Red River Thresher

A 30-60 Hart-Parr and a 32-52 Red River Thresher was taken at Kelso, Saskatchewan, Canada in 1928.

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1961 West-Side Drive, Rochester 1, New York


Dear Editor:

It has occurred to me that some of the readers of your fine magazine might be interested in an introductory discussion of the ordinary flyball governor as used on the steam engine and in the terminology of control instrument engineering. To accentuate the control instrument point of view, I have capitalized terminology common to the process control field.

For the same governor setting, it is well known that within the limits of engine operation with the governor valve being neither fully opened nor practically closed, the heavier the load, the slower the speed. Speed is the CONTROLLED VARIABLE in this PROCESS. When a LOAD CHANGE occurs, say the load is increased, the speed falls because the governor must open the steam valve wider to permit the passage of more steam to handle the extra load; and the steam valve will be opened wider by the governor only as the speed is reduced.

Conversely, a reduction in load will cause the speed to be increased to a higher value. With the system IN BALANCE so the speed is again steady, the deviation of the new value is known as OFFSET. Should the exact same speed be required with the different loading, the operator would have to 'touch-up' the governor setting. The control instrument trade would call the means of 'touching up' the setting MANUAL RESET.

This throttling type of control in which the CORRECTIVE ACTION (repositioning of the governor actuated steam valve) is proportional to the DEVIATION OF THE PROCESS (speed change) from the CONTROL POINT SETTING (governor adjustment) is known as PROPORTIONAL CONTROL. In proportional control, there is no single exact speed for one setting of the governor. Instead, there is a CONTROL BAND which consists of all engine speeds that the engine will assume, as dictated by the loads, within the limits of the governor-controlled steam valve being neither fully opened nor practically closed.

Incidentally, if the control band is relatively wide, the action of the governor is sluggish and it is said to have LOW SENSITIVITY. If the band is narrow, the action of the governor is quick and the engine is 'up on the bit.' A governor so designed is said to have HIGH SENSITIVITY. However, if the control band is too narrow with resultant too high sensitivity, there will be excessive correction of the steam valve position for the change in speed incurred and there will be a tendency to HUNT. In others words, the engine will tend to alternate between running too slowly and too rapidly and we have INSTABILITY.

When the load is so heavy that the engine cannot maintain the speed corresponding to the fully opened position of the governor valve, the process is said to be OUT OF CONTROL. The process is also out of control whenever the engine speed is higher than that corresponding to the practically closed position of the governor valve as has occurred in descending steep hills at too high a speed. The governor valve would be closed because of the speed, and no steam would be available for reverse braking. The engine would run down the hill, literally out of control a situation that would evoke something like 'quick John, off the governor belt!'

* Project Engineer for The Taylor Instrument Companies at Rochester, New York; in charge of pressure standards and author of 'The Engine Wasn't There,' 'The Boiler Is Leaking, Dad' and other articles.