Editing Stepping switch (section)

==Single-axis stepping switches==
[[File:Uniselector Stepper detail.jpg|thumb|350px|An example of a Western Electric 7A Rotary (Bird-cage) Line Finder assembly. The horizontal shaft is driven by a gear and when the Line Finder's electromagnet is energized, a flexible disc at the base of the Line Finder's brush carriage is engaged through friction to the horizontal shaft's driving disc, causing the brush carriage to rotate.]]
Stepping switches were widely used in telephony and industrial control systems when electromechanical technology was paramount.

A basic stepping switch is an electrically operated [[rotary switch]] with a single (typically input) terminal, and multiple (typically output) terminals. Like other typical rotary switches, the single terminal connects to one of the multiple terminals by rotating a contact arm, sometimes called a wiper, to the desired position. Moving from one position to the next is called stepping, hence the name of the mechanism. Using traditional terminology, this is a single-pole, multi-position switch.

While some stepping switches have only one pole (layer of contacts), a typical switch has more; in the latter case, all wipers are aligned and move together. Hence, one input with multiple wires could be connected to one of multiple outputs, based on the receipt of a single set of pulses. In this configuration, the rotating contacts resembled the head support arms in a modern hard disk drive. Multipole switches were common; some had perhaps as many as a dozen poles, but those were less common.

Most switches have a bank of stationary contacts extending over half a cylinder, while some have only a third of a cylinder. The typical "half-cylinder" switch has two sets of wiper contacts opposite each other, while the "third of a cylinder" type has three sets, equally spaced. For any given level, both or all three wipers are connected, so it makes no difference which of the two (or three) is connecting. When access to more outlets was required, the rotor had two sets of wipers opposite each other but offset vertically: on the first half rotation one set of outlets was accessed; the second set of outlets was accessed on the second half rotation.

An electromagnet advances (steps) the wipers to the next position when fed with a pulse of DC. The magnet's armature (spring-loaded) operates a pawl that advances a ratchet. When the pawl reaches its full stroke, it blocks the ratchet so it and the wipers will not overshoot. When power to the coil disconnects, the spring retracts the pawl. Another pawl, sometimes called a detent spring, pivoted on the frame ensures that the wipers do not move backward; contact friction keeps them in place. Some uniselector designs step on application of the operate pulse; others step on its removal.

[[File:Panel GCO District Sender Selector Frame.jpg|thumb|alt=An array of stepping switches|An array of uniselector stepping switches as installed in a telephone exchange. The silver dials show the current position of the moving wipers. The fixed feeder brushes are barely visible.]]

In most applications, such as telephony, it is desirable to be able to return the wipers to a "home" position; this is at the beginning of rotation, at one end of the array of fixed contacts. Some switches have a cam attached to the wiper shaft. This cam operates a set of contacts when the wiper is at home position, which is at the beginning of the span of rotation. Other circuit designs used one level (pole) of the contacts to home the wipers, so the separate homing contacts were not needed.

Typical stepping switches have contacts directly operated by the stepping magnet's armature; these contacts can serve to make the magnet cycle ("self-step") and advance the wipers as long as power is applied. The external control circuits remove power when the wipers reach the desired position; that could be the home position.

Most stepping switches rotate the wipers in only one direction, but some are bidirectional; the latter have a second magnet to rotate the wipers the other way. A third variety "winds" a spring as the wiper steps progressively, and a ratchet holds the wipers from returning to home position. When the circuit is no longer needed, another electromagnet releases the holding pawl; the spring then returns the wipers to their home position.

Stepping switches were quite noisy in operation (especially when self-stepping), because their mechanisms accelerated and stopped quickly to minimize operating time. One could compare their sound to that of some snap-action mechanisms. Nevertheless, they were engineered for long life, given periodic maintenance; they were quite reliable.

Single-axis stepping switches are sometimes known as uniselectors.

<gallery>
File:WE 206A stepping switch front.agr.jpg|Several views of a Type 206A stepping relay
File:WE 206A wiring terminals.agr.jpg|Wiring terminals, unit is a 22-position, six-layer switch
File:WE 206A contacts and wipers right.agr.jpg|Internal contacts
File:WE 206A stepping switch wipers.agr.jpg|Solenoid and wiper arm
File:WE 206A stepping switch top.agr.jpg|Top view, ratchet wheel, pawl and detent spring on the left
</gallery>