Lead Screw

What is a Lead Screw

A lead screw, also called a power screw or translation screw, is a threaded shaft paired with a nut that converts the rotation of the shaft into linear travel of the nut, or the rotation of the nut into linear travel of the shaft. The defining characteristic is sliding contact: the load is carried directly on the thread flanks, which rub against the nut as the assembly moves. This is the mechanical opposite of a ball screw, where a closed circuit of steel balls rolls between the screw and nut and removes almost all of the sliding friction. That single difference, sliding versus rolling, drives every downstream property of efficiency, life, cost, noise, and self-locking behaviour.

Two terms describe how far the nut moves per turn. Pitch is the axial distance between adjacent thread crests. Lead is the axial distance the nut travels in one full revolution of the screw. For a single-start thread, lead equals pitch. For a multi-start thread, lead equals pitch multiplied by the number of starts, so a two-start screw advances twice as fast per turn as a single-start screw of the same pitch. Multi-start screws raise the lead angle, which increases speed and efficiency but removes the self-locking property. Choosing the lead is one of the first selection decisions because it sets the trade-off between travel speed, resolution, and required input torque.

The power screw is one of the oldest machine elements, traced to the water-lifting screw attributed to Archimedes and to screw presses used since antiquity. The trapezoidal and Acme thread forms were standardized in the industrial era to replace the older square thread, which is harder to cut and to mate cleanly. The Acme form, with its 29 degree flank angle, became the North American standard under ASME B1.5, while the 30 degree metric trapezoidal form was codified in the ISO 2901 to 2904 series and the German DIN 103 standard. These flank angles make the threads easier to manufacture and to engage than a square thread while keeping good axial load capacity.

In application scale, lead screws span from sub-millimetre micro-actuators in medical and optical instruments up to large screw jacks lifting many tonnes in stage machinery, dam gates, and industrial lifting tables. Diameters in common catalog stock run from roughly 6 mm (about 1/4 inch) to 80 mm or more, with Thomson BSA, for example, offering 300-series stainless screws from 3/16 inch up to 3 inch diameter. Across this entire range the same physics applies: a sliding helical wedge trades input torque for output thrust, and the geometry of that wedge fixes the efficiency.

Four engineering properties separate a good lead screw selection from a poor one: thread form and lead (which set speed and self-locking), nut material (which sets load capacity and lubrication needs), lead accuracy and backlash (which set positioning precision), and the speed and column limits (which constrain how long and how fast the screw can run). The chapters below address each in turn so that a requirement can be mapped to a specific screw, nut, and accuracy grade.

Thread Types and Standards

Lead screw threads divide first by profile and then by measurement system. The two dominant power-thread profiles are Acme (inch, 29 degree flank angle, ASME B1.5) and trapezoidal (metric, 30 degree flank angle, ISO 2901 to 2904 and DIN 103). Older square threads and the V-thread variant also exist, but the trapezoidal family dominates industrial practice because it balances manufacturability, strength, and ease of nut engagement. The table below summarizes the main thread standards a buyer will encounter.

Acme threads are the North American standard for power screws. ASME B1.5 defines a 29 degree included flank angle and provides two applications: General Purpose, which has clearance on all diameters for free running, in classes 2G, 3G, and 4G; and Centralizing, which controls the major diameter to keep the screw and nut concentric for higher precision. The shallower Stub Acme form, covered by ASME B1.8, uses the same 29 degree angle but a reduced thread depth for cases where a full-depth thread is unnecessary or where the root must stay stronger. Acme sizes are written as nominal diameter and threads per inch, such as 1/2-10 Acme.

Trapezoidal threads are the metric counterpart. The ISO 2901 standard fixes the basic profile, ISO 2902 the general plan of sizes, ISO 2903 the tolerances, and ISO 2904 the basic dimensions; DIN 103 provides the German nominal-dimension tables that European drawings most often cite. The included flank angle is 30 degrees rather than 29, and threads are dimensioned in millimetres with the designation Tr followed by nominal diameter and lead, for example Tr 40x7 for a 40 mm diameter, 7 mm lead screw. The one degree flank difference means Acme and trapezoidal parts are not interchangeable, even at similar nominal sizes.

