11-09-2020, 07:29 AM
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.
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.