A through-hole fiber optic rotary joint is not simply a standard FORJ with a larger housing. The first engineering question is whether the center of the rotating axis must remain physically open after the complete rotary interface is assembled.
If a shaft, tube, fluid passage, RF path, cable bundle, or other mechanical element must continue through the axis, a conventional on-axis fiber optic rotary joint may not solve the problem because its optical path normally occupies the centerline. In that case, the project may require an off-axis or true through-bore fiber optic rotary joint, or a different arrangement that separates the optical joint from the clear center passage.
This guide focuses on that architecture decision. It does not replace a general FORJ selection guide. Instead, it shows how to define the open bore, mechanical envelope, optical link, hybrid services, and dynamic acceptance test before choosing a rotary interface.
First Decide What "Through-Hole FORJ" Means in Your Machine
The term "through-hole" can describe several different assemblies. They may look similar in a system drawing but leave very different space at the center of rotation.
| Architecture | What Occupies the Rotation Axis? | Is the Final Center Bore Open? | When to Evaluate It |
|---|---|---|---|
| Standard on-axis FORJ | The optical path and optical elements are arranged around the rotation axis. | No clear mechanical passage should be assumed. | Optical transfer is required and nothing else must pass through the center. |
| Electrical through-hole slip ring with a FORJ in the center | The electrical slip ring has a bore, but a separate FORJ is installed in that bore. | The electrical component has a bore, but the completed assembly may no longer have a usable open center. | Power or signals and fiber must share one axis, but the center does not need to remain open after integration. |
| True through-bore / off-axis FORJ | The optical transfer is moved away from the central mechanical passage. | Yes, subject to the confirmed finished drawing. | A shaft, pipe, fluid path, RF component, cable bundle, or other service must pass through the center. |
Patent literature distinguishes on-axis and off-axis FORJ architectures in the same way: an on-axis design uses the central region for the optical path, while an off-axis or through-bore design is intended to preserve routing space along the rotation axis. See the technical description in US8554029B2.
This distinction matters because a specification such as "50 mm through-hole slip ring with fiber" is incomplete. It does not say whether 50 mm is the bore of an electrical slip ring before a FORJ is installed, or the minimum clear opening that must remain available in the finished assembly.
Keep Geometry, Rotation Mode, and Optical Architecture Separate
"Through-bore" describes a geometric requirement. It does not, by itself, prove that a particular design supports unlimited continuous rotation, and it does not tell you whether the optical interface is fully passive or includes active electro-optical elements. Patent literature includes through-bore concepts with different operating principles, including designs intended for limited rotation and other off-axis approaches.
| Architecture Gate | Question to Put in the Specification | Why It Must Be Separate |
|---|---|---|
| Clear bore | What minimum passage must remain physically open after final assembly? | This is a packaging and machine-interface requirement. |
| Rotation mode | Is the motion unlimited continuous rotation, oscillation, or a defined number of turns? | A through-bore geometry does not establish the permitted rotation mode. |
| Optical architecture | Must the link remain passive fiber-to-fiber, or are active electro-optical elements acceptable? | Different architectures can have different interfaces, power needs, diagnostics, latency, serviceability, and qualification requirements. |
If the application requires all three conditions-an open center, unlimited continuous rotation, and a passive end-to-end optical path-state all three. Do not expect the phrase "through-hole FORJ" to communicate them automatically.
Define the Rotary Interface Before Selecting a Product
Start with the machine, not the catalog. Draw one boundary between the stationary side and the rotating side, then list every service that crosses it.
