Dewhurst’s big switch…

A man walks into a bar. “Ow!” he says…

Time to clean The Forward Off Reverse Switch (FORS). Here’s how it looked before…

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You can see some blackening of a contact top right and brown staining on another to the left. There was also some muck – actually mainly wood dust – in and amongst the gubbins…

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As I took it apart, I took photos to aid in replacing all the components in their original places. Then I cleaned them all in a bowl of GREEN GUNK and laid them out. The bright metal rods – called “drum contacts” – were polished up by putting them in a drill chuck like this…

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…switching it on at low speed and holding Wet and Dry to it…

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Here’s that blackened piece from earlier, now with the black removed. You can see the pitting. This is because it contacts the incoming live contact plates. Obviously sparking just before it makes contact with it’s plate…

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I found a PDF leaflet online showing wiring diagrams and a parts list for this switch. Here’s the parts list..

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Here’s all the cleaned parts posing for a team photo…

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Top row from left: the body, the centre rod, 4 short and 2 long drum contacts, the handle, 3 drum contact clips and some nuts, the stepping arm spring and the cable conduit. Below them are the 5 moulded drum sections. Below them are the stepping arms, an end washer and some nuts and bolts. In the front are the 8 contact fingers, 8 backing springs and 2 long dark terminal strips. To the right of those are the 8 terminal strip nuts and bolts. The zig zag paper strips seem to be for insulation. Here’s the polished up stepping arms resting on the centre rod…

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Reassembly. The terminal strips were loaded up in this order: zig zag paper, contact fingers, backing spring and fixed with nut and bolt. This is the result, with the earth contact attached top right of the right hand terminal strip…

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Each of the 5 moulded drum section has 3 recesses on each face, spaced every 90 degrees and one hole. The recess is for the short drum contact and the hole is for the long drum contact. To help visualise the correct layout I laid them out…

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It was impossible to keep them all together to insert them back into the body, so I assembled them in a vertical stack and taped them together…

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…then I dropped the assembly into position, inserted the centre rod through and bolted it tight…

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…then I could remove the tape. The drum contact clips were then clipped into position. The stepping arms and their spring were next to be fitted, then the conduit, the knob and finally the wire retaining washers and nuts. Here’s some pictures of the finished, clean FORS…

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In Off position…

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…and in Reverse…

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If you look closely at some of these pictures you can see numbers, 1 to 4 on one side and 5 to 8 on the other. These help to understand the wiring diagrams and therefore the logic and, of course, how they should be connected to the motor. Here’s some enlargements of the wiring diagrams…

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Works nice n smooth like. Next, new cable for the motor…

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