The ball screw support bearing arrangement determines how the screw is located, how axial thrust is transferred to the machine structure, and how the shaft responds to rotation, deflection, vibration, and thermal expansion. Even a high-accuracy ground ball screw cannot deliver the expected positioning performance if its support bearings are incorrectly selected or installed.
The most common ball screw bearing arrangements are fixed–fixed, fixed–floating, and fixed–free. Each configuration provides a different balance of axial rigidity, allowable speed, buckling resistance, thermal behavior, installation difficulty, and cost.
What Does a Ball Screw Support Bearing Arrangement Do?
A ball screw support bearing arrangement locates and supports the rotating screw shaft. Its main functions include:
- Locating the screw axially and radially
- Transferring axial thrust to the machine structure
- Maintaining a stable rotational center
- Limiting unwanted axial displacement
- Increasing the axial rigidity of the feed system
- Controlling shaft deflection and vibration
- Accommodating or controlling thermal expansion
The support bearings do not normally carry the machine table's main radial load or overturning moment. These loads should generally be supported by the linear guideways, while the ball screw provides the axial force required to move the load.
Why Bearing Support Matters on a Ground Ball Screw
The same support principles apply to both rolled and ground ball screws. However, bearing selection and installation become more critical when using a precision ground ball screw because bearing clearance, shaft-end runout, housing deformation, and misalignment can reduce the benefit of the screw's tighter lead accuracy.
The positioning performance of a complete ball screw axis depends on several related factors:
- Ball screw lead accuracy
- Ball nut preload or axial clearance
- Support-bearing rigidity and preload
- Shaft-end machining accuracy
- Bearing-seat and housing accuracy
- Alignment between the screw, nut, and linear guideways
- Thermal expansion during operation
- Feedback and control method
A high-accuracy ground ball screw cannot compensate for excessive bearing clearance or poor installation alignment.
Fixed-End Bearings in a Ball Screw System
The fixed end establishes the axial position of the ball screw. It commonly uses a matched pair of angular contact bearings capable of carrying axial loads in both directions. Purpose-designed ball screw support bearings may also be selected according to load, speed, rigidity, and service-life requirements.
Bearing preload helps reduce internal clearance and increase axial rigidity. However, excessive preload can increase friction, running torque, heat generation, and bearing wear. Preload should therefore match the actual operating requirements rather than simply being made as high as possible.
Common fixed-side support units include BK, FK, and EK types. The appropriate unit depends on the screw diameter, machined shaft-end dimensions, mounting direction, and required rigidity.
Floating-End Bearings in a Ball Screw System
A floating bearing arrangement does not necessarily mean that the end of the screw is left unsupported. In most fixed–floating systems, the floating end contains a radial bearing that maintains the rotational alignment of the screw while allowing axial movement.
This axial freedom allows the screw to expand or contract as its temperature changes without generating the additional internal force that may occur when both ends are rigidly constrained.
BF, FF, and EF are common floating-side support units used with BK, FK, and EK fixed-side units. For example, BK/BF is a frequently used combination for standard ball screw assemblies.
The floating end should not be installed in a way that unintentionally clamps the screw axially. Otherwise, the system may behave like an incorrectly assembled fixed–fixed arrangement and develop excessive load as the screw temperature rises.
Fixed–Fixed Ball Screw Bearing Arrangement
In a fixed–fixed ball screw bearing arrangement, both ends of the screw provide axial constraint. When the screw is correctly tensioned between the two bearing groups, this configuration provides high axial rigidity and greater resistance to shaft deflection.
The stronger end restraint can also increase the critical-speed potential of the screw. Fixed–fixed support is therefore frequently considered for long screws, high-speed axes, precision machine tools, and applications requiring high dynamic rigidity.
Advantages of fixed–fixed support
- High axial rigidity
- Greater resistance to buckling under compression
- Higher critical-speed potential
- Better control of long or heavily loaded screws
- Suitable for high-dynamic and precision feed axes
Installation considerations
The two bearing groups must be accurately aligned. Installation errors can produce abnormal torque, vibration, heat, excessive internal load, and premature bearing wear.
Thermal expansion must also be considered. Because both ends are constrained, an increase in screw temperature can create additional axial force. Precision machinery may use calculated screw pretension, temperature control, position compensation, or another engineered method to manage this effect.
Uncontrolled bearing clearance should not be used as a substitute for proper thermal design because it can reduce axial rigidity and reversal accuracy.
Fixed–Floating Ball Screw Bearing Arrangement
In a fixed and floating bearing arrangement, the fixed end controls the screw's axial position and carries axial thrust. The floating end supports the screw radially while allowing axial movement caused by thermal expansion.
This configuration is also called a fixed–supported arrangement because the floating end is still radially supported. The terms "floating end" and "supported end" therefore often describe the same function in a conventional ball screw assembly.
The fixed–floating arrangement provides a practical balance of rigidity, speed capability, thermal freedom, installation tolerance, and cost. It is widely used in CNC equipment, industrial automation, linear modules, handling systems, and other machine axes.
Advantages of fixed–floating support
- Better shaft support than a fixed–free arrangement
- Controlled axial positioning at the fixed end
- Allows axial thermal movement at the floating end
- Lower installation sensitivity than fixed–fixed support
- Suitable for many standard machine axes
Its axial rigidity and critical-speed potential are generally lower than those of a correctly designed fixed–fixed arrangement. However, it is suitable for many medium-length and moderate-speed ball screw systems.
