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A support unit is not an accessory bolted onto the end of your ball screw. It is the engineering decision that governs the positioning accuracy, axial stiffness, and service life of the entire drive axis. Choose wrong and it won't fail on the test bench — it will quietly consume your accuracy budget through drift, noise, and premature wear, months after the machine ships.

1. Why Support Unit Selection Is the Most Underrated Decision on the Bench

On a servo-driven ball screw axis, an engineer's attention almost always concentrates on three things: motor torque, screw lead and accuracy grade, and linear guide stiffness. The support unit — the bearing block at each end of the screw — is typically the lowest-cost line item on the BOM and the last component anyone specifies.

That ranking is wildly out of proportion to the support unit's actual influence on system performance. The support unit carries the screw's entire axial load, defines its positioning datum, and provides rotational support. Its bearing arrangement, preload, stiffness, and mounting method directly determine three things: the screw's critical speed, repeatable positioning accuracy, and how that accuracy degrades over years of high-duty-cycle operation.

Put plainly: whether the money you spend on the motor and the screw actually converts into accuracy you can measure at the work point is, in large part, decided by how you specify the support unit.

What makes this worse is that a mis-specified support unit rarely fails immediately. It fails gradually: preload relaxes and backlash creeps in, bearing heat builds thermal displacement, and insufficient stiffness lets the axis drift under high acceleration. These symptoms often surface months after commissioning — and by then, replacing the support unit means tearing down and re-aligning the entire axis.

This guide exists to turn "how do I choose a support unit" from a fuzzy rule of thumb into a decision sequence you can follow. We work through four questions in order: do I need a unit at both ends? Square or round? How do I set accuracy and surface treatment? And how do I read the part code so I order it right the first time?

2. Decision One: Fixed End, Floating End, and the Four Mounting Configurations

The first and most consequential decision in support unit selection is understanding what role each end of the screw plays. This determines which class of support unit you need and whether you mount one at both ends.

Fixed (Locating) End: the Axial-Load Datum

The fixed end — SYK's BK, FK, EK, AK and LK series — uses an angular-contact bearing pair in a back-to-back (DF) arrangement under preload. It carries axial load in both directions and serves as the screw's axial datum, ensuring the screw does not shift axially between thrust and pull. It is the root of positioning accuracy, and the motor is normally coupled at this end.

Floating (Support) End: Radial Support Only, Free to Grow

The floating end — SYK's BF, FF, EF, AF and LF series — typically uses a deep-groove ball bearing that provides radial support only and does not constrain axial movement. Its essential job is to let the screw expand freely as it heats up. If you fix both ends without designing for thermal growth, the screw has nowhere to expand, preload spikes to abnormal levels, and bearing life collapses.

The Four Configurations: Choose by Stroke, Speed, and Accuracy

How you combine the two ends falls into four industry-standard configurations. Select based on stroke length, maximum speed, and positioning accuracy:

In practice, more than 80% of precision positioning applications use the Fixed–Supported configuration. It strikes the best balance between stiffness, critical speed, and thermal compensation — and it is the configuration SYK covers most completely as standard: pick a BK or FK for the fixed end and the matching BF or FF for the floating end to build a complete set.

3. Decision Two: Square or Round? The SYK Support Unit Family Explained

With the configuration set, the next step is choosing the body style and series. SYK's lineup is organized by fixed/floating end and by square/round/low-profile/heavy-duty, and the part numbers match up into complete sets.

Square Series: BK (fixed) / BF, HF (floating)

Square units bolt down on four points on the base face, giving high mounting stiffness and strong resistance to twisting — the mainstay for machine tools and heavy cutting. SYK's BK series spans BK10 through BK40, covering screw shaft diameters of roughly ø12–ø50, with the floating end handled by the BF and HF series. When the system sees significant radial load or demands high mounting rigidity, square is the first choice.

Round Series: FK, FKA (fixed) / FF (floating)

Round units have a compact profile and high concentricity between machining datums, ideal for tight spaces or applications demanding high mounting accuracy. SYK's FK series spans FK06 through FK30, paired with angular-contact bearings from 706A, 708A and 7000A up to the 7206 — all in a preloaded DF arrangement with zero axial clearance. The floating end is handled by the FF series (FF06–FF30, paired with 606ZZ through 6206ZZ deep-groove bearings). Semiconductor, inspection, and precision automation applications frequently choose round.

