Gear Box Features

1. Introduction
The vast majority of manually operated automobile gear boxes use hardened steel gears on shafts supported by ball or (tapered) roller bearings with a casing cast in a permanent mould. While the majority of designs are 2 shaft layouts, see diagram below, one recent design uses a dual clutch, 3 shaft layout which can give faster changes (ref 1). This type of gearbox is available in the 3.2 litre Audi TT, the VW Golf R32 and in some Porsche models.

2. Reverse Engineering
Important areas of the reverse engineering are:

  • Gears: sizes, angles, tooth surface roughness (comparator), strength / depth of hardening (Rc and VPN at different loads)
  • Shafts: diameters, radii at changes of section, splines
  • Casing: strength (hardness tests)
  • Bearings: types and dimensions
  • Shift mechanism: arrangement

3. Procedure
It is suggested that you should follow the outline procedure given below, but note that this is an iterative process and you will probably have to go through the process more than once.

  • decide the layout, including that of the shift mechanism
  • from the maximum power and speed derive the input torque
  • decide on speed ratios
  • derive torque on each gear
  • size each gear, evaluate shaft and bearing loads
  • design the shafts
  • select / specify the bearings
  • design casing

In the example above, the gears on the input shaft are integral with it. The gears on the output shaft are continuously in mesh with those on the input shaft and in neutral they all freewheel on the output shaft.
The gear synchroniser consists of a shifting sleeve, synchroniser body, 2 cones or rings (1 shown in diag.) and (not shown) 3 blocker bars, balls and springs.
The synchroniser body bore is splined to the output shaft.
The shifting sleeve is splined to the synchroniser body. Sliding a selctor towards either gear presses a sychronising cone against the freewheeling gear and changes its speed to match that of the sychroniser and output shaft. Once the speed is sychronised the shifting sleeve can slide accross transmitting drive from the gear via teeth on the side of the gear.

1. 'Professional Engineering', 3rd September 2003 Vol.16, No. 15. page 27-28.

David J Grieve, Revised: 18 February 2010, 8th October 2003, Original: 2nd October 2001.