How a Mechanical Governor Works
Most engine-powered applications, from lawn mowers to tractors to standby power systems must be constrolled, automatically, by a “governor”.
The frequency and suddenness of load changes in most of these applications would require the full attention of-and probably overwhelm- a human operator. Among a few exceptions is the automobile, where the driver manages the speed control. (…unless he/she is using cruise control- which is… a governor.)
The oldest and most widely used governor is “mechanical.” It gets its energy to control the throttle from “flyweights,” driven by the engine. The centrifuge energy created by these flyweights varies with the speed of the engine and is always “working to CLOSE the throttle.” But,
somewhere in the linkae to the throttle, this flyweight energy comes up against a spring that is always “working to OPEN the throttle.” The working speed is determined by the amount of energy (tension) applied by the spring.
Incidentally, many small engines use the airflow created by the cooling fins of the flywheel, instead of internal flyweights. This air flow blows against a “vane” inside the flywheel cover. This vane “works to CLOSE” the throttle against a spring that “works to OPEN” to the throttle. The principle is the same.
Flyweight governors evolved from the “Ball Ranger” governors seen spinning in the open on steam engines, seen at fairs and in the movies.
Initially, the flyweights, or “balls”, “ranged” against gravity. Later models replaced gravity with a leaf spring. When engines were invented, the need for automatic load control was immediately recognized by worn-out operators, hence, the “governor.”
In variable-speed applications- tractors, loaders, etc- the governor spring tension is controlled by a human operator through a hand
control. He/she sets the speed and it stays there. In constant-speed applications- lawn mowers, pumps, welders, generator sets, air compressors, etc- the spring tension is preset and untouched by a human operator.
An important characteristic of any mechanical governor is its degree of “regulation.” This is the amount the engine slows, or “droops” under load. Most mechanical governors will “droop” no more than 10%. For example, a lawn mower that is set to run at 1800 RPM when it is started will probably “droop” to now lower than 1600-1750 RPM while mowing. This degree of regulation is suitable for mowers, pumps, chippers, etc.
Generator sets, welders, etc.,require closer regulation, like 3% to 5%. A generator set, for instance, might be set to run 1860 RPM, (62 Hz) “no-load”, and hold at 1770 RPM, (59 Hz) under “full-load”. In that industry, “full load” is defined as 80% of the maximum engine power at that speed.
That’s a lotta load!
If regulation is too broad, the engine will fall under load, perhaps to the point of stalling. If regulation is set too tight, the engine will pick up the load faster, but might tend to “surge” at no-load.
On small engines, with built-in, governors flyweights (or the air vane), the regulation adjustment (if-any) is a series of holes in the external “throttle lever” of the governor. Moving the governor spring to a hole closer to the governor rocker shaft will make the governor sharper. Moving the spring to a hole father from the rocker shaft will make the governor less sensitive and less apt to surge, but it will “droop” more under load.
Larger engines, generally, use an externally-mounted governor assembly that is either belt- or gear-driven. Many times, this type of governor will have an “eye-screw” to which the spring is attached. Lengthening or shortening that eye-scew will change the governor regulation in the same manner- the closer the spring is to the governor rocker shaft, the sharper the regulation. On this type of governor, if you have a no-load surge, it can be carefully “dampened out” by a “bumper screw” usually located in the back of the governor housing.
By the way, the externally-mounted governor is designed to create A LOT more energy than the internal governors of smaller engines. Hence, a MUCH
heavier spring is used. This extra energy is needed to control the higher air/fuel velocity forces encountered in bigger carburetors and other fuel
Every mechanical governor is designed to operate in a certain speed range (RPM). This is established by the size of the weights and the rate of the spring. Operating at lower governor shaft speeds might result in sluggish governor action. Operating at a higher governor shaft speed might damage the governor internally.
NOTE: Governors “hate friction, with a passion!”
Some unavoidable friction in the necessary linkage- bearings, seals, ball joints, rods, etc- is what causes the governor to “droop” in the first plce. Don’t add to it by allowing debris, hoses, or wires to inhibit free movement of all throttle linkage.