Pneumatic rodless cylinders in cleanroom applications
1st Quarter 2018, Pneumatic systems & components
With industry subjected to increasing demands for product miniaturisation, a major challenge is manufacturing in cleanroom conditions. This applies especially to sectors such as pharmaceutical, medical, food, microsystems and microelectronics, where even the smallest amount of air pollution or contaminant can lead to high and costly reject rates. As a result, there is an increasing need for linear drives that meet the requirements of cleanroom applications, while at the same time providing customers with an attractive and viable cost-to-performance ratio.
Tasks and requirements
The principal requirement of cleanroom technology is to aid production without compromising air purity. The design of a cleanroom is governed by the required air purity class. As a rule, cleanrooms are served by vacuum systems operating in suction mode, whereby the air is fed to one of the room’s boundary walls or ceiling in order to create a room-filling, unidirectional flow.
This ideal scenario occurs if there is no place within the whole cleanroom where air can rest, and if there are no sources of interference positioned at right angles to the direction of flow. Moreover, relatively slow motions are required in order to avoid any additional whirling of particles and so avoid conversion from laminar to turbulent flow. The permissible particle concentrations in the air of cleanrooms are graded according to purity classes defined in various standards such as VDI 2083, US Federal Standard 209E and DIN EN ISO 14644-1.
Rodless pneumatic cylinders
Standard rodless pneumatic cylinders are normally unsuitable for use in cleanroom environments as they emit tiny abrasive particles, fine lubricant particles and oil mist during operation. They can also stir up contaminant particles when operating at high speeds. In terms of design, the majority of rodless pneumatic cylinders use an axially slotted cylinder barrel. Here, power transmission through the slot outwards to the carriage is form-fit. The slot is sealed on the inside by an internal stainless steel sealing strip along the cylinder barrel wall that prevents the penetration of contaminants.
Cleanroom-compatible rodless pneumatic cylinders differ from their standard counterparts in a number of ways. For instance, a partial vacuum is created via a vacuum line in the space between the inner and outer sealing band. This feature helps to reduce possible emissions, such as abrasive particles from the piston seals or slide elements, using suction to remove and feed them into downstream filters. Special vacuum connections at both ends of the barrel are used for this purpose. The vacuum generates a suction flow from the cylinder that ensures emissions are extracted and no particles are released into the cleanroom atmosphere.
Parker’s Origa System Plus (OSP-P) cleanroom cylinder offers this functionality. It combines the efficiency of the Origa cylinder slot seal system with vacuum protection against progressive wear and contamination from sliding components. A partial vacuum drawn between the inner and outer sealing bands prevents emission into the clean room. To generate the vacuum, a suction volume of approximately 4 m³/h is required.
Benefits for cleanrooms
Cleanroom-compatible rodless pneumatic cylinders, such as the OSP-P, offer extensive customer benefits with a very good price/performance ratio that is achieved by demonstrating high cleanliness and cleanroom suitability in practical applications. Specific performance attributes such as excellent low speed running characteristics are delivered thanks to the use of special piston seals, and this is supported further by stick-slip-free operation at low speeds of around 0,005 m/s.
Tests in a class 1 cleanroom provide an idea of the increased performance levels offered by cleanroom-compatible rodless pneumatic cylinders in comparison with standard versions. While a standard cylinder will show counts of up to 3,4 million particles/m3 in operation at a velocity of 0,5 m/s, a cleanroom-compatible rodless pneumatic cylinder featuring extraction and aspiration in the slot area between inner and outer sealing band will statistically show that it emanates no cleanroom particles. Values measured in the direct vicinity of the cylinder will be equivalent to those of the ambient air in the cleanroom. Based on these performance levels, a cleanroom-compatible rodless pneumatic cylinder can be used dependent on the velocity in ISO class 4 and 5 cleanrooms.
The Origa OSP-P rodless pneumatic cylinders also provide many additional advantages. For instance, double-action force can be delivered due to their duplex design, a configuration that also offers rigidity in high load and rotational movement operations. Cleanroom cylinders are available in 16, 25 and 32 mm diameter. The design principle also offers a cost-effective alternative to electrical linear drives in cleanroom applications.
For more information contact Lisa de Beer, Parker Hannifin SA, +27 (0)11 961 0700, firstname.lastname@example.org, www.parker.com/za