By Mark Wakeham, Key Account Manager, and Mark Pritchard, Senior Sales and Applications Engineer, at The Barden Corporation
Semiconductors are an integral part of society and demand is raising thanks to the increasing use and consumption of consumer electronic devices coupled with the rapidly growing use of AI within industry. Furthermore, today’s requirement for more renewable energy sources and therefore semiconductors for solar cells is contributing significantly to the industry’s overall growth.
Manufacturing semiconductors requires an ultra-high vacuum environment lower than 10-7mbar to guarantee there is no risk of contamination and vacuum pumps are a critical piece of equipment to create the right atmosphere. A key component of the pump design is the bearing system. This article explores four key trends that are driving the design of new vacuum pumps and considerations for bearings.
There are typically two types of pump – Fore vacuum pumps, which take the pressure from atmosphere down to 10-3 mbar and then turbomolecular pumps (TMP) typically operate at ultra high vacuum levels down to 10-9 mbar. In this latter application the bearings are often required to work at high speed. However, regardless of the pump type there are some common challenges:
In a bid to reduce energy consumption and operate as efficiently as possible, pump manufacturers are looking at designing pumps with increased power density. However, parasitic losses which take energy from the system need to be taken in to consideration. Parasitic losses in a bearing system are mainly down to friction torque generated by the friction between the balls and the raceway and the lubrication in the bearing. They can be reduced by downsizing the bearing and reducing the number of balls, which also increases the speedability of the bearing, and thus the pump maybe able to run faster or more efficiently.
However, this poses a challenge because the loads on the bearing are the same but the ability of the bearing to support those loads is reduced. This can be overcome by looking at alternative materials with improved properties, namely fatigue strength and wear resistance – such as SV30®, a martensitic through-hardened, high nitrogen, corrosion-resistant steel.
Reliability is the foundation for a long life time and minimal downtime. Using a combination of specialist materials, heat treatments and surface coatings, bearings can be designed that typically run for five years without failures in these harsh environments. Current “greased-for-life” bearing technology can consistently give 30,000+ hour life at high speeds in excess of 800,000 ndm.
Advanced coatings and surface treatments can be applied to bearings to combat friction, prevent corrosion and reduce wear. Heat treatment is especially important. At high temperatures retained austenite in a bearing steel negatively affects the dimensional stability of the bearing, causing it to expand, and this can lead to premature failure. Retained austentite in a bearing steel that is not heat treated is approximately 3 – 4 %, but with special heat treatment the level of retained austentite can be reduced to just 0.1%. This delivers better performance at high temperatures and ultimately a more reliable system with a longer life.
Another material that delivers superior reliable long life performance is ceramic (silicon nitride) balls, thanks to the material’s inherent mechanical properties. Use of ceramic balls in place of steel balls can radically improve bearing performance in several ways. Because ceramic balls are 60% lighter than steel balls, and because their surface finish is almost perfectly smooth, they exhibit vibration levels two to seven times lower than conventional steel ball bearings. Ceramic hybrid bearings also run at significantly lower operating temperatures, allowing running speeds to increase by as much as 40% to 50%. Lower operating temperatures help extend lubricant life. Bearings with ceramic balls have been proven to last up to five times longer than conventional steel ball bearings. Systems equipped with ceramic hybrids show higher rigidity and higher natural frequency making them less sensitive to vibration.
Bearing precision also plays a big part in efficiency and reliability and there is a direct relationship between precision class and bearing life. Bearings with the highest tolerances offer the highest precision levels and a longer life. Pump manufacturers should look out for bearings classified as at least ISO P4S (ABEC 7) as standard to ensure they deliver the best performance.
Speed to market
Manufacturers are under pressure to design and bring their pumps to market as quickly as possible in order to remain competitive and at the forefront of their market. One way to ensure that the process runs swiftly is to review the length of time taken for bearings to be designed.
The industry standard lead time for specialist precision bearings is between 40 – 60 weeks. However, this time has been drastically cut in half and bespoke bearings can now be delivered in a little as 20 weeks, enabling pumps to be introduced more quickly. What’s more, this time is expected to be reduced even further, to just 15 weeks for tailored solutions.
Partnership and support
Pump manufacturers are increasingly requiring more than just a product. They are ever more in need of a partnership with their suppliers. Pumping systems are complex with many parameters and true partners for the bearing element can deliver full support in calculating and modelling the system. For example, once a bearing enters the qualification stage and tested it can be returned to the supplier for examination and running evidence of its performance. Leaving the modelling to bearing experts also frees up time for the pump designers to focus on their strengths and day-to-day tasks.
With increased demands for semiconductors and the current economic climate there are various requirements on pumps namely increasing efficiency and ensuring reliability. Selecting specialist bearings with a longer service life compared to conventional bearings means pumps such as dry pumps and TMP pumps can operate to their maximum performance levels and over many years.
Precision bearings also contribute to an overall reduction in the total system costs. When calculating the indirect costs of frequent bearing replacement — which include not just inventory, but machine down time, lost productivity and labour — the cost savings potential become significant.
Working with bearing experts that can design bearings with the right geometries to deliver the best performance and speeds for the environmental conditions, as well as deliver quickly can make all the difference to a manufacturer’s productivity and profitability. Seeking out partners that combine heritage and knowledge with a flexible approach such as that offered by The Barden Corporation and its partnership with HQW Precision means pumps can keep running at the right pressure levels time and time again without fail. For more information visit www.bardenbearings.co.uk or www.hqw.gmbh.