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P4 and P5 are rolling-bearing precision classes defined by ISO 492 and JIS B 1514; P4 carries tighter dimensional and running-accuracy tolerances than P5. For ball screw support units, the working rule is that the bearing grade must match the screw grade: a C3 precision-ground ball screw should be paired with P5-class or better angular contact bearings (P4 recommended for heavy-load precision applications), while C5 to C10 transmission screws are well served by P5 or P0 grades. Grade, however, is only the entry ticket. The contact angle design, the DF duplex pairing, and the quality of factory preload setting together determine how the support unit actually performs once it is bolted onto your machine.

Support unit accuracy is not bought - it is assembled. The bearing grade is the entry ticket; preload setting and bore grinding quality decide what your machine actually delivers.

Many buyers compare support unit quotations by reading nothing more than the model code - BK20, FK25 - and then ranking suppliers by price. What that comparison misses is that two suppliers can ship the same nominal model with entirely different bearing grades, different contact angles, or bearings that were never matched and preloaded as a pair. This article unpacks the first item on any serious procurement checklist for ball screw support units: what P0, P5, and P4 actually mean, why the 60-degree TAC contact angle is the purpose-built design for ball screw support, what factory-completed preload means for your assembly line, and finally a selection workflow you can apply directly to your next RFQ.

What Actually Determines Support Unit Accuracy? Three Layers, None Optional

The fixed-side support unit is the foundation of a ball screw drive. Motor torque enters the screw through the coupling, and every axial load in the system is ultimately carried by the fixed-side bearings. If the foundation has clearance or deflects under load, even the finest C3 screw will never deliver C3 positioning on the machine. In practice, support unit accuracy is the product of three stacked layers:

1. Bearing precision class (P0 / P5 / P4): sets the dimensional tolerances of the rings and the upper limits of radial and axial runout in rotation.
2. Contact angle and duplex arrangement (60-degree TAC, DF pairing): determines axial rigidity and bidirectional load capacity, which directly govern backlash and deflection under load.
3. Preload setting and housing machining: the micron-level grinding of the bearing bore, the perpendicularity of the locating faces, and whether the preload was matched and set in a temperature-controlled environment. This third layer is the real source of the price differences you see between nominally identical products.

Put bluntly: printing 'P4 bearing' on a datasheet is easy. If the housing bore roundness is out, or preload is set by feel on the assembly bench, the paper precision of a P4 bearing is never realised. This is why SYK has kept turning, milling, precision grinding, surface treatment, assembly, and quality control inside a single factory since 1989: accuracy is the product of an unbroken process chain, and every outsourced step adds an uncontrolled variable.

P0, P5, P4 Compared: ISO/JIS Classes and Where Each Belongs

Rolling-bearing precision classes are governed by ISO 492 and JIS B 1514; the smaller the number, the tighter the class. For support unit selection, the point is not to memorise tolerance tables but to understand the application threshold each class corresponds to:

Here is a correspondence that is routinely overlooked: the bearing grade has to be worthy of the screw grade. Take SYK's FK round-flange fixed-side series as a worked example from the catalogue. The standard FK25 is fitted with P5-class bearings and covers ball screws from C3 through C10. The FK25_C5 variant is fitted with P0-class bearings, and its applicable range narrows to C5 - C10 accordingly. If your machine runs a C3 ground screw but procurement saves a few dollars by ordering the C5-spec unit with P0 bearings, the foundation is now less precise than the screw it supports - a saving made in exactly the wrong place.

The reverse is equally true. Pairing a C7 rolled screw with P4 bearings wastes money in the other direction: the bearing's precision margin is consumed by the screw's lead error before it can benefit positioning. The correct procurement logic is to fix the screw grade and the application's positioning requirement first, then derive the bearing grade - not 'the higher the better' and not 'the cheaper the better'.

The 60-Degree TAC Contact Angle: Purpose-Built for Ball Screw Support

Fixed-side support units do not use general-purpose 15-degree or 30-degree angular contact bearings. They use TAC-type angular contact thrust ball bearings with a 60-degree contact angle. The larger the contact angle, the more the ball-to-raceway contact line tilts toward the axial direction - and the higher the axial rigidity and axial load capacity. That matches the load profile of a ball screw exactly: the load is almost entirely axial push-pull, with very little radial component.

