Overhead Stirrers

<h2>What Are Overhead Stirrers?</h2>
<p>&nbsp;</p>
<p>An overhead stirrer is a laboratory mixing instrument that mounts a motor unit above the sample vessel on a stand or clamp, driving a rotating shaft and impeller that is immersed directly into the liquid. Unlike magnetic stirrers, which rely on a rotating magnetic field to spin a small bar inside the vessel, overhead stirrers use direct mechanical drive: the motor connects to the impeller shaft through a chuck or collet, allowing the motor to deliver consistent torque to the sample regardless of viscosity changes during mixing. This makes overhead stirrers the correct choice for applications where a magnetic stir bar would stall or decouple: polymerization reactions, emulsion preparation, gel dispersion, cell suspension in bioreactors, slurry processing, and large-volume buffer and media preparation. Digital overhead stirrers with brushless DC motors, torque display, programmable speed ramps, reversible rotation, and overload protection are standard research-grade configurations covering volumes to 200 L. Switch from a magnetic stirrer to an overhead stirrer when the sample volume exceeds 2 L, viscosity exceeds roughly 2,000 mPas (similar to glycerol), or the stir bar stalls during the mixing process.</p>
<p>&nbsp;</p>
<h2 dir="ltr">What You Will Find:</h2>
<div class="" data-turn-id-container="285cb5a4-0dac-4f8b-a570-84aea5821575" data-is-intersecting="true">&nbsp;</div>
<div class="" data-turn-id-container="request-WEB:3e5641be-0405-48bf-955e-e718573469fb-4" data-is-intersecting="true">
<section class="text-token-text-primary w-full focus:outline-none has-data-writing-block:pointer-events-none [&amp;:has([data-writing-block])&gt;*]:pointer-events-auto R6Vx5W_threadScrollVars scroll-mb-[calc(var(--scroll-root-safe-area-inset-bottom,0px)+var(--thread-response-height))] scroll-mt-[calc(var(--header-height)+min(200px,max(70px,20svh)))]" dir="auto" data-turn-id="request-WEB:3e5641be-0405-48bf-955e-e718573469fb-4" data-turn-id-container="request-WEB:3e5641be-0405-48bf-955e-e718573469fb-4" data-testid="conversation-turn-10" data-turn="assistant">
<div class="text-base my-auto mx-auto pb-10 [--thread-content-margin:var(--thread-content-margin-xs,calc(var(--spacing)*4))] @w-sm/main:[--thread-content-margin:var(--thread-content-margin-sm,calc(var(--spacing)*6))] @w-lg/main:[--thread-content-margin:var(--thread-content-margin-lg,calc(var(--spacing)*16))] px-(--thread-content-margin)">
<div class="[--thread-content-max-width:40rem] @w-lg/main:[--thread-content-max-width:48rem] mx-auto max-w-(--thread-content-max-width) flex-1 group/turn-messages focus-visible:outline-hidden relative flex w-full min-w-0 flex-col agent-turn" data-conversation-screenshot-content="">
<div class="flex max-w-full flex-col gap-4 grow">
<div class="min-h-8 text-message relative flex w-full flex-col items-end gap-2 text-start break-words whitespace-normal outline-none keyboard-focused:focus-ring [.text-message+&amp;]:mt-1" dir="auto" tabindex="0" data-message-author-role="assistant" data-message-id="36dcb2b0-e8ee-474e-a18c-91cce46497c4" data-message-model-slug="gpt-5-5" data-turn-start-message="true">
<div class="flex w-full flex-col gap-1 empty:hidden">
<div class="markdown prose dark:prose-invert wrap-break-word w-full light markdown-new-styling">
<ul>
<li>Digital overhead stirrers for laboratory mixing and sample preparation applications.</li>
<li>Stainless steel propellers available in one-line, four-arm, paddle with flat holes, and centrifugal impeller designs.</li>
<li>PTFE (Teflon) propellers offered in one-line, four-arm, paddle with flat holes, and centrifugal impeller configurations.</li>
<li>Optional interchangeable propellers allow optimization of mixing performance for different sample types and viscosities.</li>
