The Problem:

A large pharmaceutical company had a catastrophic failure on a fan unit. The failed bearing and shaft assembly was brought into our shop and a Failure Analysis was conducted. The drive end (DE) bearing was a 2 15/16”, solid housed, spherical roller pillow block with a set screw locking device. The inner ring of the bearing had fractured. Most bearing inner rings are through hardened which makes them very brittle.  This unit had obviously seen some violent movement which caused it to fail.

The Analysis:

Based on the visual findings, we noted this bearing was subject to intense impacting. We collected background information on this application by speaking directly to Troy technicians who had worked on and/or performed vibration analysis on this fan.  As expected, there was history of this fan going to high vibration levels at 1210rpm and higher operating speeds. They also have a similar fan very close in proximity not showing similar behavior.

We discovered the fan had a history of premature bearing failure, and also found the client had added two more rooms to the system which increased demand on the original design. Although the base structure was modified, the loads and mounting configuration of the bearings needed to be improved. We also felt the need to introduce an expansion bearing on the opposite drive end to accommodate any thermal growth in the system.

The Solution:

A recommendation was made to retrofit the bearings with Dodge Imperial Units. This was based on a comparison of bearing load capacities and shaft mounting designs. Since we did not have access to the original design criteria, this analysis was done without any details of the Equivalent Radial or Thrust Loads required for this application.

For comparison all bearing loads were given as follows:

Allowable Equivalent Loads Ratings, (ELR) in pounds at 1500rpm and 100,000 L10 Life. ELR is a baseline calculation provided from the manufacturer of the bearings. There will be slight variances between manufacturers in the ELR.

Current:

Opposite Drive End (ODE) Bearing: 1 7/16”, SC 107 PB Dodge, ELR – 275lbs
Drive End (DE) Bearing: 2 15/16”, PB22447H Linkbelt/Rexnord, ELR – 2720lbs

Redesign:

ODE Bearing: 2 15/16”, P2B-IP-215LE (expansion) Dodge, ELR – 2700lbs
DE Bearing: 2 15/16”, P2B-IP-215L (fixed) Dodge, ELR – 2700lbs

Troy was able to improve efficiencies by:

• Increasing ELR (Equivalent Load Rating) on the ODE bearing from 275lbs to 2700lbs
• Introducing an expansion unit to handle future thermal growth
• Improving concentricity in the mounting design to reduce inherent vibration
• Developing better shaft fit between the bearing and the shaft to reduce eccentricity
• Increase L10 life to reduce costly and unplanned replacements of the bearings and shaft

The Dodge Imperial Bearing design allows for maximum speeds up to 3400rpm using the ELR criteria outlined for this application, whereas the Linkbelt unit is limited at 1500rpm. This increased speed capability is attributed to the unique adapter design of the Dodge Imperial Units – eliminating the setscrew mounting methods. Our experience has shown a growing trend to use cold rolled shafting vs. turned, ground and polished (TGP). The use of set screw locking devices with cold rolled shafting will distort the shaft fit and increase eccentricity which is then amplified as the velocity or speed of the shaft increases – causing stress on the system.

So next time you are in the conference room, listen for that fan echoing…it could be costing you.

If you would like more information on this solution contact Dave Barcomb: [email protected] or 518-272-4920.