What's a "final drive"?
Most diesel hydraulic or mechanical locomotives have transmissions which rotate in one direction, i.e. that of the engine which is 'anti-clockwise viewed on flywheel'. In order to go forwards or backwards they require a gearbox that can reverse that rotation, and this is generally carried out in a "final drive reversing gearbox" which is either jackshaft or axle-mounted and turns the drive additionally through 90 degrees.
(Some locos do not need reversing final drives - RSH built some locos with Crossley 2-stroke engines where the engine itself was stopped and reversed in order to change direction, in this case the final drive was just a 90 degree function. Similarly locos using hydrostatic drives can reverse the hydraulic motors like a diesel electric traction motor. UK hydrostatics are mostly low hp n.g. types, but larger locos have been built on the continent, and UK examples include the Autoloc and Zephyr locos at Mountsorrel, and the 6 axle Vollert at Tarmac/Lafarge at Tunstead.
Shaft drive locos, such as the R-R/TH "Steelman" employ a non-reversing final drive mounted on each axle. The reversing function is therefore carried out in a transfer box, or in some continental locos in a fully-reversing Voith transmission, where it is possible to use opposite direction converters to create braking force without using friction brakes.
Finally, a lot of 1st generation DMUs use reversing final drives - but because of their higher speed requirement, these are single-stage reduction with ratios around 3:1.
So let's look at the main final drive gearboxes found in industrial and BR shunters...
Self-Changing Gears Ltd, type RF11
SCG, based in Coventry, manufactured a range of transmission products that although expensive, were rugged units of exceptional build-quality. The design appeared around the 1940s and continued until SCG's demise in the 1990s, though by then the price of a one-off RF11 was over £20,000.
There were broadly two types. The standard box was jointed in two places on the horizontal, and the 3 shafts (the top shaft, intermediate and output) were in a vertical line. The heavy-duty gearbox (referred to officially as the "Z'ed" but colloquially as a "Bulging Bertie") had the intermediate shaft offset to the rear and the intermediate section of the casing had a domed raised section that covered the gears. In this form, it was rated as suitable for up to 640hp or 60ton or loco weight, the limit being the tooth load resulting from torque or adhesion. Thomas Hills pushed the limit with SCG building a 70ton 0-6-0DH with only 427bhp - SCG accepted it under pressure but it is of academic interest that the loco suffered 2 final drive gear teeth failures during its career.
The RF11 had a number of variations in the casings depending how the gearbox was being mounted - for example in BR 03 and 04 class locos, the jackshaft output is catered for by a pad either side which is bolted through the frame (or subframe) - play in these bolts results in the gearbox tipping slightly under load and this can be seen when the wingnut on the top starts scraping the underside of the cab floor! Other manufacturers used torque reaction arms to keep the box vertical and SCG produced versions with provision for such arms either on the sides, back or top.
The beauty of the RF11 lies in its accessibility - for a problem with the top shaft, the top of the casing comes off - to take the axle out for reprofiling, the upper 2/3 of the box can be left in the loco by removing the bottom. In addition, the RF11 incorporates an oil pump which creates a positive oil flow to lubricate the gears in motion, and a manual handle and locking peg so that the loco can be towed with the transmission dis-engaged.
The RF11, by having two different input/crown wheel ratios and a number of top-shaft to intermediate shaft combinations, had a wide range of potential ratios to suit a loco for different duties. Most common was 12.4:1, but standard boxes came up to 6:1 and Z'ed up to 15.32:1.
Thomas Hill and Sentinel were almost exclusive SCG customers, but RF11s were also used by Yorkshire, Barclays, Drewry, English Electric (Stephenson), Bagnall, etc.
SCG also produced smaller version of the RF11, the RF25, used in a lot of smaller Thomas Hill locos ("Noddy's") which was approximately 2/3 the size of the RF11 and only available in a standard form.
Hunslet had their own gear-making machinery and having made their own fully-mechanical transmissions, continued to make their own final drives through to the late 1980s. The most common gearbox, the "350" could be axle or jackshaft mounted and was of similar layout in gear form to the RF11 but otherwise is very different. For starters, the casing splits vertically, so any heavy attention it requires neccessitates taking the gearbox out of the loco and laying it down. The gears are straight cut and splash lubricated, so additional thin trays are incorporated to 'catch' the oil and direct it to the gear trains and bearings. Finally, whereas on an RF11 the gears are operated by internal air cylinders where the air is held "on" to hold the gearbox in mesh, the Hunslet uses an external cylinder with seperate latching pistons which engage with grooves in the selector shaft, i.e. once a direction is engaged the air is (must be) turned off to the operating cylinder else the gearbox becomes pressurised and pumps the oil out. Reversing the box requires that the latching piston is first raised by its cylinder and then the selector shaft moved across.
The most common 350 ratio is 14.42:1.
Hunslet made a number of other final drives - the "600" in some 60Ton class locos, and the "650" (solely for the BR class 14s) - but the "350" is the most common in 0-4-0 and 0-6-0 standard gauge locos.
Wiseman RLGB series
Alfred Wiseman, a major gear specialist, made a number of locomotive final drives under the 'RLGB' suffix (reversing locomotive gear box) with a two digit prefix indicating the size of the unit, in construction and appearance very similar to the RF11. Not as common as either of the above types, they were utilised in locos by Barclays, Bagnall, Clayton and Baguley.
Rolls-Royce's erstwhile Rail Traction Department at Shrewsbury, although using RF11 gearboxes on the Sentinel range and Wiseman boxes on Steelman, nonetheless designed their own final drive gearboxes as part of their long-term plans for developing the business. A CGF310 gearbox was a 3.5:1 final drive for dmu applications, but the CGF610 was a really heavy duty loco gearbox intended to be used where the RF11 had insufficient strength. Only a small number were built, ironically not in a Sentinel locomotive but in Yorkshire locos starting with the "Taurus"/"Indus" designs for which it was probably developed. (Taurus, trialled but not bought by BR, was an 0-8-0DH with two R-R C8 engines, for which one was used during shunting and the second brought in to boost the total horsepower to trip. In performance it was similar to the Class 14 and probably of similar price, but aside from Western Region no BR region saw a need for this type of loco and as Taurus was trialled on Eastern Region, nothing developed. Taurus was only built for export. "Indus", the industrial version, used both engines running continuously with a different driveline but the same final drive, only 2 were built.)
Other locomotive manufacturers own
Aside from Hunslet, latterday Baguley locos and railcars used a simple box designed and built by Baguley, and Hudswell Clarke started manufacturing their own final drives for their 35ton 0-4-0DHs. The Yorkshire Engine Company also made their own gearbox, used for example on their industrial 0-4-0DH which was classed by BR as an "02". This box was unique in that the input shaft was not designed horizontal - rather the entire drive line was angled at about 11 degrees. Three ratios, 9.05, 11.5 and 13.2, were available.
The Yorkshire final drive, as employed on the 02s