Slip-ring variations abound — and the advanced components come in bore sizes from half an inch to many feet in diameter — for medical-imaging machines designed to service livestock, for example. But options go beyond mere scale. So how do OEMs and design engineers choose between standard and custom options … and what are other options beyond the most widespread variations of electromechanical and fiber-optic slip rings?  Let’s explore.

Standard slip-ring options for OEMs

Some slip rings are sold as standard product offerings. These slip rings are easy to add to machine builds — with harnesses for simple connectivity as well as standard mounting interfaces. The latter can be in the form of a flange, threaded rotor, mounting on a shaft, or fitted into a shaft’s internal bore.

Standard hardware can serve a secondary purpose — ensuring slip-ring installation alignment to within tolerances for proper operation. Required shaft alignment is sometimes expressed to within some value of runout; slip-ring face perpendicularity to the axis is usually held to even tighter tolerances.

Video snippet courtesy Deublin. Click to watch the full version on YouTube.

Circuitry options include various standard contacts to satisfy amperage and voltage ranges — ac or dc.

For example, the manufacturer Deublin (known for supplying customs to the industry) is now getting into offering more off-the-shelf slip rings in standard product lines. This is an interesting departure from the general trend towards customization in industrial components.

Where might standard slip rings work? Well, an engineer might know he or she needs ten channels or ten individual circuits over a slip ring … and an off-the-shelf item will get this job done. Of course, there are other design considerations — and an engineer that chooses a standard slip ring may ultimately end up with a slip ring that’s a bit larger than needed … but in the end, the benefits of an off-the-shelf selection maybe worth the overbuild.

In other cases, engineers don’t always know upfront the number of dedicated circuits required — or Ethernet connections, for example.

EtherCAT image courtesy Beckhoff

Note: One factor related to Ethernet that often confounds engineers is that one basic cable has four 100-megabit Ethernet runs — containing only four individual wires. But the outside of that cable is metal braid for shielding. That metal braid counts as an additional circuit. So slip-ring manufacturer count five circuits or traces minimum for a standard Ethernet line … though many design engineers will think it’s only four.

Ultimately, issues like these underscore the importance of market education and communication between manufacturers and the design engineer on how to fit designs with slip rings.

It also illustrates the need for both off-the-shelf and custom slip-ring options … to serve design engineers with tight design cycles and those who need more involvement to develop very specific builds.

Custom slip rings

In many cases, trying to fit a standard slip ring to a design ranges from challenging to impossible. That’s in part because there are just so many things to consider in regard to slip-ring design … far more to consider than most engineers initially assume. Custom-designed slip rings are specific to design engineers’ specifications — pun intended.

Some customized designs that incorporate slip rings take different formats … for example, some slip rings ultimately come in unexpected housings … assemblies with square footprints, for example. Deublin supplied one such slip ring with an inner diameter of about 13 in. and the sides of the outer box were about 24 in. each … not a morphology most engineers associate with slip-ring designs. These are truly custom subsystems designed for a tight fit into the machine.

In fact, custom slip rings can include combinations of connection and trace types; encoders and other feedback devices; heaters, if needed; shielding for EMC and subcomponents rated for high temperatures, if those are concerns; and mixed-signal handling. Some manufacturers can deliver custom slip rings in a few weeks.

Slip-ring customization can include integration into rotary unions. Image courtesy Deublin

Note: Slip rings fully integrated into rotary unions can take customization a step further … to simplify installation and minimize maintenance. That’s in part because the connectors for electric and fluid-power hoses and cables can be designed to join and unplug together —with quick-connect hardware.

Don’t discount how complicated these subcomponents can be. Visit Design World’s connectortips.com site for more on what advanced connectors look like.

Example courtesy Deublin • Visit deublin.com/product-support/electrical-slip-rings for details.

Now let’s explore three less common slip-ring design iterations that provide still more options to design engineers looking to connect rotating and stationary machine-build segments.

Wireless alternative #1 — inductive slip rings

Not as widespread as electromechanical and fiber-optic slip rings but certainly commercialized, some slip rings use inductive coupling for the mode of transmission. Benefits include long-life friction-free operation with no arcing — helpful where settings include flammable materials.

How do they work? Well, inductive assemblies act as transformers but with relative movement between coils — called the primary and secondary in this context. Cores (usually of layered silicon steel) couple the coils. (By the way, the core material choice is to minimize the generation of heat from eddy currents.) The transmission of electrical power is constant, regardless of rpm or the position of the stationary and rotating coils. Some versions boost power transfer efficiency with self-harmonizing functions — useful in the face of fluctuating loads or settings.

One caveat: Most inductive designs are more suitable for the transmission of power or low-frequency signals (in the case of data transmission) than they are for high-frequency signal conveyance.

Where do slip rings based on inductive coupling work? They are found in the wind-power industry. Read more about this on Design World’s sister site windpowerengineering.com, which discusses iterations specific to turbines. Inductive coupling slip rings are also useful on packaging applications where axes run at high rpm.

