DF, DB, DFD or DFF? Selecting a Heavy-Duty Ball Screw Fixed-End Arrangement

Published:2026-09-02 09:00:00

Heavy-duty fixed-end bearing arrangements

Heavy duty is not “more bearing rows.” It is a load-direction, rigidity and thermal design.

DF, DB, DFD and DFF change more than row count. The arrangement changes capacity by direction, axial rigidity, preload, starting torque, heat and alignment sensitivity. Map the load directions and duty first, then select the arrangement and housing family.

Direction firstone dominant load or two heavy directionsThen dutypeak, duration, speed and reversalFinally the assemblyarrangement, housing, interface and heat

HZMotion heavy-duty ball screw fixed-end support units — Real HZMotion heavy-duty products. Similar housings can contain different row counts, preload and operating limits.

01 / Load map

Do not start from an arrangement code—map both axial directions first

Duty information	Minimum input	Why it changes the arrangement
Load direction	Peak in both directions, duration and reversal frequency	Determines symmetric capacity versus extra rows in one dominant direction
Duty cycle	Acceleration, cutting, hold, return and dwell fractions	Peak controls structure; repeated load and time control life and heat
Speed	Maximum/common speed and acceleration frequency	Rows, preload, seals and lubrication affect loss and limiting speed
External moment	Load offset, guide span, base and nut-bracket rigidity	Separates true axial load from parasitic moment caused by geometry
Thermal environment	Ambient, continuous time, cooling and permitted drift	High-preload multi-row sets can heat faster; fixed-fixed systems couple growth and preload

Code boundary: DF and DB identify contact-line orientation; DFD and DFF are multi-row combinations. The letters alone do not provide ratings. Use the complete model and current catalog.

02 / Four arrangements

“Heavy duty” can still mean different load directions and alignment sensitivity

Arrangement	Typical meaning	Better fit	Verify together
DF, face-to-face	Two rows provide general bidirectional axial support; shorter effective load-center span	Similar loads in both directions with a balance of envelope and installation tolerance	Both direction ratings, preload, speed and housing rigidity
DB, back-to-back	Two rows spread outward; normally a wider load-center span and greater moment rigidity	Higher moment rigidity where base, shaft-end geometry and alignment are controlled	Misalignment sensitivity, thermal differential, fits and model availability
DFD, three rows	Two rows support one direction and one row the opposite direction	A clear dominant cutting/pressing direction with smaller return load	Installation orientation and the weaker reverse direction
DFF, four rows	Two rows in each direction create symmetric reinforcement	High load and axial rigidity in both directions	Starting torque, temperature, width, cost, speed and lubrication

WBK DF DFD DFF heavy-duty arrangement drawings — Current HZMotion WBK drawings show DF, DFD and DFF options; the complete model controls configuration and length.

FBSA DB DF and multi-row heavy-duty arrangement drawings — FBSA includes DB/DF and multi-row options. Internal bearings cannot be inferred from housing shape.

03 / Same-size catalog example

As rows are added, load, rigidity, preload and torque move together

The data below is a HIWIN WBK25 catalog example used only to show the direction of change. It is not an HZMotion rating and must not be used to select an HZMotion model.

HIWIN WBK25 example	Normal-direction permissible axial load	Opposite-direction permissible axial load	Preload	Axial rigidity	Reference friction torque
DF	40.5 kN	40.5 kN	3.15 kN	1000 N/μm	0.29 N·m
DFD	81.5 kN	40.5 kN	4.3 kN	1470 N/μm	0.39 N·m
DFF	81.5 kN	81.5 kN	6.3 kN	1960 N/μm	0.49 N·m

What the example actually shows: DFD concentrates the gain in one direction; DFF increases both directions, while preload and friction torque continue to rise. Do not scale these values to another size or supplier.

04 / Rigidity and deflection

Catalog bearing rigidity is not complete feed-axis rigidity

δ ≈ F_a / k_sysfirst-order axial elastic deflection

δ: axial elastic displacement
F_a: axial load in the current condition
k_sys: combined bearing, screw, housing, bracket, joint and base rigidity

1/k_sys ≈ Σ(1/k_i)simplified series-compliance check

Bearing: arrangement, preload and bearing axial rigidity
Screw: effective length, diameter, tension/compression and temperature
Joints: housing, nut bracket, bolted interfaces and local base deformation

If the base, joint or long screw dominates system compliance, replacing DF with DFF will not improve machine positioning in proportion to the bearing catalog number. Build the compliance budget first.

