Nick Dowding discusses how they can play a pivotal role in reducing whole life costs of dry vacuum pumps applications such as semi-conductor manufacturing where a clean environment is vital to avoid contamination of the product.
Typical vacuum pump assembly: higher operating life can be achieved
if the bearings are correctly designed, assembled and maintained.
Dry vacuum pumps play a key role in a variety of industrial environments. Production processes for semi-conductors, 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. As a result, working chambers are normally flooded with gas and evacuated using dry vacuum pumps to create a working vacuum.
In semi-conductor manufacturing, the 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. So it is common to see rows of dry vacuum pumps installed under production floors in many semi-conductor-manufacturing plants.
For end users of dry vacuum pumps, highest possible pump reliability is required. The pumps must not, under any circumstances fail due to unreliable bearings, as this would cause costly stoppages to production processes. 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 pumps (and other 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 they are correctly designed, assembled and maintained.
Bearing solutions
At the Barden Corporation (UK) 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, including screw type and Roots Claw vacuum pumps. For example, in a recent project, Barden was asked by a manufacturer of a Roots Claw type vacuum pump to devise a bearing solution that satisfied multiple criteria. These included allowing the pump to operate at higher speeds (100Hz rather than 60Hz). The bearings would also need to operate reliably in poor lubrication conditions with heavy 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. This caused costly production stoppages for the end user.
In order 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, cut noise and reduced heat generation. The steel balls, for example, were replaced with ceramic versions to avoid asperity welding and reduce the running temperature of the bearings. These ceramic balls also provided higher resistance to ball damage from contaminants. The original cage was also removed in order to make a more ‘failsafe’ design.
A typical dry vacuum pump.
Heat treatment
For the high temperature conditions, Barden also applied a special heat treatment to the steel in order 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 of the new bearing solution was dramatic: the MTBF improved 20-fold, increasing from three months to five years. Also, the improved bearing solution now runs 30 °C cooler than the original design. A more ‘tolerant’ bearing has also led to a decrease in power consumption.
The decision in favour of a specific bearing solution 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 – despite an overall higher bearing price.
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 solution 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, maintenance costs and downtime often more than outweigh the initial higher purchase price of the advanced bearing solution.
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. The whole design process requires a 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 between 50Hz and 150Hz. But 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. Other engineered bearings can add value by reducing the customer’s overall pump assembly costs and pump component count.
In addition to the underlying technology benefits, a decision in favour of a specific bearing solution for a dry vacuum pump (or other process pump) should only be taken after examining the whole life cost and not merely on the basis of the bearing purchase price alone.
If advanced bearing technology is employed correctly, cost reductions can be achieved despite a higher bearing price which in turn offers advantages for the pump end user and pump manufacturer. These same whole life cost principles can be applied to other types of pump, including general vacuum pumps and other process pumps.
