	2
T8 variant	2 mm	8 mm	4

The 2 mm pitch / 8 mm lead version implies four starts and nominally advances 8 mm per revolution, not 2 mm. These options appeared on a generic T8 marketplace listing checked September 3, 2026; the page did not provide a governing standard, dimensional tolerances or a technical drawing.

Hand determines travel direction. For the same observed rotation, right-hand and left-hand screws move a constrained nut in opposite axial directions. Reverse-thread arrangements may combine left- and right-hand sections so that two nuts move toward or away from one another.

Lead describes nominal travel, not positioning quality. Dividing lead by motor steps gives a nominal commanded increment, but it does not account for:

• Backlash between screw and nut
• Lead error along the screw
• Runout or rod curvature
• Bearing clearance
• Coupling error or slip
• Misalignment
• Structural deflection
• Lost motor steps or control errors
• Thermal change and wear

Accuracy, repeatability, resolution and backlash should therefore be specified separately.

If one is missing, the designation is incomplete for compatibility purposes.

A practical selection workflow for scratch-built machines

Selecting diameter first is tempting because diameter is prominent in catalogs. It is usually better to define the job first and then use manufacturer calculations and product data to identify candidate sizes.

1. Define the motion and load

Write down:

• Maximum and normal axial load
• Whether the load pulls, pushes or reverses
• Required stroke
• Desired linear travel per revolution
• Required linear speed
• Duty cycle and expected starts and stops
• Horizontal, vertical or inclined installation
• Available motor torque and speed
• Ambient temperature, moisture, dust, chips and chemicals
• Required positioning accuracy
• Required repeatability
• Allowable backlash
• Whether the axis must hold its load with power removed

Do not hide unknowns behind phrases such as “medium duty” or “fairly accurate.” Put a number, operating range, defined case or explicit “to be determined” beside each requirement.

2. Choose lead around the motion objective

A larger lead produces more travel per revolution. As a general system tendency, that favors linear speed at a given rotational speed but provides less mechanical advantage and can make back-driving or power-off holding more important.

A smaller lead produces less travel per revolution and generally provides finer nominal increments and more mechanical advantage, but it requires more shaft revolutions to cover the same stroke.

This is a system trade-off, not a rule that the finest pitch is best. Motor speed, available torque, cycle time, friction and screw geometry must be evaluated together using data for the selected assembly.

3. Separate the positioning requirements

Use distinct entries for:

• Resolution: The smallest nominal command increment
• Accuracy: Closeness to the commanded or true position
• Repeatability: Ability to return to the same position
• Backlash: Lost axial motion when direction reverses

A drive can have tiny nominal motor increments and still position poorly. Conversely, a modest mechanism may repeat acceptably when approached from one direction while remaining inaccurate over its full travel.

4. Identify the governing structural and speed checks

A screw loaded in compression requires an application-specific buckling evaluation. The supplied catalog evidence does not establish a calculation method or safe limit, so obtain a manufacturer calculation, an authoritative engineering analysis or a qualified review for the actual installation.

A long rotating screw also requires a critical-speed or resonance evaluation. Manufacturer guidance states that usable length depends on factors including diameter, speed, load and installation position, and warns that longer screws can encounter resonance at higher rotational speeds (igus lead-screw overview).

These checks are separate from thread strength and nut capacity. A thread may appear adequate while the overall shaft and support arrangement remain unsuitable.

5. Treat load holding as a deliberate function

Self-locking is not guaranteed by the word “trapezoidal.” Holding and back-driving behavior depend on the selected geometry and the friction present in the actual screw-and-nut assembly. Nut material, surface condition, lubrication, wear, vibration and load can affect that behavior.

For a vertical lift or any axis whose uncontrolled movement could cause injury or damage, obtain verified load-holding information for the selected assembly. Use an appropriate brake, counterbalance, mechanical prop or secondary restraint where the risk assessment requires one. Manufacturer comparisons likewise warn that low-friction ball screws may back-drive and may require a brake to hold a load (Duff-Norton comparison).

