FH Series Hollow-Shaft Strain Wave Gear

The FH Series is a hollow-shaft strain wave gear with a large central through-bore that allows cables, air lines, vacuum tubes, or laser beam paths to pass directly through the gear center. This eliminates external cable management around rotating joints, reducing wear points and simplifying multi-axis robot wiring architecture.

All FH Series models share the same cup-type flexspline architecture and multi-segment tooth profile as the FS Series, with zero backlash and 15,000-hour service life. For engineering teams evaluating Harmonic Drive SHG, SHF, or SHD models, the FH Series provides a compatible hollow-shaft alternative with equivalent or higher torque ratings.

Through-Bore Design Advantages

In a conventional solid-shaft strain wave gear, cables and hoses must route externally around the joint, adding service loops, drag chains, and failure-prone flex points.

The FH Series through-bore design helps solve this challenge: cables pass through the gear center and exit at the next joint, staying protected inside the robot arm structure.

Measurable benefits of through-bore routing:

Longer cable life: Internal routing eliminates repetitive bending at the joint pivot, one of the major causes of cable fatigue failure in multi-axis robots
Cleaner robot exterior: No external cable bundles or drag chains, reducing snag risk in confined workspaces
Simplified assembly: Cables pre-threaded through the bore during arm build, eliminating post-assembly cable dressing labor
Multi-utility pass-through: A single bore carries power cables, signal lines, pneumatic tubing, and fiber optics simultaneously

Through-Bore Design Advantages FH Series — Product Overview

FHD

FHD Ultra‑Short (Sizes 14–32)
Axial length reduced by 50% (FHD‑I) vs standard SHD
Thin robot wrists
Vacuum chambers
Tight spaces

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FHN

FHN Lightweight (Sizes 14–40)
20% weight reduction vs standard FH
No direct HD equivalent
Collaborative robots
Mobile platforms
UAV gimbals

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FHG

FHG High‑Torque (Sizes 14–58)
30% higher torque + 43% longer service life vs SHG
Heavy‑duty joints
Press tending
Continuous operation

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Hollow type FHT

FHT Large‑Bore (Sizes 14–40)
Maximized through‑bore diameter for bundled cable pass‑through
Multi‑cable routing
Hydraulic lines
Fiber optics

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Lightweight Build and High Torque Output

FHN Lightweight Build

FHN models Strain wave gears mass by approximately 20% compared with equivalent-size standard FH models Weight savings come from thin-wall flexspline design, optimized housing geometry, and material selection, without sacrificing the rated torque capacity. For collaborative robots and mobile platforms where every kilogram affects battery life and dynamic response, this weight reduction directly improves the payload-to-weight ratio of the arm.

FHG High Torque Output

FHG models deliver 30% higher rated torque than standard FH at the same frame size, and extend service life by 43%. The gains come from upgraded flexspline material and deeper tooth engagement that distributes contact stress across a larger surface area. Designed for heavy-duty joints operating near their torque ceiling under continuous duty cycles.

Spec Overview Table

Parameter	FHD Ultra‑Short	FHN Lightweight	FHG High‑Torque	FHT Large‑Bore
Key Advantage	Axial length -50% (FHD‑I)	Weight -20% vs standard FH	Torque +30% Life +43%	Max bore diameter
Size Range	14 to 32	14 to 40	14 to 58	14 to 40
Reduction Ratio	50:1 to 100:1	50:1 to 160:1	50:1 to 160:1	301 to 160:1
Rated Torque @2000rpm	Equivalent to FSD class	Standard FH class (20% lighter)	Equivalent to FSG class (+30%)	Standard FH class
Backlash	≤20 arc sec	≤20 arc sec	≤20 arc sec	≤20 arc sec
Bore Diameter	Standard bore	Standard bore	Standard bore	Maximized bore
Design Life	9,000~10,000h	10,000~15,000h	10,000~15,000h	10,000~15,000h

Note: Complete specifications on individual sub-model pages.

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Applications

Multi-Axis Robot Arms

Each wrist joint on a 6-axis welding or painting robot routes power, signal, shielding gas, and wire feed through the hollow shaft, eliminating external cable bundles that snag on fixtures.

Frequently Asked Questions

When should I choose a hollow-shaft strain wave gear over a solid-shaft type?

Choose a hollow-shaft strain wave gears when cables, hoses, or optical fibers need to pass through the joint rotation axis. This is common in multi-axis robot arms (welding, painting, assembly), medical scanners, and rotary tables where external cable routing would create snag hazards or require complex drag chain systems.

What is the difference between FHD-I and FHD-III?

FHD-I achieves maximum axial compression (up to 50% shorter than conventional hollow-shaft designs) by integrating the output flange and bearing seat into a single component. FHD-III reduces length by 15% while retaining a larger through-bore diameter. Choose FHD-I when depth is the primary constraint (thin robot wrists, vacuum chamber manipulators). Choose FHD-III when you need both depth reduction and a bore large enough for multiple cable bundles.

How much weight does the FHN save versus standard FH models?

FHN models reduce mass by approximately 20% compared with equivalent-size standard FH models Weight savings come from thin-wall flexspline design, optimized housing geometry, and material selection. The torque rating is maintained at the standard level — the weight reduction is not at the expense of load capacity.

What does the FHG +43% service life extension mean?

Under identical load and speed conditions, an FHG unit is rated for 43% more operating hours than the standard FH model of the same size. For a typical industrial robot operating 16 hours per day, this translates to several additional months of production before scheduled replacement. The extension comes from upgraded flexspline material and a deeper tooth engagement profile.

Can I use the FH Series as a drop-in replacement for Harmonic Drive SHG/SHF/SHD?

The FH Series is designed for dimensional compatibility with SHG, SHF, and SHD hollow-shaft models at corresponding size points. Mounting bolt circles, output flange patterns, and input shaft interfaces follow industry conventions. Through-bore diameters may differ slightly by variant. Contact Laifual engineering with your current part number for a detailed interchange verification report.

Is the hollow shaft sealed against contamination?

The through-bore is not sealed by default — it is designed as an open passage for cables and lines. If your application requires a sealed bore (for vacuum or cleanroom use), Laifual offers custom sealing solutions including rotary shaft seals and protective bellows. Specify your sealing requirement at the RFQ stage.

What is the maximum cable bundle diameter that fits through the bore?

Through-bore diameter varies by reducer size. General guide: size 14 accommodates up to 5 mm bundle, size 20 up to 8 mm, size 32 up to 14 mm, size 40 up to 18 mm. These are clear-bore dimensions; for reliable cable routing, allow at least 1 mm radial clearance. Full bore dimension drawings are available for download on each sub-model page.