Positioning drift, lost motion during direction reversals, and high-frequency servo noise rarely originate in the motor encoder. In CNC machine tools and precision positioning stages, axis backlash usually traces back to loose locknuts on fixed-end bearings, un-preloaded single nuts under alternating cutting forces, or thermal expansion along the screw shaft. When a ball screw assembly lacks axial rigidity, reversal error accumulates, ruining tool path accuracy and causing surface chatter during milling passes.
SFU single-nut ball screws use internal deflector return tubes, providing a compact footprint for standard drive axes. However, under heavy or reversing loads, single nuts exhibit micro-clearance between the steel balls and raceways. Swapping to a DFU double-nut design eliminates this backlash. A precision shim fitted between two nut bodies pushes the ball circuits in opposite directions, removing axial play while keeping the screw diameter the same. For high-speed setups crossing 3,000 RPM-where standard return tubes get noisy and vibrate-SFS end-cap nuts provide a cleaner alternative by routing balls smoothly through internal end caps.
Even the best nut design cannot compensate for a skewed axis, so the screw must sit dead parallel to the guide rails. Mount and pin the fixed-end bearing block (BK or FK) first, then slide the nut housing along the shaft to verify runout before torquing the floating-end block (BF or FF). Misalignment between the ball nut housing and the motor mount flexes the screw shaft, causing cyclic current spikes every revolution and destroying the angular contact bearings. Never spin a ball nut off the screw shaft without a dummy sleeve; once recirculating balls spill, repacking them by hand risks misarranging spacer balls, which jams the recirculation circuit.
Field Integration FAQ
Why select DFU double-nut ball screws over single-nut SFU units for dynamic axes?
Switch to DFU double nuts whenever an axis faces heavy reversing loads, rigid tapping, or requires sub-micron positioning repeatability. The internal shim in DFU nuts maintains zero backlash under bidirectional forces, whereas single SFU nuts rely solely on oversized balls, which wear faster under heavy thrust.
What causes a ball screw shaft to bind or vibrate at specific positions along its stroke?
Local binding usually stems from mounting misalignment or screw shaft runout. If the support blocks (BK/BF) are not co-axial with the linear guide rails, the screw bends as the nut approaches the stroke ends. This bending increases torque demand on the motor and causes premature fatigue in the raceways.
How do you properly adjust BK fixed-end bearing locknuts to eliminate end-play?
Tighten the ground locknut on the BK housing using a hook spanner while measuring axial float with a dial indicator on the shaft end. Lock the set screws against the brass pads only after torqueing the nut to spec. Leaving this locknut loose lets the angular contact bearings move axially, creating backlash that no drive parameter can compensate for.
Technical Performance Matrix
| Parameter | Engineering Specification |
| Screw Shaft Material & Hardness | GCr15 / SUJ2 Bearing Steel, Induction Hardened to HRC 58–62 |
| Precision Class Options | C7 Grade Rolled (<= 0.050 mm / 300 mm), C5 Grade Ground (<= 0.018 mm / 300 mm) |
| Nut Configuration | SFU Single Nut (Internal Deflector), DFU Double Nut (Preload Washer), SFS High-Speed Nut (End-Cap) |
| Support Block Pairing | BK Fixed End (Matched Angular Contact Bearings) + BF Floating End (Deep Groove Ball Bearing) |
| Preload Level | P0 (Standard Clearance), P1 (Light Preload), P2 (Heavy Preload for DFU) |
| Axial Runout Limit | <= 0.010 mm over total stroke length |
Maintaining precise feed accuracy in ball screw drives depends on choosing the correct nut preload configuration, maintaining strict alignment between the shaft and guide rails, and securing the fixed-end bearing pack against axial deflection.