Motor-Mount Drive Housings: MBC and MB Series

When you need to integrate the motor, coupling space, and fixed-end bearing support into a single structural part, you need a drive housing. SYK's MBC series (including MBCA, MBCB, MBCE, MBCF, MBCS and MBCK variants) combines the fixed-end bearing block with the motor mounting face, sparing the engineer the work of designing a motor bracket and managing alignment. The companion MB series adds a narrow-rail type (MBA), a built-in type (MBB), a low narrow-rail type (MBL), and a flat-base type (MBH) to suit height limits and packaging needs. SYK also provides a motor cross-reference table to match flange dimensions across major motor brands.

Heavy-Duty Series: WBK / MBK / SBK

Beyond general applications, SYK offers heavy-duty support units in the WBK, MBK and SBK series, using P4-grade 60° angular-contact (TAC) bearings in DF/DFD arrangements to carry higher axial loads and moments — suited to large machine tools, heavy automation, and long-stroke high-load axes.

Low-Profile Series: LK / LF

When centerline height is constrained and the screw mounting height must be kept low, SYK's low-profile fixed-end LK and floating-end LF/LFA series offer a shorter body height. The SBK/SBF series additionally offers a "zero centerline height" version to lower the mounting height further.

4. Decision Three: Accuracy Grade, Bearing Arrangement, and Surface Treatment

With body style and series set, three remaining parameters decide whether the unit can actually hold up to your accuracy requirement and working environment.

Accuracy Grade: How to Choose Between C3, C5, C7

The accuracy grade reflects the level of bearing matching and preload control inside the unit. Lower numbers mean higher accuracy (C3 beats C5 beats C7 beats C10). The selection logic:

Selection principle: don't over-spec on reflex. Spend the budget where the application actually needs it — semiconductor and high-speed machining earn C3, but a general conveyor running C3 is wasted money. SYK offers C3, C5 and C10 within the same model, so you can grade each axis appropriately.

Bearing Arrangement: DF Pairing, P4/P5, Taiwan or Japan

SYK's fixed ends use angular-contact bearings in a back-to-back (DF) pair under preload, achieving zero axial clearance — the key to carrying bidirectional axial load. Standard units use P5-grade bearings (Taiwan-made or Japanese NSK); the heavy-duty series uses P4-grade 60° TAC bearings. If nothing is specified at order, the fixed end defaults to Taiwan-made P5; note Japanese bearings in the part code if required.

Surface Treatment: Black Oxide vs. Electroless Nickel

Surface treatment decides what environment the unit can run in long-term — the dividing line between a cleanroom and a general factory:

For cleanrooms, specify an electroless-nickel body (part code N) plus low-particle grease (part code C). SYK's cleanroom-grade surface treatment capability is the same option chosen by the semiconductor fab customers discussed in our April and May articles.

5. Decision Four: Read the Part Code and Order It Right the First Time

SYK's part code encodes every decision above into a single string. Read it, and you can state the full specification once at RFQ time and avoid back-and-forth. Using a fixed end as the example:

Fixed-end ordering example: BK30B_C5 G N C A S M

Drive housings follow the same logic — for example, MBCB12-D N C J: MBCB12 is the model, D the servo-motor spec code, N electroless nickel, C low-particle grease, and J Japanese P5 bearings.

The Five-Step Selection Sequence (Use This Directly)

Compress the whole process into five questions; answer them in order and you have your spec:

1. Configuration? Based on stroke / speed / accuracy, choose Fixed–Supported (most common), Fixed–Fixed (high speed, high stiffness), or Fixed–Free (short stroke, low speed).
2. Body style? High mounting rigidity / heavy cutting → square (BK/BF); tight space / high concentricity → round (FK/FF); integrated motor → drive housing (MBC); high load → WBK/MBK/SBK.
3. Shaft size? Match the screw shaft-end diameter to the model (e.g., BK10–BK40 cover ø12–ø50).
4. Accuracy and environment? Precision positioning → C3/C5; cleanroom → electroless nickel (N) + low-particle grease (C).
5. Assemble the code? Combine the above into a complete part code, and RFQ the fixed and floating ends as a matched set.

6. Conclusion: The Support Unit Is the Gatekeeper of Your Accuracy Budget

Back to the opening premise: a support unit is not an accessory on the end of your ball screw — it is the engineering decision that governs the positioning accuracy, stiffness, and service life of the entire drive axis.

