Application · Powertrain

Measuring the torque of an accessory belt pulley with an end-of-shaft slip ring

Pulley torque
Rotational speed
Angular position

An end-of-shaft slip ring on a belt-driven shaft. The signal wires leave from the stationary side.

In brief

The torque of an accessory belt pulley is measured with strain gages placed on the rotating pulley. An end-of-shaft slip ring on the pulley carries the signal to the data acquisition system. The method applies to the alternator, the air compressor and the other components driven by the belt. The S-Series slip ring mounts with two screws and turns at up to 12 000 rpm.

12 000 rpm: standard maximum speed, S-Series and Short S-Series
0.1 Ω: maximum peak noise of the slip ring
22 mm: shortest Short S-Series slip ring

One belt drives every accessory

An accessory drive belt, also called a serpentine belt, drives one or several devices of an engine at the same time. It replaces the earlier layout of one belt per component.

If that belt fails, every component it drives stops. Belt tension also affects the mechanical efficiency of the drive. The torque on each pulley is measured for both reasons.

Where the method applies

The test applies to any product with an engine or a belt-driven system. Typical test objects:

Vehicle alternators
Air compressors
Tractors
Air conditioners

Gages on the pulley, slip ring on the shaft end

A slip ring assembly has a rotating side and a stationary side. The rotating side connects to the strain gages. The stationary side connects to the data acquisition system.

Short S-Series end-of-shaft slip ring, with solder terminals on its flange — A Short S-Series slip ring.

01

Place strain gages on the rotating pulley

The strain gages go on the pulley itself. Michigan Scientific applies strain gages to production parts.
Custom strain gage transducers
02

Mount the slip ring on the end of the shaft

The S-Series mounts with two screws and needs 33 mm of length. The sensor wires run on the surface of the shaft or inside it, through a groove milled across the shaft end or an adapter.
S-Series end-of-shaft slip rings
03

Amplify the bridge on the rotating side, if needed

A miniature amplifier between the gages and the slip ring reduces the errors from long lead wires, connector resistance variations and electromagnetic interference. It holds the bridge excitation supply and a shunt calibration resistor, switched remotely.
AMP-SG-M1 strain gage amplifier
04

Route the stationary wires to the recorder

The wires from the stationary side go to the data acquisition system.

Choosing the slip ring for the pulley

The choice depends on the length available in front of the pulley and on whether speed and angle are recorded with the torque.

S-Series	4, 6, 8 or 10 circuits. 12 000 rpm standard. Rotor diameter 41.28 mm. Mounts with two screws.
Short S-Series	4, 6, 8 or 10 circuits. 12 000 rpm. 22 to 37 mm long, 55 to 120 g. Some models carry an embedded amplifier.
SR/ERT Series	10, 20 or 36 circuits, with an optional encoder or resolver. 10 000 rpm with 10 circuits. 80 to 115 mm long.

A low-level bridge signal through the rings

A strain gage bridge gives a low-level signal. The brushes and the rings of the S-Series and the Short S-Series are in precious metals, which keeps the resistance variation low enough for these signals. The maximum peak noise is 0.1 Ω.

The AMP-SG-M1 amplifier weighs 14 g and mounts on the rotating part. It serves one bridge, with ± 2.5 V or ± 5 V excitation, a ± 10 V output and a 20 kHz frequency response (−3 dB).

With the rotation sensor of the SR/ERT Series, rotational velocity, angular position and direction of rotation are recorded with the torque. The sensor uses none of the slip ring circuits.

What you get

The torque on the pulley, measured on the rotating part while the accessory drive runs. The slip ring sits on the shaft end and needs no routine cleaning or maintenance on the S-Series. With a rotation sensor, speed and angle are recorded with the torque.

Frequent questions

How is torque measured on a serpentine belt pulley?

Strain gages are placed on the rotating pulley and an end-of-shaft slip ring carries their signal to the data acquisition system.

Which slip ring fits on the end of an accessory pulley?

An end-of-shaft slip ring such as the S-Series or the Short S-Series. Both have 4 to 10 circuits and turn at 12 000 rpm. The Short S-Series is 22 to 37 mm long.

Is an amplifier needed between the strain gages and the slip ring?

Not always. The precious-metal rings and brushes keep resistance variation low enough for a bridge signal. An amplifier on the rotating side, such as the AMP-SG-M1, reduces the errors from long lead wires, connector resistance variations and electromagnetic interference.

Can pulley speed be recorded with the torque?

Yes, with a slip ring that has a rotation sensor. The SR/ERT Series measures rotational velocity, angular position and direction of rotation without using any of its slip ring circuits.