Sectional view of gearbox

[Tony Griffiths]

A restored cover showing the 4-speed crash gearbox.

[bob46320]

that's a nice start image.

[Bob Culver]

An excellent picture. Thanks Tony.
If those that know grit their teeth, those that don’t might be interested in an explanation....
Most cars in the 1920s had straight cut gears throughout, as on the left. The gear is move sideways to engage. (Moving to the left gives 1st and to the right 2nd. Sliding the one piece pair of gears in the background rightward so both enter into mesh gives reverse.) Straight cut gears are prone to graunching and whine in operation. Around 1930 there was a mania for “silent third”. The input gear and third gear was of the quiet helical pattern. Such gears produce heavy end thrust. The flanges and washers to resist this are a common weakness in many cars. Single helical gears also tend to tilt although this seems to matter little. To avoid thrust and tilt the technically very correct double helical style was adopted by several. The gears remain always in constant mesh but are locked or released from one shaft, in this case by moving the pair bodily sideways so side teeth are engaged or disengaged. Moving the helical pair to the left locks to the secondary shaft and engages 3rd. Constant mesh gears still require speeds to be matched but any graunching is spread over many side teeth and is less damaging.
But GM bought out synchromesh in 1929 and rendered constant mesh old fashioned, so the type was short lived. Synchromesh is same as constant mesh except that there is a spring loaded cone clutch to bring the gear speeds to match before the side teeth engage. With the added complication and space taken double helical gears were generally discontinued, and the engaging teeth are on a hub and this is moved instead of the actual gears.
In 4th gear or top the top set of side teeth enage with the input. There is no power drive through the gears.
The 1933 gearbox had constant mesh on 2nd but single helical so used a sliding hub to engage the side teeth.
(The very refined large Austin gearbox in the factory film recently referred to combined double helical gears and synchro)
Magnificent examples of the technically ideal double helical are the final reduction gears for steam and gas turbine ships.

[Parazine]

  Citroen-logo-2009-640x550.jpg (Size: 51.09 KB / Downloads: 342)

The Citroen logo is a stylised representation of the herringbone gears, reflecting Andre Citroen's involvement with them.

They were very expensive to manufacture, requiring a complex machining process.

[Bob Culver]

Hi Parazine
Somewhere I got the idea some Citroen had a sort of double helical bevel gear. Maybe I am dreaming but irecall seeing a set in a wreckers office 55 years ago.
The gears of the cruiser Belfast in the Thames can be seen. Because of their speed cruisers transmit very high power.

[Howard Wright]

Hi Tony

Thanks for the editing work on this and other covers and posters that you have done.  I hope that somehow these can form a collection in the archive?

Keep them coming....

Cheers

Howard

[Stuart Joseph]

Good morning all. I'm in awe of the skill and ability of the technical illustrator in the days before CAD. The sectional gearbox is a fine example of their work with few aids other than french curves and ellipse aids.
The cut away drawings in the Eagle influenced my later career and I cut them out and still have them. One year the Eagle annual had an article on the building of an A7 special which coincided with my Super Accessories special which my father bought for me to complete to keep my off motorcycles whilst still at school. Was the article written about John Haynes?

Regards from the creative county - Staffordshire

Stuart

[Reckless Rat]

Even though I stripped and restored the 4 speed crash box on my RP a few years ago ( i replaced the input output and intershaft bearings) there was no apparent wear to any of the gears even after 80+ years, even to the 1st/2nd gears and those on the layshaft. What is interesting about this particular box is the fact that the 3rd/4th gears both on the mainshaft and layshaft are effectively locked together by the chevron gears and meshing is done by the gear dogs not the teeth. Both the layshaft chevron gear and mainshaft chevron gears move as a pair dependant on the selector movement. The input chevron gears do not move and the design removes all the thrust which sadly wasn't carried over to the later sunchromesh gearbox. Layshaft thrust and subsequent wear was a source of subsequent failure of the later boxes.

[Mike Costigan]

This is an example of Citroen's double-helical gear form:

   

[Tony Griffiths]

The Citroen logo is a stylised representation of the herringbone gears, reflecting Andre Citroen's involvement with them.

They were very expensive to manufacture, requiring a complex machining process.

Yes, the story of how Citroen came to use the logo is interesting and well worth looking up.

---

Hi Tony

Thanks for the editing work on this and other covers and posters that you have done.  I hope that somehow these can form a collection in the archive?

Keep them coming....

Cheers

Howard

Yes, I'll get Mike to assemble them into a folder in the Archive. Some previous examples in the forum still have their link to a high-resolution download intact; once in the Archive, I'll make sure the links are all intact.