Do not make safety depend on an unverified friction condition.

Selection worksheet

Requirement	Project value
Normal and maximum axial load
Load direction: tension/compression/reversing
Stroke
Desired travel per revolution
Linear speed
Duty cycle
Installation orientation
Environment and temperature
Accuracy requirement
Repeatability requirement
Allowable backlash
Unsupported screw length
End-support arrangement
Available drive torque and speed
Power-off load-holding requirement
Brake or secondary restraint
Required documentation or certification

This process exposes the right questions, but it cannot produce a defensible final diameter, load rating, drive torque, buckling limit or critical speed without geometry, boundary conditions and product-specific data.

Choose the nut, materials and backlash strategy as part of the system

The nut is not a generic accessory. Its geometry, engagement, material and clearance often determine friction, backlash, wear, noise and lubrication requirements.

Supplier catalogs list steel, stainless-steel and aluminum screws. Nut and accessory categories include brass, bronze, iron, steel and several polymers. Because some retailer pages mix rods, nuts, bearings and accessories, a material shown in a filter should not be assumed available for every screw diameter, pitch, hand or nut style.

Nut arrangements

• Standard-clearance nut: Provides deliberate running clearance. It is simple and tolerant of some variation, but it permits backlash.
• Split or adjustable nut: Uses a divided body or adjustment to remove some axial clearance.
• Reduced-clearance nut: Is manufactured or selected for less play than a standard version.
• Preloaded or anti-backlash nut: Uses opposing contact, often through two nut elements or an elastic preload, to maintain contact during reversals.

Reducing clearance can improve reversal behavior, but it is not free. More preload or tighter fit may increase friction, required drive torque, heat, wear, adjustment needs and sensitivity to misalignment or contamination. The community discussion cited earlier also notes increased friction when opposed nuts are preloaded.

Anti-backlash hardware does not correct screw lead error, a bent rod, inadequate bearing support or a flexible structure.

For a budget positioning mechanism, approaching every target from one direction can keep the same thread flanks in contact and work around reversal clearance. This may improve repeatability in a particular arrangement, but it neither removes the physical clearance nor corrects lead error.

Material pairing and lubrication

Polymer nuts may suit systems designed for lower noise or manufacturer-approved dry running. Dry operation belongs to a matched screw-and-nut design; it is not an inherent property of trapezoidal rods. Igus, for example, bases its lubrication-free claims on systems using self-lubricating plastic nuts and offers reduced-clearance variants within that product family, not as a universal rule for any rod-and-nut combination.

Metal nuts are common in loaded mechanisms, but “metal” is not a complete specification. Bronze, brass, steel and iron options involve different friction, wear, corrosion and lubrication considerations. No one material is categorically best for every load and environment.

Confirm all of the following with the supplier:

• Nut engagement length
• Fit, running clearance or preload
• Screw and nut material compatibility
• Approved lubricant and application method
• Whether dry operation is permitted
• Contamination and corrosion resistance
• Operating temperature range
• Wear-adjustment method, if any
• Replacement and inspection criteria

Screw-and-nut compatibility checklist

Both components must match in:

• [ ] Thread family and standard
• [ ] Nominal diameter
• [ ] Pitch
• [ ] Lead
• [ ] Number of starts
• [ ] Hand
• [ ] Profile geometry
• [ ] Fit or clearance requirement

A nut labeled merely “T8,” “8 mm,” “Acme” or “trapezoidal” has not supplied enough information.

Plan the bearings, coupling, alignment and maintenance

A lead-screw mechanism needs controlled support. The nut transmits axial force between screw and carriage, but it should not automatically be asked to serve as the sole radial bearing, thrust bearing and linear guide.