| Requirement Group | Questions to Define | Why It Changes the Architecture |
|---|---|---|
| Mechanical pass-through | What must physically pass through the center: shaft, tube, cable bundle, fluid line, RF hardware, or another component? | Determines whether a true clear bore is mandatory. |
| Optical link | Fiber type, wavelength, channel count, connectors, allowed insertion loss, rotational loss variation, and return-loss requirement. | Defines the optical interface and acceptance criteria. |
| Electrical services | Power circuits, control signals, encoders, sensors, copper data links, grounding, and shielding. | May require a separate or integrated electrical slip ring around the optical/mechanical architecture. |
| Other media | Air, gas, vacuum, coolant, hydraulic fluid, or RF/coaxial paths. | May require a rotary union or RF rotary joint and additional sealing or spacing. |
| Motion | Continuous rotation or oscillation, normal and maximum speed, direction changes, duty cycle, acceleration, runout, and vibration. | Changes bearing, alignment, cable-management, and dynamic optical requirements. |
| Environment | Temperature, dust, moisture, condensation, washdown, salt, pressure, vibration, and service access. | Changes materials, sealing, connectors, cable jackets, and qualification scope. |
If the project also carries power, control signals, or other media, review the complete rotary interface rather than treating the FORJ as a standalone component. ByTune's FORJ and slip ring integration guide explains how optical, electrical, mechanical, and test requirements interact in a combined assembly.
Specify a Functional Clear Bore, Not Just a Nominal Hole Diameter
The bore requirement should describe the usable space that must remain after the complete assembly is installed. A nominal hole diameter by itself does not capture shaft runout, tube fittings, cable bend space, assembly tolerances, or service clearance.
A useful mechanical definition includes:
- The minimum clear diameter required by the object or service passing through the axis.
- The maximum diameter of the shaft, tube, cable bundle, or fitting during assembly and operation.
- Concentricity, runout, and alignment conditions that can reduce practical clearance.
- Whether the pass-through component is stationary, rotating, or moves axially.
- Required clearance for connectors, couplings, clamps, or maintenance tools.
- Maximum allowed outside diameter and axial length of the rotary assembly.
- Mounting flange, pilot, shaft interface, and anti-rotation arrangement.
- Fiber and electrical cable exit direction, bend space, and strain relief.
Do not copy a bore range from a product-family page into a project specification. Bore size, outside diameter, length, mounting, speed, channel count, and sealing are linked mechanical decisions. The approved dimensions must come from the actual model drawing or custom design.
If the center only needs to carry a shaft or cable bundle and the optical joint can occupy the center, an electrical through-hole slip ring with a separate or integrated FORJ may be sufficient. If the center itself must remain open after the optical hardware is installed, the requirement is different and should be stated explicitly as a clear-bore requirement.
Define the Optical Link Before You Define the FORJ
A through-bore requirement does not reduce the importance of the optical system. The FORJ still has to fit inside the complete link budget and remain stable while rotating.
At minimum, specify the following for every optical path:
| Optical Field | What to Provide | Selection Purpose |
|---|---|---|
| Fiber type | Single-mode or multimode, matched to the installed system. | Prevents an incompatible rotary section from being inserted into the link. |
| Operating wavelength | The actual wavelength or wavelength range used by the transceiver system. | Optical performance must be evaluated at the operating wavelength. |
| Channel count | Number of independent optical paths required by the system architecture. | Changes optical complexity and packaging. |
| Connector / termination | Connector family, polish where relevant, pigtail length, jacket, and exit direction. | Controls mating compatibility, routing, cleaning, and service access. |
| Maximum insertion loss | Project-specific maximum at the stated wavelength and measurement boundary. | Consumes part of the system optical budget. |
| Insertion-loss variation during rotation | Permitted dynamic variation and the speed, direction, duration, and sampling conditions of the test. | Checks whether acceptable static loss remains stable in motion. |
| Return loss | Minimum requirement when it matters to the source, receiver, connector system, or application. | Controls reflected optical power within the complete link. |
Avoid specifying the rotary joint only by a headline data rate. The transceivers, fiber, connectors, fixed cable loss, adapters, FORJ loss, dynamic variation, and engineering margin all contribute to link performance. If the optical power budget is not yet defined, establish it before approving the rotary component.
For a broader treatment of channel count, optical budget, insertion loss, return loss, and dynamic variation, use the fiber optic rotary joint integration and testing guide. The purpose of the present page is to keep those optical requirements tied to the clear-bore architecture.
Choose the Architecture From the Center-Out
Once the bore and optical requirements are known, compare the possible arrangements from the center of the machine outward.