Single-start versus multi-start is an orthogonal choice that applies to both Acme and trapezoidal forms. A single-start thread advances one pitch per revolution and tends to be self-locking. A multi-start thread (two, three, or four starts) advances by lead equal to pitch times the number of starts, giving fast traverse and higher efficiency, but it raises the lead angle past the self-locking threshold so the axis will backdrive under load. Designers reach for multi-start, high-helix screws when speed matters more than holding position, for example in pick-and-place and fast Z-axes, and pair them with a motor brake if the axis must hold.

Manufacturing method also belongs in the thread discussion because it sets accuracy and cost. Rolled threads are formed by pressing the blank between dies, which is fast and economical and work-hardens the surface, but holds looser lead tolerance. Cut or milled threads are more accurate, and ground threads are the most accurate and most expensive, used for precision positioning. Nook Industries, for example, produces precision Acme screws by rolling, milling, or grinding depending on the accuracy grade required.

Nut Technologies and Materials

The nut, not the screw, usually determines load capacity, life, lubrication strategy, and backlash. A lead screw nut runs in continuous sliding contact, so its material must balance load capacity, friction, wear rate, and operating temperature. The three mainstream nut families are bronze, plastic (acetal and internally lubricated polymers), and steel, plus specialty self-lubricating composites. The table below compares the key engineering trade-offs.

Bronze nuts are the traditional choice for power screws. Bronze carries higher loads than plastic, resists impact and shock, and tolerates higher temperature, which makes it the default for screw jacks, lifting tables, and heavy intermittent duty. Its disadvantage is that it relies on an external film of grease or oil; run dry it will gall and wear both the nut and the screw. Bronze is also heavier and more expensive than plastic. Larger Acme screws to suit bronze nuts are made from medium-carbon steel above about 1-3/4 inch diameter and low-carbon steel below that, with stainless options for corrosion service.

Plastic nuts, most commonly acetal (POM, sold as Delrin and similar grades), are self-lubricating, run dry and clean, are quieter, and resist corrosion. Acetal has a low coefficient of friction and can operate to roughly 90 degrees Celsius continuously. Internally lubricated polymer nuts, such as those used in igus dryspin assemblies, embed solid lubricant in the matrix so the pairing runs without any external lubrication, which suits washdown, food, medical, and maintenance-free OEM equipment. Plastic nuts often deliver slightly higher efficiency than bronze because of lower friction, but they are limited in load capacity, temperature, and PV (the product of contact pressure and sliding velocity), so they are not for heavy continuous lifting.

Anti-backlash nuts address the lost motion inherent in any clearance-fit thread. A standard nut has a manufacturing clearance, so when the screw reverses, the nut does not move until the flanks re-engage on the opposite side, typically 0.05 to 0.2 mm of axial play that grows with wear. An anti-backlash nut removes this with preload: a split-nut design uses a compression spring to push two nut halves onto opposite thread flanks, while a wear-compensating polymer nut uses an internal spring or interference geometry to take up clearance automatically as it wears. The cost is added drag torque and faster wear, so anti-backlash designs trade efficiency and life for repeatability that can reach a few microns.

Self-lubricating composites such as PTFE-based bearing materials with reinforcing fillers can substitute for bronze where dry running and low friction are needed but plastic load capacity is insufficient. These materials combine the low-friction, wear-resistant nature of PTFE with the rigidity added by fillers, sitting between acetal and bronze in capability. Whatever the family, the nut should be chosen against the actual duty cycle: peak load, continuous load, sliding speed, temperature, contamination, and whether the axis must hold position without power.

