Page "Clutch" Paragraph 55

Single-revolution clutches in teleprinters were of this type.

Basically the spring was kept expanded ( details below ) and mostly out of contact with the driving sleeve, but nevertheless close to it.

One end of the spring was attached to a sleeve surrounding the spring.

The other end of the spring was attached to the driven member inside which the drive shaft could rotate freely.

The sleeve had a projecting tooth, like a ratchet tooth.

A spring-loaded pawl pressed against the sleeve and kept it from rotating.

The wrap spring's torque kept the sleeve's tooth pressing against the pawl.

To engage the clutch, an electromagnet attracted the pawl away from the sleeve.

The wrap spring's torque rotated the sleeve which permitted the spring to contract and wrap tightly around the driving sleeve.

Load torque tightened the wrap so it did not slip once engaged.

If the pawl were held away from the sleeve the clutch would continue to drive the load without slipping.

When the clutch was to disengage power was disconnected from the electromagnet and the pawl moved close to the sleeve.

When the sleeve's tooth contacted the pawl the sleeve and the load's inertia unwrapped the spring to disengage the clutch.

Considering that the drive motors in some of these ( such as teleprinters for news wire services ) ran 24 hours a day for years the spring could not be allowed to stay in close contact with the driving cylinder ; wear would be excessive.

The other end of the spring was fastened to a thick disc attached to the driven member.

When the clutch locked up the driven mechanism coasted and its inertia rotated the disc until a tooth on it engaged a pawl that kept it from reversing.

Together with the restraint at the other end of the spring created by the trip pawl and sleeve tooth, this kept the spring expanded to minimize contact with the driving cylinder.

These clutches were lubricated with conventional oil, but the wrap was so effective that the lubricant did not defeat the grip.

These clutches had long operating lives, cycling for tens, maybe hundreds of millions of cycles without need of maintenance other than occasional lubrication with recommended oil.