• High axial rigidity: the 60-degree design delivers several times the axial rigidity of standard angular contact bearings, meaning less elastic deflection under load and better positioning repeatability.
• Bidirectional load capacity: paired in a DF duplex arrangement, the set carries axial force in both directions, so reciprocating motion produces no directional clearance.
• Controlled starting torque: from SYK's catalogue test values, maximum starting torque is 370 gf-cm for FK17, 730 gf-cm for FK25, and 1,050 gf-cm for BK30. Balancing preload against friction is a matter of bearing-matching technique, not luck.

Every SYK fixed-side support unit uses 60-degree TAC bearings from leading Taiwanese and Japanese bearing manufacturers (model numbers shown in blue in the catalogue denote Taiwanese-made P5), and the WBK / MBK / SBK heavy-load series are fitted with P4-class 60-degree TAC bearings as standard. For buyers, this means the 'bearing' column on the datasheet is verifiable: asking every supplier to disclose the bearing model and grade is the fastest way to explain a price gap in an RFQ.

DF Duplex Pairing and Factory Preload: What 'Zero Axial Clearance' Means for Your Assembly Line

Fixed-side bearings are paired in a DF duplex arrangement and preloaded at the factory to achieve zero axial clearance. That single line of catalogue fine print is the highest-value sentence in the entire procurement evaluation, because it directly determines how many hours your assembly line spends per axis:

• No on-site preload adjustment: preload is set in the factory by a controlled procedure. Your technicians torque the unit down to specification and move on - no tap-and-check alignment rituals, no dependence on a senior fitter's feel.
• Backlash eliminated at the source: zero axial clearance means no lost motion at the instant the screw reverses. For AOI inspection, laser processing, and any high-frequency reciprocating positioning task, this is a hard requirement, not a nice-to-have.
• Batch-to-batch repeatability: every unit's preload is controlled by the same process and the same inspection standard, so production machines do not develop individual personalities - 'this one runs free, that one runs tight'.

A note from the factory floor, because this is where experience earns its keep. At SYK, bearing matching and preload setting are performed by dedicated technicians in a temperature-controlled environment, and every preloaded assembly is verified against a starting-torque inspection - the maximum starting torque printed beside each model in the catalogue is precisely that inspection criterion. Our long-running analysis of returned competitor units tells a consistent story: the 'same model, 30% cheaper' support unit, once opened, most often reveals unmatched bearings and preload gambled on shim thickness. It looks fine at installation; three to six months later the reversal backlash starts to drift, and by then the downtime and re-calibration cost has long since exceeded the original saving.

One practical caution: BK, FK, EK and related series ship as completed, preloaded assemblies. Do not disassemble them - doing so voids the accuracy guarantee. If you need the grease nipple option (G code), specify it at order time rather than machining it afterwards.

SYK Series Bearing Configuration and Lead Time Quick Reference

The table below summarises the bearing configuration logic across SYK's main fixed-side series, for buyers and design engineers preparing an RFQ (full dimensions are in the catalogue; 2D/3D CAD files are available on request):

Two ordering-code rules that directly change the bearing grade you receive: a '_C5' suffix denotes P0-class bearings (for C5 - C10 screws), '_C3' denotes the C3-screw-dedicated configuration, and a 'B' denotes the higher-capacity B contact-angle bearing option. The same BK25 housing therefore exists as BK25, BK25_C5, BK25B_C5, and BK25_C3 - four different bearing configurations under one family name. Always quote the complete ordering code in your RFQ, or you will be comparing suppliers on 'same name, different contents'. (See: [internal link: Ordering Code Explained].)

On availability: BK / FK / EK standard items are predominantly ex-stock with 1-3 day dispatch; electroless nickel (N) and certain C3 configurations ship in 3 days; custom modifications ship in 5-7 days. There is no minimum order quantity - single-piece prototype orders ship just like production orders, which matters when you are validating a new axis design or rescuing a stopped machine.

