Tatham's dynamometer is a historical transmission dynamometer: power continues to the driven machine while a balanced lever indicates reaction moment. The calculator below performs ideal entered-value torque and rotational-power arithmetic.
Resources for separate linear-actuator projects
The linked resources below concern separate linear-actuator projects. This dynamometer tool does not size an actuator or calibrate a force-measurement system. An actuator linkage needs its own mounting geometry and load analysis.
With leverage, force is traded against travel. A bracket change that improves force can also increase required stroke, slow the output motion, or move the actuator into a poor mounting angle.
If extra force is added as a margin, confirm that the frame, hinge, pins, and brackets can take the load. A stronger actuator can protect the motor from stalling while quietly moving the failure point into the structure.
- Calculate actuator force at the worst bracket angle, not just at mid-stroke.
- Check stroke, retracted length, and extended length against the mechanism envelope.
- Confirm the structure can accept the higher loads created by a stronger actuator.
For a FIRGELLI actuator application, run the FIRGELLI actuator force calculator, then compare available stroke, force, and mounting envelope in the FIRGELLI linear actuator range. If the goal is to replace an existing unit, use the linear actuator replacement finder before assuming the new actuator is a direct fit.
Tatham's Dynamometer Interactive Calculator
Enter two pulley diameters, the balancing force, its perpendicular lever arm and shaft speed. Results are ideal static and steady-speed quantities, not an instrument calibration.
Equation Used
- The entered force acts perpendicular to the entered lever arm.
- The lever is in static balance and its reaction moment represents shaft torque in the ideal model.
- The inferred belt-tension difference uses the entered pulley-diameter difference as its effective differential diameter.
- Speed is steady; friction, slip, tare, compliance, runout, vibration, heat and uncertainty are omitted.
Historical Transmission-dynamometer Principle
A transmission dynamometer is placed in a power path so useful power continues to the load while a restrained member indicates reaction torque. In the illustrated Tatham arrangement, unequal effective pulley diameters create a measurable reaction that is balanced by a force on a lever.
The historical drawing establishes the mechanism concept, not universal dimensions, accuracy, bearing clearances, materials or maintenance limits.
What This Explanatory Calculator Does
The calculator converts an entered perpendicular force and lever arm into ideal reaction moment, then combines that moment with entered shaft speed to obtain ideal rotational power. It also rearranges the ideal differential-diameter relation to show the corresponding belt-tension difference.
It does not calibrate a dynamometer, establish accuracy, size a belt or certify machine power.
Ideal Moment and Power Relations
W is entered balance force, L is its perpendicular lever arm, n is shaft speed in rev/min and D1−D2 is the entered effective differential diameter. ΔT is undefined when the two diameters are equal. Mechanical horsepower uses the NIST conversion 1 hp = 745.69987158227022 W.
Worked Ideal Scenario
For D1 = 305 mm, D2 = 295 mm, W = 100 N, L = 0.50 m and n = 300 rev/min, reaction moment is 50 N·m, inferred tension difference is 10,000 N, and ideal shaft power is about 1,571 W or 2.11 hp.
The large inferred tension difference reflects the small entered 10 mm differential diameter; it is not a belt selection or instrument-validity result.
Model Boundary
- Calculated: ideal reaction moment, an algebraically inferred tension difference, steady rotational power and diameter difference.
- Historical arrangement: Tatham's device is a transmission-dynamometer arrangement.
- Not calculated: actual effective belt diameter, belt tension distribution, slip, creep, wrap, bearing or pivot friction, lever tare, calibration, runout, compliance, heat, transient load, uncertainty, accuracy, capacity, allowable speed or safety.
Frequently Asked Questions
Is W times L always the shaft torque?
Only under the stated ideal balance assumption and after any real tare and friction corrections established by calibration.
Why does a small diameter difference produce a large inferred tension difference?
The same moment divided by a smaller differential radius requires a larger ideal force difference.
What happens when D1 equals D2?
The ideal inversion is undefined because the differential lever arm is zero.
Does the horsepower result include losses?
No. It is ideal power from entered moment and speed.
Can the calculator certify measurement accuracy?
No. No calibration data or uncertainty model is included.
Can it size pulleys, belts or bearings?
No. Use applicable component data and qualified engineering for the real machine.
References
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