David C. Droege
OEM Product Specialist
No single type of air compressor or vacuum pump works best across the board. But wobblepiston pumps represent what is arguably the most versatile pressure and vacuum technology on the market today.
Wobble pumps combine the pressure, vacuum, and flow capabilities of conventional piston pumps with the compact size, quiet operation, and clean airflow of diaphragm pumps. Plus they are more efficient and simpler to manufacture than either piston or diaphragm versions.
This combination of features lets wobble technology capture a large share of the pressure and vacuum market in many different industries. For instance, wobble pumps are found in medical nebulizers, oxygen concentrators, blood analyzers, and pneumatic hospital beds. Laboratory devices that take advantage of this design include water aspirators, vacuum filters, mass spectrometers, and gas analyzers. On vehicles they are used with air-operated suspensions, seats, horns, and clutches. And they are a key component in vending and business machines, environmental and safety devices, printing equipment, and many industrial applications.
Despite their versatility, however, selecting appropriate wobble-piston pumps is not a job for the uninformed. No matter how much designers know about vacuum and pressure technology, it's a good bet a pump manufacturer's engineers know even more. Don't underutilize this resource. Manufacturers typically offer a wide array of standard pumps. And, in some cases, modified or even allnew designs specific to an application might be the best choice, given sufficient time for design, prototyping, testing, and production.
The point is, don't let preconceived notions lead to a pump that's overdesigned and too expensive or underdesigned and a recipe for failure. Here are a number of factors to consider when selecting the best pump for an application at the lowest possible cost.
Working range. First, define the maximum pressure or vacuum for the application. Be sure to take into account the maximum allowable pressure or vacuum tolerated by every device in the system — especially connecting lines and hoses. Define airflow requirements at the highest pressure or vacuum, because manufacturers usually rate full flow at zero pressure and vacuum. Performance curves typically graph pressure or vacuum that pumps deliver at various flows.
Most applications do not require maximum flow and pressure or vacuum all the time, so also define a typical working point and provide some idea of the frequency and duration the pump must operate at peak flow, vacuum, or pressure. This helps avoid oversizing and potentially reduces cost. Also specify if equipment must stop and restart under pressure or vacuum.
Motor requirements. Next define the power source. If it's direct current, specify voltage and source — for example, rectified ac, battery, solar, or dc generator. If using alternating current, define voltage and frequency. List power-consumption and current-draw limitations, and thermal-overloadprotection requirements. For constantspeed motors, define how precisely speed must be maintained. Or specify variable flow with an adjustable-speed motor and controller. Finally, determine the duty cycle. If intermittent, indicate a pattern of "minutes on, minutes off."
Unit envelope. List maximum height, width, and length available to mount the pump. Also, determine free air space around the unit. This helps the pump manufacturer ascertain whether there is sufficient cooling air or the application requires additional fans and other cooling mechanisms.
Operating life. Be straightforward about expectations for pump life and maintenance intervals. Wobble-piston pumps may be designed for anywhere from 500 to more than 30,000 hr of service-free life, depending on ambient temperature, operating speed, motor type, and various other factors.
Piston cups and stationary seals are the major wear parts, and many larger units can be serviced to extend life. Let the manufacturer know maintenance requirements so the pump can be specified accordingly.
Environment. Typical ambient temperatures for wobble-piston pumps range from 50 to 104°F. However, special designs operate in temperatures as low as 40°F or as high as 212°F. The quality of air surrounding the pump, whether clean, dusty, humid, dry, or containing volatile gases, determines filtration and sealing requirements.
Media. Chemicals and moisture in the air, as well as media temperature, affect pump sizing and construction. For example, piston-cup materials for general-purpose applications rapidly disintegrate if exposed to condensed moisture. Pump manufacturers conduct extensive life testing under various combinations of temperature, pressure, stroke, and humidity with hundreds of different composite blends, resulting in specialized piston-cup materials for many applications.
Noise and vibration. Sound and vibration present a complex challenge when installing a pump. All reciprocating pumps inherently vibrate due to torque pulsations when the motor speeds up and slows down as pressure builds up and subsequently discharges in each cylinder. Unwanted vibrations and acoustic noise also stem from gas pulsations, valve action, rolling-element bearings, and various flow-path restrictions downstream of the pump.
Isolating the compressor will minimize vibrations making their way to surrounding structures and enclosures. Piping or hose connections need to be flexible enough to provide sufficient isolation. Otherwise they transmit vibration to connected components that, in turn, emit noise. System resonances can also greatly amplify vibrations and noise even in well-balanced compressors.
Pump mounting is another important consideration. Larger pumps typically sit on an isolation system with elastomeric members that provide some damping and snubbing capability. In smaller pumps, standard elastomeric mounts often provide a measure of isolation and protection against shock loads.
Because wobble pumps do not use liquid lubricants, they can run in any orientation. This gives designers leeway in placing them inside equipment. Capturing the pump inside an enclosure with elastomeric components or springs provides lowcost mounting and isolation. High-volume applications permit tooling of custom mounts. For lower production volumes, standard off-the-shelf isolators can be threaded into mounting feet for most pumps.