Common assembly elements include:

• Radial bearings to locate the rotating shaft
• Thrust or angular-contact bearings to react axial load
• A fixed and floating support arrangement where appropriate
• Couplings between motor and screw
• Collars, shoulders, locknuts or retaining rings
• Machined bearing seats and coupling journals
• Separate linear guides to resist carriage rotation and external side load

Decide early whether the screw or nut rotates. That choice changes bearing placement, rotating inertia, end machining, motor mounting and critical-speed concerns. Packaging and validated calculations should drive the decision; there is no universal architecture.

Alignment and end machining

The motor shaft, coupling, screw, nut and linear guides must share the intended axis closely enough for the chosen components. Misalignment can create tight spots, cyclic torque variation and unintended side loads. A flexible coupling may accommodate limited error, but it should not conceal a poor bearing or guide layout.

Specify end details before cutting the rod:

• Bearing-seat diameters and lengths
• Shoulders and fillets
• Threads for retaining nuts
• Snap-ring or circlip grooves
• Coupling journal and key, flat or clamp area
• Overall and threaded lengths
• Which end controls axial location
• Surface-finish and runout requirements at bearing seats

Cutting first and designing supports afterward can leave too little material for a proper shoulder, bearing fit or retention feature.

Lubrication and service

For metal-on-metal service, use the lubricant and maintenance procedure specified for the selected screw-and-nut assembly. Compatibility with the nut material, temperature, contamination and surrounding seals matters.

Do not confuse machining lubricant with operating lubricant. Cutting oil used while turning, tapping or machining a journal answers a tooling question. The grease or oil used after assembly answers an operating and maintenance question.

An official SawStop cabinet-saw service procedure provides an application-specific example. It instructs the technician to support the elevation assembly before removing the rod, inspect and grease reusable bearings, orient the bearing races correctly, tighten the locknut without causing bearing bind, and test the mechanism through its full travel. Those instructions apply to that machine, not to every lead-screw installation, but they illustrate why support, cleanliness, component orientation and final testing matter.

Installation checklist

• [ ] Inspect the screw threads, nut and bearings for damage
• [ ] Clean the components and remove chips, grit and old debris
• [ ] Verify bearing and race orientation
• [ ] Confirm that shoulders, spacers, collars and retainers are present
• [ ] Align the motor, coupling, screw, nut and linear guides
• [ ] Set manufacturer-specified bearing retention or preload without binding
• [ ] Rotate the mechanism by hand before applying power
• [ ] Test the complete stroke slowly
• [ ] Check retention hardware again
• [ ] Recheck alignment under a representative load
• [ ] Apply only the specified operating lubricant

Troubleshooting prompts

Backlash: Is the play in the nut, thrust bearings, coupling, collars or structure?

Tight spots: Does resistance repeat once per screw revolution, or does it occur at one carriage position? The pattern can help separate screw runout from guide alignment or localized thread damage.

Noise: Is it generated by dry sliding, a bearing, coupling misalignment, contamination or resonance?

Wear debris or galling: Are the materials, lubricant and alignment appropriate? Inspect the assembly and consult the component limits before continued operation damages both screw and nut.

Resonance: Does vibration appear only above a particular rotational speed or at a particular extension?

Inconsistent positioning: Separate reversal error from lead error, missed drive motion, coupling slip, bearing movement and structural compliance.

Do not invent a tolerance or replacement interval to make diagnosis look precise. Use the manufacturer’s limits and inspect against the application’s actual positioning and safety requirements.

When to use ordinary threaded rod, a trapezoidal screw or a ball screw

Each system converts rotation into linear motion, but it does so with different contact geometry and design priorities. The comparisons below are general tendencies, not product-level performance guarantees.