<li><a href="https://mbpinc.net/equipments/temperature-control-equipment/hotplate-and-stirrers/overhead-stirrers/overhead-stirrer-accessories"><strong>Overhead stirrer accessories</strong></a> support routine laboratory stirring, blending, and homogenization tasks.</li>
</ul>
</div>
</div>
</div>
</div>
</div>
</div>
</section>
</div>
<p>&nbsp;</p>
<h2>How to Choose an Overhead Stirrer</h2>
<p>&nbsp;</p>
<p><strong>Torque capacity</strong></p>
<p>Torque (measured in N &middot; cm) is the most important selection criterion for overhead stirrers. The motor must maintain its setpoint rpm against the resistance of the sample throughout the mixing process, including at points of highest viscosity (e.g., during emulsion formation or cooling of a polymer melt). Common torque classes are 20 Ncm, 40 Ncm, and 75 Ncm for lab-scale models. High-viscosity applications (pastes, gels above 10,000 mPas) require 40 Ncm or more. Models with a real-time torque display let the operator monitor process state during mixing; torque reaching a setpoint can signal batch completion.</p>
<p><strong>Speed range</strong></p>
<p>Low-speed overhead stirrers (12 to 300 rpm) are used for gentle laminar flow mixing and tangential agitation of high-viscosity materials to prevent air entrainment. Mid-range models (60 to 2,000 rpm) handle general stirring through turbulent flow. High-speed models (up to 6,000 rpm) are used for homogenization, emulsification, and dispersion at small to medium scales. Match the speed range to the flow regime required by the application.</p>
<p><strong>Volume capacity</strong></p>
<p>Overhead stirrers specify maximum volume based on water viscosity at the stated rpm. Actual volume limits for viscous samples are lower. Benchtop models cover 2 L to 15 L; mid-range floor-stand models reach 80 L; large-scale process-development stirrers handle up to 200 L. Confirm that the impeller diameter is appropriate for the vessel diameter in use.</p>
<p><strong>Impeller type</strong></p>
<p>Impeller geometry determines the mixing regime: propeller-type impellers generate axial flow for general liquid blending; paddle and anchor impellers provide tangential flow for high-viscosity materials; dispersing disc impellers create radial high-shear flow for emulsification and dispersion; and ribbon impellers handle very high-viscosity pastes and thickening batches. Impeller shafts and heads are commonly made from stainless steel or PTFE for chemical resistance.</p>
<p><strong>Digital vs. analog control</strong></p>
<p>Digital overhead stirrers display speed in rpm, maintain setpoint through closed-loop motor control, and include overload protection, reversible rotation, and data logging capability. Analog models use a dial for speed setting and provide no readout. For regulated or reproducible workflows, digital control is strongly recommended.</p>
<p>&nbsp;</p>
<h2>Specifications Context</h2>
<p>&nbsp;</p>
<p>Motor type affects long-term reliability: brushless DC (BLDC) motors require no brush replacement, operate quietly, and maintain consistent torque at low speed, which is important for laminar flow applications. Overload protection (automatic speed reduction or shutdown when torque exceeds a setpoint) protects both the motor and the sample from damage during unexpected viscosity increases. Shaft and impeller material should be confirmed for chemical compatibility: stainless steel (grades 304 or 316) suits aqueous and mild organic media; PTFE shafts are required for corrosive acids, bases, and halogenated solvents. Support stands should be rigid enough to prevent shaft wobble at speed; confirm the stand's load rating and reach for the vessel configuration in use.&nbsp;</p>
<p dir="ltr">&nbsp;</p>
<p dir="ltr">Ready to power through your toughest mixing projects? Take a look at our overhead stirrer range and <strong><a href="https://mbpinc.net/contact-us">reach out to the MBP team</a> </strong>for a friendly quote today.</p>