The four decisions an engineer makes at selection — configuration, body style, accuracy and surface treatment, and part code — each write themselves directly into the machine's positioning accuracy and maintenance frequency for years to come. Choose right, and the support unit is invisible. Choose wrong, and it consumes, through gradual drift and noise, every bit of the accuracy budget you spent on the motor and the screw.

This is exactly why "askability" and "sampleability" matter so much at the selection stage. With 35 years focused on precision motion components, SYK offers the complete family — BK/FK fixed ends, BF/FF floating ends, MBC drive housings, and heavy-duty series — all vertically integrated under one roof in Taiwan: turning, milling, grinding, heat treatment, surface treatment, and bearing assembly. Standard items ship in 1–3 days, custom items in 5–7 days, with no MOQ — so you can sample a single unit and iterate fast during design validation, instead of waiting weeks just to confirm one spec.

FAQ | Design & Selection Questions

Q1: What's the difference between the fixed end and the floating end? Do I need both?

The fixed end carries bidirectional axial load and serves as the positioning datum; the floating end provides radial support only and lets the screw grow thermally. Whether you mount both depends on configuration: short, low-speed strokes can use Fixed–Free (fixed end only); most medium-to-long precision strokes use Fixed–Supported (both ends); high-speed, high-stiffness axes can use Fixed–Fixed. Over 80% of precision positioning uses Fixed–Supported.

Q2: How do I choose between square (BK/BF) and round (FK/FF) units?

Square units bolt down on four base points, offering high mounting rigidity and twist resistance — ideal for machine tools and heavy cutting. Round units are compact with high concentricity, ideal for tight spaces or high mounting accuracy, such as semiconductor and inspection equipment. With no special space limit and a priority on rigidity, square is the safe choice; for tight packaging or profile precision, choose round.

Q3: What's the difference between C3, C5, and C7? Which does my application need?

Lower numbers mean higher accuracy (C3 > C5 > C7), corresponding to less runout and greater stiffness. Semiconductor, precision inspection, and high-speed machining warrant C3; general CNC and automation mechanisms are well served by C5; conveying and low-speed lifts can use C7. Don't over-spec on reflex — spend the budget on the axes that truly need accuracy.

Q4: When do I need a drive housing (MBC), and how does it differ from a plain fixed end?

Use a drive housing (MBC series) when you want to integrate the motor mounting face, coupling space, and fixed-end bearing block into one structural part. It eliminates designing a motor bracket and managing alignment, and SYK's motor cross-reference table helps match flange dimensions across brands. A plain fixed end (BK/FK) handles only the screw-end axial support; the motor bracket must be designed separately.

Q5: How do I choose between black oxide and electroless nickel? Is nickel mandatory for cleanrooms?

Black oxide is fine for general industrial environments — economical, with adequate rust protection. For clean/cleanroom environments, electroless nickel is recommended for higher corrosion resistance and lower particle contamination, paired with low-particle grease. For Class 100 / Class 10 cleanrooms, specify an electroless-nickel body (code N) plus low-particle grease (code C). SYK offers cleanroom-grade surface treatment.

Q6: What does a code like BK30B_C5 G N C mean, and how do I order?

Take BK30B_C5 G N C: BK30B is the model and bearing angle, C5 the accuracy grade, G the grease-nipple type, N electroless nickel, and C low-particle grease. To order, confirm the spec in sequence — configuration, body style, shaft size, accuracy and environment, part code — and RFQ the fixed and floating ends as a matched set. SYK has no MOQ; a single unit can be ordered, with standard items shipping in 1–3 days.

About SYK | 35 Years of Precision Motion Components

Founded in 1989 and headquartered in Taiwan, SYK has spent 35 years focused on the design and manufacture of ball screw support units and precision motion components. The lineup spans BK/FK fixed ends, BF/FF floating ends, MBC drive housings, and WBK/MBK/SBK heavy-duty series, in C3/C5/C7 accuracy grades with black-oxide and electroless-nickel surface treatments. A single vertically integrated facility combines turning, milling, precision grinding, heat treatment, surface treatment, bearing assembly, and quality inspection — delivering industry-leading lead times of 1–3 days for standard items and 5–7 days for custom, with no MOQ. SYK components serve CNC machine tools, semiconductor equipment, PCB processing, Logistics 4.0 automation, and precision inspection across demanding high-accuracy fields.