-07-June-2006

Miniature diaphragm pumps meet the performance challenge

System designers are being challenged to advance their product technology by specifying fluidic modules that can achieve increased performance capabilities while fitting in ever-smaller envelopes. Dan Schimelman from Hargraves Technology explains how the BTC IIS has been designed to outlast most of the systems it will be integrated into.

Since customers demand quality, reliability and high system up-time, Hargraves’ development engineers were tasked to incorporate fluidic and pneumatic components that can exceed 20,000 hours of operation under demanding loads, higher temperature environments and strict noise levels.

Increasing the performance of miniature pressure and vacuum diaphragm pumps has traditionally been accomplished by increasing the size and stroke of the diaphragm, increasing the volume of the pump chamber, and increasing the motor size to generate the additional required torque. Increasing the performance while shrinking the size of the miniature diaphragm

pump has posed many interesting challenges. Several companies, whose development engineers could not find miniature diaphragm pumps to meet their demanding performance and long life requirements, challenged Hargraves Technology to think out-of-the-box to advance this critical fluidic component technology.

Miniature diaphragm pumps have become popular with system engineers to provide the pressure and vacuum transport of their fluid media in a cost efficient manner. Diaphragm pumps exhibit excellent gas tightness and offer the advantage of the fluid chamber being totally sealed from the pumping mechanisms. An eccentric connecting rod mechanically flexes a diaphragm inside the closed chamber to create a pressure or vacuum. Unlike piston pumps, miniature diaphragm pumps do not require lubricants in the pump’s stroking mechanism. On a similar note, rotary vane pumps are prone to the vanes wearing and spewing debris in the flow path. Therefore, miniature diaphragm vacuum pumps and compressors ensure an oil-less, contaminant free fluid pathway. This is critical for many medical device, analytical instrumentation, clinical chemistry, gas analyzer, fuel cell and other advanced technology applications.

Advanced EPDM elastomers

The Hargraves product development team set out to clearly define their customers’ application operating parameters, while benchmarking existing miniature diaphragm air pumps available in the industry to understand their limitations. It was found that the miniature diaphragm pump and compressor technology available experienced both diaphragm rips and tears and motor failures limiting pump life to around 3,000 hours. Diaphragm failures had become so commonplace that manufacturer replacement diaphragm kits were an accepted practice and added cost.

To extend diaphragm life under ‘real world’ operating conditions, the development team searched throughout the elastomer industry for a material that would endure these rigorous demands at extended life cycles. Standard EPDM elastomers are typically rated only up to 40°C and have limited elastic properties to endure the rigorous cyclic stretching required for higher output applications. So the team took the initiative to develop an increased performance diaphragm material. Since typical operating environments for fluidic modules see far higher temperatures, the engineers hunted for an elastomer that could withstand 70°C with improved mechanical capabilities. This research project resulted in the development of an Advanced EPDM (AEPDM), a proprietary material configuration that has been tested to last ten times longer than those used by other diaphragm pump manufacturers. The life of the Hargraves’ AEPDM diaphragms can even exceed 20,000 hours, depending on the application.

The shape of the diaphragm itself was evaluated and optimised to improve vacuum, pressure and flow performance efficiencies. Typical flat diaphragms are performance limited by the amount that they can be stretched. High performance air and gas pumps require increased pump stroke beyond the stretch limits of the flat diaphragm. Higher vacuum or higher flow performance requires that either a larger flat diaphragm be used (requiring a larger pump head design) or an increased diaphragm surface area by using a shaped diaphragm. Shaped diaphragms allow the pump stroke to increase by as much as 80%. The Hargraves team became very innovative in designing optimised diaphragm shapes. As a result, a significantly increased performance output was achieved, in a much smaller, compact envelope size.

Brush motor technology limits

The motor driving the diaphragm vacuum pump or compressor is an important factor. It affects the overall performance and expected operational life. DC brush motors have been common with many diaphragm pressure and vacuum pump applications when operational life is not critical. Iron core brush motors typically use carbon brushes to conduct the electrical input from the lead wires to the motor’s commutator. The constant rubbing of the brushes on the commutator causes the brushes to wear down, like lead in a pencil. Brush motors are designed to last from 500 hours to 6,000 hours, depending on the quality of the motor and how it is used.

Motor brushes experience an electrical arcing at each start up. Frequent arcing will heat up the carbon brushes causing them to wear out more rapidly. Therefore, brush motors that experience frequent, daily, on/off cycles wear out more quickly. A top quality brush motor can be expected to last 3,000 hours with frequent on/off cycles. Brush motors used in high duty applications, with more continuous operation, can last longer. But few applications allow a pump to run continuously. Frequent starts and stops are the norm. Occasional cycling may lead to motor stall, due to carbon dust build up between the brush base and commutator. Tapping the outer housing to clear these deposits from the brush tips can usually restart the motor. In addition to limited life, brush motors can introduce unwanted electrical or RFI noise into a system’s circuitry.

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