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For a detailed comparison of performance and dimensional specs, see this spreadsheet:
Gearhead Crossover.xls
In comparing the torque output of the gearhead, note that the life rating of the Zenith gearheads at catalog torque and speed ratings is not published and unfortunately is not available. Bayside life ratings for the gearheads are published in the catalog pages at the nominal output torque at the rated speed.
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As a reference for Zenith Gearhead catalog specs:
Zenith Gearheads Sales Brochure
If needing a mechanical dimensional drop-in replacement, customs are readily available for the Bayside gearheads. Custom ratios are also available. Please contact your Automation Technology Center distributor for a custom gearhead quote if interested.