Ball Screw Components Explained: Parts and What They Do

What a complete ball screw drive contains

Before sending an RFQ, it helps to know what a "ball screw assembly" actually includes. A shaft alone does not make a machine axis: the ball nut, the bearing blocks at both ends, the coupling to the motor, and the way the nut is mounted all decide how the axis performs.

Inside the screw and nut: raceway, balls, recirculation, and sealing

Many buyers see the screw and nut as "a shaft plus a flanged part," yet the details that decide performance sit hidden inside. The biggest difference from an ordinary lead screw is a train of balls that recirculate between the screw raceway and the nut raceway, turning sliding friction into rolling friction:

• Raceway and balls: the balls carry load on the loaded side and return through a return channel, forming a continuous circuit. Balls are normally made of hardened bearing steel; ceramic balls appear on high-speed, electrically insulating, or corrosion-resistant axes. The ball diameter and the raceway must match to micrometer level, which is why the screw and nut are supplied and matched as a set — mixing brands is not recommended.
• Recirculation style: the return path can be external (a return tube outside the nut, the most common style on standard nuts), internal (a return channel machined inside the nut body, more compact), or end-cap (end caps guide the balls back at both ends). The style affects nut size, noise, and the maximum running speed.
• Screw material and raceway: screws are normally made of high-carbon chromium bearing steel (for example SUJ2 or GCr15) with a hardened, wear-resistant raceway; stainless steel screws suit humid, food, or clean-room environments, at the cost of somewhat lower load capacity and life.
• Single nut versus double nut: a single nut is simple and economical, while a double nut applies preload between two nut halves to remove axial clearance, giving less backlash and higher rigidity for frequent reversing and precise positioning. More preload is not always better — see ball screw nut preload explained for the trade-off.
• Sealing and dust protection: both ends of the nut usually carry rubber seals or metal wipers to keep chips and dust out of the raceway. In dusty woodworking or stone-cutting environments, state the protection requirement in the RFQ — see dust protection for a woodworking CNC ball screw RFQ.

Most of these details do not need to be listed line by line in the RFQ, but once you understand them, you can state the points that actually move price and service life — such as whether preload is needed and whether the environment is dusty.

Couplings: rigidity, accuracy, and misalignment

The coupling transfers torque from the motor to the screw. In practice the choice is a trade-off between torsional rigidity and how much shaft misalignment the coupling can tolerate:

• Jaw or spider couplings use a flexible insert that compensates radial and angular misalignment and absorbs small shocks. They are economical and suit low- to medium-accuracy axes such as slides and feeding units.
• Diaphragm couplings rely on elastic flex of metal diaphragms. They offer high torsional rigidity with low backlash, which is why machining centers and other accuracy-sensitive axes use them.
• Rigid couplings have no flexible element. They are the stiffest option but demand accurate alignment between motor and screw, so they fit machines built to tight tolerances.

If you do not need the coupling, say so. Many buyers order only the screw and nut and fit their own coupling.

Fixed-side and support-side bearing blocks

A ball screw is usually held by two bearing blocks with different jobs. Mixing them up is a common reason for poor accuracy or short bearing life:

• Fixed side (BK type): houses a pair of angular contact ball bearings that take both radial and axial load. It locates the screw in the axial direction, controls end float and backlash, and therefore defines the positioning accuracy of the axis.
• Support side (EK type): uses deep groove ball bearings and carries radial load only. It lets the shaft expand and contract slightly with temperature, so thermal stress does not load the bearings.
• Compact and larger variants such as RBK and FK follow the same split but suit tighter spaces or bigger shaft diameters. The number after BK or EK, like 12 or 15, refers to the shaft end size the block fits, not the screw diameter.

Nut styles: speed, load, and noise

The ball nut is where the drive earns its speed, thrust, and life. Nuts differ in length, lead, and ball recirculation design. Ordering by a full model such as SFU1605 is the fastest route, but the style has to match the application:

If you know the function but not the model, state the load, speed, and travel in the RFQ and let the supplier propose the nut style.

