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:
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.
Axial motor shaft movement (end float).
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.
Thru-bore encoders with flex mounts.
When deciding whether or not a modular or bearing encoder is the best solution for your application, consider these factors:
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 |
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.
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.