Option	Intended use	Cost tendency	Friction and backlash	Speed and duty tendency	Holding behavior	Specification availability
Ordinary metric threaded rod	Fastening; occasional budget motion experiments	Usually lowest	Sliding contact; nut clearance and lead quality may be poorly controlled	Best limited to modest, noncritical use unless validated	Must be tested; do not assume load holding	Fastener specifications may be available, but motion data often are not
Metric trapezoidal lead screw	Purpose-made sliding motion and power transmission	Moderate	Sliding friction; standard, split and reduced-clearance nuts may be available	Broad manual and machine-drive use; validate higher speed and duty	May resist back-driving under some conditions, but not universally	Often sold by diameter, pitch, hand and material; accuracy data varies
Acme screw	Imperial sliding motion and power transmission	Moderate	Similar system-level trade-offs to metric trapezoidal, with different geometry and standards	Common in vises, lathes, actuators and loaded motion	Often considered where resistance to back-driving matters, but must be verified	Multiple leads, starts, hands and fits are commercially available
Ball screw	Efficient rolling-contact linear motion	Usually higher	Low friction; preload strategies can manage backlash	Common where higher speed, duty or powered-motion efficiency matters	More susceptible to back-driving; load holding may require a brake	Usually supplied with detailed system-specific selection data

Ordinary threaded rod can be a rational prototype choice when motion is slow, loads are modest, consequences of failure are minor and imperfect backlash or lead quality is acceptable. It is not equivalent to a specified lead screw. A community report of good repeatability in one carefully arranged mechanism remains anecdotal and cannot predict another build.

Metric trapezoidal and Acme systems are widely used sliding-screw arrangements for vises, lathes and other loaded mechanisms. Manufacturers may describe them as less expensive, quieter or more resistant to back-driving than ball screws. These are family-level tendencies that depend on lead, material, lubrication, fit and operating conditions.

Ball screws replace sliding contact with recirculating rolling elements. They are broadly favored where lower friction, higher speed or higher duty matters. That low friction can make back-driving and power-off holding more important, particularly on vertical axes. Duff-Norton’s manufacturer comparison frames the choice around holding, positioning, force, speed, duty cycle, life and cost; its broad performance comparisons should not be treated as guaranteed values for an untested assembly.

Choose the system according to verified load, speed, duty cycle, backlash, accuracy, holding, maintenance, packaging and budget requirements—not because one type is marketed as “precision,” “self-locking” or “heavy duty.”

Sizes, sourcing paths and the RFQ checklist

There are three practical sourcing routes. They serve different levels of cost, documentation and customization.

The supplier examples below were checked September 3, 2026. They are catalog snapshots, not promises of continuing price, stock or availability.

1. Commodity T8 screw-and-nut sets

These are common in small printers, positioning devices and hobby mechanisms. Listings often provide many lengths, public prices and an included brass or polymer nut. They may omit:

• Rod material and grade
• Governing thread standard
• Thread tolerance or fit
• Lead accuracy
• Straightness and runout
• Backlash
• Load capacity
• Surface finish
• Inspection documentation

That can be acceptable for an experiment if the buyer is prepared to measure, test and replace components. It is a weak basis for a loaded or safety-related design.

2. Stocked specialist metric products

Specialist suppliers offer standard-style size-and-pitch combinations, matching nuts and support accessories. Examples show advertised range rather than recommended sizes:

• Motedis lists Tr8×1.5, Tr10×2, Tr12×3 and Tr16×4, with displayed length filters of 500, 1,000 and 1,980 mm in its trapezoidal spindle and accessory category.
• Metric & Multistandard displays supplier-designated steel combinations from M10×3 through M50×8, with RFQ-only pricing, on its trapezoidal-rod page. Because the displayed notation uses M rather than a complete Tr designation, buyers should obtain written confirmation of profile, standard, lead, starts, hand and compatible nut.
• Igus advertises trapezoidal products from 6 to 50 mm outside diameter and pitches from 1.5 to 8 mm, including right-hand, left-hand, reverse, single-start and multi-start options on its threaded-rod range page.

These ranges show what suppliers advertised when checked, not that every size, material, hand and length combination was stocked. Confirm the individual product drawing and availability.

3. Custom rolled or machined bars

Keystone advertises 3, 6 and 12 ft stock lengths and custom lengths up to 50 ft, as well as multiple materials, hands, starts and fit options on its custom trapezoidal threaded-bar page.

A capabilities page does not confirm that every option can be combined. A formal quotation and approved drawing should define what is actually offered.