Product Literature

Optional propeller, stainless steel paddle with flat holes

Optional propeller, stainless steel centrifugal impeller

Optional propeller, stainless steel one line

Optional propeller, Teflon paddle with flat holes

Optional propeller, Teflon centrifugal impeller

Included propeller, stainless steel 4 arm

Benchmark Scientific Overhead Stirrers

<p>IPS2050-20 by Benchmark Scientific is the OS-20L digital overhead stirrer with a 20-litre mixing capacity, 30–2,200 rpm speed range, 40 Ncm maximum torque, and a DC brushless motor rated for up to 10,000 mPas viscosity. The tool-free 0.5–10 mm feed-through chuck, LCD display, universal 100–240V power input, and IP42 protection make the OS-20L a practical overhead stirrer for large-volume buffer preparation, viscous polymer blending, and bioreactor vessel agitation in research and pilot-scale labs.</p>

Model

Mixing Capacity (H20)

Maximum Torque

Input Power

Output Power

Net Weight (Mixer)

Maximum Viscosity

Speed Range

Motor Type

Chuck Diameter Range

Exterior Dims.

Protection Mode

Ambient Operating Range

Electrical

Warranty

Manufacture

OS20L Overhead Stirrer, 20L includes stand, and 4 arm propeller (IPS2050-P-S4), 100 to 240V with US Plug

OS20L Overhead Stirrer, 20L includes stand, and 4 arm propeller (IPS2050-P-S4), 100 to 240V with EU Plug

OS40L Overhead Stirrer, 40L includes stand, and 4 arm propeller (IPS2050-P-S4), 100 to 240V with US Plug

OS40L Overhead Stirrer, 40L includes stand, and 4 arm propeller (IPS2050-P-S4), 100 to 240V with EU Plug

Overhead Stirrers Accessories

<h2>What Are Overhead Stirrer Accessories?</h2>
<p>&nbsp;</p>
<p>Overhead stirrer accessories are modular components that attach to an overhead stirrer motor unit to adapt it for specific applications, vessel formats, and chemical conditions. The core accessories are impellers (the mixing head), drive shafts (which connect the motor to the impeller), chuck or collet adapters (which secure the shaft in the motor's output), and support stands with clamps or crossbar assemblies that position the motor above the vessel at the correct height. Additional accessories include vessel holders and clamp stands for securing round-bottom or Erlenmeyer flasks during mixing, PTFE sleeves for shaft sealing in closed-vessel applications, and digital speed display controllers that retrofit onto analog motor units. Because overhead stirrer motor chucks accept shafts within a defined diameter range (commonly 6 mm, 8 mm, 10 mm, or 12 mm), confirm shaft diameter compatibility with the motor before ordering any impeller-and-shaft combination. Before ordering any accessory, confirm three things: motor chuck bore diameter, required shaft length for your vessel depth, and whether the impeller geometry matches the flow regime your application needs.</p>
<p>&nbsp;</p>
<h2>What You Will Find:</h2>
<p>&nbsp;</p>
<ul>
<li>Stainless steel and PTFE (Teflon&reg;) propellers compatible with overhead stirrers for a variety of mixing applications.</li>
<li>One-line propellers designed for efficient agitation in low- to medium-viscosity liquids.</li>
<li>Paddle propellers with flat holes for enhanced fluid circulation and blending performance.</li>
<li>Centrifugal impellers that promote radial flow and effective mixing of liquid samples.</li>
<li>Four-arm propellers available in stainless steel or PTFE for versatile stirring across diverse laboratory workflows.</li>
</ul>
<p>&nbsp;</p>
<h2>How to Choose Overhead Stirrer Accessories</h2>
<p>&nbsp;</p>
<p><strong>Impeller geometry</strong></p>
<p>Impeller type determines mixing regime and is the primary accessory selection decision. Propeller impellers (two- or three-blade marine propeller) generate axial flow, drawing fluid from above and directing it downward, suited for general liquid blending, cell suspension, and low-viscosity mixing. Paddle and anchor impellers sweep tangentially near the vessel wall, appropriate for high-viscosity materials that resist axial flow. Dispersing disc impellers (flat disc with serrated edge or teeth) generate radial high-shear flow for emulsification, pigment dispersion, and fine particle suspension. Ribbon impellers are used for highly viscous, thickening batches where conventional impellers fail to move material from the vessel walls.</p>
<p><strong>Shaft length and diameter</strong></p>
<p>The shaft length must extend from the motor chuck to the correct immersion depth in the vessel. Standard lengths range from 200 mm to 600 mm; longer shafts are available for deep vessels or pilot-scale bioreactors. Shaft diameter must fit within the motor's chuck bore (confirm before ordering). Thicker shafts (10 mm to 12 mm) are more rigid and reduce wobble at high speed; thin shafts (6 mm) suit small-volume vessels and lower-torque motors.</p>
<p><strong>Material: stainless steel vs. PTFE</strong></p>
<p>Stainless steel (316L is preferred for chemical resistance over 304) is the standard for shafts and impellers in aqueous buffers, culture media, alcohols, and mild organic solvents. PTFE shafts and impellers are required for corrosive acids, bases (concentrated NaOH, HCl, HF), halogenated solvents, and any application where metal ion contamination of the sample must be avoided. Stainless steel and PTFE impellers are not interchangeable from a torque-transfer standpoint; PTFE is softer and suitable for lower-torque applications.</p>
<p><strong>Support stands and clamps.</strong></p>
<p>Support stands must be rigid enough to prevent shaft wobble at maximum operating speed. Confirm that the stand's vertical travel range accommodates the vessel height plus the motor unit. Crossbar clamp assemblies allow the motor unit to be raised and lowered without removing the shaft from the vessel. Heavy-duty stands with a wide base footprint are recommended for motors above 40 Ncm and for operation above 500 rpm.</p>
<p><strong>Chuck and collet adapters</strong></p>
<p>Motors accept shafts with a specific bore range. Collet adapters reduce or increase the effective chuck bore to fit a different shaft diameter. Confirm the motor's chuck diameter (manufacturer-specific) and the shaft outer diameter before ordering a collet or chuck adapter; a mismatched fit can cause shaft slippage during mixing.</p>
<p>&nbsp;</p>
<h2>Specifications Context</h2>
<p>&nbsp;</p>
<p>Impeller diameter should be 30 to 50% of the vessel inner diameter for most mixing regimes. A blade diameter that is too small leaves dead zones at the vessel wall; too large generates excessive shear and can damage shear-sensitive samples. For autoclave-compatible setups, confirm that the shaft, impeller, and any sealing sleeve are rated for 121 degrees C steam sterilization. Shaft straightness should be verified at the stated operating speed; a bent or unbalanced shaft generates vibration that compounds with any sample viscosity increase to stress the motor bearings.&nbsp;</p>
<p dir="ltr">&nbsp;</p>
<p dir="ltr">Looking for the right propeller for your process? Explore our overhead stirrer accessories and <strong><a href="https://mbpinc.net/contact-us">reach out to the MBP team</a> </strong>for a quote today.</p>

