Look beyond the purchase price

Dry vacuum pumps play a key role in a variety of industrial environments. Production processes for semiconductors, flat panel displays, and vacuum degassing applications in metal processing, all require a clean environment, free from any contaminants that may adversely affect the process. Therefore, working chambers are normally flooded with gas and evacuated using dry vacuum pumps to create a working vacuum.

In semiconductor manufacturing, etching tools need to operate in a vacuum chamber whilst they lay down miniature grids of electronic circuitry to silicon wafers. This process is so intricate that all contaminants must be removed from the production chamber before work can begin. It is therefore common to see rows of dry vacuum pumps installed under production floors in many semiconductor-manufacturing plants.

For users of dry vacuum pumps, highest possible pump reliability is required. A dry vacuum pump typically requires between four and eight bearings. The average operating life of these bearings varies from one year to five years; a higher operating life can be achieved if bearings are correctly designed, assembled and maintained.

Barden engineers use special bearing analysis and calculation software in combination with other design software such as kinematics and FEA analysis to model and simulate the function of the proposed bearing in a vacuum pump. This results in bearing solutions that achieve the required performance, operating life and lubrication characteristics, whilst also optimising the cost of the solution.

Barden works closely with manufacturers of every type of dry vacuum pump. For example, in a recent project, it was asked by a manufacturer of a Roots Claw type vacuum pump to devise a solution that would allow the pump to operate at higher speeds (100Hz rather than 60Hz). The bearings would also need to operate reliably in poor lubrication conditions, high contamination levels and high temperatures.

The original bearings were of the standard steel ball-steel cage type design. To operate more reliably at higher speeds, Barden engineered a super precision bearing solution 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 provided higher resistance to ball damage from contaminants. The original cage was also removed in order to make a more ‘failsafe’ design.

Barden applied a special heat treatment to the steel to avoid any long-term growth or softening. By the use of ceramic balls and by removing the cage, a balance of Austenite levels was retained to provide the best possible compromise between temperature capability and contamination.

The result was dramatic: The MTBF improved 20-fold, increasing from three months to five years. The improved bearing solution now runs 30°C cooler than the original design and a more ‘tolerant’ bearing has led to a decrease in power consumption.

Whole life costs

The decision in favour of a specific bearing solution should always be taken after analysing the overall cost/benefit issue – not merely on purchase price. For a bearing designed/selected for a given application, the life cycle cost 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 purchase price of an advanced bearing solution for a dry vacuum pump may be between three and five times higher than a standard bearing, potential savings in the form of reduced energy costs, maintenance costs and downtime, often more than outweigh the initial higher price. 

Simplified 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 entire design process requires a close working partnership between 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 50 to 150Hz. 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 bearing solutions can be devised that increase the reliability even further by, for example, replacing the standard steel ball elements with ceramic balls, which improves lubrication.