Hiscox's plumb-bob governor for a gas engine is an inertia mechanism carried by the reciprocating exhaust-valve push rod. The bob and its rigid pick blade respond to the rod's changing motion so that the blade can meet the valve spindle. It is not the rotating conical-pendulum governor previously modeled on this page.
Plumb-Bob Push-Rod Kinematics Explorer
Enter a declared push-rod stroke and cycle rate plus bob-arm length and mass. The worksheet reports ideal sinusoidal push-rod speed and acceleration scales, bob inertia-force scale, and bob weight; it does not predict governor setpoint or trip adjustment.
Equation Used
- Push-rod displacement is prescribed as sinusoidal with amplitude S/2.
- Cycle rate n is push-rod cycles per minute, not engine crankshaft RPM unless the actual drive makes them equal.
- The inertia-force output is the peak mass-times-prescribed-acceleration scale.
- Bob arm length changes only the illustrated rigid arm length; it is not used to infer a trip angle.
- No spring setting, contact clearance, impact, friction, wear, valve load, engine speed regulation, droop, or safe adjustment is predicted.
Replaces the incorrect rotating conical-pendulum setpoint model with the historical reciprocating push-rod inertia topology and explicitly bounded ideal kinematics.
The bob rides with the reciprocating push rod
Hiscox item 439 describes the plumb bob A as pivoted in a box attached to the exhaust-valve push rod. On the back motion of that rod, bob inertia produces a forward relative motion and a downward motion of pick blade C toward valve spindle D. The source also shows spring-end screws E and F for adjustment, but it does not supply dimensions, a speed equation, or a universal setting procedure.
A historical gas-engine valve mechanism
The sourced subject is specifically a plumb-bob governor for a gas engine acting through the exhaust-valve push rod. This page therefore does not generalize the drawing to named engine makes, throttle butterflies, marine auxiliaries, tractors, or restoration specifications without device-specific documentation.
The worksheet quantifies only declared push-rod kinematics
OpenStax gives the standard simple-harmonic-motion identities used here. Multiplying the declared bob mass by peak prescribed acceleration gives an inertia-force scale; multiplying by standard gravity gives bob weight. Neither quantity solves the governor's transient swing or contact event.
A transparent declared-motion example
For a 30 mm stroke at 240 cycles per minute, amplitude is 15 mm and angular frequency is about 25.13 rad/s. Peak push-rod speed is about 377 mm/s and peak acceleration is about 9.47 m/s squared. A declared 300 g bob gives a peak mass-times-acceleration scale of about 2.84 N and a weight of about 2.94 N.
What this ideal check omits
The historical plate does not provide mass distribution, pivot friction, spring rates, contact clearances, push-rod motion law, valve load, impact restitution, damping, engine-cycle ratio, or a calibrated speed setpoint. Those missing properties control the actual trip and reset behavior. The former conical-pendulum RPM formula, universal droop figures, lost-motion rules, maintenance tolerances, engine examples, diagnostic claims, and adjustment advice are removed.
Plumb-bob governor worksheet questions
Is this a rotating flyball governor?
No. Hiscox shows a bob pivoted on a reciprocating exhaust-valve push-rod box.
Does the calculator predict engine RPM?
No. Cycle rate is a declared push-rod input.
Does the animation calculate the trip point?
No. It shows the documented connection and sequence schematically.
Can I use these numbers to adjust a running engine?
No. Use device-specific documentation and appropriate qualified inspection.
Plumb-bob governor and motion references
- Gardner D. Hiscox, Mechanical Movements, item 439: primary historical description of the push-rod box, plumb bob A, pick blade C, valve spindle D, and adjustment screws E and F.
- OpenStax University Physics, 15.1: position, peak velocity, and peak acceleration identities for prescribed simple harmonic motion.
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
