Dover "squirrel on limb" : LUSENET : Elevator Problem Discussion : One Thread

What is the typical operation & construction of a Dover "squirrel on limb"? It is usually driven by a cable winding drum or a tape sheave.It is usually on top of the controller.

-- MATHEW H.E.BAILEY (, May 24, 2000


The device you describe is a selector machine. It provides a position signal to the control and logic systems. Basically, it is like a miniature elevator, that moves at a proportionate but slower rate than the elevator.

These devices have been around for many years; in most versions, the movement is vertical. In the DOVER "Composite Control" System, it is horizontal. The principal is the same. A screw is rotated by a cable, chain, tape, (whatever) driven by the movement of the elevator car. The screw drives a carriage which is analogous to the elevator car. The carriage makes up contacts as it moves, which produce the position signals. If leveling is accomplished from the carriage, a "step up" mechanism is provided to lower the ratio for improved accuracy.

-- John Brannon (, May 26, 2000.


It resembles an Otis 6850 selector mounted horizontally on top of the controller...And yes it is driven by 1/8" an airplane already stated...called "composite"...


-- Joe M... (, May 26, 2000.

Dover Selector

Although it is hard to imagine the similaty of this device and a "squirrel of a stick", in combination with the plethora of other metaphors, e.g. "monkey on a stick" for the Westinghouse SN selector, I will accept the analogy.

The typical operation of the composite is manyfold. There are two ratio's of selectors, 70 to 1 and 111 to 1. This can be discerned by the color of the leveling cams. Blue cams are 70 to 1, black cams are 111 to 1. Very common error, replacing the cams and having interference problems.

The cam blocks several IR lights as it passes between the leveling switches providing level and slowdown information to relays controlling generator field excitation (speed) and series field excitation (compounding). Older versions of the same selector used mechanical microswitches and an engagement solenoid to get running clearance during multi-floor runs. The solenoid would drop out allowing the leveling ring to line up with the microswitches to provide the slowdown and stopping signals after the last step of high speed deceleration.

Additionally, the High speed acceleration and deceleration signals are on the bottom of the carriage, the signals being generated by the on and off contact of carbon brush contacts as the moving selector passes over the different lanes. Each lane is dedicated to particular steps of accel and decel.

Other lanes signal call cancellation, Hall Lantern and Position Indication.

The entire assembly is driven most commonly by a 1/8th aircraft cable. This cable has three or four extra turn from the ends of the cable on the drum. The cars movement is then geared down to provide the rotation of the leveling worm gear, with a linear bearing or travelling nut to drive the carriage. The Main Gear has a vernier dial that is typically adjusted so that the middle floor (or one of the middle floors) is set as the master floor. The master floor is set to "0" on the vernier when the car is floor level, EXACTLY. This has now registered the vernier to the actual position of the elevator. From this point, all other floors are marked on the vernier dial with a scribe, marked on the plate when the car is EXACTLY floor level with the floors. These markings become the reference for the setting of the slowdown and acceleration dimensions, indicated by a placard on the side of the selector with the major slowdown distances (in marks). Typically an adjuster wrote any variations to the standard dimensions on the placard. All adjustments are from these reference markings plus and minus for up and down dimensioning of the function involved, measured in marks of the vernier dial. The dial is divided into 100 marks.

Degradation of the carbon switches are the first cause of inaccuracy in slowdown, the carbons will typically last the lifetime of the elevator when properly adjusted, if the carbon has worn off, readjusting the carbons can always be done to get proper distancing. To adjust them, the selector needs to be seperated from the main gear by locating the car (typically at the master floor, so that the vernier is set at "0" to the pointer, the FPS switch turned off to prevent the car from moving and then the two bolts on the end of the main shaft taken off. If this has not been done for awhile, it may seem very tight. WD 40 the joint until it is free to move. The main shaft sould be free to turn by hand which in turn spins the leveling worm, moving the leveling carriage. Using a bell and battery, turn the main shaft up and down until the switches go onto the stationary contacts at the appropriate distance. Readjustment of the floor bars should never be necessary as they will never move normally.

One failure condition is the main shaft moving through its bearings. This would show up as the cams hitting the leveling switches, usually blowing fuse 6, the 48VDC supply, and all the floor bars seeming to be shifted in one direction. It is a mechanical failure of the bearing connection on both ends of the main shaft. Don't chase your tail adjusting the floor bars only to have to do it again later when the shaft moves some more. You should never have to move a floor bar.

That is the basic operation and construction of the Composite selector.

-- John Koshak (, May 30, 2000.

Moderation questions? read the FAQ