The Pickering Governor is a spring-loaded centrifugal speed regulator used on small steam engines, where three flyweights mounted on flat leaf springs swing outward as engine speed rises. It solved the problem of building a compact, high-speed governor light enough for printing presses and small mill engines that could not carry the bulky weight of a Watt-style pendulum governor. As the weights fly out, the springs flex and lift a sleeve that throttles the steam supply. Tens of thousands shipped on Pickering & Davis engines from the 1880s onward.
How the Pickering Governor Actually Works
The Pickering Governor, also called the Pickering governor (spring balls) in older steam-trade catalogues, replaces the gravity-loaded balls of a Watt governor with three small steel balls cantilevered on flat leaf springs. Spin the spindle and the balls fly outward — but instead of lifting against gravity through a linkage, they flex the springs directly. The spring stiffness sets the equilibrium speed, not the weight of the balls. That is the whole trick, and it is why the unit can run at 300-400 RPM on a small engine where a Watt governor would need a 6:1 step-up gear.
The leaf springs are clamped at the top into a hub on the spindle and at the bottom into a moving collar. As the balls swing outward, the springs bow, and the collar rises along the spindle. A simple bell-crank or direct linkage off that collar pulls a throttle valve closed. Drop the load on the engine, speed climbs, balls fly further, throttle closes, speed drops back. The deadband is small — typically 2-4% droop — because the springs respond instantly without the friction of a pendulum joint.
Get the spring temper wrong and the whole thing falls apart. Springs too soft and the governor hunts — surging open and closed once a second as the engine overcorrects. Springs too stiff and you lose sensitivity, the engine runs 5-8% over set speed before the throttle moves. Original Pickering springs were ground to 0.040 inch thickness with a tolerance band you could not exceed by more than 0.002 inch without rebalancing. If you notice the governor oscillating at start-up, the usual cause is mismatched springs or a worn collar bushing letting the sleeve cock sideways.
Key Components
- Flyweight balls: Three small steel balls, typically 1.5-2.5 inch diameter on a stationary engine governor, mounted at the free end of the leaf springs. Mass directly affects the equilibrium speed — a 10% mass change shifts set speed by roughly 5%.
- Leaf springs: Flat spring-steel blades, usually 0.040-0.060 inch thick, clamped at the spindle hub and at the lower collar. Their stiffness sets the regulated speed. Springs must be matched within 2% stiffness across the three legs or the governor will wobble.
- Spindle: Vertical shaft driven by belt or gear from the engine crankshaft, typically at 1:1 to 4:1 step-up. Runs in two bronze or ball bearings. Any spindle runout above 0.005 inch causes vibration that masks real speed deviations.
- Sliding collar: A bronze sleeve at the lower spring clamp that translates spring deflection into linear motion along the spindle. Travel is small — usually 3/8 to 5/8 inch full stroke. The collar must slide freely with no more than 0.002 inch radial slop.
- Throttle linkage: Bell-crank or direct lift rod from the collar to the steam throttle valve. Linkage ratio sets governor sensitivity — too much travel ratio and you get hunting, too little and you get steady-state droop.
Industries That Rely on the Pickering Governor
The Pickering Governor showed up wherever an engineer needed compact speed regulation on a high-speed engine and could not afford the size or cost of a Porter or Corliss flyball setup. Printing presses were the killer app. The governor solved a real industry pain point — small print shops running 250-400 RPM engines that needed tight speed control to keep ink density consistent across a sheet, where even a 3% speed swing showed up as visible banding in the print.
- Commercial printing: Pickering & Davis high-speed steam engines driving Hoe and Goss flatbed printing presses through the 1890s — the spring-ball governor held press speed within 2% to keep ink lay consistent.
- Small textile mills: Single-cylinder mill engines under 50 HP at woolen mills in New England, where the Pickering's quick response handled abrupt loom-loading transients better than a Watt governor.
- Machine shops: Line-shaft drive engines in jobbing shops running lathes and milling machines from a common belt — the governor kept shaft RPM steady as operators engaged and disengaged tools throughout the day.
- Yacht and launch propulsion: Small marine compound steam engines in steam launches built by Herreshoff and similar yards in the 1890s, where the compact spring-ball governor fit inside engine-room envelopes that ruled out a flyball governor.
