An Unconventional Upgrade

(aka an experiment with whitemetal bearings)

A client's hydrogen fan operating at 200 deg C, built by Howden, has a MTBF of its rolling element bearings every 3-9 months. After each failure, the fan requires overhauling with new bearings and re-balancing of the rotor.  This is costly and multiple fan sets are in use for quick change overs.

Several OEMs were invited to provide solutions, but there was no positive response. Oiltech Bearings was approached (on an experimental basis), so we took up the challenge.

The fan unit would benefit from whitemetal bearings; providing damping to the rotor, easier maintenance (without re-balancing each time) and hopefully increasing reliability. Designing a whitemetal bearing assembly for this application is not conventional, here’s why:

1. Load is not uniformly applied – the pulley will always create shaft misalignment.
2. The loads are almost horizontal.
3. The impeller operates above 200 deg.C and forced lubrication should be avoided as this introduces more failure modes. The client did not want to increase instrumentation and protection systems around the fan. Heat transfer is likely to be a problem!

Our Scope:

We started with FEA of the oil films, making the assumption the bearings are uniformly loaded. Since this wouldn’t be the case, a large margin of error was added into the design process.

A simple design for initial testing was developed, using two simple cylindrical bearings with offset split lines to maximise the uninterrupted running arc of the bearing. The oil sump was designed as large as possible which is constrained by the pedestal. This limitation on size impacts the potential heat dissipation from the bearing assembly and heat "capacity" of the oil sump.

Oiltech Bearings designed and manufactured the bearing assembly in 10 weeks, installed PT100 sensors and delivered it for initial testing. 

Test runs were conducted on a spare fan unit, monitoring temperatures of both NDE and DE bearings. On the first test run, heat generated in the DE bearing (with location faces) was excessive within 2 hours of running. Oiltech Bearings modified (overnight) the plain location faces to a taper land location face on both ends for another test run the next day. The Oil temperature was much lower and misalignment wasn’t a problem. After several hours on test, heat dissipation from the bearing became an issue and it finally stabilised at 95 deg.C, far too high to used in operation.

Two more modifications are now proposed to the design. Firstly, spherically backed journal bearings will reduce misalignment of the shaft. It is worth noting misalignment cannot be avoided in this application.  Secondly, increasing the heat dissipation rate without the use of forced lubrication. An estimated 1kW of heat needs to be transferred from the bearing assembly. Testing has not been done in operation with the hot processes gases, so a large safety factor needs to be considered for possible heat transfer down the rotor.

The additional work is underway and we will share the results when completed. Oiltech Bearings have decades of experience in whitemetal bearing design and analysis. Visit us at oiltechbrg.com to see what we can do.