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Electric and Motor Cradle 

The motor is contained in a cylindrical casing closed by end caps at both ends. Protruding from each end cap are two square supporting/locating lugs, which rest on the clutch and motor cradle. The motor is secured to the cradle by two opposed securing screws and two opposed dowels. The left hand end of the motor casing (around its periphery) is cut-away to provide access to the motor brushes. These cut-aways are enclosed by a one piece cover, clamped together by two securing screws. The motor drive shaft protrudes from both ends of the casing and rotates on ball bearings (lubricated for life). Secured to the left hand end of the drive shaft is a coupling, which provides a housing/location for the handset when inserted into the DPM. Keyed to the opposite end of the drive shaft is a small gear pinion, which acts as the “driver” gear. Three threaded studs protrude from the right hand end of the motor and provide terminal posts for the incoming electrical wiring.

Fig 1 – Electrical Motor


1 Cover Securing Screws 4 Supporting/Locating Lugs
2 Motor Casing 5 Terminal Post
3 Gear Pinion (Driver) 6 Cover


Transfer and Reduction Gears 

The transfer/reduction gears comprise: small (driver) and large (driven) gear pinions, worm and wormwheel. The small gear pinion is keyed and pinned to the motor drive shaft, which drives the larger gear pinion (84 teeth). The larger gear pinion is keyed and pinned to the right hand end of the wormshaft. A cast aluminum cover shrouds both gears to provide protection to personnel during hand operation of the machine. The cover is hinged by an axis bolt secured to the right hand side of the clutch and motor cradle. The worm is an integral part of the wormshaft, which is supported by two plain bearings (one at either end) housed in horizontal borings in the clutch and motor cradle. The wormwheel, which forms part of the clutch mechanism, is in mesh with the worm and “retained” by the clutch friction band, clutch levers and spring. The clutch is supported and mounted on the clutch-driving disc.

Fig 2.– Transfer and Reduction Gears

  1. Motor Drive Shaft;  2. Wormshaft Bearings; 3. Worm; 4. Wormwheel;  5. Large (Driver) Pinion;  6. Small (Driver) Pinion

Combined Clutch and Reduction Mechanism 

The combined clutch and drive mechanism consists of the following sub-assemblies:

  • Locking Plate

A circular cast iron plate with a shoulder to accommodate the unlocking cam ring. The plate has a centre boring to house the top bush of the clutch axis pin. Two grease nipples, in recessed holes, are fitted to provide a means of lubricating the interior of the clutch mechanism.

  • Unlocking Cam Plate

A slightly dished machined steel ring. Relieved both internally and externally to form cams. Its inside face locates on the underside of the locking plate shoulder. Two circular steel studs protrude vertically from the underside of the plate, opposed by 270º.

  • Clutch Friction Band

A “C” shaped thin steel metal band. The break in the band is to accommodate the two operating tongues of the clutch levers. The outer face of the band is ribbed and in contact with the inner face of the wormwheel.

Fig. 3. – Combined Clutch and Drive Mechanism

1 Locking Plate 7 Retentive Spring
2 Grease Nipples 8 Left and Right Clutch Levers
3 Unlocking Cam Ring 9 Clutch Lever Spring
4 Clutch Friction Band 10 Driving Disc
5 Wormwheel 11 Clutch Axis Pin Bushes
6 Retentive Spring Plungers 12 Clutch Axis Pin


  • Clutch Levers and Spring

Composed of two levers (left and right) mounted one on top if each other. Both have a central boring to accommodate the clutch axis pin. The end of each lever is a shaped operating tongue, which when in the normal operating position bear against the two open ends of the clutch friction band. In the assembled state, the compression spring is retained between the two inside faces of the operating tongues. The levers are retained in the center of the clutch mechanism by the clutch axis pin passing through central borings in the levers.

  • Wormwheel

A substantially constructed steel ring with its periphery cut to form the wormwheel teeth. Its machined inner face accommodates the clutch friction band.

  • Driving Disc

A multi-contoured cast steel disc, its underside formed with a gear pinion (10 teeth) its center has a boring to house the lower axis pin bush. Two projections on top of the disc provide a location and housing for the retentive spring. A raised semi-circular ridge on the underside of the disc provides a stop face for the movement of the complete clutch mechanism.

  • Retentive Spring

This heavy compression spring is retained between two plungers, which are threaded together. The ends of the pistons are shaped to locate in recesses in the driving disc and locking plate.

  • Clutch Axis Pin and Bushes

A round steel pin passes through two phosphor bronze bushes. The lower bush is housed in the driving disc whilst the top bush is housed in the locking plate. The bottom of the pin locates in a boring of the lower casing of DPM, whilst the top of the pin locates in a boring in the clutch and motor cradle. The axis pin is prevented from rotating by a locking plate secured to the top of the clutch and motor cradle. A boring through the center of the pin acts as a lubrication channel, serviced by a grease nipple retained in the top of the axis pin.

Drive Bar 

The drive bar consists of a machined, flat steel bar with gear teeth (rack) machined along one edge for approximately half its length. These teeth mesh with the pinion of the driving disc. The opposite end is bored to house a steel bush to which the switchblade rodding is connected. The bar is supported by the lowercasing extension and a machined face on the inside bottom of the lower casing.

Detector Slides

Both detector slides (upper and lower) are manufactured from flat steel bar with machined notches to accommodate the detector pawls. Each detector slide has two notches, one to detect that the switchblade is closed and the other to detect that the switchblade is open. The open switchblade notch is ramped to permit free movement of the open switchblade and this permit unlocking of the close switchblade. During operation of the DPM, the detector slides move in and out of the lower casing extension, which also provide support for the slides. Secured to the front end of each slide by a dowel, two securing screws and a locking plate, is fork end connector. This in turn is connected by a horizontally positioned connecting pin to the switch rail rodding.

Fig. 4. – Drive Bar and Detector Slides

1 Lower Casing 5 Extension
2 Detector Slides (Upper and Lower) 6 Stop Plate (Detector Slide)
3 Pinion (Driving Disc) 7 Fork end Connectors
4 Drive Bar 8 Connecting Pins


Editor:  Merabi Chaladze