Hollow-shaft strain wave gear units are strain wave reducers whose flexspline cup and circular spline share a coaxial central bore sized to pass power cables, signal lines, pneumatic tubing, fiber optics, or laser beam paths directly through the rotational axis, eliminating the external service loops and drag chains that traditionally wrap multi-axis robot joints [S1][S4][S7].
The architecture preserves the standard strain wave operating principle: a wave generator deforms a thin-walled flexspline into a two-tooth-difference elliptical mesh against a rigid circular spline, producing single-stage ratios commonly in the 50:1 to 160:1 band with zero mechanical backlash [S1][S4]. Frame sizes currently offered by multiple OEMs run from size 14 (sub-30 mm bore) up to size 58, with rated torque scaling into the multi-kilonewton-metre range [S1][S5].
Through-bore geometry and how it changes the joint design
Routing media through the center of a strain wave gear removes the external cable bundle that otherwise has to flex every time the joint rotates, and the bore size is what governs how many lines and what diameter shaft can be passed [S1][S7].
Laifual's FHT "Large-Bore" variant is explicitly optimised for this, maximising the through-bore diameter for bundled multi-cable routing, hydraulic lines, and fiber optics in sizes 14 to 40, while the related FHG high-torque derivative claims 30% higher torque and 43% longer service life than the SHG reference [S1]. By contrast, maxon's strain wave gearhead page states only that the hollow shaft "can be designed large" in combination with a suitable motor, leaving bore diameter as a project-specific design variable rather than a published headline number [S4]. The geometry matters because the bore competes with the flexspline cup wall thickness for radial space, so maximum bore is a trade-off against rated torque and torsional stiffness in any given frame size.
Torque range, accuracy, and service life across the product class
Strain wave gearing in general is specified where zero backlash and 1 arc-minute repeatability matter more than absolute efficiency, and hollow-shaft variants inherit those numbers while adding the bore [S4][S5][S6].
Schaeffler's Ultra Precision Drives datasheet lists 10 Nm to over 7,000 Nm of rated torque across its precision strain wave and precision planetary lines, giving a realistic bracket for the upper end of what a single hollow-shaft stage can deliver in production hardware [S5]. Laifual rates the FH family for a 15,000-hour design life with the same cup-type flexspline and multi-segment tooth profile as its solid-shaft FS series, which functions as a useful order-of-magnitude benchmark for maintenance planning [S1]. Harmonic Drive's hollow-shaft gearhead line, including the HPF and equivalent zero-backlash units, is published at 1 arc-min positioning accuracy with bore diameters designed to take shafts, wires, tubing, or laser beam paths through the rotation axis [S6][S7].
Sub-lineup comparison: FH vs FHD vs FHN vs FHG vs FHT
Engineers selecting a hollow-shaft strain wave unit should match the sub-lineup to the dominant constraint, because the FH platform is split by bore size, torque, weight, and axial length rather than by a single "bore diameter" spec [S1].
Comparison on the four criteria that drive robot-joint selection: (1) Torque: FHG high-torque is the strongest, claiming 30% higher torque than SHG reference units; FH, FHD, and FHN are aligned with the SHF/SHD family in their respective sizes. (2) Weight: FHN lightweight cuts roughly 20% versus standard FH and targets cobots, mobile platforms, and UAV gimbals where mass matters more than peak torque. (3) Axial length: FHD ultra-short reduces axial length by about 50% versus the equivalent SHD, suiting thin robot wrists and vacuum chambers. (4) Bore capacity: FHT large-bore is the explicit choice for bundled multi-cable, hydraulic, and fiber-optic routing, accepting a likely torque penalty in the same size to enlarge the through-bore [S1]. Nidec's two hollow-shaft entries, the WPU-SDH flat type and the WPS-SD series, compete in the same space, with the WPS-SD adding a cross-roller bearing output for high-accuracy motion control and the WPU-SDH emphasising compact high-torque output with integrated FLEXWAVE gearing [S2][S3]. Harmonic Drive's HPF, by contrast, combines the large coaxial bore with a high-stiffness output flange so the driven load can be bolted directly to the reducer face [S7].
