The Hartnell Governor

Stationary steam engines, traction engines, portable and semi-portable engines all require somemethod of steam regulation to enable the operator to maintain adequate control over the ma-chine. A device known as a governor is the piece of equipment used for this purpose.

Numerous types of governors have been designed and put into use. It is widely accepted that theWatt governor may be one of the oldest, if not the oldest, design which can still be found in useon numerous applications around the world today.

This article, although introducing the Hartnell Governor to the best of my ability, assumes thereader to have an understanding of the basic functions of governors in general.

Mr. Hartnell* designed his governor circa 1875 and at the time it was considered to be of supe-rior design to other governors of the day. It rotated between 500 and 600rpm and by utilisingsmaller fly balls and less parts, the internal friction was less than that of others.

His original design consisted of the previously accepted method of a spring loaded rotatingsleeve which altered the position of a bell crank and valve rod connected directlyto a control valve fitted into the steam inlet pipe. The control valve either reduced or increasedsteam admission to the engine as required. When the steam engine increased in speed, possiblydue to reduced loading of the engine, the fly balls would be forced out by centrifugal force thusmoving the sleeve against, as in this case, spring tension. As the fly balls moved out, the centralsleeve would be forced upwards which in turn moved the bell crank, valve rod and control valvein the correct direction to minimise steam flow and therefore decrease the speed, of the slightlyover revving engine, back to its normal operating speed.

Although the above description is reasonablytypical for the majority of fly ball type governors,the Hartnell Governor had better stability against itsstrong spring, increased sensitivity due to its highspeed, much less hunting (trying to maintain theengine at its correct speed without overshooting)and the fly balls moved roughly in a straight lineparallel to the ground..With his original governor, as with other governorsof the day, the steam stop valve would be openedand closed to maintain correct operating speed; i.e.,the amount of available steam flowing into theengine was either increased or decreased.

Hartnell was not satisfied with this arrangement anddesigned an improvement which allowed the steamstop valve to be left fully opened at all times whilesteam distribution to the engine was altered by aseparate cut off valve sliding on the back of themain valve. In effect, it was a governor with auto-matic steam cut off. He called it “AutomaticExpansion”.

To achieve this he added a locomotive typeexpansion link. An eccentric drove the mainvalve in the normal manner while a secondeccentric drove the expansion link which in turndrove the cut off valve rod and the cut off valve.The cut off valve rod was attached to the normaltype of spring loaded sleeve which was alwaysoperated from the fly balls.

As the engine changed speed due to variations inloading, the valve rod moved up or down theexpansion link due to the action of the springloaded sleeve and appropriate linkage. This inturn repositioned the cut off valve rod as well asthe cut off valve, thus changing steam distribu-tion to the engine in much the same manner aswith a railway locomotive; e.g., less load re-quired less steam admission while the mainsteam cut off valve (or throttle) remained in theone position; which is the method used toprovide greater efficiency in steam usage. Whenin operation, at normal engine speed, the valvecut off rod is roughly in the central position ofthe expansion link depending on the internalfriction of the engine and drive belts etc.

Watching the automatic cut off Hartnell governor, with its two eccentrics operating together, isvery fascinating.

Obviously, when the engine is at rest the governor is not turning. Due to spring tension, the flyballs are as far inwards as they can go and the cut off valve rod is at the bottom of the expansionlink. In this position, the cut off valve rod has maximum travel thus allowing maximum steam tobe admitted to the engine due to the relative positions between the cut off valve and main valve.When steam is admitted to the unloaded engine, the governor senses that the engine has (appar-ently) slowed dramatically and needs to change things quickly to ensure the engine returns to itsnormal operating speed. To do so, more steam needs to be admitted to the engine. Initially thefly balls remain in the inwards position and the cut off valve rod, positioned at the bottom of theexpansion link, allows the cut off valve to admit as much steam as is available for the engine togain speed.

As the engine speed increases, the fly balls move further outwards which in turn continuallyrepositions the cut off valve rod so it moves slowly upwards in the expansion link. When thevalve rod is moved upwards, its travel is gradually decreased causing the relationship betweenthe two slide valves to change and increase the amount of cut off. Eventually, due to engine overspeed, the cut off valve rod rises above the centre point of the expansion link and because ofattaining this position it has now moved the cut off valve to a point where steam is being cut offenough to cause the engine to slow down. As it slows down, the fly balls move slightly inwardslowering the valve cut off rod. Admission of steam is re-adjusted by the cut off valve and theengine quickly stabilizes to its normal operating speed with the valve cut off rod now roughly inthe centre of the expansion link.

After a load is applied, the engine will slow down. The fly balls now move inwards reposition-ing the valve cut off rod to a lower point on the expansion link which in turn gives greater travelto the cut off valve and allows more steam to be admitted to the engine to restore correct operat-ing speed. As the load on the engine is continually being altered, due to work being carried out,the entire process of adjusting the steam cut off via the governor and cut off valve is repeated tomaintain correct speed.

Another interesting part of the design is the angles at which the eccentrics are positioned on theshaft. The main eccentric leads the crankpin by 115° and the cut off eccentric leads the maineccentric by 30°; i.e., the cut off eccentric leads the crankpin by 145°. Both eccentrics havedifferent throws and need to be designed to meet the requirements of the engine to which thegovernor is being applied; e.g., on the 3” by 4½” model mill engine I am making, the maineccentric has a total throw of 0.9375” which provides the main valve with a travel of the samedistance. The cut off eccentric has a total throw of 0.71875” and due to the continual changingposition of the cut off valve rod in the expansion link the travel of the cut off valve also varies asrequired to maintain correct engine speed.

All in all, a very fascinating governor.

*To date this author has not been able to discover anything about Mr. Hartnell, his origin, or histechnical background.