Nut housing material: aluminum or steel

The nut housing fixes the ball nut and carries the linear motion to the table or arm:

• Aluminum housings are light, easy to machine, and adequate for light loads, a common choice on small 3D printer or laboratory axes.
• Steel housings add rigidity and tolerate heavy loads and shocks, so machine tools and robots tend to use them.

If the nut has its own flange, a separate housing may not be needed at all.

Why shaft end machining decides how well the assembly fits

The screw ends must be machined to match the bearing blocks and coupling. This is where a seemingly identical screw becomes one that fits your machine or one that does not:

• Fixed-side end: the thread, journal tolerance, and face squareness must hold the bearing pair concentric with the screw axis, or the bearings wear unevenly and accuracy drifts.
• Support-side end: the journal diameter and fitted length control how the support bearing sits and leave room for thermal expansion.
• Surface finish and geometric tolerances on the ends show up directly as runout and repeatability at the axis. Suppliers with end machining can produce the screw to your drawing, so provide the end drawing or dimensions with the RFQ.

Typical applications and what they ask of the assembly

• CNC machine tools: feed axes with micrometer-level positioning, where accuracy class and preload matter most.
• Industrial robots: vertical and linear axes such as a SCARA Z column need low backlash and smooth motion.
• Measuring instruments: coordinate measuring machines rely on smooth, repeatable travel to keep measurements trustworthy.
• Automation lines: transfer and sorting slides run at high speed over long cycles, so life and lubrication win.

Efficiency and accuracy advantages

Compared with a trapezoid lead screw, a ball screw rolls instead of sliding. Efficiency is typically above 90 percent against roughly 30 to 40 percent for a trapezoid screw, so the same motor moves the load faster and wastes less heat. Rolling contact also wears slowly and, with a suitable drive, an axis can move in 0.001 mm steps. That is why precision machining and optical assembly rely on ball screws. Another often-missed difference is direction: a ball screw drive is reversible. Motor rotation becomes linear motion, and a linear push can also turn the screw — which is why electric cylinders and similar linear actuators are built around ball screws. A trapezoid screw has much higher friction and is usually self-locking, so the load cannot easily drive it backwards.

What to include in your RFQ

A clear RFQ removes a round of clarifications. Cover these points:

• Scope: screw and nut only, or with bearing blocks, nut housing, and coupling.
• Screw specification: diameter, lead, overall length, working travel, and accuracy class such as C5 or C7.
• Nut: full model (for example SFU1605) or flange dimensions, plus preload if backlash matters.
• Bearing blocks: fixed- and support-side models such as BK12 and EK15.
• End machining: drawing or dimensions of both ends, at least the coupling-end diameter and lock thread.
• Quantity, export packing, and target delivery time.

FAQ

Do I need to buy the whole assembly, or can I order the screw and nut alone?

Both are common. Standard screws and nuts ship on their own, BK/EK bearing blocks can be added to the same order, and buyers usually fit their own coupling.

Is BK12 a screw diameter?

No. BK12 is a fixed-side bearing block whose number indicates the shaft end size it fits. Always check the journal diameter against the block model before ordering.

Does a high-lead nut give more speed?

Yes, at the cost of thrust. A high-lead nut moves further per motor turn, so linear speed rises for the same RPM while the load it can push falls.

Do you machine the shaft ends before shipping?

Yes, end machining is a separate step. Send the end drawing or dimensions with the RFQ so the fixed side, support side, and coupling all fit.

Single nut or double nut — which should I choose?

It depends on the load and how often the axis reverses. A single nut is simple and cheaper, and it suits conveying and positioning axes that are not sensitive to clearance. A double nut removes axial clearance through preload, improving rigidity, repeatability, and backlash behavior, at the cost of price and heat. When in doubt, state load direction, reversing frequency, and allowable clearance in the RFQ and let the factory recommend a solution for the application.

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