Choose a hollow ball screw when internal forced cooling is required and acceptable stiffness loss can be controlled; THK offers 20, 25, and 32 mm hollow shafts with 6 or 8 mm bores.
The decision is not simply hollow versus solid: cooling at the nut can deliver capacity equal to or greater than a hollow-shaft arrangement without sliding seals or rotary joints [S1].
Cooling architecture and selection thresholds
Forced cooling requires a liquid path through the screw, so the shaft bore, seals, coolant compatibility, pump arrangement, and heat rejection must be designed together. THK’s hollow-shaft New N series lists screw outer diameters of 20, 25, and 32 mm, internal diameters of 6, 8, and 8 mm, maximum shaft lengths of 2,000, 3,000, and 3,000 mm, and maximum thread lengths of 1,500, 2,000, and 2,000 mm respectively [S2].
Those dimensions define the practical cooling envelope. The listed series is available in caged-ball types SBN-V, SDA-V, and SDAN-V, plus full-ball types SDA-VZ, SDAN-VX, BIF-V, BNFN-V, BNF-V, EBB-V, and EPB-V, with C3, C5, and C7 lead accuracy classes [S2].
Cooling at the nut is the cleaner starting point when a hollow shaft is not justified. NSK states that its nut-cooled ball screws achieve cooling capacity greater than or equal to existing hollow-shaft ball screws, with no sliding seals or rotary joints required [S1]. This configuration is preferable where the thermal load can be removed at the nut, the machine does not need internal screw circulation, and avoiding rotating fluid connections is a priority.
Structural performance: stiffness versus inertia
A hollow shaft removes material from the region that contributes strongly to bending stiffness and torsional rigidity, so cooling gain must be purchased with a mechanical penalty. For the same outside diameter, a solid shaft has maximum torsional rigidity and load-bearing capacity; the hollow design reduces mass but can lower stiffness [S4].
The mechanism matters in practice. Bending resistance depends strongly on material near the outer surface, while axial thrust capacity uses the net section. Torsional rigidity depends on the polar section modulus, which falls sharply as the bore grows. A hollow shaft can therefore be appropriate when axial load dominates and high speed, rapid traverse, or repeated acceleration makes inertia important, but it is a poor choice when bending stiffness is the governing constraint.
Published comparisons of large hollow and solid spindles do not support a universal statement that hollow shafts are stiffer. One technical comparison describes hollow shafts as lower in mass and usually lower in rigidity than corresponding solid spindles [S7], while another discussion emphasizes the fundamental tradeoff between rigidity and rotational inertia [S3]. For design work, calculate section properties and bearing reactions rather than treating “hollow” as inherently lighter, stronger, or more rigid.
Thermal performance and positioning accuracy
Internal cooling is most valuable when friction heat causes measurable axial growth in a precision feed drive. A finite-element study of a high-speed ball-screw system modeled heat generation in the screw, nut, and bearings and used a maximum screw speed of 40 m/min. Its results showed that effective cooling countermeasures improved positioning accuracy by reducing thermal deformation [S5].
That matters because thermal expansion along the screw axis moves the nut relative to the commanded position. Internal cooling also allows coolant to be delivered directly to the region generating heat instead of relying only on external heat rejection from the shaft surface.
Cooling does not eliminate the need for thermal design. The study modeled the screw and nut deformation using linear elasticity with the material assumed to be elastic, homogeneous, and isotropic, and used a three-dimensional unsteady heat-conduction model with heat generation from the main sources, including the ball screw shaft, nut, and bearings, to determine temperature rises and axial thermal deformations under operating conditions [S5]. Actual results depend on coolant flow, inlet temperature, heat-transfer coefficients, bearing heat, preload, duty cycle, and the machine structure. A hollow bore is a means of controlling temperature, not a guarantee of a specified positioning error.
