When specifying a rotary encoder, should you opt for a Hollow shaft or a Thru-bore housing? Usually, either design can work equally well. However, in some situations there are important reasons to choose one over the other.
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In the parlance of Encoder Products Company, a Thru-Bore (or Through-bore) housing has a bore opening that passes completely through the encoder body. The encoder is more or less a donut.
With a Hollow Bore (or Blind-Hollow bore) housing the bore does not pass completely through the encoder, with the shaft end residing inside the housing. The depth of the bore varies from model to model, with some units only having a cap or cover on one side of the housing.
A Thru-Bore housing offers the most flexibility for shaft attachment. Provided a means of securing the anti-rotation tether is readily available, a Thru-Bore encoder can be affixed to any point on the shaft in question. Of course it's a good practice to confirm that runout and endplay limits are within the published tolerances of the encoder.
Here are three factors that favor use of a Hollow Bore encoder:
1. Environmental seal. If your application is in an area where dirt, dust and moisture are in abundance, then consider applying a Hollow Bore encoder. A Thru-Bore encoder has two exposed shaft seals that offer potential paths of contaminant ingress. By opting for a Hollow Bore solution, one seal is protected from contaminants and potential leakage. If the encoder is exposed to washdown or direct water spray, we recommend a Hollow Bore housing if possible.
2. Starting Torque. Again, because a sealed Thru-Bore encoder has two shaft seals, the friction created by the additional shaft-seal interface is usually greater than that of a Hollow Bore housing. This can translate into greater starting torque for Thru-bore encoders vs. Hollow Bore. For example, the EPC Model 260 with an IP64 seal has 2.50 oz-in for Thru-Bore vs. 2.0 oz-in for the Hollow Bore.
3. Safety. A Hollow Bore encoder minimizes hazards presented by an exposed rotating shaft. Be sure to consult your relevant regulatory authorities regarding safety requirements.
For most applications the items above may be relatively minor points to consider when specifying a rotary encoder. However, failure to properly address them could contribute to less than optimum encoder longevity and performance, especially when an application pushes encoder performance requirements toward the limits.
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