Getting ready for 2016's London to Brighton, this car entered the workshop with some gearbox problems: The high gear was slipping when disengaged, and continued to do so when engaged.
In this case, it's the gearbox of a 1902 Crestmobile. It's a design commonly found in veteran American gearboxes like in early Cadillac's and REO's, just in a little different form. It's a cast iron housing which contains oil, in which 3 planetary gear sets run. 2 brake bands hold 2 different brake drums stationairy, which uses either 1 set of planetary gears for a low forward speed, or 2 sets of planetary gears for a low reverse speed, and there's 1 brake set that holds the drum stationary to the shaft, making the whole assembly "stiff" creating what on a modern gearbox is referred to as a "prise direct".The gearbox is all cast iron parts, and the brake bands are all bronze castings, causing the brake bands to wear first. Since the 2 low gears rely on the principle of "choosing the path of least resistance", they work quite well even when worn down. The high gear however has to hold the whole assembly firm, because allowing slip will cause the gearbox to start turning freely and the car loses acceleration.
In this case, the brake band had worn down about 0,75mm causing it to grip fairly bad, I was able to turn the outgoing shaft by hand when holding the incoming shaft still. It was at the end of adjustment so something needed to be done.I've had several ideas on how to approach this:
- Turn the brake round and sleeve the drum with a cast iron liner
- Turn the brake smaller and line it with leather or friction material
- Knurl the surface of the brake to make it grip harder and rise up the surface
In the end, I chose boring both the drum and the brake round and lining the brake band. The whole process can be seen in my photo album here. I think it turned out really well.
I anyone needs similar work done, or any other machining job on their vintage or veteran machine, you can contact me via the editor: office@prewarcar.com
Article and work by Jos van Genugten.