Prices, inventory and shipping estimates change. Category filters can also combine screws, nuts, bearings, collars and unrelated thread forms. Record a supplier part number and revision-controlled drawing rather than relying on a screenshot or search title.

Complete RFQ template

A useful request for quotation can read:

Please quote a complete compatible trapezoidal screw-and-nut assembly with the following requirements:

• Governing thread standard and profile:
• Nominal diameter:
• Pitch:
• Lead:
• Number of starts:
• Thread hand:
• Screw material, grade and condition:
• Nut type, material and configuration:
• Nut engagement length:
• Overall screw length:
• Threaded length:
• End-machining details:
• Thread fit, tolerance or clearance:
• Lead-accuracy requirement and measurement basis:
• Straightness requirement:
• Runout requirement and datum:
• Thread and journal surface finish:
• Backlash or preload requirement:
• Approved lubricant or dry-running condition:
• Operating axial load and direction:
• Operating and maximum rotational speed:
• Duty cycle:
• Installation orientation:
• Unsupported length and end supports:
• Environment and temperature:
• Quantity:
• Required technical drawing:
• Required inspection report:
• Required material certification:
• Required load, buckling, critical-speed and load-holding data:

Please confirm in writing that the quoted screw and nut are mutually compatible. Identify whether each quoted dimension applies to the screw, nut, collar, bearing or complete assembly.

Review the response for omissions. “Standard trapezoidal,” “T8,” “M20×4” or “DIN threaded rod” may still leave lead, starts, hand, fit or motion quality unresolved.

Do not approve an industrial or safety-critical order if the supplier cannot provide the drawings, tolerances, load data or calculation inputs needed to verify the application.

Frequently asked questions

Are trapezoidal and Acme threaded rods interchangeable?

No—not by default. Metric trapezoidal threads are commonly described as having a 30-degree included profile, while standard imperial Acme uses 29 degrees. Their dimensions, designations and governing standards also differ. Match the thread family, standard, diameter, pitch or threads per inch, lead, starts, hand, fit and nut geometry.

What is the difference between pitch and lead on a trapezoidal screw?

Pitch is the axial spacing between adjacent thread forms. Lead is the axial travel produced by one revolution.

On a single-start screw, lead equals pitch. On a multi-start screw, lead equals pitch multiplied by the number of starts. A screw with a 2 mm pitch and four starts therefore has an 8 mm lead and nominally travels 8 mm per revolution.

Can ordinary metric threaded rod be used as a lead screw?

Sometimes. It may be adequate for a slow, inexpensive, noncritical mechanism if its load, backlash, lead quality, supports and repeatability are acceptable.

Ordinary metric rod has a 60-degree fastener thread and commonly lacks the motion-performance data supplied with a purpose-made lead screw. Test results or community anecdotes from one arrangement should not be generalized to another.

Are trapezoidal threaded rods always self-locking?

No. Self-locking is a system condition influenced by screw geometry, friction, nut material, lubrication, wear, vibration and load. Multi-start and higher-lead designs are especially poor candidates for assumptions based only on thread shape.

For a vertical or safety-critical mechanism, obtain verified load-holding data and provide an appropriate brake or secondary restraint where required.

What specifications are needed to order a compatible rod and nut?

At minimum, state the governing thread family or standard, nominal diameter, pitch, lead, number of starts, hand, fit and matching profile.

Also specify screw and nut materials, nut engagement, length, end machining, lead accuracy, straightness, runout, backlash, surface finish, lubrication, operating conditions and required documentation.

Never order a trapezoidal threaded rod by diameter or shorthand alone. Confirm the governing thread form, diameter, pitch, lead, starts, hand, nut, materials, length, end machining, fit, accuracy, straightness, lubrication, supports and documentation as one system.

For any loaded, long, fast, vertical or safety-critical mechanism, obtain manufacturer data and complete application-specific torque, buckling, critical-speed, load-holding and guarding checks before building.