Tuesday, June 9, 2015

How to Install an Interference Fit Coupling

Interference Fit Hub Gear Coupling - by Lovejoy, Inc.
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.

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