Advanced bearings cut life costs of dry vacuum pumps
10 October 2015
Bearings are key components in dry vacuum pumps; their reliability and operating life have a significant effect on pump operating costs. Nick Dowding discusses how bearings can play a pivotal role in reducing whole life costs of these pumps.
Dry vacuum pumps play an important role in a variety of industrial environments. Production processes for semiconductors, solar panels, flat panel displays, as well as vacuum degassing applications in metal processing, all require a clean environment that is free of any contaminants that may adversely affect the process. Therefore, working chambers are normally flooded with gas and evacuated using dry vacuum pumps in order to create a working vacuum.
For dry vacuum pumps, reliability is critical. The pumps must not, under any circumstances fail due to unreliable bearings, as this would cause costly stoppages to production processes. In semiconductor manufacturing, for example, these types of breakdowns can result in thousands of pounds worth of lost production.
But as the pace of bearing technology accelerates, opportunities are being created for bearings to play a much wider role in minimising whole life costs of dry vacuum and other types of process pumps.
A dry vacuum pump typically requires between four and eight bearings. The average operating life of these bearings varies considerably, from one year to five years, but an even higher operating life can be achieved if the bearings are correctly designed, assembled and maintained.
The Barden Corporation (UK) reached an important milestone earlier this year. In January 2014, the company’s production plant in Plymouth produced its one-millionth deep groove ball bearing for use in dry vacuum pumps. Barden engineers use special bearing analysis and calculation software in combination with other design software such as kinematics and FEA analysis, in order to model and simulate the function of a bearing destined for use in a vacuum pump.
The company works closely with manufacturers of every type of dry vacuum pump, including screw type and Roots Claw vacuum pumps. For example, in a recent project, it was asked by a manufacturer of a Roots Claw type vacuum pump to create a bearing that would allow the pump to operate at higher speeds (100Hz rather than 60Hz), as well as operating reliably in poor lubrication conditions, high contamination levels and high temperatures. The original vacuum pump bearings were of the standard steel ball-steel cage type design, whose mean-time-to-bearing-failure (MTBF) was less than three months.
In order to operate more reliably at higher speeds, Barden engineered a super precision bearing specifically for the customer’s pump, which provided improved running accuracy, reduced noise and reduced heat generation. The steel balls, for example, were replaced with ceramic balls to avoid asperity welding and to reduce the running temperature of the bearings.
These ceramic balls also provided higher resistance to ball damage from contaminants. The original cage was replaced with a more ‘failsafe’ design, and for the high temperature conditions, Barden applied a special heat treatment to the steel in order to avoid any long-term growth or softening.
The results proved dramatic: the MTBF improved 20-fold, increasing from three months to five years. Moreover, the improved bearing runs 30°C cooler than the original design and has reduced the pump’s power consumption.
Whole life costs
The decision in favour of a specific bearing development should always be taken after analysing the overall cost/benefit issue and not merely on the basis of purchase price alone. Today’s high technology bearings offer many improved features that enable whole life cost reductions to be achieved, providing benefits to both pump manufacturers and end users – albeit at a higher cost.
For a bearing designed/selected for a given application, the life cycle cost (LCC) is equivalent to the sum of the following: initial cost/purchase price + installation/commissioning costs + energy costs + operation cost + maintenance cost (routine and planned) + downtime costs + environmental costs + decommissioning/disposal costs.
Whilst the initial purchase price of an advanced bearing developed specifically for a dry vacuum pump may be between three and five times higher than a standard bearing, the potential savings that can be achieved in the form of reduced energy costs, maintenance costs and downtime, often more than outweigh the bearing’s initial higher purchase price.
Simplified mechanical design
The process of whole life cost reduction begins at the design stage of the pump and exploits the possibilities for simplifying the mechanical design. This, in turn, simplifies the pump assembly and the subsequent maintenance tasks, resulting in both direct and indirect cost savings. This whole design process requires a very close working partnership between the bearing designer and pump manufacturer.
Whilst the speed requirements on dry vacuum pump bearings are normally lower than usual, other factors such as temperature, contamination and reliability mean that a special bearing design is necessary in order to meet the application requirements. Bearings for dry vacuum pumps tend to operate at speeds of 50Hz to 150Hz. However, despite the slower speeds, the bearings must operate reliably and provide dimensional stability at high temperatures, as well as being able to continue to operate effectively even under poor-lubrication conditions.
If required, a solution can be engineered for the customer that includes a special heat treatment or coating for the bearings, which will provide temperature stability and high resistance to contaminants. Other bearings can be devised that increase reliability even further: for example, by replacing the standard steel ball elements with ceramic balls, which improves lubrication. Other engineered bearings can add value by reducing the customer’s overall pump assembly costs and pump component count.
Nick Dowding is business development manager, The Barden Corporation (UK)
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