This calculator compares the gravitational torques of two point masses on opposite sides of a fulcrum. Static balance occurs when m1gL1 = m2gL2.
Motion-control checks for See-saw Mechanism
The practical actuator question in see-saw mechanism is where the force is applied and how the moment arm changes. That is why a simple load value should be checked against the installed bracket positions before choosing stroke or force.
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.
The actuator stroke should also be checked against the useful motion, not just against the visible travel. End clearance, hard stops, bracket rotation, and wire routing can all reduce the usable stroke in the final build.
- 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.
See-saw Static Torque Balance
Compare two point-load torques about a fulcrum and solve the right-arm distance required for static balance.
Equation Used
- The beam is rigid, horizontal and massless.
- Each mass is a stationary point load at its stated perpendicular distance.
- Gravity is 9.80665 m/s² and acts vertically.
- Pivot friction, stops, dynamics, strength and safety are excluded.
Positive net torque is counterclockwise under the stated sign convention.
First-class lever equilibrium
Each downward weight acts at a selected perpendicular distance from the fulcrum. The left and right forces create opposite-sense torques. The fulcrum reaction has zero lever arm about the fulcrum and therefore contributes no torque about that point.
What the calculator determines
Select two masses and their horizontal distances from the fulcrum. The calculator reports each gravitational torque, the right-arm distance required for balance and the signed net torque.
OpenStax uses this same mass-and-distance seesaw model to illustrate the rotational-equilibrium condition Στ = 0.
Static torque equations
With downward forces perpendicular to a horizontal beam, τL = m1gL1 and τR = m2gL2. Taking counterclockwise as positive, τnet = τL − τR. Balance requires L2,bal = m1L1/m2.
Selected-input arithmetic
For 25 kg at 1.20 m and 45 kg at 0.67 m, the left torque is about 294.2 N·m and the right torque is about 295.7 N·m. Exact balance would place the 45 kg mass about 0.667 m from the fulcrum.
Model boundary
This is a static point-load model with a massless rigid beam and vertical gravity. It does not include beam mass or center of gravity, seat dimensions, distributed loads, pivot friction, stops, flexure, occupant motion, angular acceleration, impact, structural stress or safety requirements.
A nonzero net torque indicates rotational tendency only. Tipping angle, speed and time require inertia and constraint data that are not provided.
Questions
Why use N·m rather than kg·m?
Torque is force times perpendicular distance; each weight force is mg, so SI torque is reported in newton-metres.
Why does gravity cancel in the balance distance?
The same gravitational acceleration multiplies both sides of the equilibrium equation.
Does zero net torque guarantee a safe seesaw?
No. Structural capacity, stability, restraints and applicable safety requirements are outside this model.
References & Further Reading
- Wikipedia contributors. Seesaw. Wikipedia
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