Option 1: Standard On-Axis FORJ
Use a conventional on-axis FORJ when optical transfer is required but no mechanical passage needs to remain open through the rotation axis. This is usually the first architecture to evaluate because it avoids creating an off-axis optical path solely for packaging reasons.
If electrical power or signals are also required, the on-axis FORJ may be combined with an electrical slip ring around it. In that arrangement the surrounding electrical slip ring can have a through-bore, but the final system center is occupied by the FORJ. The resulting assembly should not be treated as a true open-bore optical interface.
Option 2: True Through-Bore / Off-Axis FORJ
Evaluate an off-axis or true through-bore FORJ when the central passage has a function that cannot be displaced. The optical transfer then has to occur outside that protected center space or by another architecture that preserves the bore.
This choice should be driven by the machine requirement, not by terminology. The RFQ should state what must pass through the bore and the minimum clear opening that must remain after final assembly. A supplier can then determine whether an off-axis optical architecture is appropriate for the required rotation, channel count, optical performance, and mechanical envelope.
Option 3: Separate Concentric or Stacked Components
Sometimes the most serviceable solution is not a single integrated housing. A machine may use separate optical, electrical, and fluid rotary components arranged concentrically or axially, provided the resulting assembly still preserves the required bore and meets alignment and cable-routing constraints.
This can simplify replacement of one technology, but it transfers more interface responsibility to the machine designer: concentricity, brackets, cable exits, anti-rotation features, bearing loads, and service clearance must all be controlled in the system drawing.
Map Hybrid Services Around the Bore
When the same axis carries more than fiber, create a channel and media map before discussing housing size. "Hybrid" is not one specification; each transmission method has its own design and validation requirements.
| Service Crossing the Interface | Define Before Design | Validation Focus |
|---|---|---|
| Fiber optic | Fiber mode, wavelength, channels, connectors, optical budget, dynamic loss limits. | Insertion loss, rotational variation, return loss where applicable, end-to-end link operation. |
| Electrical power | Voltage, continuous current, peak/inrush current, duty, conductor and connector requirements. | Temperature rise, insulation, voltage drop, dynamic contact behavior as applicable. |
| Low-level signals | Signal function, level, grounding, shielding, cable type, and noise sensitivity. | Noise, continuity, crosstalk, and real sensor/encoder operation. |
| Copper data | Protocol and physical layer, data rate, cable/pair geometry, shielding, connector, nearby power circuits. | Real communication test during rotation, not continuity alone. |
| RF / coaxial | Frequency range, power, impedance, connector, insertion/return loss requirements. | RF performance across rotation and operating conditions. |
| Air / gas / vacuum / liquid | Actual medium, pressure, temperature, flow, ports, materials, leakage requirement. | Leakage, pressure, compatibility, speed, and environmental test. |
If the center bore is reserved for one of these services, mark it on the mechanical drawing before the optical architecture is selected. Do not assume the remaining radial space will automatically accommodate the electrical rings, optical hardware, seals, bearings, and cable exits.
Projects that require a purpose-built combination can use ByTune's custom slip ring page as the commercial starting point. The technical RFQ should still define each medium separately.
Rotation Can Change an Optical Design That Looks Fine at Standstill
A rotary optical interface should be evaluated as a moving system. A stationary insertion-loss measurement is useful as a baseline, but it does not demonstrate that the link remains stable at operating speed or through the machine's full motion profile.
Include the following motion conditions in the specification:
- Normal operating speed and maximum required speed.
- Continuous rotation, oscillation, or a limited number of turns.
- Clockwise, counterclockwise, and reversal behavior.
- Acceleration and deceleration if they are significant to the mechanism.
- Expected duty cycle and test duration.
- Shaft runout, vibration, and shock conditions that can disturb alignment.
- External cable routing and strain relief so the fiber does not apply unintended load.
- Operating temperature and any environment that can change alignment, seals, or cable behavior.
This is especially important for a true through-bore architecture because the optical system, mechanical bore, bearings, and any surrounding hybrid channels compete for the same radial package. The accepted design has to work as one assembly, not as independent catalog specifications.