Fixed–Free Ball Screw Bearing Arrangement
In a fixed–free arrangement, only one end of the screw is supported and axially located. The other end remains unsupported.
This arrangement is mechanically simple and allows the screw to expand freely. However, it provides the lowest resistance to bending and vibration among the three configurations. Its critical speed and compression-load capability are also limited.
Fixed–free support is generally used for short screws, low-speed mechanisms, light loads, and applications where compact construction is more important than maximum rigidity.
A fixed–free arrangement should not be confused with a fixed–floating arrangement. The floating end of a fixed–floating system normally has radial bearing support, while the free end of a fixed–free system does not.
How the Bearing Arrangement Affects Critical Speed
A rotating ball screw can begin to vibrate when its operating speed approaches its critical speed. The critical speed is influenced by the screw root diameter, unsupported length, end-bearing arrangement, installation accuracy, and rotational speed.
As a general comparison:
- Fixed–fixed provides the strongest end restraint and the highest critical-speed potential.
- Fixed–floating provides moderate restraint and is suitable for many conventional machine axes.
- Fixed–free provides the least restraint and is normally limited to shorter and slower screws.
The working speed should remain safely below the calculated critical speed. Ball circulation limits, bearing speed, lubrication, heat generation, and the ball screw's DN value must also be considered. Critical speed is not the only limit on operating speed.
How the Arrangement Affects Buckling
When a ball screw pushes a load, part of the screw shaft may be subjected to compression. A long and slender screw can buckle before the screw-and-nut assembly reaches its nominal dynamic load capacity.
The support condition has a direct effect on buckling resistance. Fixed–fixed support generally provides greater resistance than fixed–floating or fixed–free support. However, the actual calculation must use the screw root diameter, unsupported length, load direction, and real mounting condition.
Where the machine structure permits, arranging the ball screw so that it works mainly in tension rather than compression can help reduce buckling risk.
How Thermal Expansion Affects the Selection
Heat can be generated by the support bearings, ball circulation, seals, lubricant, motor transmission, and the surrounding machine. As the temperature of the screw increases, the screw becomes longer.
In a fixed–floating bearing arrangement, the screw can expand toward the floating end. This prevents the floating-side bearing from rigidly opposing the change in length. However, screw elongation may still affect the positioning accuracy of the axis.
In a fixed–fixed arrangement, correctly applied screw tension can improve rigidity and help control thermal behavior. The initial tension, operating temperature, bearing load, and expected expansion must be considered together to avoid overloading the screw or bearings.
High-accuracy systems may use one or more of the following measures:
- Calculated screw pretension
- Temperature stabilization
- Controlled lubrication
- Position compensation
- Linear-scale feedback
- Reduced unnecessary friction and preload
Comparison of Ball Screw Bearing Arrangements
| Arrangement | Relative rigidity | Thermal behavior | Typical application |
|---|---|---|---|
| Fixed–fixed | High | Requires calculated control of pretension and thermal expansion | Long, fast, rigid, or precision axes |
| Fixed–floating / fixed–supported | Medium | Allows axial movement at the floating end | General CNC and automation axes |
| Fixed–free | Low | Allows free expansion | Short, slow, and lightly loaded mechanisms |
Choosing a Ball Screw Bearing Arrangement
The correct ball screw bearing arrangement depends on the complete operating conditions rather than the screw accuracy grade alone. Before selecting the support method, confirm:
- Screw nominal and root diameter
- Total and unsupported length
- Screw lead
- Maximum rotational speed
- Axial load and load direction
- Mounting orientation
- Required axial rigidity
- Expected temperature change
- Positioning and repeatability requirements
- Available installation space
A fixed–fixed arrangement provides higher rigidity and critical-speed potential but requires accurate alignment and proper thermal control. A fixed and floating bearing arrangement offers a practical balance of support and thermal freedom for many CNC and automation axes. A fixed–free arrangement is simpler but is normally limited to short, low-speed, and lightly loaded screws.
Installation Details That Affect Performance
Even the correct bearing arrangement may perform poorly if the assembly is misaligned. Important installation checks include:
- Concentricity of the machined shaft ends
- Squareness of the bearing-housing mounting surfaces
- Alignment between the screw axis and linear guideways
- Correct orientation and preload of the fixed-end bearings
- Correct tightening of the bearing locknut
- Axial freedom at the floating end
- Uniform running torque over the complete stroke
- Correct lubrication before operation
The ball screw should rotate smoothly after assembly. Binding, abnormal torque variation, repeated noise, or rapid temperature rise may indicate misalignment, excessive bearing preload, incorrect bearing installation, or axial interference at the floating end.
DLY Ball Screw and Support Options
DLY supplies rolled and ground ball screws together with corresponding shaft-end machining and support-unit options. Available fixed- and floating-side units include common BK/BF, FK/FF, and EK/EF combinations.
View DLY's ball screw support units or explore the complete ball screw range.
For a project review, provide the screw model, diameter, lead, length, maximum speed, axial load, accuracy grade, proposed bearing arrangement, and shaft-end drawing. The ball screw, support bearings, and end machining can then be evaluated as one assembly.
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