Through Bore Slip Rings vs. “Wireless” Rotary Joints: An In-Depth Comparison

Rotary joints are tools used to transfer power or data in moving systems, like robots. This article compares through-bore slip rings and wireless rotary joints. You’ll get to know how they work, when to use them, and their upsides and downsides. This will help you choose the right one.

Overview Of Through‑Bore Slip Rings

Core structure & features

A through-bore slip ring sends data or electricity through rotating parts. Its defining feature is the hole in the middle. This hole allows routing shafts, fibers, or tubes to pass through. The diameters of these bores range from inches to tens of centimeters. Most of the designs allow modular multi-circuit stacks.

Most through-bore slip rings use multiple ring and brush pairs. These are stacked inside the housing. Some use fiber-brush or precious-metal contact assemblies. These reduce friction and dust. Some of these versions are also sealed against dust and water, which have different IP ratings.

Capabilities

Power

Through-bore slip rings are very good at handling power transmissions. A single assembly can have rings rated for 10 A. and support voltages that are up to 380 volts AC even higher. It is also possible to combine many rings in one unit. Some models carry 24 power circuits and 72 signal circuits at once.

Data Transmission

When it comes to data transmission, many through-bore slip rings support various protocols. They can send Ethernet, CAN, and other low-level control signals. The data speeds can reach up to 1 Gb/s with advanced brush technology.

At high RPMs, friction and wear can reduce data quality. That’s why maximum rotational speeds are limited to around 200–500 RPM.

Environmental Constraints

These slip rings can be susceptible to dust and debris. Different contaminants can get inside and block the contact path. In addition to external dust and debris, internal electric brushes and rings wear can also produce abrasive particles, so it is necessary to clean the interior dust regularly. The life-time of components largely depends on speed, total wear cycle and maintenance.

Overview Of Wireless (Contactless / RF) Rotary Joints

Wireless or rotary joints are known to send power or data without touching parts.They make use of optical，magnetic or electric fields to work.

Transmission Technologies

Magnetic Coupling

This makes use of magnetic fields to transfer power or data. It is more suitable for moderate power and low-speed data ( <100 Mbps ). It is common in Oil drilling system and marine systems.

Capacitive Coupling

This technology uses electric fields to send high-speed data. Usually the transmission speed can reach 800M/s-300G/s. It also offer a very low bit-error rate (<1 ×10^-12),and a very low latency (< 1 μs).

RF

This type sends data using radio waves, and it works at very high speeds. RF systems with 60 GHz millimeter-wave chips can hit 1 to 5 gigabits per second. They also offer a very low bit-error rate (<1 ×10⁻¹²). These systems also tolerate misalignments.

System design & features

A wireless joint is a two-part system. It consists of a stationary transmitter and a rotating receiver. These parts are usually separated by a small air gap. This gap can range from 1–2 mm to several centimeters, depending on the technology.

No Upkeep Required

This design enables a maintenance-free operation. This means no brushed or sliding contacts that wear out. With this, the tool gets a very long lifetime with high cycle counts. It also means that there is minimal friction, low torque, and no carbon dust generation.

Environmental Tolerances

Wireless joints are less sensitive to dust, moisture, and vibration. Most meet IP65 standards or better. They work very well across wide temperature ranges, typically between –40 °C to +85 °C.

Power Handling

The power handling capability of magnetic coupling wireless Rotary Joints is excellent, with transmission from a few watts to several hundred KWs. It should be noted that the transmission efficiency of this solution is generally between 80% and 95%, and some heat is generated, so heat dissipation should be considered during use.

Head‑to‑Head Technical Comparison

Pros & Cons Summary

When to favor Through‑Bore Slip Rings

Choose through-bore slip rings when:

● You need to send data and power through a central shaft.

● Your system uses moderate speeds ≤ 500 RPM.

● You need a lower upfront cost and can manage maintenance every few months.

When Wireless Rotary Joints are better

Wireless Rotary Joints are more suitable when:

● You need high data speeds of 1 Gb/s or more.

● Your system needs a long life-time with zero maintenance.

● You need very low or negligible torque.

● You work in a dusty or wet environment.

Hybrid approaches

Some applications benefit from a hybrid solution. They could use both:

● Slip rings for power

● Wireless joints for data

It keeps the currents strong while keeping signals clean. This approach offers the best solution for very complex systems.

Application Scenarios & Decision Matrix

Wind Turbine Blade Control

Wind turbine blades need a strong and stable power delivery for pitch control motors.Usually regular through-hole slip rings are used for onshore wind turbines. Due to the difficulty of maintenance and higher reliability requirements for offshore wind turbines, hybrid slip rings are more suitable.

Medical CT Scanner Gantry

For medica CT Scanner, Imaging systems require extremely high data throughput. These systems also need slip rings to reliably transmit large currents and control signals. Typically, a hybrid slip ring combining capacitive coupling wireless transmission for data transmission and contact transmission for current is used, and sometimes the slip ring also needs to integrate an encoder.

Packaging or rotating test rigs, and cable reels

Cable reels and test rigs usually carry light to mid-range power and some data. A through-bore slip ring is very good for these setups, especially with IP-rated seals.

Conclusion

The best rotary joint to use is usually the one that gives your system what it needs.

If the upfront cost is low and regular maintenance is acceptable, through-hole slip ring is a suitable choice. If regular maintenance is not allowed, large data transmission is required, higher reliability requirements, longer life-time requirements, faster speed requirements, wireless slip ring is more suitable.

Always pick the one that can survive your setup.