Tuesday, October 7, 2014

Clearance vs Interference Fit Couplings - Which Hub Fit is Better?

Coupling hubs being mated to a shaft are generally categorized and specified to have either a clearance fit or interference fit. Which is better? Well, of course it depends...

To quickly first define the two... clearance fit is where a coupling hub can easily slide onto the shaft without doing anything particularly special (i.e. - adding heat) to the hub. The fit is generally snug, but the hub will slide on.

Interference fit is where the bore of the coupling hub is actually slightly smaller than the shaft outside diameter... and the hub must be heated (generally in an oven or oil bath) and/or the shaft cooled (less common)... for the hub to get onto the shaft.

(To put it out there... a third option, transition fit, where the bore and shaft outside diameter are basically the same is available through some manufacturers. However, this option is much less popular because it is not known in advance if the hub will be slightly interference or slightly clearance fit... creating unknowns, and making coupling hub installations and removals more complex than they likely need to be.)

Clearance fits are popular because they allow for easy coupling assembly and removal, and work well as long as the set screws, shaft collars, or other holding feature is strong enough to resist sliding (pushing in or pulling out) of each hub relative to its position on a shaft. Clearance fits are extremely popular on smaller couplings... with interference fit becoming more dominant in the 4 inch (100 mm) to 6 inch (150 mm) diameter range. Above 6 inch diameter bores, clearance fit hubs are almost exclusively used.

One key drawback of clearance fit coupling hubs is that they can introduce a bit of unbalance to the coupling... with pressure from the setscrew or clamping mechanism applying a non-uniform force against the shaft... moving the center of rotation of the hub out of alignment with the shaft center of rotation.

By applying force evenly to the shaft, interference fit couplings do not have the unbalance concerns of clearance fit couplings... thus making this fit ideal for both high torque and high speed applications (where balancing issues become much more severe). Unlike clearance fit hubs (which must have either a keyway, square bore, or spline to keep them from rotating), interference fit hubs also have the option of not necessarily having a keyway (or keyways) to transmit shaft toque (though many still will).

The major drawback of interference fit hubs is the complexity of putting them on (not terribly fun) and taking them off a shaft (even less fun). Putting them on generally requires heating the hub in a large oven or oil bath (blow torches may be really cool to play with, but should be avoided to heat the hub as the heating is so uneven), and moving the hot coupling into place while it is still very hot. Because you can't simply re-heat the hub without also heating the shaft the hub is attached to (given metal on metal heat transfer)... the recommended method of removal for this type fit is a mechanical or hydraulic puller device.

While putting the coupling on is no less easy, removal of interference fit hubs can be improved (at a cost) by requesting your coupling supplier incorporate hydraulic ports and grooves into the coupling hub designs prior to installation... as this will allow the hub to be hydraulically expanded if/when removal is eventually required. (Should interference fit coupling removal be of particular interest to you, and excellent whitepaper on the issue, titled Hydraulic Removal of Coupling Hubs – Keyed and Keyless by Michael M. Calistrat, James R. Morris, and Robert Parker from the Proceedings of the Twenty-Second Turbomachinery Symposium, is readily available.)

In addition to installation and removal hassles, a secondary issue to consider (and primarily for your manufacturer to consider) is the strain that will be put on an interference fit hub (as not all materials deal well with this type of load). Regardless of the material, it is also important to go with a well-accepted industry bore standard so the interference fit is not either too severe or non-existent/transition fit.

Big picture... consider clearance fit as long lower torques & RPM levels permit (your maintenance team will thank you)... and consider interference fit when required (higher torques & highly balanced applications). Should you have any further questions, please do not hesitate to contact your favorite coupling applications expert.

Recommended Follow-On Reading: To learn about current coupling bore and keyway industry standards (inclusive of clearance and interference fit bores), please check out the following blog article: Active Coupling Bore and Keyway Standards - What Are They & Where Can I Find Them?

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