Going Nowhere: For greater stability, the disk table of Turck's T8.A02H encoder is mounted on the hub and the bearings mechanically locked in place.
Turck's newest encoder, the T8.A02H, is a heavy-duty version for industry's standard 3.5-inch, hollow-shaft applications. The design distinguishes itself by addressing improved bearing life—one of the most common mechanical reasons for encoder failures.
Encoders are frequently designed to address low-cost requirements that typically trade off performance and reliability to achieve cost objectives. "The heart of any encoder is the disk table," says Jerry Memoli, Turck's business development manager for encoders. "The disk table supports the disk." The combination of the disk and the optics create the output waveforms of the encoder, so having a mechanically stable disk table is essential.
Typically, encoders have their bearings pressed into a hub assembly. Under normal operation the bearing assembly can loosen up, causing misalignment of the disk with the optics. This can produce erratic signals, half-tolerance signals that cause poor servo performance, loss of an index channel that causes a fault, intermittent operation, or a complete encoder failure.
Locked in place
In Turck's T8.A02H encoder, the disk table is mounted on the hub where the bearings are mechanically locked in place by a tolerance fit and a high temperature adhesive. Shoulders inside the housing complete the locking mechanism. Memoli says the result is a bearing-disk table that will never move over time. To address another potential failure mode, a rubber lip seal keeps out dirt.
The unit is designed for any variable frequency drive ac motor with the industry standard 7272 line driver, providing A-A, B-B, and Z-Z square wave outputs. Handling input voltages from 5 to 30V input with short circuit and reverse polarity protection, the unit provides a pulse rate of up to 5,000 pulses per revolution (ppr) and operating frequency (push-pull) of 300 kHz or sine wave 180 kHz. Shock and vibration are specified at up to 200g and 10g maximum.
Designed to endure the operating environment of a steel mill, applications include ac vector motors used in elevators. The encoder can also be applied in harsh conditions such as those with operating temperatures of up to 85C. In addition, it is compatible with higher operating speeds, or any application where downtime would cause significant problems.