Wireless alternative #2 — Slip rings that use capacitance

Note that there are also wireless slip rings that work by leveraging detection of capacitance. In contrast with inductive variations using magnetic fields from conductive segments, capacitive variations employ electric fields.

Mercury-wetted slip rings

Yet another type of slip ring is a mercury-wetted slip ring. Here, pools of liquid metal make the rotating connections. Mercury is a toxic heavy metal so not suitable everywhere — certainly not where food and beverage products are processed. But the material gives slip rings long life for both data and power transmission — along with low-noise operation.

Resistance through the contacts is less than one mΩ. Compare that to traditional brush-and-ring setups of 10 to 20 times that. Life is also longer with mercury-wetted slip rings — to a billion revolutions compared to tens of millions from brush-and-ring setups.

Design engineers identify the need for a slip ring in the very first stages of the design process — as usually there are no viable alternative solutions.

Of course, while engineers know near the beginning of the design process whether they need a slip ring, the slip-ring design details are usually developed and finalized with the manufacturer.

The most common slip-ring assemblies use metal rings or traces on one subcomponent. Then these interface with graphite or wire brushes that ride the rings’ surfaces. So as that metal ring spins, transmissions conduct via the ring to the brush or vice versa.

Where the application needs multiple contacts — which is usually the case — more ring-brush pairs stack along the Z axis. In some cases, manufacturers make their designs more compact by stacking the brushes on two sides of the slip-ring assembly.

A webinar on this topic was presented live on Tuesday, December 18, 2018. Click below to watch it on demand.

Metal rings for signal transmission are sometimes plated with silver, rhodium, or gold; those for power transmission are sometimes brass or (for higher rpms) phosphor-bronze.

Slip rings that use fiber-optic technology

Fiber-optic slip rings are usually called fiber optic rotary joints or FORJs for short. Some manufacturers partner with suppliers to combine fiber-optic rotary joints with other technologies that are suitable for the transmission of high power … and recognize the technology’s place in complementing other slip-ring power and data transfer options.

Data signals transmit well over FORJs — with 100 Gbps a typical speed. Plus data transfer isn’t affected at all by EMI from high frequencies. FORJs also excel where data loss can arise from resistance — and on axes with exceptionally high rpm.

Remotely operated vehicle (ROV) example courtesy NOAA. Click the gif to watch the full video.

FORJs are not without challenges … in suboptimal or compromised installations exhibiting connector issues; signal degradation in the form of polarization, wavelength, return, and insertion-related losses; more challenging engineering for multiple channels (to avoid crosstalk and other issues); and the need for higher levels of protection should the slip ring operate in harsh settings. As always, the application will dictate which technology is most suitable.

Fiber-optic slip rings often allow multiplexing or muxing — the transmission of multiple signals or data streams at one trace and at once. That allows for the full use of the optical lines’ high speed. Systems combining signals often use time-division multiplexers, which gives each independent signal a time window at an output. Other systems often use what’s called wavelength-division multiplexing — which gives each independent signal on the line a set light wavelength for transmission. This in turn boosts line capacity, speed, and scalability.

Pancake slip ring designs — common morphology for FORJs

Sometimes FORJs take the form of pancake slip rings — also called face slip rings or just flat slip rings. In this arrangement, the rings of the assembly are like tracks on a record. Using this analogy, the brushes ride the platter as a record-player needle. When discussing slip-ring morphology, the design we usually assume is the common drum design … not pancake slip rings. We usually take this morphology for granted.

Pancake slip rings based on contacting electromagnetic operation work for relatively low-current applications. They can be less power dense than comparable drum-shaped options — exhibiting more capacitance and crosstalk — and must be protected from debris on its working surface. Brushes are usually optimized for low height, but that can mean the brushes have little travel or exert relative light pressure on the rings. The outermost rings exhibit faster wear than inner rings, so can be the limiting factor on life. But pancake slip rings obviously have shorter axial lengths than traditional slip rings with a comparable number of circuits. No wonder that slip ring designs that incorporate both fiber-optic data transmission and other noncontact electromagnetic power transmission have pancake morphology.

Side note on slip rings incorporated into rotary unions

It’s common for OEMs to specify slip rings with rotary unions where a machine build needs both … as they serve similar purposes albeit with different media. Recall that a rotary union joins a stationary inlet with a rotating outlet and contains fluid under pressure or vacuum to maintain that fluid connection.

Deublin supplies rotary unions that incorporate slip rings.

Sometimes called swivel joints, these rotary unions integrate slip-ring subsystems quite well. One off-highway equipment example is a rotary union with integrated slip ring that transmits hydraulic fluid between an excavator’s track drive and its cab — even though the cab of the vehicle may turn 360° without limit.

Rotary unions on off-highway equipment accommodate the transmission of signals and fluid power between cab and track base. Excavator image courtesy Caterpillar