The adjustable sheave illustrated here changes a V-belt’s running pitch diameter by moving one groove flange axially. Closing the groove raises the belt to a larger effective diameter; opening it lowers the belt. With this sheave on the driver, a larger diameter increases the ideal driven speed. Explore that ratio using an entered diameter change per adjustment turn.
Adjustable Universal Sheave Interactive Calculator
Vary sheave turns, pitch-diameter change, motor speed, and driven pulley size to see driven RPM and alignment shift.
Equation Used
The adjustable motor sheave pitch diameter is estimated from the open setting plus turns closed times the diameter change per turn. Driven shaft speed follows the V-belt speed ratio, so a larger motor sheave pitch diameter increases driven RPM. Centerline shift is estimated from a fixed flange and an 8 TPI movable flange thread.
- Adjustable sheave is mounted on the motor shaft.
- Pitch diameter changes linearly with turns.
- Driven sheave pitch diameter is fixed.
- Belt slip is ignored.
- Centerline shift estimate uses an 8 TPI movable flange thread.
How moving the flange changes speed
The opposed groove faces support the belt’s sides. Moving one face toward the other narrows the groove and changes the radial position at which the belt runs. The pitch diameter represents the belt’s effective running diameter, not simply the flange’s outside diameter.
A threaded adjustment and locking arrangement establishes the setting in the illustrated type. Available settings, compatible belt sections, locking details and rated speed depend on the specific sheave. This manually set arrangement is distinct from a continuously controlled variable-speed pulley system.
When only one face moves, the groove’s axial center changes. Alignment and belt tension must be checked using the relevant product instructions after a setting change. A centerline shift is not the same as a change in distance between the two shafts.
Principal parts
- Fixed and movable groove flanges: define the belt contact geometry.
- Hub and shaft connection: transmit torque.
- Adjustment thread: sets flange position.
- Locking parts: retain the approved setting.
- Companion sheave and belt: complete the drive.
The animation uses a slow illustrative speed and shows an enlarged groove section. Its take-up movement maintains the drawn belt path; belt length, tension and a physical take-up design are not calculated.
Trimming a belt-drive ratio
Adjustable-pitch sheaves can provide a setup adjustment in suitable belt-driven fans and other machinery. They allow the drive ratio to be changed within the product’s approved range without selecting a different fixed sheave for every setting.
Speed is only one part of the equipment selection. Driven-machine load, available motor power, belt rating, wrap, shaft loads and maximum permissible speeds also need assessment. The tool does not determine airflow, pump performance or motor current.
Follow the manufacturer’s procedure for stopping, securing and adjusting the actual drive. The moving illustration does not imply that a manually locked sheave can be adjusted while operating.
Pitch diameter and ideal speed ratio
The estimated driver pitch diameter is D = Dopen + nk, where n is turns closed and k is the entered diameter change per turn. Use the product’s actual setting data for a real calculation. A constant k is an approximation, not a universal calibration or an approved adjustment range.
With no belt slip, output speed is Nout = NmotorD/Ddriven. Both diameters must use the same pitch-diameter convention and length units. Use actual motor speed for the condition of interest; a nameplate speed is only a reference.
Speed Gain compares the calculated output with the output at the entered open diameter. For nonzero motor speed it is 100(D/Dopen−1). The tool displays zero gain when motor speed is zero, where a speed percentage relative to a stopped drive is not defined.
CL Shift uses the separate assumption of an 8-TPI single-start moving flange and one fixed flange. Axial flange travel is n/8 in, and the groove midpoint shifts half as much: n/16 in. This assumption is not derived from k. Both values must be consistent with the actual groove and thread before being used as hardware dimensions.
The calculation excludes belt slip, belt length and tension, load capacity, efficiency and thermal behavior. It does not predict motor amps or the operating point of a fan or pump.
Worked example: three diameter settings
Use a hypothetical driver speed of 1,750 rpm, open driver pitch diameter 3.0 in, diameter change 0.20 in per turn and fixed driven pitch diameter 8.0 in. These are illustrative values rather than a specification for a named rooftop unit or sheave.
At zero turns, driver diameter is 3.0 in and output is 1,750 × 3/8 = 656.25 rpm. At 2.5 turns, diameter is 3.5 in and output is 765.625 rpm, a 16.667% increase from the open setting. At 5 turns, diameter is 4.0 in and output is 875 rpm, a 33.333% increase from open.
Under the separate 8-TPI assumption, CL Shift is 2.5/16 = 0.15625 in at 2.5 turns and 5/16 = 0.3125 in at 5 turns. These values describe the assumed groove midpoint shift, not a required motor-base displacement.
The example provides no airflow, pressure or current result. Even where fan similarity laws are applicable, they require a defined reference operating point and appropriate system assumptions; motor current is not simply the cube of fan speed.
Comparing ways to change speed
| Approach | What changes | Selection considerations |
|---|---|---|
| Adjustable-pitch sheave | Belt running diameter and drive ratio. | Approved settings, belt compatibility, alignment and tension. |
| Fixed-pitch sheaves | Ratio remains fixed until components are changed. | Correct ratio, belt rating and installation. |
| Variable-frequency motor drive | Motor speed within the approved motor/drive system range. | Motor cooling, electrical compatibility, torque requirements and machine limits. |
No universal efficiency, speed turndown, cost or service life applies to these categories. Mechanical and electronic speed changes must both stay within the limits of the complete drive.
Adjustable-sheave questions
No. Diameter change depends on the sheave, groove geometry and belt section. Enter the applicable calibration or setting data; the tool uses a linear estimate.
The belt runs at a larger driver pitch diameter. At a fixed driver rpm this increases belt speed, and the fixed driven sheave therefore turns faster in the no-slip model.
Use compatible pitch diameters, not flange outside diameters. The belt’s running position depends on the groove and belt, so those dimensions are not interchangeable.
No. It estimates axial groove-midpoint movement from a separate 8-TPI, one-moving-flange assumption. Shaft center distance and required belt length are not calculated.
Actual motor speed, pitch-diameter estimates, belt behavior, slip and measurement conditions can differ from the inputs and assumptions. The discrepancy alone does not diagnose one fault or establish an acceptable amount of slip.
No. Use the exact sheave and belt manufacturers’ instructions, including locking, approved adjustment positions and tensioning requirements.
Compatibility depends on the sheave, belt, motor, drive and machine operating range. This ratio tool neither approves nor rules out that combination and does not prescribe a universal frequency limit.
References and further reading
TB Wood’s VP FHP adjustable-sheave installation instructions gives model-specific adjustment, locking, alignment and belt-installation requirements. TB Wood’s belted-drive selection program overview describes evaluating adjustable drives across speed settings.
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