In stand-alone tests with intake and exhaust piped away, fractional-hp wobble pumps typically generate about 40 to 60 dBA. With inlet mufflers, isolation mountings, and other techniques, pump suppliers can often reduce noise levels to the low 30-dBA range.
Regulatory requirements. UL, CSA, TUV, FDA, ISO, and other regulatory agencies require that pumps and motors meet rigorous safety standards. Suppliers with certified labs can test and evaluate products to ensure compliance with appropriate standards. For custom designs, make sure suppliers address regulatory issues in parallel with product development. Failure to do so can delay production.
Altitude. Height above sea level, barometric pressure, and temperature all affect air density — a major variable in sizing pumps to meet flow requirements. Day-to-day changes in barometric pressure and temperature affect pump performance, but generally within acceptable tolerance ranges. Altitude, or more specifically its effect on lowering atmospheric pressure, is therefore a critical factor.
High pressures or vacuums magnify altitude's effect on pump flow. A compressor at free flow (high cfm) is not affected much at all, while one operating close to full, deadhead pressure can lose virtually all flow at high altitude. Effects on vacuum applications are even more pronounced. As the pump approaches maximum vacuum, flow drops off steeply. In general, the clearance-volume to totalvolume ratio, or compression ratio, determines pump or compressor sensitivity to altitude.
Cost. Cost plays a role in determining the type of pump to buy and who to buy it from. Sometimes cost plays a secondary role, sometimes primary, but it's always present and should be a key consideration. However, when analyzing cost, consider all elements, including purchase price and installation costs, life-cycle operating and maintenance costs, cost of failure, and so on. Let the manufacturer know which of these elements has top priority.
In simple terms, a wobble pump is mechanically a cross between a diaphragm and an articulating-piston (or reciprocating-piston) pump — the shared mechanical element being an eccentric connecting rod. Wobble designs eliminate a connecting wrist pin, which significantly reduces size, weight, and mechanical complexity.
Directly coupling a unitary piston rod and crank without a wrist pin introduces a characteristic wobbling motion to the piston. At the bottom of its stroke, the piston is perpendicular to the cylinder wall. As it moves up, the piston tilts proportional to the ratio of the stroke to the overall rod length, and reaches perfect alignment again at the stroke's top. The downstroke produces the reverse motion.
To guide the piston in the cylinder bore and provide a seal (similar to piston rings) between the wobbling piston and stationary cylinder walls, the piston rides within a flanged polymer cup. Air pressure on the upward stroke of a pressure pump or the downward stroke of a vacuum pump expands the cup against the cylinder wall, sealing while compensating for the wobble action. Made of a PTFE composite, the cup produces minimal friction, requires no lubrication, and generates relatively little heat.
While the basic design of wobble pumps hasn't changed since its introduction 30 years ago, engineering enhancements have improved capabilities and opened up new applications. These enhancements include:
CHARACTERISTICS OF STANDARD DESIGNS
PERFORMANCE COMPARISON FOR STANDARD DESIGNS
ASSESSING AIR-PUMP TECHNOLOGIES
A good initial step in sorting out the best air pump for a job is to review flow, pressure, and vacuum charts commonly available in pump manufacturers' literature. Here are the chief characteristics of some commonly used pumps.
Wobble-piston pumps have high pressure and vacuum capabilities relative to their compact size and light weight. They feature moderate to high airflows, depending on design, and are more efficient than similarly sized diaphragm pumps. The units are relatively quiet, easily serviceable, and dry-running (oilless) versions offer clean output. Modern seal materials and a simple design contribute to long, service-free life, especially at lower pressures. Intake air must be filtered and should be dry. They are not suitable for full-pressure restarts.
Rotary-vane pumps have the highest airflow relative to physical size, but are not suitable for high-pressure applications. Simple designs, with or without oil lubrication, contribute to long life. While less efficient than piston or diaphragm pumps, they have smooth, pulsation-free airflow. The pumps produce a characteristic "whine," especially in smaller sizes, and vane debris can contaminate output air. They cannot handle full-pressure restarts.
Articulated-piston pumps are generally chosen for heavy industrial applications requiring long life, especially those that require full-pressure restarts. They generate high pressure and vacuum with high flows and can be oil lubricated or oilless. Noise can be an issue. They are relatively heavy, higher priced, and intake air must be filtered and dry.
Diaphragm pumps are best for lower pressure or moderate vacuum at lower airflows. They tolerate aggressive media and liquids, run quietly, and may have lower pulsations than some piston pumps. They are generally oilless, offer clean airflow, and can be designed for full-pressure restarts. A range of sizes and price points provides application flexibility.
Linear pumps fit applications requiring moderate flow with low pressure or vacuum. The units are efficient and feature long life, low power consumption, and low pulsation with large integral exhaust volumes. Some types can pump liquids.