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White Paper | WP-2014

Seven Selection Criteria

Modular Rotary Encoders Vs. Bearing Encoders

Axial Motor Shaft Movement (End Float or End Play)

End float refers to the amount of axial movement in the motor shaft. There are a number of factors that can contribute to axial motor shaft movement including part tolerances, bearing pre-load method, thermal expansion, and bearing wear over time. When a modular encoder is mounted, the amount of end float directly affects the encoder’s air-gap (the distance between the sensor and the disk).

It can be difficult to obtain end float specifications from the motor manufacture, and even when you do, the information may not be completely accurate. Why? Some motor designs mechanically lock the shaft’s axial movement on the feedback end so that end float is minimal. However, other motor designs often use a wave spring washer to take up any excess play and provide a pre-load force to the bearings. In this case, you cannot assume that the end float has been removed by the wave washer, since this is only true until an opposing axial force on the end of the shaft overcomes the spring force of the wave washer allowing the shaft to move. The results on a modular encoder mounted to this shaft could be disastrous, potentially causing the encoder disc to hit the sensor. Two examples where this situation can occur are:

  • If a motor shaft is connected to a ball screw and the motor changes direction, the force from the ball screw will also change directions. This alternating force may cause the motor shaft to move axially.
  • If a sprocket, pulley, or gear with some side wobble is mounted on the motor shaft, the wobble may cause an alternating axial load to the motor shaft.

Total Indicated Runout (TIR)

Total Indicated Runout (TIR) measures the radial range of shaft movement about its centerline. If an encoder is to be mounted on the motor shaft, TIR should be measured at the point that represents the furthest extent of the encoder case. For example, if the encoder is one inch thick, TIR should be measured about 1” from the motor face. Although many encoders with newer sensor technology will continue to operate as TIR increases beyond the specified tolerance, accuracy will be sacrificed.

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Axial motor shaft movement (end float).

Encoders with Bearings

Bearing-style optical encoders include internal bearings. With bearings, the amount of axial play is typically controlled to less than 0.0005” In addition, the disk is carefully aligned to the optics as part of the calibration procedure to keep radial run out less than 0.0002” typical. In this manner, the critical factors of end float and TIR are controlled and will not be affected by the motor shaft’s end float or TIR in normal operation. A stainless steel flex mount allows the encoder to tolerate increased TIR and end float from the motor without sacrificing encoder performance or damaging its sensor, disk or bearings.

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Thru-bore encoders with flex mounts.

Selection Criteria

When deciding whether or not a modular or bearing encoder is the best solution for your application, consider these factors:

  1. First and foremost, shaft end float and TIR must be within the encoder’s specifications. This is so important that if you don’t have (or can’t get) this information, or don’t trust what you have, then an encoder with bearings is strongly recommended, since it will be a much safer choice.
  2. Modular encoders can be a good choice for high-speed applications, those above 10,000 RPM, because there are no speed limitations dictated by encoder bearings. For example, EPC’s Accu-Coder® Model 121 Modular Encoder has been successfully operated at speeds in excess of 40,000 RPM. The speed limiting factor is the maximum frequency of the encoder, which is a function of disk resolution, RPMs and the signal processing circuitry. Most encoder manufacturers include maximum frequency in product specifications.
  3. If the motor is to be used under considerable mechanical load, where the motor bearings could experience extra wear, then an encoder with bearings would be the better choice. Remember, the bearings of the host device, serve as the bearings of the modular encoder.
  4. Modular encoders are difficult to seal. If your application requires wash-down, or if the operating environment is dirty, dusty or wet, then an encoder with bearings and seals should be your first consideration. Such environments effectively rule out modular encoders, unless external protection, such as an IP sealed motor cover, is used.
  5. If your application requirements combine high maximum frequency (>200kHz), high temperature (>100°C), and higher resolution (>2048 CPR), then an encoder with bearings is recommended. For long term reliability, this combination of factors requires the air-gap between the disk and sensor to be very narrow and tightly controlled. An encoder with bearings simply provides a more stable optical platform.
  6. Lower resolutions (up to 1024 CPR) are more forgiving of end float and TIR, and are often well-suited for modular applications if the operating environment is appropriate.
  7. If you plan to use numerous encoders, then the relatively lower price of a modular encoder could save you some money. On the other hand, the greater durability and easier installation of an encoder with bearings might be worth a slightly higher unit price. In any case, carefully weigh the factors of long term support costs versus lower acquisition costs before making your final decision.

Quick Selection Chart

Parameter Attribute Use Modular Use Encoder with Bearings
Motor shaft end float and TIR Within the encoder manufacturer's specifications Yes Yes
Motor shaft end float and TIR Outside the encoder manufacturer's specifications No Yes
Motor shaft end float and TIR Don't have the information or don't trust Not suggested Suggested
High-speed applications Above 10,000 RPM Good possibility Not suggested
Severe duty application Motor bearings have extra load and wear Not suggested Suggested
Dirty environment May need seals Not suggested Suggested
Combination of high frequency response, temperature, CPR

> 200 kHz

> 100° C

>2048 CPR

Not suggested Suggested
Lower resolution requirement < 1024 CPR Good possibility Good
Number of units needed Acquisition cost vs. life cycle cost Consider if large volume Good

Self-Aligning Encoder

If you do decide that a modular encoder is the right choice for your application, consider the Accu-Coder™ Model 121 self-aligning encoder. The Model 121 has an innovative design that eliminates the installation and mounting hassles typical of other modular encoders. As a result, the Model 121 does not require calibration gapping or the use of special tools to install. Its three step installation is simple and quick. Also, the Model 121’s all-metal construction will not warp or deflect, unlike plastic modular designs. These features combine to make the Model 121 a durable, accurate and cost-effective solution for many applications where a modular encoder is the preferred choice.

Conclusion

Modular encoders can be an acceptable and effective feedback solution for many applications. However, in others, an encoder with bearings is the best choice in terms of performance and reliability. By assessing and addressing pertinent mechanical, electrical and environmental requirements, you can avoid costly and troublesome encoder failure or substandard performance.

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Seven Selection Criteria: Modular Vs. Bearing Encoders