Selection criteria: who benefits and who does not
Hollow-shaft strain wave units are the right call when the joint rotates continuously or repetitively and the cable bundle cannot survive external flexing, but they are not the right call when maximum torque density in a small envelope is the only goal [S1][S4][S7].
The clearest fit is articulated robot arms: six- and seven-axis robots where every joint is a cable-bending hazard, surgical robotics where a sterile, snag-free exterior is mandatory, semiconductor and inspection equipment that needs to feed light or laser beam paths through a rotary axis, and collaborative robots where a lighter FHN-style unit trims payload mass at the wrist [S1][S4]. The wrong fit is any application that does not actually need to pass something through the axis, such as a fixed-base rotary table driven from the side, where a standard solid-shaft strain wave gear or a precision planetary will deliver higher torque density for the same envelope [S5]. For background on the underlying gear type itself, see the strain wave gearing primer and related entries on precision gearhead shaft coupling and shaft fastening practices that govern how the driven load is attached to the hollow output.
Application notes from OEM product pages
OEM-published use cases for hollow-shaft strain wave gearheads cluster around three sectors: robotics, medical/imaging, and semiconductor or optical handling, with the bore acting as a service channel rather than as a power-transmission feature [S1][S4][S7].
Laifual's FH through-bore design is documented to "carry power cables, signal lines, pneumatic tubing, and fiber optics simultaneously" inside a single bore, which simplifies multi-axis arm assembly because cables are pre-threaded during the arm build rather than dressed after the fact [S1]. Harmonic Drive's HPF is aimed at the same problem from a different angle: the large coaxial bore accepts cables, shafts, ball screws, or lasers through the axis, while the integrated output flange takes the reaction loads of the driven mechanism without an additional bearing block [S7]. Maxon's strain wave gearhead page lists Robotics, Aerospace, Medical technology, and Optical systems as the target markets, explicitly citing "media and cables fed through the drive unit" as a core advantage in those segments [S4]. A separate Laifual page (FHD) extends the same platform into vacuum chambers and tight spaces, where eliminating external cable outgassing sources is as important as reducing snag points [S1].
Limitations, failure modes, and integration trade-offs
Adding a through-bore is not free: the bore reduces the available wall thickness for the flexspline cup, which in turn caps rated torque and torsional stiffness in any given frame size, and the bore itself becomes a tolerance-critical surface that must be sealed against contamination [S1][S5].
Specific failure modes to design around: (a) cable fatigue at the bore exit, if the bore is too small or the cable jacket is too stiff, the flexure point simply moves from the joint exterior to the bore interior; (b) contamination ingress, because the bore is a direct path from the joint exterior to the wave generator bearing; (c) torque derating, where moving from an FH standard to an FHT large-bore in the same size sacrifices peak torque for bore diameter; (d) torsional stiffness reduction, a known consequence of thinning the flexspline cup wall, which can matter for high-acceleration point-to-point moves [S1][S5]. For related context on how feedback devices and encoder selection interact with strain wave joint design, see the Hall versus optical encoder feedback trade-off in linear actuators and the broader single-stage harmonic reducer ratio selection guide.
Standards, sourcing, and what to verify before purchase
No public ISO or IEC standard governs strain wave gear geometry directly, so the meaningful compliance signals are the OEM-published test data and the supplier's own quality certifications rather than a third-party harmonised standard number [S4][S5].
Specifications to confirm on a vendor datasheet before signing a purchase order: rated torque at the operating speed, not just the catalogue peak; 15,000-hour or equivalent life rating with the documented load profile; backlash figure stated in arc-minutes and the test method used; available bore diameter and the recommended maximum cable bundle cross-section; cross-roller or output bearing type and its moment stiffness; and any conformal coating or sealing option if the unit will sit near vacuum, washdown, or cleanroom environments [S1][S2][S5]. For the wider system design around the joint, the shaft key and shaft collar references cover the mechanical interface conventions that the hollow output still has to honour on the load side. Track the next design review against (1) the FHT large-bore variant's published maximum bore per frame size, since this is the most application-defining number and (2) any cross-roller versus standard output bearing update in the WPS-SD or HPF datasheets, because bearing type changes the moment load the reducer can absorb without an external bearing block [S1][S3][S7].