Overhead Stirrer Accessories

<p>IPS2050-P-S4 by Benchmark Scientific is the 316 stainless steel 4-arm propeller (400 mm length, 50 mm diameter, 8 mm shaft) included as standard with the Benchmark IPS2050 OS-20L and OS-40L overhead stirrers. Designed for mid to high-speed mixing, it generates strong radial flow across the IPS2050's 30–2,200 rpm operating range.</p>

Length

Diameter

Shaft Diameter

Material

Optional propeller, Teflon 4 arm

<p>IPS2050-P-T4 by Benchmark Scientific is a PTFE-coated (Teflon) 4-arm propeller for the Benchmark™ OS-20L and OS-40L overhead stirrers — 300 mm total length, 65 mm propeller diameter, 6 mm shaft. Designed for mid to high-speed mixing of corrosive acids, bases, and solvents where metal or plastic propellers are not suitable. </p>

Optional propeller, Teflon one line

<p>The IPS2050-P-T1 by Benchmark Scientific is a PTFE-coated, single-line propeller (350 mm length, 70 mm blade diameter, 6 mm shaft) designed for low-viscosity, mid-to-high-speed mixing. Compatible with Benchmark OS-20L and OS-40L overhead stirrers, it suits buffer prep, dissolution studies, and solvent-based mixing in research labs worldwide.</p>

<p>IPS2050-P-T3 by Benchmark Scientific is a PTFE-coated flat paddle propeller with three holes, measuring 350 mm long with a 68 mm blade diameter and a 6 mm shaft, compatible with the Benchmark OS-20L and OS-40L overhead stirrers. The flat-hole paddle geometry produces low-shear axial flow for gentle to mid-speed mixing of corrosive, protein-containing, or foam-sensitive fluids where metal-free contact and minimal turbulence are required.</p>

<p>The IPS2050-P-S2 by Benchmark Scientific is a 316 stainless-steel centrifugal impeller propeller (400 mm length, 90 mm diameter) for the Benchmark Scientific OS-20L and OS-40L overhead stirrers, designed for mid- to high-speed mixing in small-mouth, round-bottom flasks.</p>

<p>IPS2050-P-S1 by Benchmark Scientific is a 316 stainless steel one-line (single flat-blade) propeller, 400 mm long with a 60 mm blade diameter and an 8 mm shaft, designed for low-viscosity, mid-to-high-speed mixing on the Benchmark OS-20L and OS-40L overhead stirrers. The single-blade axial flow pattern suits rapid buffer homogenisation, reagent dissolution, and suspension maintenance in aqueous process fluids up to 20 L capacity.</p>

<p>IPS2050-P-S3 by Benchmark Scientific is an optional stainless steel flat paddle propeller with 3 flat holes for the Benchmark IPS2050 OS-20L and OS-40L overhead stirrers. Designed for gentle, mid to low-speed mixing, its paddle geometry minimizes shear stress — suited for fragile cell suspensions, viscous samples, and slow-dissolution applications where the standard 4-arm propeller would cause damage or foaming.</p>

<p>IPS2050-P-T2 by Benchmark Scientific is a PTFE-coated centrifugal impeller measuring 350 mm long with an 85 mm open diameter and a 6 mm shaft, designed for mid-to-high-speed mixing on the Benchmark OS-20L and OS-40L overhead stirrers. The centrifugal open-blade disc geometry is optimised for small-mouth round flasks and corrosive solvent applications where a paddle propeller cannot enter the flask neck and metal-free fluid contact is required.</p>