Use a Dynamic Acceptance Test, Not Only a Datasheet Check
Define the test method before ordering. The goal is to verify the installed requirement under rotation rather than to collect a list of typical component values.
- Define the measurement boundary. State whether the result includes the FORJ pigtails, adapters, patch cables, panel connectors, or only the rotary component.
- Establish a stationary optical reference. Use the required fiber type, wavelength, connector arrangement, and reference method.
- Measure while rotating. Test at the relevant speeds and directions and record optical power or insertion loss over angle, time, or revolutions.
- Check the worst rotational variation. Compare the measured variation with the project acceptance limit, not with an unrelated catalog target.
- Verify the real optical link. Where practical, run the intended transceivers or communication equipment through the rotary interface in addition to component-level optical measurements.
- Test hybrid functions together when interaction matters. If electrical power, data, RF, or fluid circuits operate at the same time, reproduce the combined operating condition defined by the system.
- Repeat under required environmental conditions. Temperature, vibration, pressure, moisture, or other exposures should be included when they are part of the project specification.
For a new design, request that the supplier and system integrator agree on the acceptance limits, test speed, measurement boundary, and reporting format before production. That reduces disputes caused by comparing a supplier's component test with a customer's different installed-system measurement.
RFQ Checklist for a Through-Hole Fiber Optic Rotary Joint
A useful RFQ should make the center-bore requirement impossible to misinterpret. Include:
- A simple drawing showing the stationary side, rotating side, rotation axis, and available envelope.
- The object or medium that must pass through the center.
- The minimum clear bore required after the full rotary interface is assembled.
- Maximum outside diameter, axial length, flange or shaft interface, and cable-exit restrictions.
- Normal and maximum speed, and whether the system requires unlimited continuous rotation, oscillation, or a defined number of turns.
- Whether the optical interface must remain passive end-to-end or whether active electro-optical conversion is acceptable.
- Direction changes, duty cycle, runout, vibration, and shock requirements.
- Single-mode or multimode fiber as required by the existing optical system.
- Operating wavelength or wavelength range.
- Number of independent optical channels.
- Connector or pigtail requirements, cable length, jacket, bend constraints, and strain relief.
- Maximum insertion loss and permitted insertion-loss variation, with measurement conditions.
- Return-loss requirement if it is relevant to the installed optical system.
- All electrical power and signal circuits with their individual ratings and functions.
- Any copper data, RF, pneumatic, hydraulic, vacuum, or other media crossing the interface.
- Temperature, ingress, corrosion, pressure, and other environmental requirements.
- Required drawings, test reports, inspection records, and acceptance procedures.
Avoid writing "custom through-hole FORJ, high speed, low loss, waterproof" as the complete request. Those words do not define the geometry, optical budget, speed condition, sealing boundary, or acceptance test.
When a True Through-Bore FORJ Is Not the Better Choice
Do not preserve a clear center bore unless the machine actually needs it. If the rotation axis can be occupied by the optical joint, a standard on-axis FORJ or a conventional hybrid assembly may provide a simpler mechanical architecture.
Use a true through-bore approach when the open center performs a real system function that cannot be relocated. If the only reason is that "through-hole sounds more flexible," the added packaging and integration constraints may provide no practical benefit.
The same rule applies to integration. If the project needs optical data plus electrical power but no open center, start with the broader hybrid FORJ selection guide. If the main challenge is combining and validating optical and electrical subsystems, use the FORJ and slip ring integration guide. The present page is for projects where preserving the center passage changes the architecture itself.
A Practical Selection Rule
Start with one question: must the center of the completed rotating interface remain open?
If the answer is no, evaluate a standard on-axis FORJ or a conventional hybrid arrangement first. If the answer is yes, define exactly what passes through the center, the minimum finished clear bore, the surrounding mechanical envelope, and every service that must cross the rotating boundary. Then add the optical requirements and agree on a dynamic acceptance test.
That sequence prevents a common integration mistake: choosing a component that has a "through-hole" somewhere in its description but discovering later that the optical hardware, connector, shaft, or hybrid module occupies the space the machine actually needed.