Selection Workflow: Four Questions That Decide Between P4 and P5

Condensing everything above into a decision path you can walk through in five minutes:

1. What grade is the screw? C3 ground screw: bearings of at least P5, with P4 (WBK / MBK / SBK series) recommended for heavy-load or semiconductor applications. C5 - C7: standard P5 units. C7 - C10: P0 (_C5 variants) is the rational, economical match.
2. How much load, how much rigidity? Heavy-load slides, gantries, high-inertia transfer axes: heavy-load series (P4 with DF / DFD / DFF multi-row arrangements). General feed axes: BK / FK standard units.
3. Cleanroom or corrosive environment? If yes: electroless nickel surface treatment (N) plus low-dust-generation grease (C), and stainless steel (S) where chemistry demands it. (See: [internal link: Cleanroom Specification Guide].)
4. Is reversal positioning accuracy critical? For high-frequency reciprocation and micron-level repeatability, confirm the supplier delivers factory-completed preload with zero axial clearance, and ask for the bearing model and starting-torque inspection value in writing.

If the four questions still leave you between two options, the most efficient path is to hand the application conditions - screw specification, load, speed, environment, cycle time - to a technically trained sales contact. SYK's sales engineers work directly with design engineers on selection, and a 35-year application database is more reliable than any general rule.

The Cost Conversation: Why Bearing Grade Is a TCO Decision, Not a Price Decision

It is worth closing the loop on the question that started this article: price. The unit-price gap between a properly specified P5 support unit and a cut-price nominal equivalent is typically modest in absolute terms - often less than the cost of one hour of machine downtime. The total-cost picture looks very different from the purchase order:

• Assembly labour: factory-preloaded units remove shim selection, tap-alignment, and trial-run cycles from every axis you build. On a multi-axis machine assembled in volume, that is measured in technician-days per month, not minutes.
• Warranty and field service: backlash drift from unmatched bearings surfaces months after shipment, on the customer's floor. The service visit, the re-calibration, and the reputational cost all trace back to a component that saved a few dollars.
• Inventory carrying cost: a supplier shipping ex-stock in 1-3 days lets you run lean replenishment instead of buffering 8-16 weeks of safety stock against an overseas lead time. The freed working capital routinely exceeds any unit-price difference within the first year.

Vertical integration is what makes this economics work. Because turning, grinding, surface treatment, bearing matching, and final inspection all happen under one roof, SYK controls quality at every step rather than inspecting it in at the end - and can still hold standard lead times of 1-3 days and custom lead times of 5-7 days with no minimum order quantity. When you evaluate suppliers on landed total cost rather than unit price, verifiable bearing grade and factory preload stop looking like premium features and start looking like the cheapest insurance available. (See: [internal link: Ball Screw Support Unit Manufacturer in Taiwan - Pillar Page].)

Frequently Asked Questions

Q: How much difference is there between P4 and P5 bearings in practice?

A: Under ISO 492 / JIS B 1514, P4 running and dimensional tolerances are roughly 30-40% tighter than P5. On a support unit this shows up as lower vibration at speed and better long-term accuracy retention. A C3 screw under normal loads is well served by P5; choose P4 for heavy loads, high-frequency positioning, or semiconductor process equipment.

Q: What is the benefit of the 60-degree TAC contact angle?

A: Axial rigidity rises with contact angle. The 60-degree TAC design was developed specifically for ball screw support, delivering far higher axial capacity and rigidity than general 15/30-degree angular contact bearings; paired in a DF duplex set it carries load in both directions and eliminates reversal clearance.

Q: Do I need to adjust the preload myself?

A: No - and you should not. SYK fixed-side support units leave the factory with bearings matched, preload set to zero axial clearance, and starting torque inspected. On site, simply tighten to the specified torque. Disassembling the unit voids the accuracy guarantee.

Q: Does a C3 screw always require P4 bearings?

A: Not necessarily. A C3 screw paired with P5-class units (standard BK / FK) will deliver C3 positioning performance. Upgrade to the P4 heavy-load series when high axial loads or extreme repeatability requirements (semiconductor, metrology equipment) coincide with the C3 screw.

Q: Is there a difference between Taiwanese and Japanese brand bearings?

A: SYK fits 60-degree TAC bearings from leading Taiwanese and Japanese manufacturers; both meet the same precision class standards and pass the same incoming-inspection and preload verification (blue model numbers in the catalogue denote Taiwanese-made P5). At equal class and equal inspection standards, performance is equivalent, while Taiwanese supply adds lead-time and cost flexibility.