- Ice and refrigeration plants: Steam-driven ammonia compressors at small ice plants where the Pickering held compressor RPM steady within 3% to prevent suction-pressure swings.
- Museum restorations: Working steam exhibits at the Henry Ford Museum and Mid-America Windmill Museum where original Pickering governors are kept in service on demonstrator engines.
The Formula Behind the Pickering Governor
The equilibrium speed of a Pickering governor sets where the throttle holds steady — too low and the engine lugs, too high and it overspeeds before the springs can respond. The relationship below ties spring stiffness, ball mass, and effective radius to the regulated angular velocity. At the low end of typical operating range (around 200 RPM for a small printing engine governor), spring deflection is barely 1/8 inch and you have very little authority over the throttle. At the nominal design point (300-350 RPM for most Pickering units), the springs sit at roughly half their travel, giving symmetric correction in either direction. Push above 400 RPM and the springs go fully bowed — any further speed increase produces no additional throttle motion, and the engine will run away under a sudden load drop.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| ω | Equilibrium angular velocity of the spindle | rad/s | rad/s |
| k | Effective radial stiffness of one leaf spring at the ball position | N/m | lbf/in |
| m | Mass of one flyweight ball | kg | lb |
| r | Radial distance from spindle axis to ball centre at equilibrium | m | in |
Worked Example: Pickering Governor in a restored Pickering & Davis printing press engine
A small letterpress shop in Philadelphia is recommissioning a 12 HP Pickering & Davis vertical steam engine that drives a Chandler & Price platen press through a 4-inch flat belt. The original Pickering governor on the engine has new leaf springs fitted, and the operator needs to verify the equilibrium spindle speed before tying the throttle linkage back into the steam chest. The ball mass is 0.18 kg each, the equilibrium radius is 0.055 m, and the measured radial spring stiffness at that radius is 2200 N/m per leg.
Given
- m = 0.18 kg
- r = 0.055 m
- k = 2200 N/m
Solution
Step 1 — at the nominal design point, compute ω2:
Step 2 — take the square root and convert to RPM:
For a real Pickering & Davis printing engine governor designed to run at 320 RPM nominal, back-solve the spring stiffness from the target speed: k = m × r × ω2. With ω = 33.5 rad/s (320 RPM), k = 0.18 × 0.055 × 1122 ≈ 11.1 N/m per leg, summed across three legs gives the effective restoring stiffness the design needs.
Step 3 — at the low end of the typical operating range (200 RPM, ω ≈ 20.9 rad/s), the springs are only deflected to about 40% of their working travel. Throttle authority is weak — a 10% load drop pushes speed up only a few RPM before the linkage barely moves, and the engine wallows.
At the high end (400 RPM, ω ≈ 41.9 rad/s), the springs sit at roughly 90% of full bow. Any further overspeed produces almost no additional throttle closure, and a sudden load loss can let the engine accelerate past 450 RPM before the governor can react. The design sweet spot is 300-340 RPM — springs at half travel, symmetric correction in either direction.
Result
Nominal regulated spindle speed for this engine sits at 320 RPM, which on a 1:1 belt to the crankshaft holds the platen press at its rated impression rate of roughly 1,200 sheets per hour. At 200 RPM the governor barely lifts the collar — the operator sees the engine surging 15-20 RPM under each press cycle because the springs are not loaded enough to pull the throttle smartly. At 400 RPM the springs are nearly bottomed in their bow and the engine has no upper margin, so a paper jam that frees the press will let it spin up dangerously before the throttle catches. If you measure equilibrium speed 8% above the predicted 320 RPM, the most common causes are: (1) leaf springs over-tempered during replacement and 10-15% stiffer than spec, (2) ball mass low because someone fitted a non-original ball set drilled out for lightness, or (3) the throttle linkage bell-crank ratio incorrect, letting the collar travel without actually closing the steam valve at the right rate.
