The essential
Use a slip ring when the shaft has a free end or can pass through a tubular slip ring, and when you need many channels or unlimited signal bandwidth. Use wireless telemetry when the shaft cannot be taken apart or modified, or when nothing may touch it. The same strain gage bridge measures the torque in both cases; only the way the signal leaves the shaft changes.
Is the end of the shaft free?
It decides the mounting. An end-of-shaft slip ring needs a free shaft end. A shaft with no free end takes a tubular slip ring, a clamp-on transmitter or a split collar.
- Free end, for example a pulley, a wheel or a gearbox shaft: S-Series or SR Series end-of-shaft slip ring. The S-Series mounts with two screws and needs 33 mm of length.
- No free end, shaft can be taken apart: B-Series tubular slip ring. The shaft passes through the bore, 30 to 81.3 mm, and the bore can be adapted to smaller shafts.
- No free end, shaft cannot be taken apart: CIT or CBT clamp-on transmitter, strapped to a shaft of 25 mm or more, or the TEL-SC split collar, which closes around shafts of 23 mm and larger.
- Free end at high speed, with no bearing wanted: Model T3 or T8 end-of-shaft telemetry.
- Solid axle inside a housing: neither a tubular slip ring nor a transmitter fits around the shaft. The axle is gaged inside a bore and the signal leaves through the hub, to a slip ring or a telemetry package on the wheel.
How long is the test, and where does the power come from?
With a slip ring, power reaches the rotor through the contacts, so the test length does not depend on a battery. With telemetry the transmitter turns with the shaft and needs its own supply: a battery, which sets the test duration, or induction, which does not.
- Battery: no coil to install, so it needs the least space. The CBT runs 48 hours with one strain gage bridge and 36 hours with two; the CBT-mini runs up to 28 hours with one bridge.
- Induction: a stationary coil powers the transmitter without contact, with no battery to change. The CIT and the TEL-SC are made for long tests.
- On a half-shaft, an induction power hoop keeps the transmitter powered while the shaft moves through the suspension travel.
- The T3 and T8 are powered by induction, with the primary coil about 3 to 5 mm from the transmitter.
How many channels?
Count the strain gage bridges and the thermocouples. A slip ring is rated in circuits, and each sensor uses several; a telemetry transmitter is rated in channels.
- End-of-shaft slip rings: 4 to 10 circuits on the S-Series, up to 36 on the SR Series, up to 60 as standard across the range.
- Tubular slip rings: 4 to 8 circuits on the B-Series.
- Clamp-on telemetry: 1 strain gage bridge on the CIT, 1 or 2 on the CBT.
- Split collar: the TEL-SC carries up to 3 channels on hoops under 76 mm, more on larger hoops, strain gage or thermocouple.
- Multi-channel telemetry: 1 to 8 strain gage or thermocouple channels on the M560, 3 on the Model T3, 8 on the Model T8.
How fast does the shaft turn?
The shaft speed must stay under the rated speed of the slip ring, which on the SR Series falls as the circuit count rises. End-of-shaft telemetry has no bearing and no wear parts.
- S-Series: 12 000 rpm as standard, up to 20 000 rpm in option.
- SR Series: 12 000 rpm for the SR10M, 4 000 rpm for the SR20M, 2 400 rpm for the SR36M. Most weatherproof versions are limited to 2 000 rpm.
- Model T3 and T8: proven at 24 000 rpm, with no upper limit established and no periodic maintenance.
- CBT on a higher-speed shaft that needs balancing: order the counterweight and mount it 180° opposite the transmitter.
What bandwidth and what signal do you need?
A slip ring passes the analog signal itself, with no limit on bandwidth. Telemetry digitizes the signal on the shaft and sends it by radio with error checking, so the bandwidth is set by the transmitter.
- Slip ring: unlimited bandwidth. Precious metal rings and brushes keep the contact resistance variation low enough for strain gages and thermocouples. An amplifier on the rotor raises the signal to a high-level voltage before the rings, for low strain levels, high speeds, vibration or electrical noise.
- CBT: 3 kHz sampling per channel, 12 bit, ±10 V analog output.
- Model T3: up to 7 kHz per channel. Model T8: 0 to 1 kHz bandwidth (−3 dB), 16 bit.
- M560: 16 bit, all channels sampled at the same time, ±10 V analog output per channel.
- A slip ring can also include an encoder for shaft speed and angle: 60 ppr on the SR/E60 and the B-Series /E60 versions, 512 ppr on the SR/PE512.
What environment, and what budget?
Both routes exist in sealed versions for road and off-road use. On cost, a slip ring costs less than most non-contact systems.
- Weatherproof slip rings: the B-Series weatherproof versions take water, mud, snow and dust on a drive shaft. Other references are for rigs and dynos.
- Telemetry housings: weatherproof, for vibration, high acceleration and contaminants. The CBT is IP67.
- Temperature: −40 to 121 °C for the S-Series and SR Series, −40 to 125 °C for the Model T8. The batteries limit the CBT to −20 to 50 °C.
- Radio range: the 2.4 GHz link reaches up to 4 m in line of sight; the CBT typically broadcasts over 1 to 2 m.
- Electric and hybrid powertrains: telemetry is used on gearbox input and output shafts, and for thermocouples in hard-to-reach places in the motor or transmission.
To remember
- Free shaft end: S-Series or SR Series slip ring. High speed on a shaft end with no bearing: Model T3 or T8.
- No free end: B-Series tubular slip ring if the shaft can be taken apart, CIT, CBT or TEL-SC if it cannot.
- Short test or no room for a coil: battery (CBT). Long test: induction (CIT, TEL-SC).
- Many channels or unlimited bandwidth: slip ring. Up to 8 channels without contact: M560 or Model T8.
- A slip ring costs less than most non-contact systems.