BK-20 Ball Screw Support Unit (Fixed end, Square, for shaft ⌀25/28 mm)
BK-20 MOSIONX support unit: Fixed end, Square, compatible shaft ⌀25/28 mm, bearing 7004ATYDFC8P5. Original MOSIONX, paired with ball screws. In stock.
Key Specifications
| Series | BK |
| Model | BK-20 |
| Type | Fixed end |
| Shape | Square |
| Compatible shaft dia | 25/28 mm |
| Bearing model | 7004ATYDFC8P5 |
Dimension Drawing
Drawing data from the original MOSIONX catalog; always use the actual mounting dimensions.
Compatibility
| Compatible shaft dia | Bearing model |
|---|---|
| 25/28 mm | 7004ATYDFC8P5 |
Model Code Explanation
BK-20 is a Fixed end (Square) support unit. During installation the fixed end should carry the main axial load; the supported end gives auxiliary support and allows slight axial float.
FAQ
Which ball screws are compatible with BK-20?
Compatible with ball screws of shaft ⌀ 25/28 mm mm; match by the catalog dimensional table.
What is the difference between fixed and supported ends?
The fixed end (BK/EK/FK) carries axial load and locates; the supported end (BF/EF/FF) gives auxiliary support and allows slight axial float.
How do I get the 3D / CAD drawing of BK-20?
In the Selector model list, click the CAD/3D link for this support unit to download the original drawing.
Mounting Dimensions & Outline (BK20 · fixed end (square))
Housing & mounting dimensions
| Item | Value (mm) |
|---|---|
| Shaft dia D1 (bearing bore) | 20 |
| Body length A | 88 |
| Mounting hole pitch B | 70 |
| Body width C | 52 |
| C1 | 19 |
| E | 8 |
| C2 | 44 |
| H1 | 50 |
| Centre height h (±0.02) | 34 |
| Overall height H | 60 |
| Overall length L | 35 |
| L1 | 8 |
| L2 | 43 |
| L3 | 8 |
| T | 30 |
| Mounting hole ØP | 6.6 |
| N | 22 |
| Set screw M | M5 |
| Through hole ØX | 9 |
| Counterbore ØY | 14 |
| Counterbore depth Z | 8.5 |
| Weight (kg) | 1.19 |
Matching screw shaft-end dimensions
| Item | Value (mm) |
|---|---|
| Suitable screw dia | ⌀25,⌀28 |
| D | 20 |
| D1 | 17 |
| B | 53 |
| E | 25 |
| Locknut thread M | M20×1 |
Dimension codes A / B / C / E / H1 / h / H / L / PCD / Dg6 follow the drawings in chapter G of the MOSIONX 2026 general catalogue; all values in mm, h to ±0.02 mm. Assembly reference: housing bore K7 for the fixed end, H7 for the supported end; coaxiality of the two housings ≤ 0.02 mm; use KM-series precision locknuts to the thread sizes listed above (external 6g / internal 6H). Always confirm against the delivered product.
BK-20 — Assembly & Lock-nut Torque (reference)
The values below are general references for precision lock nuts. Always follow the manufacturer's technical manual; tighten symmetrically and step-by-step.
| Lock nut thread | Reference torque |
|---|---|
| M10 | 18 N·m |
| M12 | 28 N·m |
| M14 | 38 N·m |
| M15 | 46 N·m |
| M16 | 55 N·m |
| M20 | 95 N·m |
| M25 | 180 N·m |
| M32 | 340 N·m |
| M40 | 620 N·m |
For this unit the compatible shaft is 25/28 mm; the fixed / supported lock nut thread is approx. M25 with a recommended reference torque of approx. 180 N·m.
BK-20 — Selection & Load Match
- Structure type: Fixed end (Square) — The fixed end carries axial force, radial force and bending moment — use a zero-clearance or preloaded bearing and lock it reliably.
- Matching bearing: 7004ATYDFC8P5, matched to the ball screw's rated speed and axial load.
- Compatible shaft 25/28 mm — keep the screw shaft within this unit's range to avoid long overhang and deflection.
- Together with the ball screw and coupling it forms the feed unit; its rigidity directly affects machine positioning accuracy and backlash.
BK-20 — Common Faults & Troubleshooting
| Symptom | Possible cause | Check & remedy |
|---|---|---|
| Bearing noise | poor lube, misalignment, damaged or contaminated bearing | re-align, re-lubricate, replace bearing if needed |
| Axial play | lock nut loose, bearing clearance too large | re-tighten per table, use zero-clearance / preloaded bearing |
| Abnormal heating | over-preload, poor lube, over-speed | adjust preload, improve lube, verify rated speed |
| Housing crack / deformation | uneven mounting face, over-tight bolts, impact | ensure face flatness, tighten to recommended torque symmetrically |
BK-20 — Typical Applications
As the end-support component of a ball-screw feed unit, BK-20 (Fixed end · Square) is widely used in CNC machine tools, automation lines, 3D printing / precision measuring equipment, semiconductor packaging and transfer robots where stable axial support and high repeatability are required.
BK-20 — Installation & Maintenance
- Mounting face: keep the housing bottom flat and fully seated on the machine frame — no forced tightening that distorts the housing.
- Alignment: keep screw-to-rail parallelism ≤ 0.02 mm/m to prevent eccentric load and abnormal wear.
- Tightening: tighten the lock nut symmetrically and step-by-step per the table above.
- Lubrication: re-grease the bearing per duty; use the right grade for high speed or clean rooms.
- Protection: in dusty / chip environments add a cover to extend bearing and screw life.
Further Reading: Support Units & Selection Guides
Need to choose shaft diameter, lead and precision grade (C5/C7/C3), or learn about mounting torque, pre-stretch and troubleshooting? We have prepared complete technical guides:
Support Units OverviewSupport Unit Selection & MountingTroubleshooting