Thermal Growth – No Problem

Recently a power plant was installing a new coupling after an extended outage. A new pump shaft and element were installed during the outage, and significant work was performed on the turbine. The original distance between shaft ends (DBSE or BSE) was 14.0 inches, and the FLEXXOR Coupling spacer was designed for this dimension. However, the correct amount of thermal growth and equipment thrust was unknown at that time.

Thermal Growth Determination

Before the installation could continue, a discussion about thermal growth and thrust position ensued. The team realized that the net result of the shaft movement would amount to a different BSE than was expected. The difference in shaft position was about 0.100 inches. Since the allowable axial movement of the coupling was +/- 0.090 inches, it was crucial to set the hubs and shafts properly so that the running BSE would not be in a position to overly stretch or compress the coupling.  

Correcting the Axial Position of the Coupling Hub

Once the correct shaft spacing was determined, the hubs needed repositioning. Since the pump side hub was already fixed in its position by the shaft taper, the adjustment needed to happen on the turbine side where the shaft was straight. Luckily CouplingCorp had designed the hubs to have some adjustability in the axial position of up to 0.250 inches. So, within minutes, the Anderson Clamp Hub was loosened, repositioned, and re-tightened in the new position. 

Thanks to the ease of use of the Anderson Clamp Hub, the mechanics overcame a last-minute hurdle with no sweat. No heat or hydraulics were needed, and one mechanic could make the adjustment very quickly.

A CouplingCorp customer came with a problem that can be common. The original shaft attachment device they were using had a set screw to hold it in place on the shaft. Over time the vibrations of the hub on the shaft caused micro fretting between the set screw and the shaft. This small movement created a weak spot on the shaft which eventually caused shaft failure.

Stress Risers in Shafts

In any shafts that carry high torsional load, a stress riser can be catastrophic.  In this particular case, the shaft was also carrying a cantilevered load which added to the stress on the outer surface of the shaft.  The micro-cracking that happened at the set screw contact point was enough of a stress riser to cause a full crack even on a shaft running at relatively low speeds.

This is a cautionary tale for any shafts that have surface irritations like this or other intentional features.  For example, keyways in shafts are often problematic if the inside corners do not have an appropriate fillet.  It’s always good to inspect shaft to avoid surprises.

The customer wanted to find a mechanical device without a set screw which would be easy to install, simple to uninstall, and, most importantly, it had to be customized to fit a unique geometry which supported a bearing. Luckily CouplingCorp was able to create the perfect solution. The Anderson Clamp Hub™ is a mechanical clamping device that has a fully customizable flange which was able to handle a tight tolerance bearing surface.

Design Flexibility

One distinct advantage of the Anderson Clamp Hub is the ability to access the loading screws from either end as chosen by the user.  Sometimes it can be very helpful to access the loading screws from the shaft end side because there is more wrench clearance.  With some other keyless devices, loading screws are typically found on the back end of the hub where they can be very close to the bearing housing which leaves very little room for access.  In other cases, it can be more helpful to have the loading screws on the other end, which is achievable with the Anderson Clamp Hub.  This design freedom gives the user a much more user-friendly device.

By working with CouplingCorp on this application, the customer was able to solve the shaft failure problem.  In the process of making that change, they were able to save millions of dollars in warranty parts on their entire installed base.

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