How to Install an Interference Fit Coupling

Interference Fit Hub Gear Coupling

The installation is of Interference Fit coupling hubs is similar to Clearance Fit hubs except that these hubs need to be heated prior to installing onto the shafts. It is important when installing a hub with an interference fit that some clearance exists over the top of the key. Otherwise, when the hub cools, the top of the hub keyway will rest on the key and produce high stresses in the keyway that could cause the coupling to fail.

For the purpose of this general educational post, we've referenced and extracted sections of Lovejoy's F Type Gear Coupling installation instructions (which use interference fit hubs as their standard fit). However, prior to completing any installation, please download and review your specific coupling installation instructions from the Installation Instructions section of the Lovejoy website. You may might also consider then further supplementing these instructions, by watching an installation video which can also be located on that same section of the website. (And if you are using a non-Lovejoy product... well... we hope you appreciate the information we are providing, and that you will look to put your faith and trust in us and our products in the future!)

Generic Step #1: Install Sleeves/Required Accessories

When installing coupling hubs, always consult with all applicable Federal, State, and local laws and regulations covering the safe operation and maintenance of equipment. This includes, without limitation, the USDOL-OSHA “Lockout/Tag-out” procedure set forth in 29 CFR 1910.147. 

Fast forwarding through safety, product inspection, and required tools that you would find on a given set of installation instructions... prior to installing any interference fit hubs, it is important to make sure that anything that needs to be put on the shafts prior to hub installation are put on... and put on correctly! In the case of gear couplings, this means the coupling sleeve (singular if continuous) or sleeves (two if flanged style) prior to hub installation.

The exact text from Lovejoy's F type installation instructions reads as follows: The sleeve will need to be slid onto the shaft with the O-ring installed prior to installing the hub allowing adequate room to mount the hub. Care should be taken to support the sleeve and prevent accidental damage if the sleeve were to slip while placing it on the shaft. Once the sleeve is on the shaft, allow room to install the hub. The sleeve will need to be supported to prevent damaging the seal. The flange face should be oriented toward the end of the shaft.

Generic Step #2: Layout/Orient Hubs

This step is largely self-explanatory, but you want to ensure you've got the hubs laid out correctly prior to heating and putting them on. (Putting a hub on the wrong way = no fun!)

Exact text for the F type: Orient the hub with the long end of the hub facing the end of the shaft. Align the keyway(s) and key in the shaft, then slide the hub onto the shaft. The key(s) should be flush with the end of the shaft and face of the hub. 

Generic Step #3: Expand the Hubs by Heating

Hubs should be heated using a uniform heat source, and use extreme caution when handling hot hubs to avoid personal injury. 

Here are a few options that one might consider to heat the hubs: 

Oil Bath Heating is usually limited to 350º F. (177º C), or less than the flash point of the oil used. Special handling devices are required to support the hub in the oil bath such as tongs, threaded rods or eye-bolts placed in puller holes, etc. The hubs should not rest on the bottom of the oil bath container at any time. (Warning: If using this method, make sure you know the flashpoint of your oil, and stay below it!)

Oven Heating offers some advantages over oil bath heating. Parts can be heated to higher temperatures, usually not to exceed 600º F (315º C). This is roughly the maximum temperature where the metal does not go through an annealing process and yet can still be handled with heat resistant gloves. When heating the hubs in an oven, place them on a rack and do not rest the hubs on the oven surface.

Induction Heating can be used as long as the temperature rise in the hub is uniform and controlled.

Open Flame Heating is typically not recommended. When heating the hub with an oxyacetylene torch, or blow torch, use an excess acetylene mixture. Mark the hub body at the top, center, and bottom along the length of the hub with heat resistant crayons, one with a 350º F (177º C) melt temperature and another with a 450º F (232º C) melt temperature. The hub should be sitting elevated on refractory bricks oriented to allow the flame to flow through the hub. With a “Blue Flame” or “Rosebud” torch, direct the flame towards the hub bore using constant motion to avoid overheating any single area. Once the heat sensitive crayon marks melt, the hub should be ready for mounting. (Warning: Do not use an open flame in a combustible atmosphere or near combustible materials!)

Generic Step #4: Mount the Hubs

Mounting instructions may vary by vendor and product type, but to cite Lovejoy's F gear coupling instructions exactly: Mount the hubs on the proper shafts and ensure the face of the hub is even with the end of the shaft. The hub should be oriented with the long end of the hub facing the end of the shaft unless this is a “Slide – FSL” style coupling (Contact Lovejoy Technical Support with questions). The key should be lined up with the end of the shaft and face of the hub. 

Once mounted on the shaft, allow the hubs to cool completely before continuing.

After interference fit hubs have been properly heated, mounted, and cooled, the coupling still needs to be assembled and aligned, which would be very similar to what you would do with a Clearance Fit coupling. 

For more information on the pros and cons of interference fit couplings, please consider reading this article, and if you would like to learn more about the actual standards behind interference coupling fits, please check out this article.

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?