Experimentation with Cycloidal Gearbox

The vibration measurements were carried out on a test stand shown in Figure 3. A Sumitomo cycloidal gearbox of a 15.00:1 ratio was selected for the experiment. CNH-609 gearbox was specifically chosen as it consists of one cycloidal disc (Ref. 17). In lieu of the second disc, which exists in a typical cycloidal reducer, this gearbox design uses a counterweight to achieve dynamic balancing (refer to Figure 1). The one-disc mechanism simplifies the analysis and allows focusing on certain frequencies. In the setup, the gearbox was driven by a DC Servo motor with an encoder feedback loop to provide a consistent speed of 1,800 rpm (30 Hz). The DC motor’s torque capacity was 2.39 Nm. The output shaft of the gearbox was connected to the asynchronous motor that acted as a brake. The torque and speed were measured using torque meters and encoders, respectively, for both input and output shafts. Three vibration sensors (sensitivity 87 mV/g at 6000 CPM) were mounted on the gearbox to collect tri-axial vibration data. All measurements were recorded using the National Instruments Data Acquisition (NI DAQ) system at a sample rate of 12.8 kHz for vibration and 10 kHz for torque measurements. For the input shaft spinning at 30 Hz, the DAQ system would generate 426 and 333 data points for vibration and torque, respectively, in one revolution of the shaft.

The gearbox was assembled with the standard lubricant using an NGLI 00 grade grease, with a predefined quantity. To confirm that all test iterations were performed at a steady temperature, the test stand was placed in a climate-controlled room with an ambient temperature of 21°C. Two temperature probes were placed to monitor the reduction housing or ‘ring gear’ housing of the gearbox. The load on the reducer output shaft was varied from 0 Nm to 15 Nm during the experimentation. The gearbox was tested for a healthy condition, meaning it had no induced errors. Then it was tested for a worn-out cycloidal disc or damaged condition by swapping out the normal disc with the one seen in Figure 4. Other than the disc, the use of the same parts in both conditions ensured that there were no other changes to influence the vibration data. It is important to highlight the fact that the induced change in the ‘damaged’ condition is relatively small. The goal of the testing was to gather vibration data and analyze it with FFT and multifractal analyses.

Referring to Figure 1, the cycloidal reducer utilized for the experiment has a cycloidal disc with 15 lobes. These lobes engage with 16 ring gear housing pins when the eccentric bearing sways the disc with its eccentricity. The ‘engaged’ disc (with pins) then generates a rotary motion while it is swaying and forces the output shaft to spin with the reduced speed in the opposite direction. The connection between the disc and the output shaft is achieved by engaging 8 output rollers within the 8 holes of the disc. This information helps to determine the mesh frequencies. The disc-pin mesh frequency is 16x order. The disc-output shaft roller mesh frequency is 16x8 = 128x order.