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Hartnell steam governor
A working Hartnell centrifugal governor with a simulated throttle valve and steam inlet.
About this creation
Please feel free to look over the images and skip the verbiage.

This spring-loaded Hartnell centrifugal governor is modeled after a successor to the iconic flyball governor James Watt developed for his steam engines ca. 1788.1



Operating the model by hand gives a better feel for the mechanisms involved.



The consistent throttle jumps in the 1st video above aren't due to a flaw in the governor per se. Rather, they reflect jumps in transformer output as the yellow dial is turned.



As seen with the centrifugal tachometer in this 3rd video, the jumps vanish when a continuously variable power supply is used.

The "Hartnell governor" presented here differs from Watt's design (i) in its use of a return spring to oppose the centrifugal force acting on the flyballs, and (ii) in its use of bell cranks to relay the centrifugal force to the central sleeve that actually operated the throttle valve the governor was meant to control.







Hartnell's improvements increased the speed at which centrifugal governors could operate. They also provided an easy way to adjust the governor's setpoint and increased the force the governor could exert on the throttle linkage. And last but not least, they offered finer control of the throttle by widening the governor's "dynamic range" (the difference between the speed at which the throttle first starts to close and the speed at which it's maximally closed).

On this page:Warning! Always wear eye protection when working or playing with high-speed LEGO® rotating machinery and keep valuables and bystanders (including pets) a safe distance away -- especially when testing new designs.




How it works

At the core of any centrifugal governor is the "spindle" -- here, the black vertical shaft passing through the silver-colored "return spring". In real steam engines, the output shaft turns the spindle through a chain drive or a system of gears and shafts.



In this model, an XL motor representing the steam engine turns the spindle with a 1:5 overdrive. (The XL was simply the only motor free at the time. An L with less overdrive would also have worked.)



Keyed to the top of the spindle is a rectangular LBG "carrier" providing pivot points for a pair of angled LBG "bell cranks" bearing the "flyballs" at their outer ends.





The cranks connect the rotating flyballs to the non-rotating cylindrical LBG "sleeve" below the spring.



Importantly, the sleeve must be free to slide up and down along the spindle while (i) the spindle turns freely inside it and (ii) the inner bell crank "fingers" rotate around it within their groove. I found it quite a challenge to arrange these relative degrees of freedom, keep friction to a minimum, and keep the mechanism reasonably robust at the same time.

The sleeve in turn connects the governor to a dummy DBG throttle plate via a rather complex 3D black "throttle linkage". The big yellow tube represents the engine's steam inlet pipe.









Matching the model governor's dynamic range to the desired 90° of throttle plate deflection took quite a bit of fiddling -- especially with spring and flyball selection and the throttle linkage. After finding all of my LEGO® springs far too stiff (and generally of the wrong length to boot), I resorted to a return spring found around the house.



To keep vibrations in check, I had to find just the right rubber feet (added after most of the photos) and fine-tune the mass, stiffness, and geometry of the black frame holding the spindle upright.

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Control dynamics (optional)

Together, a steam engine and its centrifugal governor form an automatic closed-loop control system based on negative feedback.2 The steam engine turns the governor, which in turn adjusts the flow of steam to the engine via a throttle valve so as to maintain a constant engine speed in the face of changing engine loads.

In the model's case, the control loop is open: The simulated steam engine (motor) turns the governor, and the governor adjusts the angle of the simulated throttle plate, but the throttle doesn't control the motor -- yet.

To first order, the outward centrifugal force on each flyball is proportional to the flyball's mass and the square of its angular speed about the spindle axis. The bell cranks convert the centrifugal forces on the flyballs into an upward force on the sleeve. The downward force on the sleeve is the sum of sleeve weight and the restoring force exerted on the sleeve by the compressed return spring. The last is proportional to the rate constant of the spring and the difference between its uncompressed and compressed lengths.



Hence, sleeve height varies roughly with the square of flyball angular speed. Throttle plate angle then varies roughly linearly with sleeve height via the throttle linkage.

To make all this a little more concrete, consider a steam locomotive at constant speed on a flat stretch of track. The engine sees a constant load and is happily chugging away at its predetermined setpoint speed. The flyballs, via the bell cranks, have compressed the return spring just enough to exactly balance the upward and downward forces on the sleeve, thus establishing a certain "equilibrium sleeve height" in the governor.

When the locomotive enounters an uphill grade, this equilibrium is transiently broken. Engine and spindle speeds initially fall under the increased engine load. The flyballs retract under the reduced centrifugal force, and the sleeve drops, thus opening the throttle to restore engine speed to its setpoint. If the same grade continues long enough, a new equilibrium sleeve height will be reached, but one lower than before.

Ideally, equilibrium between the upward and downward forces acting on the sleeve would enforce a smooth one-to-one correspondence between flyball angular speed and sleeve height. The model, however, exhibits a mild hysteresis, in that the sleeve height obtained when a given speed is approached from above differs slightly from the height obtained when the same speed is approached from below. Friction is the most likely culprit.

