Electrical Notes: Making Slip Rings
These lines can carry signals or many amps of power, depending on the details of implementation.
The ability to interact with motors, lighting and sensors on rotating structures opens up a lot of possibilities.
Although excellent slip rings are available commercially, (left) they may not fit well into your design. And, as with any exotic component, be ready for shocking prices and long delays.
Many kinetic art projects have modest slip ring requirements, and warrant a DIY approach (right). It's pretty easy to make them.
RINGS AND BRUSHES
The 2 key elements of a slip ring assembly are a ring and a "brush" (actually, 2 or more brushes are typically employed per ring). The ring rotates with respect to the brush which rides on the ring with a slight amount of downward force. Both the ring and the brush must, of course, be electrical conductors. And, the ring and brush must not destroy each other as they live a long life of sliding contact.
These 2 requirements, conductivity and sliding contact compatibility, sharply limit the materials selection.
The industry favorite is coin silver for rings and sintered silver-graphite for brushes (right top). Gold on gold is also used in some critical applications such as spacecraft. Graphite, either in spring-loaded block form or "miniature broom" form is also used for brushes.
A lower cost, easy-material-availability alternative (example below right) which I've used with good results is brass for the rings; and for the brushes either hard-temper stainless steel wire (for low currents) or hard-temper silver wire (for higher currents).
Hard wire bent into a "U" is an easy dual brush to make, and it has intrinsic downward force. By making the spacing of the 2 legs of the "U" slightly smaller than the groove diameter, controllable downward force is achieved.
A plastic block holds the brushes in place. The block pictured at right is left-right position adjustable which in effect allows adjustment of brush force. If the brushes are squeaking, the downward force is too high and the block should be backed off. Once the proper force is determined, WD-40 should be applied to reduce friction and keep the system quiet.
Slip ring assembly for naval missile launcher (above) and naval tracking radar (below).
Humble home-made slip ring assembly with 3 electrical lines capable of 1A each. Brushes are intrinsic-sprung hard stainless steel .029" wire (McMaster 8908K28). Rings are brass.
In the electrical design process, it would be nice to ignore slip rings - to consider them as just another "piece of wire" that electricity goes through - but things are a bit more nuanced. There are three areas of concern: current limitations, low voltage "skipping", and arcing.
CURRENT CONCERNS. If stainless steel brushes are used, current is impeded by the material's significant resistance. The .029" brushes pictured above work great mechanically, but have about 2 ohms per foot. Power = I squared x R, so 1 amp produces 2 watts of heat per foot - hot, but probably manageable without melting the plastic block. 2 amps produces 8 watts per foot - well into the melty danger zone.
Silver has much less resistance, so current handling is not a problem. However, silver, in addition to increased cost, wears faster than stainless steel. Copper wears and corrodes quickly, making it a poor brush material.
LOW VOLTAGE SKIPPING. When moving, especially if a lubricant such as WD-40 is present, the brushes may "fly" slightly above the rings. Such tiny separations inhibit low voltage (e.g. under 1 volt) current flow. Higher
voltages (e.g. 12VDC) can easily jump the gap. The electrical design should take this into consideration. For example, say we are powering an LED through a slip ring: it would be better to use 15V power and a 1K resistor at the LED than 5V and a 330 ohm resistor. The lower the voltage, the more likely that skips will occur in current flow (LED going dark for milliseconds). Another important point about skipping is that the more brushes per ring, the less it will occur.
Skipping can be a non-issue (e.g. powering incandescent lamps), or it can be a THE issue (e.g. passing digital signals). If critical signals are being passed, some combination of brush redundancy, filtering, and voltage level shifting may be required, or consider buying. The serpentine arm carries RS-485 serial signals with no errors using sprung wrap-around brushes.
ARCING. Under certain high current conditions, brief microscopic welding arcs can occur between a brush and it's ring. This roughs-up both surfaces which accelerates wear, resulting in premature slip ring assembly death. The cure to arcing is brush redundancy which insures that there is never a full skip (a brush may skip, but all won't at the same time). Cleanliness is also a factor: micro-particulate obstacles in the ring groove cause brushes to jump.
RING STACK CONSTRUCTION
The slip ring stack construction process pictured below shows the typical steps. What's nice about custom slip ring assemblies like this one for Orbit 3 kinetic sculpture is the design flexibility: you get to choose the number of electrical lines, outer and inner diameters, etc. And the core can be integrated with other parts of the structure, saving considerable space. This unit has 5 rings and a 1.75" outer diameter. The rings are .125" thick alloy 360 brass, spaced .250" on center.
RINGS. It is possible to use large brass washers as the ring starting point, but in this case a sheet of brass was used. Rough blanks were center drilled 3/8" and band sawn out. These were arbor mounted and lathed to the initial outer diameter. Now with smooth extrados, they could be held in the lathe's 3 jaw chuck and be bored to correct inner diameter. A notch was milled into each inner surface to accommodate wires. Then wires were soldered onto each ring with a torch.
RING MOUNTING. The brass rings must not, of course, make contact with the inner aluminum (in this case) structure. 5 strips of multi-layer tape were attached to the aluminum first. This pretty well centers the rings and makes it impossible for them to touch the aluminum. The rings are laid in and wires placed (well thought out wire paths are important, and another bonus of custom slip rings). Then the ring spacing is controlled with dowel pins. Now the rings are centered, electrically isolated, and parallel. 5 minute epoxy is applied to hold it all in place for now.
RING POTTING. To permanently unify this whole collection, Tap Marine slow epoxy is used. This stuff has minimal shrinkage, excellent strength and adhesion. A housing is needed to contain the liquid epoxy, an acrylic tube and delrin base is employed. The epoxy cures for about 48 hours, 36 of which are under lamps (it must be fully cured to withstand lathing).
FINISHING. The housing is pryed or broken away. The final outer diameter must now be lathed. It is critical to lathe this surface (and the grooves) concentric with the inner structure, so a 4 jaw chuck and dial indicator is used. Finally, the lovely brass surfaces are polished by steel-wooling then buffing. The brushes will be happy in these smooth tracks. It's ready for integration to Orbit 3 kinetic sculpture.
Commercial slip ring assembly (Fabricast 0908) with 8 lines capable of 5A each. Brushes are silver-graphite blocks on leaf springs holders, rated 5A, 3500 ft/min max. Rings are coin silver.
Slip rings allow electrical lines to traverse continuously rotating mechanical joints
Trabant Kinetic Sculpture (above). Trabant's slip ring stack (which is also its main structural axis) before installation (below).