When to Use a Pickering Governor and When Not To
Pick the wrong governor architecture for a small high-speed engine and you will spend the rest of the engine's working life chasing hunting, droop, or runaway. The Pickering governor (spring balls) competes with the Watt pendulum governor and the Porter loaded-flyball governor — each makes a different trade between speed range, accuracy, and physical size.
| Property | Pickering Governor | Watt Governor | Porter Governor |
|---|---|---|---|
| Typical operating speed | 250-450 RPM | 40-120 RPM | 80-200 RPM |
| Speed regulation accuracy (droop) | 2-4% | 5-10% | 3-5% |
| Physical envelope (height) | 6-10 inch | 18-30 inch | 14-22 inch |
| Response time to load change | 0.3-0.6 s | 1-2 s | 0.5-1 s |
| Sensitivity to spring tolerance | High — ±2% spring match required | Low — gravity sets equilibrium | Medium — central weight dominates |
| Best application fit | Small high-speed engines, presses, launches | Slow mill engines, beam engines | Mid-size stationary engines, generators |
| Cost to rebuild today | High — custom-tempered springs | Low — gravity weights only | Medium — central weight casting |
Frequently Asked Questions About Pickering Governor
Spring steel changes modulus slightly with temperature, but the bigger effect is bushing clearance. A cold sliding collar with a tight bronze bushing binds intermittently, so the throttle does not move smoothly with spring deflection — the engine overshoots, then catches up, then overshoots again. As the bushing warms and the lubricant thins, the collar slides freely and the hunt disappears.
Pull the collar and check radial clearance with feeler gauges. You want 0.001-0.002 inch cold. If it is below 0.001 inch, hone the bushing slightly. If it is above 0.003 inch, the collar will cock under spring load and you will get a different problem — sticky throttle response on one side of centre.
250 RPM sits in the overlap zone where either will work, so the deciding factors are envelope and load type. If the engine drives a constant load like a generator, the Porter governor's central weight gives steadier regulation with cheaper rebuild cost — gravity weights do not need precision tempering. If the engine drives a transient load like a press or compressor, the Pickering's faster response (0.3-0.6 s vs 0.5-1 s) catches load changes before they show up as speed swings.
Also check the engine bed pad. A Porter needs 14-22 inches of vertical clearance above the pad. A Pickering fits in 6-10 inches. On many marine and skid-mounted engines that decision is made for you by the available headroom.
This is almost always heat-treat variation. Original Pickering springs were oil-quenched and tempered to a specific Rockwell range — modern shops often air-cool or temper to a different hardness, leaving the spring 5-10% stiffer than spec. Equilibrium speed scales with √k, so a 16% stiffness error gives you exactly the 8% speed error you are seeing.
The fix is to re-temper the springs at a slightly higher draw temperature, typically 25-50°F above the heat-treater's first attempt, and re-measure radial stiffness on a fixture before reinstalling. Match the three legs to within 2% of each other or the governor will wobble at speed.
A pure Pickering will always droop — equilibrium speed depends on spring force balancing centrifugal force, and the spring is stiffer the more it is deflected, so higher load (more throttle open, less collar lift) means lower speed. That is droop by definition.
You can approach isochronous behaviour by adding a compensating spring or a small dashpot to the linkage, which is what some late-model Pickering variants did around 1905-1915. But you trade response speed for flatness — the dashpot slows the throttle response by 0.2-0.4 seconds, which defeats the main reason you picked a Pickering in the first place.
On a healthy unit running at 300-350 RPM nominal, full-stroke collar travel is typically 3/8 to 5/8 inch. Between no-load and full-load on a press or mill engine, you should see 40-60% of that travel used — call it 0.20-0.35 inch of collar movement.
If you only see 0.05-0.10 inch of travel under big load swings, the linkage ratio is wrong or the throttle valve is sticky and the springs are doing the work but not getting the message to the steam chest. If you see the collar slamming end to end on every load change, the linkage is over-leveraged and you will hunt continuously.
Centrifugal force scales with ω2. At 60 RPM the balls produce roughly 1/45 the force they do at 400 RPM. To get usable spring deflection at low speed, you would need either much heavier balls (defeating the compact-package advantage) or much softer springs (which then cannot hold the collar steady under any vibration).
This is the fundamental reason the Pickering occupies its 250-450 RPM niche. Below that, gravity-loaded governors win on force-per-unit-size. Above that, the springs saturate and you lose top-end authority.
References & Further Reading
- Wikipedia contributors. Centrifugal governor. Wikipedia
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