Liebert has denied any problems with design and they might be right, but we thought it worth discussing. The Liebert units force air under the raised floor using a blower fan assembly mounted on a drive shaft (could be one fan or two fans depending on your model number) assembly. The drive shaft rides on two bearings bolted to the frame. On the left side shaft is a pulley. This pulley and its counterpart connected to the electric drive motor is what spins the blower assembly. See figure 1A for an idea of how the unit is designed.

Figure 1A – Inside the Liebert Chilled Water series CRAC unit.

It is to the best of our understanding that Liebert ships these units with a two belt pulley system that is adjustable and variable. This two belt system allows some slippage on motor start-up. However, this slippage also contributes to a belt-dust problem. Liebert knows about the belt-dust problem and sells what they deem a “low dust” belt for the computer room environments. See figure 2A for a close up of this pulley and belt system.

Figure 2A – Liebert pulley and belt system for chilled water CRAC units.

I say “it is our understanding” for two reasons. Liebert has so far refused to comment on this directly (as well as TechPlan, who provides parts) even after several requests. Our HVAC company was able to get some information from their support resources, but nothing so far in writing. Feel free to comment below if you have more in-depth knowledge about this issue.

Again, it is our understanding that the systems is designed to allow slippage of the belt. In late 2006 our building maintenance staff (who managed the CRACs at the time) decided to eliminate this belt dust problem by using a three belt pulley on the Liebert-CWs. This three belt system drastically reduced the slippage of the belts resulting in a cleaner compute environment. All was well until March of 2010. We had two separate Liebert unit’s drive shafts break just hours apart.

Our HVAC company was able to determine according to Techplan that the increased start stop torque provided by the three belt system caused them to weaken and snap over time. The Liebert shafts are hollow core foam filled shafts, and therefore not designed for this type of torque without a variable speed start relay.

Our building maintenance team continues to stand by their statement that Liebert not only authorized but requested we move to a three belt system in order to limit the dust; “Belt Dust” – which is an entirely different issue we will discuss in a future post.

We have seven Liebert CRACs in our data center and after further investigation all seven shaft were showing signs of fracturing. We quickly went into prevention mode and replaced the shaft in every remaining unit. The shafts cost slightly less than $300 from TechPlan, plus a hundred bucks or so for installation. I like to be safe rather than sorry when it comes to data center environmental services. It would be nice to know whether this is a design flaw or a mistake on our part, but alas we may never know.

We were offered the option of upgrading the Lieberts to variable start relays, but the cost would have been several thousand dollars. We may break a shaft every 5 years or so, but that’s a small cost compared to the cost of converting to variable start. You might be asking why we don’t just go back to the two belt system. Good question and we will discuss in a future post. The short answer: Belt Dust is really bad for your computers, tapes, fans, disk drives and more!

See the gallery of pictures below for more Liebert snapped shafts.

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