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Specifications

Overall dimensions:156x144x286 mm (LxWxH) excluding power supply
Mass:437 g excluding power supply
Maximum speed:865 RPM
Motor:One XL
Electrical power supply:9V train transformer
Modified LEGO® parts:None
Non-LEGO® parts:Return spring and washer
Credits:Original MOC
See also:Centrifugal tachometer, SBrick-based 9V continuously variable power supply, and Power dog

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Footnotes

1 Subtract the return spring from the flyball mechanism on my centrifugal tachometer below, and you'll have a good model of Watt's original centrifugal governor.



Hartnell's improved version again for comparison:



2 The late 18th century centrifugal governor was a major milestone in the history of technology for a least 2 reasons: (i) It vastly broadened the applications of James Watt's recently developed steam engine. Indeed, the Industrial Revolution would have been much less a revolution without it. (ii) It kick-started the modern engineering discipline of control theory, which enables much of modern mechanical technology -- automotive, aerospace, marine, robotics, manufacturing, you name it.

The centrifugal governor was one of the earliest forays into error-controlled mechanical regulation. "Error" in this context is engineering jargon for any departure from a predetermined setpoint such as steam engine speed. The earliest known centrifugal governors were used to control windmills and watermills, but the technology really took off with the introduction of Watt's rotary steam engine.



Error-controlled regulation also finds use in the LEGO® realm. The self-regulating yellow and black air compressor (lower center) in my remote control floating-arm trebuchet below is a good example.

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Recommended reading

Denny, Mark, 2007, Ingenium: Five machines that changed the world, The Johns Hopkins University Press, Baltimore (highly recommended)

Denny, Mark, 2002, Watt steam governor stability, European Journal of Physics, v.23, p.339-351.

Fasol, K.H., 2002, A short history of hydropower control, IEEE Control Systems Magazine, August, p.68-76.

Wikipedia, 2016, Centrifugal governor

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Comments

 I made it 
  January 28, 2016
Quoting jds 7777 Great job! Love raw mechanics in action! This could be used in an automatic transmission, perhaps?
Thanks, JDS! Interesting idea.
 I like it 
  January 28, 2016
Great job! Love raw mechanics in action! This could be used in an automatic transmission, perhaps?
 I made it 
  January 26, 2016
Quoting Henrik Jensen Nice Work Jeremy! Great to see your steam govenor in this "real" control set-up for the steam valve. There`s nothing like real mechanics.
Thanks, Henrik! The next step is to have the governor operate a LEGO speed control so as to adjust motor speed and thereby close the control loop. For that, however, there will have to be more torque and angular throw at what's now the throttle plate axle. That's a pretty tall order. Heavier flyballs on longer bell crank arms and a throttle linkage with more gain will likely be involved, and vibration suppression will likely become even more challenging.
 I like it 
  January 26, 2016
Nice Work Jeremy! Great to see your steam govenor in this "real" control set-up for the steam valve. There`s nothing like real mechanics.
 I made it 
  January 26, 2016
Quoting Walter Lee Awesome MOC. Great photos. Superb write up!
Many thanks, Walter! Especially glad you liked the write-up, as it was harder than usual to write for some reason.
 I made it 
  January 26, 2016
Quoting Oliver Becker Watt a MOC, Jeremy! Another superb LEGO- version of a great technical invention! I believe, most people here know steam only in connection with steam- punk... LOL So poor me is again left stunning about your great presentation! LEGOtechnic has to stay here on the pages! These models would be good for education, I never had such nice visuals in school... ;)
Many thanks, my friend! I see that you're still in pun mode from Yuri's contest. You're probably right about the steam thing. Most of my builds with educational potential started out as efforts to educate myself about the subject. Nothing beats hands-on when it come to mechanical learning.
 I like it 
  January 26, 2016
Awesome MOC. Great photos. Superb write up!
 I like it 
  January 25, 2016
Watt a MOC, Jeremy! Another superb LEGO- version of a great technical invention! I believe, most people here know steam only in connection with steam- punk... LOL So poor me is again left stunning about your great presentation! LEGOtechnic has to stay here on the pages! These models would be good for education, I never had such nice visuals in school... ;)
Jeremy McCreary
 I like it 
Matt Bace
  January 24, 2016
Very neat! I've seen a real one in action, and it is great to see one reproduced so faithfully in LEGO parts.
 I made it 
  January 24, 2016
Quoting Matt Bace Very neat! I've seen a real one in action, and it is great to see one reproduced so faithfully in LEGO parts.
Thanks, Matt!
 I made it 
  January 24, 2016
Quoting Gabor Pauler Great design. I just wondered whether the spring was from a shock absorber part, but then I found the answer.
Thanks, Gabor! Curious as to how you found this new post given that e-mail notifications are still down?
 I like it 
  January 24, 2016
Great design. I just wondered whether the spring was from a shock absorber part, but then I found the answer.
 
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LEGO models my own creation MOCpages toys shop Hartnell steam governorTechnic


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