Efficiency, Self-Locking, and Limits

The geometry of the thread fixes three physical behaviours that dominate lead screw selection: mechanical efficiency, self-locking, and the speed and length limits set by critical speed and column buckling. These are not catalog options to tick; they follow directly from the lead angle, the coefficient of friction, and the screw dimensions, so an engineer who understands them can predict performance before buying.

Efficiency is low because the load slides on the thread flanks. Acme lead screws typically reach 20 to 40 percent mechanical efficiency, depending on lead angle, friction coefficient, and nut material; trapezoidal screws span roughly 20 to 70 percent, with the higher figures coming from steeper leads and low-friction plastic nuts. By contrast, ball screws reach 70 to 95 percent because rolling replaces sliding. The practical consequence is motor sizing: at 30 percent efficiency, the drive must supply more than three times the ideal torque to deliver a given thrust, and the lost power becomes heat in the nut, which is itself a limit on continuous duty.

Self-locking is the useful flip side of low efficiency. A screw is self-locking when the lead angle is smaller than the friction angle, where the friction angle is the arctangent of the coefficient of friction. In practice, single-start Acme and trapezoidal screws with a lead angle of about 5 degrees or less tend to self-lock under static load, meaning an axial force on the nut cannot rotate the screw, so the axis holds position with the motor off. This is why lead screws are favoured for vertical axes and screw jacks. The caution is that self-locking is a static property that weakens under vibration, shock, and changing lubrication, so it must never be relied on as a certified safety brake for life-critical lifting; an independent safety nut or brake is required there.

The table below contrasts the lead screw against its rolling-element alternative on the properties that most often decide a design, so the trade-off is explicit before selection.

Critical speed limits how fast a long screw may spin. As rotational speed rises, a slender screw eventually resonates and whirls like a skipping rope; running near this critical speed causes vibration, noise, and rapid wear, so designers stay below roughly 80 percent of it. Critical speed falls with the square of the unsupported length and rises with the root diameter and the end-fixity factor, so a fixed-fixed mounting allows far higher speed than a fixed-free one. Most simple Acme and trapezoidal systems run under about 150 rpm for this reason.

Column buckling limits how much thrust a long screw can push. Under compressive load a long thin screw behaves like a column and can suddenly bow sideways; the maximum safe load is the critical buckling load, governed by the Euler relation in which the load also falls with the square of length and rises with the fourth power of root diameter and with end fixity. When one screw cannot meet both the speed and the buckling limit for a given travel, the usual fixes are to increase diameter, add an intermediate support to halve the unsupported span, stiffen the end bearings, or adopt a rotating-nut layout in which the screw does not spin.

Key Specification Parameters

Reading a lead screw spec sheet means separating geometry, accuracy, and capacity. The same assembly may list a dozen numbers, but only a handful actually drive the selection: nominal diameter and lead, thread standard, lead accuracy grade, backlash, dynamic load and PV, efficiency, and critical-speed and buckling ratings. Each is explained below so a drawing requirement can be mapped to a catalog number.

Nominal diameter and lead are the primary geometry. Diameter sets stiffness, buckling resistance, and critical speed; lead sets how far the nut moves per turn and therefore the resolution and the speed at a given rpm. A small lead gives fine resolution and strong self-locking but slow traverse; a large or multi-start lead gives fast traverse and higher efficiency but loses self-locking. The designation captures both: 1/2-10 Acme means 1/2 inch diameter at 10 threads per inch, while Tr 40x7 means 40 mm diameter with a 7 mm lead.

Lead accuracy describes how closely the actual travel matches the nominal lead over the length of the screw, and it is the dominant term in absolute positioning. Rolled screws are economical and typically hold lead accuracy on the order of 0.05 to 0.1 mm per 300 mm of travel; precision-ground screws are far tighter, on the order of 0.01 to 0.025 mm per 300 mm, and are specified by an accuracy grade. Always read whether the figure is cumulative over the full travel or a local error over a short span, because the two differ substantially.