05 / Starting torque and heat

The cost of “more rigid” appears in motor torque and thermal balance

P_loss ≈ M_f · 2πn / 60first-order friction-loss power

M_f: assembly friction torque, affected by starting state, seals, lubrication and temperature
n: screw speed in r/min
P_loss: loss that is mainly converted to heat and must leave through the housing/environment

Boundary often missed	Engineering effect	How to verify
Starting torque is not steady torque	Cold grease, seals and dwell can change the starting peak	Check motor start margin, low-speed stability and hot operation separately
More preload normally means more heat	Temperature can become the limit before static capacity at continuous high speed	Use model speed, duty and supplier friction data
Fixed-fixed couples thermal growth	Screw growth and end-to-end temperature difference change internal load	Review pretension, cooling, base and both housing temperature trends
Maximum axial load is not a life load	A structural limit cannot be substituted directly into fatigue life	Use supplier equivalent load, dynamic rating and life method

The current HZMotion manual gives approximately 20% of maximum axial load as a quick average-load life guideline. It is not a universal cross-brand life formula; formal review still needs dynamic rating, equivalent load, speed and duty.

06 / HZMotion heavy-duty families

Filter the mounting interface first, then lock the arrangement and complete model

Family	Typical arrangements in current drawings	Architecture	Selection reminder
WBK	DF, DFD, DFF	Axial/end-face-mounted heavy fixed end	Configuration changes row count and length; verify suffix and shaft-end length
SBK / SBK-U	DF, DFD	Block-type heavy fixed end	DFD is directional; orient the reinforced direction with the dominant load
FBSA	DB, DF and QBC/QFC multi-row options	Flanged large-size heavy fixed end	Do not treat every four-row option as the same arrangement; verify width and heat
MBK	DF, DFF	Compact heavy fixed end	When increasing bidirectional capacity in a small envelope, still check torque and cooling

HZMotion WBK heavy-duty fixed-end lineup — WBK: use the complete model for size, precision and arrangement.

HZMotion SBK heavy-duty fixed-end lineup — SBK/SBK-U: block architecture; DFD direction must match the load.

HZMotion FBSA heavy-duty fixed-end lineup — FBSA: larger sizes and multiple two-/four-row options.

HZMotion MBK heavy-duty fixed-end lineup — MBK: compact housing with DF/DFF options.

07 / Seven selection steps

Place the arrangement code inside the full engineering workflow

Build a two-direction load spectrumList peaks, duration, return load, shock, reversals and safety conditions.
Define the support architectureFixed-supported, fixed-fixed or pretensioned, including critical speed and thermal growth.
Filter the housing familyUse shaft-end drawing, center height, holes, envelope and maintenance direction.
Choose arrangement by directionStart from DF for general bidirectional duty; assess DFD for a dominant direction, DFF for two heavy directions and DB only where the model and geometry support it.
Verify rigidity and lifeUse both direction ratings, dynamic rating, preload, rigidity and supplier life method—not only maximum axial load.
Check motor and thermal marginInclude starting torque, steady friction, speed, duty, lubrication, seals and cooling.
Freeze the complete model and acceptanceConfirm orientation, accuracy, bearing detail, shaft-end drawing and locking, then establish runout, clearance, temperature and current baselines.

View heavy-duty fixed ends Download the support-unit manual

08 / Common mistakes

Comparing maximum axial load alone can produce a rigid but incorrect axis

Treating DFF as the default upgrade

It raises bidirectional capability, but also preload, torque, heat, width and cost.

Installing DFD backwards

The strong and weak directions differ; orientation belongs on the drawing and process.

Guessing internals from the housing

Similar housings can carry different rows, classes and preload.

Using a structural maximum as life load

Fatigue life needs equivalent load, dynamic rating, speed and cycle duty.

Ignoring base and joint rigidity

A stiffer bearing cannot correct compliance dominated by a long screw or weak joint.

Ignoring start and heat

A static-load-compliant multi-row set may still fail motor-margin or continuous-temperature checks.

09 / Quick answers

Heavy-duty arrangement FAQ

Is DFD always better than DF?

No. It fits a clear dominant direction. If reverse load is also high, the weaker direction can govern, while friction, width and cost rise.

Is DB always more rigid than DF?

DB normally has a wider load-center span and greater moment rigidity, but is more sensitive to geometry and temperature. Axial rigidity and ratings remain model-specific.

Can DFF halve axial deflection?

Not directly. Bearing rigidity is only one element in series with the screw, housing, joints and base.

Is a life calculation required if maximum axial load passes?

Yes. Maximum permissible load is a boundary check, not cyclic fatigue life. Use the load spectrum, equivalent load, dynamic rating and speed.

Why can multi-row arrangements run hotter?

More loaded rows and higher preload normally increase friction torque. At the same speed this raises loss, while seals, grease and cooling affect the result.