The wide variety of lab-shaker designs on the market reflects the increasing diversity of scientific experimentation. Labs now use a greater range of sample sizes than ever before, from liters to microliters. And while replicate and combinatorial studies increase the number of samples, requirements for environmental control create yet a third dimension that shaker designers must consider.
But variety can sometimes present a confusing array of choices.
Jeio Tech (Woburn, MA) categorizes shakers according to size (benchtop and floor models), shaking mechanism (e.g., rocking, waving, dual-action), or incubation capability. “Due to growth in biotech and pharma industries, experimental applications for shakers have broadened,” says Eric Stimac, sales and service manager. “Vendors serving these industries must offer a wide array of products.”
One might also categorize shakers according to shaking motion, number of shaking dimensions, size, capacity, and other characteristics. A useful classification might include one-dimensional orbital shakers, twodimensional seesaw or rocking shakers, three-dimensional gyratory or nutating shakers, wrist-action shakers that duplicate the action of a hand shaking a test tube, and incubator shakers. Several subcategories—related to speed range, orbit size, load capacity, and vessel type—exist within these groupings.
The 1D orbital shakers provide a circular shaking motion in a single horizontal plane and may be further subdivided into low-speed, high-speed, and vortexing styles as well as a variation that moves the sample back and forth along a line instead of in two dimensions within the plane. A further subtype that overlays all these designs is the incubator shaker, which heats or cools samples, sometimes under a carbon dioxide atmosphere.
The 2D, or seesaw, rocking shakers, tend to operate at slow speeds (3 rpm to 50 rpm or six tilts to 100 tilts per minute) and are employed for applications that require gentle rocking (e.g., washing blots, staining gels, or cell culture). Wave action helps ensure coverage of the sample with low volumes of wash/ process liquid. Rocking shakers are often found in incubators and cold rooms.
The 3D gyratory, or nutating (think head-nodding), shakers combine the actions of 1D orbital and 2D rocking shakers. These combined motions produce a sort of rotating wave or washing action within the vessel. One of the most famous of these shakers is the aptly named Belly Dancer™ model manufactured by Stovall (Greensboro, NC) and sold by Sigma-Aldrich (St. Louis, MO), SPI (West Chester, PA), and other distributors. Three-dimensional shakers tend to operate at low speeds (3 rpm to 50 rpm).
Like its large competitors, IKA (Wilmington, NC) sells the gamut of shaker types and is planning to introduce an expanded product range at ACHEMA 2012 in Frankfurt, Germany, next year. In addition to standard orbital and horizontal models, IKA will debut new roller-shakers, rockers, and overhead rotators targeted at medical and biological applications. “All the new models will be available in a basic and a digital version,” explains product manager Oliver Vogelsang. This choice provides a price-point difference for the same basic model that will appeal to many customers.
Motor trade-offs: AC or DC?
High-speed shakers primarily use brushless DC motors, belt systems, and digital electronics to achieve long motor life and reliability under continuous operation. Designers must employ large counterweights in these designs to offset the range of loads and to add stability. Otherwise, left to their own devices, high-speed shakers will shake their way off the benchtop and onto the floor. “Users must balance their loads when using these shakers to prevent ‘walking,’” notes Michael Revesz, product manager at Southwest Science (Bordentown, NJ). Southwest designs and manufactures shakers under its own brand and, relabeled, for several of the largest distributors.
While DC brushless motors produce long life and high reliability, they do not provide the option of running smoothly at low speeds. The problem lies in the torque required to generate the shaking motion, which is achieved from brushless DC motors— only at higher motor speeds. “Speed controls prevent settings below about 50 rpm,” Mr. Revesz adds.
By contrast, low-speed 1D, 2D, and 3D shakers typically use brushed DC motors and analog speed controls to achieve smooth shaking action while keeping manufacturing costs down. Brushed DC gear motors provide smooth acceleration, deceleration, and operation at speeds below 100 rpm. Their drawback is that the carbon brushes wear, creating carbon dust inside the motor and eventually clogging the motor’s electrical contacts.
Carbon-dust clogging causes slow operation and sometimes breakdowns requiring a motor change. The problem becomes acute for shakers operating around the clock or in a cold room.
“To overcome brush wear, designers have tried changeable brushes, harder brush materials, and larger motors (with larger brushes), but noise generated by the shaker often becomes objectionable when harder brushes or larger motors are used,” Mr. Revesz tells Lab Manager Magazine. “That is why the newest low-speed shaker designs now use brushless AC motors with variablespeed electronics to eliminate the carbon- brush dust problem while keeping noise levels down.”
What to look for
Jeio’s Eric Stimac suggests that purchasers consider the speed ranges for their particular samples, whether the shaker can support the sample weights, and included or optional accessories. “These factors must be balanced, because oftentimes the sample weights will reduce or dampen the shaking speed,” he said. As secondary must-have features, Mr. Stimac suggests looking into imbalance sensing (a safety feature), capability of clockwise and counterclockwise motions, and speed controls.
“Shakers are designed for continuous operation over long time periods,” explains IKA’s Oliver Vogelsang. Customers should therefore keep an eye open for construction that can withstand continued, repeated stresses. Other desirables on Mr. Vogelsang’s list include digital display, straightforward interface, and a timer, “especially if the experiments must be reproducible and documented, for example, under Good Laboratory Practice Standards.”