Backlash and repeatability are separate from lead accuracy. Backlash is the lost motion on reversal, typically 0.05 to 0.2 mm for a standard nut and near zero for an anti-backlash nut. Repeatability is the scatter when returning to the same position from the same direction, and it can be excellent even on a low-cost rolled screw if the approach is always one-directional. A common cost-saving insight is that a single-direction approach removes the need for an expensive anti-backlash nut when the application only requires repeatability rather than bidirectional absolute accuracy.

Load, PV, and efficiency set the mechanical capacity. Dynamic and static load ratings come from the nut and thread, while PV (the product of contact pressure and sliding velocity) caps the continuous duty of plastic nuts because it predicts frictional heating. Efficiency, decoded in Chapter 4, then sets the required drive torque: input torque rises as efficiency falls, so a low-efficiency screw needs a larger motor for the same thrust. The most common spec parameters are summarized below.

Critical speed and buckling ratings close the spec sheet for long axes. A reputable catalog provides charts of allowable speed against unsupported length for each end-fixity case, and allowable compressive load against length for the same cases. These are not optional for long, fast, or heavily loaded screws; ignoring them is the most common cause of whirl, vibration, and field failure. When the charts show that a single screw cannot serve the travel, the design must change diameter, span, fixity, or configuration as described in Chapter 4.

Selection Decision Factors

To turn the preceding five chapters into a specific part number, follow the decision sequence below. Most selection mistakes come not from a single wrong number but from deciding the wrong thing first, for example fixing a brand before confirming whether the axis must self-lock. These eight steps can serve as a fixed RFQ template.

1. Travel, load, and duty cycle: Fix the required stroke, the peak and continuous axial load, the orientation (vertical axes need holding force), and how often the axis moves. These set whether a lead screw is appropriate at all, or whether a ball screw is justified by speed and life.
2. Thread standard and form: Choose Acme (ASME B1.5, inch, 29 degrees) for North American supply and inch tooling, or trapezoidal (ISO 2904 / DIN 103, metric, 30 degrees) for metric drawings. Acme and trapezoidal parts are not interchangeable, so commit early.
3. Lead and start count: Pick the lead from the resolution and speed needed, and decide single-start (self-locking, slower) versus multi-start (fast, backdrives). If the axis must hold without power, keep the lead angle low or add a brake.
4. Nut material and technology: Bronze for high load, shock, and temperature; acetal or internally lubricated polymer for dry, clean, light to moderate duty; anti-backlash split or wear-compensating nuts where bidirectional accuracy matters. Match nut PV to the continuous duty.
5. Accuracy grade and backlash budget: Rolled screw for economy and one-directional repeatability; ground screw plus anti-backlash nut for precision bidirectional positioning. Specify lead accuracy as cumulative over the real travel, not a short local span.
6. Speed and column limits: Check critical speed and buckling against the unsupported length and end fixity using the maker charts. Increase diameter, shorten the span with intermediate supports, stiffen the ends, or use a rotating nut if a single screw fails either limit.
7. Mounting, ends, and environment: Define journal ends and bearing fixity (fixed-fixed, fixed-supported, fixed-free), corrosion exposure (stainless versus carbon steel), and contamination or washdown needs (favouring self-lubricating polymer nuts).
8. Total cost of ownership: Add purchase price, drive sizing penalty from low efficiency, lubrication and maintenance, and nut replacement interval. A cheap screw that needs a larger motor and frequent nut changes can cost more over its life than a better-matched assembly bought once.

One last commonly overlooked dimension is serviceability and supply continuity: whether replacement nuts are a standard catalog item, whether the screw can be re-cut or replaced from stock, and whether the supplier covers your thread standard at the accuracy grade you need. Established suppliers including Thomson (Thomson BSA), Helix Linear Technologies, Roton Products, Nook Industries, Haydon Kerk Pittman, and igus (dryspin), along with broad distributors such as MISUMI and McMaster-Carr, cover Acme, trapezoidal, and precision lead screws, so matching the supplier to your thread standard, accuracy grade, and nut technology, rather than to brand alone, keeps the axis serviceable for the life of the machine.