Flyball Overspeed governor SCHINDLER KRK15

(Translated & Co-authored by Nicola Imbimbo)


The advent of steam engine in the early 1700s, changed completely the industrial landscape. Unfortunately these early applications were highly inefficient. There was an urgent need for improvement.

In the early days of the industrial revolution, the need was to keep the steam engine turning at a constant speed, irrespective of the load on the axle. At that time, the lower the load applied, the higher the speed, and vice versa. This was not practical.

This is exactly the point in which James Watt comes into play around 1788-89. He started to think around the method to obtain this adjustment automatically. His solution was the flyball governor, centered on the concept of the retroactive closed feedback control loop.

How flyball overspeed governor works

Put in a nutshell, the concept could be developed this way: two masses on a form of a ball, are linked together and hinged to the shaft of the steam engine. The faster the shaft rotate, the more the two hinged masses are distancing each other. Thanks to the linkage they are hinged on, the throttle on the steam engine will be partially closed, and consequently the speed of the engine reduced, and the two masses comes close together. At this point the throttle reopen partially and the speed increase accordingly. This opening and closing the throttle, cause the speed to be adjusted and maintained constant at a prefixed value.

When all that is well designed, the flyball governor keeps the engine speed at a fairly constant speed, no matter which is the load.

James Watt innovation

This type of governor it is also known as the Watt’s governor, although it was not James Watt that invented this apparatus. But it was he who applied the concept in large machines (steam engines) to make them more efficient and with a feedback loop control.

Early elevators were steam engine driven, and this type of governor was in use both for the steam engine and for the elevator speed. It resisted over the introduction of electric motors, and could be seen in operation in elevators installed in the late 1800s as well as early 1900s, and by the way, few of them, are still in service.


Following the Watt’s principle, SCHINDLER realized their governor KRK15. It is made out of two masses in a form of two little steel barrels, rotating around a central axis.

This overspeed uses the centrifugal force to move the steel barrels away from the vertical rotation axis: they are hinged with a lever on the rotating disk.

Once the calibrated activation speed has been reached, the two weights lift a cam just enough to release a spring system that allows jaws to engage with the wire rope running on his own shave.

This overspeed governor, has been in production from 1920 till 1974.

Characteristics of KRK15

The activation speed of the governor was set from 0,6 m/s up to 3,5 m/s, thanks to specific calibrated sprigs used to counteract the centrifugal force on the two barrels.

Based on the nominal speed of the elevator, also the crown-pinion bevel gear was selected accordingly, cause it had a predefined ratio.

For the conventional range of possible speeds, the commonly used wire ropes were those of ø5 mm or ø6 mm. In the faster range up to 3,5 m/s, the ropes diameter was ø6.5 mm.

Depending on the rope chosen, and the nominal speed selected, the pulley was supplied with the most suitable groove geometry.

Whichever was the case, the jaws were dimensioned to hold the rope with a maximum force of 150kg, suitable to activate the safety gear accordingly.

These governor were calibrated at the manufacturing so as to have an activation speed equal to 1.25 times the nominal speed.

The maintenance periodicity for the governor was set at 12 months by the manufacturer. After that, it was necessary to re-lubricate the gears and bearings of both the governor and tensioning pulley. Furthermore, it was requested to check of the positioning of the tensioning pulley, controlling the functionality and operation for the switch, that had to cut power to the system before reaching the activation speed. Finally an operationally check for the entire linkage both with the system standstill and at nominal speed.

Dismantling the overspeed governor KRK15

To better grasp the working operation of the device, and to subject it to a conservative maintenance, it has been completely disassembled. This operation put furthermore in evidence the extreme simplicity on which the device was designed.

For the operation to be performed, it was necessary to use a limited list of tools: two wrenches, a pliers, a hammer and awl (to extract the various pins) and finally, a very common flat-blade screwdriver.

The pulley rests on a shaft assembled on two ball bearings. The vertical shaft rests on ball bearings and on a lower bushing integral with the axis of the pulley. The crown is made of cast iron while the bevel pinion is made of elastomeric material (most probably it should be Teflon). The upper supports of the vertical shaft and that of the lever mechanism that unlocks the jaws, incorporate bushings, the upper one lubricated by means of a grease nipple. The other grease nipple is located on the lower bushing of the vertical shaft and at the same time also greases the axis of the pulley.

Linings are riveted to the two jaws (they look like they are made of rock wool or something similar). The whole block of the jaws is held in position by four nuts and two pins, thus avoiding even minimal relative displacements of the jaws and of the release device with respect to the vertical arms. All shafts are held in place by pins or by washers and cotter pins.

Thanks to the grease, that has created a protective layer over the entire governor, the conservation state is excellent.

The governor has been donated by Nova Elex.

Technical information and drawings granted by Schindler through Mr. Lucio Toffanin

Bibliographic references