G-Factor in Hydro Extractors: Calculation, Limits, and Fabric Implications
G-factor is the single most useful number for comparing extraction performance across machines of different sizes and speeds. It expresses the centrifugal acceleration at the basket wall as a multiple of gravitational acceleration, and it determines how strongly water is driven outward through the fabric and out of the perforated basket during an extraction cycle. Understanding G-factor — how it is calculated, what values are typical for different machine types and applications, and where the practical and mechanical limits lie — is essential for anyone specifying or operating centrifugal extraction equipment in an industrial laundry or garment processing plant.
Published June 26, 2026 — Stalwart Engineering Technical NotesA hydro extractor works by spinning a perforated basket loaded with wet fabric at high speed. The centrifugal force acting on the water in the fabric drives it radially outward through the fabric structure, through the basket perforations, and onto the inner wall of the outer casing, from which it drains to the sump and pump. The force available to drive this process depends on how fast the basket rotates and how large its diameter is. Two machines with the same basket diameter but different rotational speeds will extract to different levels of residual moisture; equally, two machines running at the same speed but with different basket diameters will produce different centrifugal forces. G-factor captures both variables in a single dimensionless quantity, allowing meaningful comparison between machines and providing a basis for predicting extraction performance.
How G-factor is calculated
The centrifugal acceleration at the inner wall of a rotating basket is the square of the angular velocity multiplied by the basket radius. Angular velocity in radians per second is calculated from the rotational speed in revolutions per minute by multiplying by two pi and dividing by sixty. G-factor is then this centrifugal acceleration divided by the standard acceleration due to gravity, 9.81 metres per second squared.
The working formula simplifies to: G = (1.118 × 10⁻³) × r × n², where r is the internal basket radius in metres and n is the rotational speed in revolutions per minute. Applied to a typical 800 mm diameter basket (radius 0.4 m) running at 700 rpm: G = 0.001118 × 0.4 × 490,000 = approximately 219. This value is at the lower end of the range for a stand-alone hydro extractor and is adequate for cotton flatwork but insufficient for thorough extraction of heavy terry or denim. The same basket running at 900 rpm produces a G-factor of approximately 362, which is appropriate for general-purpose laundry application.
G-factor as calculated represents the acceleration at the basket wall — at the outermost layer of fabric. The inner layers of the fabric load experience lower centrifugal acceleration because they are at a smaller radius. For a heavily loaded basket where the fabric occupies significant radial depth, the G-factor at the inner surface of the fabric mass may be substantially lower than the nominal figure. This is why load density affects extraction performance and why overloading a hydro extractor reduces extraction efficiency even when the stated G-factor is maintained: the inner fabric layers do not experience adequate centrifugal force to drive water outward.
Typical G-factor ranges by application
The G-factor required for adequate extraction varies considerably by fabric type, reflecting the resistance the fabric structure presents to water being displaced outward. Loosely woven cotton fabrics with large interstitial spaces — flatwork sheets, pillowcases, lightweight uniforms — extract well at G-factors of 200 to 300. Terry towelling, with its looped pile trapping water in tight channels between fibres, requires G-factors of 350 to 450 to reach comparable residual moisture content. Heavy canvas workwear, denim, and synthetic knits vary widely; polyester-cotton blends with their lower water retention may extract adequately at 250 G, while thick polar fleece or multilayer industrial garments may require 400 G or above.
For stand-alone hydro extractors that receive wet fabric transferred from washer-extractors or paddle dyeing machines, G-factors of 300 to 500 are common for general laundry applications. The integrated extraction cycle of a washer-extractor uses a lower G-factor, typically 100 to 300 G, because the machine is designed primarily for washing and its structural mass, bearing sizing, and foundation requirements are calibrated for that duty. Specialist hydro extractors for knitwear and delicate garments run at lower G-factors — 200 to 300 G — to prevent mechanical distortion of unstable fabric structures under centrifugal loading.
Fixed-basket versus suspended-basket machines
Stand-alone hydro extractors are manufactured in two configurations that affect achievable G-factor and the forces transmitted to the building structure. Fixed-basket machines bolt the basket directly to a rigid frame, which is anchored to a concrete floor slab. Any load imbalance — inevitable because fabric cannot be distributed with perfect symmetry — creates a rotating imbalance force that transmits directly to the floor. This limits the practical load imbalance that can be tolerated at high speed and requires robust civil foundations. Fixed-basket machines are structurally simpler and less expensive to build, appropriate where loads can be pre-balanced reasonably well, as in continuous batch laundries processing uniform flatwork.
Suspended-basket machines mount the basket and drive assembly on springs or rubber isolators, allowing the spinning assembly to precess around the true centre of mass of the unbalanced load. The isolators absorb and dissipate imbalance vibration rather than transmitting it to the floor. This allows suspended machines to handle more severely imbalanced loads at equivalent G-factors and to run at higher G-factors without generating damaging floor forces. Most garment dyeing and knitwear laundries specify suspended-basket hydro extractors because the irregular shapes and varying weights of garments make consistent pre-balancing impractical.
G-factor limits and mechanical constraints
The maximum achievable G-factor is constrained by several mechanical factors. The basket must withstand centrifugal stress generated by its own rotating mass in addition to the loading of fabric and retained water inside it. For a perforated stainless steel basket, the hoop stress in the basket wall increases with the square of rotational speed and linearly with basket radius. At very high G-factors — above 800 G in large machines — hoop stress approaches the fatigue limits of the basket material, requiring thicker wall sections or higher-grade steel alloy, both of which add to machine mass and cost.
Drive motor power is a further constraint. The power required to accelerate the loaded basket to extraction speed increases with total rotating mass and the square of the target speed. A large basket with a heavy fabric load requires a significantly larger motor to reach its rated G-factor within an acceptable cycle time. Motor starting current and the thermal loading of windings during repeated acceleration cycles are practical limits in continuous production environments where the extractor runs many cycles per hour.
Bearing life is directly affected by G-factor, particularly in fixed-basket machines where imbalance forces are carried entirely by the basket bearings and housings. Bearing manufacturers rate service life in terms of equivalent dynamic load, accounting for both the static radial load of the basket assembly and the cyclic imbalance forces at extraction speed. Running at G-factors above the machine design rating accelerates bearing fatigue and reduces the interval between replacements. Operators who routinely push hydro extractors beyond rated G-factor to improve extraction results should monitor bearing temperature and vibration level as early indicators of degradation.
G-factor and residual moisture content in practice
The relationship between G-factor and residual moisture content of extracted fabric is not linear. Water removal rates decrease as extraction progresses because the easiest-to-remove water — held in large interstitial spaces — exits first, leaving behind water bound in finer capillaries that resist centrifugal displacement at a given G-factor. Doubling G-factor does not halve residual moisture content; the improvement is real but diminishing. For cotton terry towelling, increasing extraction G-factor from 300 to 450 typically reduces residual moisture from approximately 55 percent to 45 percent — a ten percentage point improvement that translates directly into drying energy savings. A further increase from 450 to 600 G may reduce residual moisture by three to five additional percentage points.
Extraction time and G-factor are therefore selected together to achieve a target residual moisture content within the available cycle time. For laundry plants where dryer capacity is the production bottleneck, maximising G-factor and extraction time to minimise residual moisture directly increases overall plant throughput by reducing the drying time required per kilogram of linen processed.