Option comparison by thermal duty and machine dynamics
For a continuous-duty precision axis with a demonstrated thermal-growth problem, hollow shafts with forced internal cooling offer the strongest direct access to the screw interior, but the bore reduces torsional rigidity and adds fluid-system interfaces [S2][S5]. For a machine with moderate duty and limited thermal growth, a solid shaft normally provides simpler construction, predictable stiffness, and fewer cooling failure modes; these benefits explain why solid shafts remain a common baseline [S4].
For high traverse rates or rapidly reversing axes, hollow construction can reduce moving mass, but the outcome depends on where material is removed and on the complete drive package. A hollow ball screw is only one part of the moving system, and support bearings, couplings, the nut, and the table still determine acceleration and positioning behavior. Compare the complete moving mass and the selected drive’s acceleration capability rather than using bore diameter alone.
For installation simplicity and leak risk, nut cooling wins when its thermal performance meets the calculated heat load. NSK specifies capacity equal to or greater than existing hollow-shaft systems without sliding seals or rotary joints [S1]. That removes rotating seals and rotary fluid joints from the comparison, but it does not eliminate the need to verify nut temperature, mounting conditions, coolant distribution, and heat rejection.
The final choice is conditional: select a hollow shaft when internal flow materially improves thermal control and the calculated stiffness margin remains acceptable. Select a solid shaft when thermal duty is moderate, structural stiffness governs, or installation simplicity dominates. Select nut cooling when it meets the heat-removal requirement and avoids the leakage and service burden of a hollow rotating circuit.
Failure modes, interfaces, and procurement checks
Forced cooling introduces failure modes absent from an uncooled solid screw. Air pockets can reduce effective heat transfer, deposits can obstruct the bore, coolant seals can age, and a pump or flow-control failure can convert an intended cooling feature into rapid thermal growth. Specify maximum coolant temperature, flow range, permissible pressure, seal material, fluid compatibility, and the response to loss of flow before releasing the axis for production. [S5]
The shaft construction also limits attachment options. A hollow shaft may complicate the installation of couplings, bearing locknuts, synchronizers, and support components because the bore occupies the shaft center. The practical-machinist discussion identifies cost and restricted drive or attachment options as reasons hollow large-diameter ball screws are not universal [S3]. For support and drive layout, check the selected ball screw coupling and bearing arrangement against the actual shaft ends before ordering.
Do not infer that a hollow shaft improves every dynamic metric. Lower mass can improve acceleration, while reduced torsional stiffness can worsen twist under load and make servo following more sensitive. Verify critical speed, maximum speed, axial load, allowable moment, life rating, and lead accuracy for the specific manufacturer’s size and grade. THK’s published hollow dimensions and accuracy classes provide a concrete starting envelope, but the product drawing and load table remain controlling documents [S2].
Sizing method and final design checks
Begin with the thermal load, not the shaft diameter. Estimate the heat that must be removed, identify the acceptable temperature rise and axial growth, then compare nut cooling with internal shaft cooling on that same basis. Internal circulation has the stronger physical case when heat must be removed from the screw itself and when the hollow design’s cooling surface and flow path provide a measurable advantage [S5].
Next, calculate the mechanical penalties using the actual outer and inner diameters. THK’s 32 mm hollow option has an 8 mm bore, while the 25 mm option also has an 8 mm bore [S2]. Those are catalog geometry facts, not proof that one diameter is stiffer or stronger; material grade, thread diameter, support span, bearing arrangement, and load direction determine the result.
Finally, treat cooling as a machine-level system. Confirm lead accuracy, maximum shaft and thread length, allowable speed, load rating, bearing support, coupling fit, coolant circulation, seal arrangement, and maintenance access. Record the selected configuration on the drawing, including whether the cooling circuit is through the shaft, through the nut, or absent, and define a verification test for temperature rise and axial positioning error before production release.
Track two acceptance signals at commissioning: screw temperature or temperature gradient and axis positioning error during the intended duty cycle. If the selected hollow-shaft system controls both without violating the stiffness margin, it is technically justified; if nut cooling meets the same limits with fewer interfaces, the added shaft bore and fluid circuit are not justified.