The South Devon Railway's Routemaster

GB32 Four Speed Epicyclic Self Changing Gearbox.
GB32 Right Hand Side GB32 Left Hand Side
Right hand side shewing EP valve block & pipes Left hand side shewing air feeds for brake bands
GB32 Rear View GB32 Lable
Output flange and ports for Speed Generator
(SG) on left and speedo/tacho on right.
Identification on Gearbox body.
Note it's self changing not automatic!
Views of a spare GB32 gearbox without the 'bus "wrapper".  This one's a lot grubbier than it should be.

The Routemaster has a four speed epicyclic gearbox with electropneumatic valves controlling pistons which actuate brake bands thereby changing gear.  Fourth gear is a lock-up clutch giving a 1:1 ratio.  The actual control of gear selection is done by the driver's gear selector, a control unit behind the driver's head and a speed sensor.  These notes are based on RM1872 which has the older CAV system with a Lucas motorcycle alternator providing the final speed detection.  It was this unit which failed when we drove RM1872 down to Devon and she's been wired to manual only operation ever since.  The speed generator has been repaired and so RM1872 will be returned to automatic at some time.  One of the reasons for the delay in doing so was the evidently poor lubrication of the gear train which may be related to the oil pump at the front of the 'box being assembled incorrectly and so failing to provide lubrication!  Time will tell.
We also developed a noise on selecting 4th gear which had to be one of the thrust bearings; probably the one in the yoke.  Examination of the gearbox proved this to be the case and the race has been replaced.
The Routemaster Owners Association suggests using 2½ imperial gallons of light turbine oil SB 2152 Gearbox or Torque Oil. We have also been told a high quality 15W40 motor oil or straight SAE30 diesel oil with no detergents are suitable. Whilst we cannot recommend a particular oil we are using the SAE30 option because we use it elsewhere on the Railway and buy it in 210 litre drums!
When in service in London the oil would be changed on an annual basis.
Here are some photographs of the repair operation.
refitting clutch plates
Refitting clutch plates (RM1872 doesn't have a pre-assembled clutch).
Clutch release springs
Clutch Release Springs
4th gear thrust race
4th Gear Thrust Bearing and yoke. Slots top left & bottom right are for the pivot pin and 4th gear piston.
Pump ecentric locator
Cover refitted.  Note extra keyway slot (indicated) on input shaft for oil pump eccentric alignment.
Oil pump feed holesOil pump indicator
Oil pump eccentric. Holes (indicated) MUST align hence keyway and indicator (face drilling on right).

The following pictures where taken during the dismantling of a scrap GB32 to see what our problem was likely to be.  They're provided here to remind us how it all went together, though it should be noted that we have now discovered the oil pump was assembled incorrectly like the one on RM1872 which probably contributed to both failures!  Aligning the extra keyway in the eccentric with the one in the shaft and ensuring the oilway lines up with the hole in the shaft has revealed that the odd drilling in the face of the eccentric has to be towards the pulley.  The shaft in the scrap 'box doesn't have the oilway input which also won't have helped!  We only dismantled the forward gears because that's all we were interested in at the time.  At some point we will strip the rest of the 'box and record what we find here.  If we rebuild this 'box then that'll be here too.
Outer thrust race
Outer Thrust race after removing pulley and oil pump; note spacer between thrust race and oil pump
Oil Pump
Oil pump assembly - note eccentric is the wrong way round (drilling in face should be to outside)
Spacer between clutch and front thrust race
Inside 4th gear cover
Inside 4th gear cover; 4th Gear piston bell crank on lower left and yoke fulcrum top right
4th gear clutch
4th Gear clutch assembly (incompatible with the one in RM1872 apart from the yoke assembly)
Gear clusters
"Exploded" view of forward gear train - output is on left
Note oilways in shaft
Gear Assembly
One of the gear clusters; planet and annulus gears are quite clear.  Sun is on the shaft.
Brake bands
View through the forward brake bands to the final gear cluster
Brake band adjusters
The forward gear brake band adjusters - Note: these come in more than one pattern (see below)
Brake band adjusters
The reverse bake band adjuster - note either type can appear in either location

Over the Thomas the Tank engine weekend we "lost" second gear which we traced to the ElectroPneumatic (E.P.) valve block so we changed it for the spare one we have.  This fixed the fault.  For those who've never seen these important items here are a few photographs.
The solenoids unbolt and contain a shuttle which acts on the spindles to select the appropriate gear.  The system is fail safe in so far as a wiring break results in no gear being selected. The E.P. valve block is two wire and should be electrically isolated from the main body.

Electro Pneumatic (E.P.) Valve Block
EP Valve Block
E.P. Valve block with spindles removed
EP Valve spindles
The E.P. Valve spindles (there are five in total).
EP Valve cover
The E.P. Valve block cover
EP Valve contacts
E.P. valve block with cover removed (contacts/relays/ports: common 1st, 2nd, 3rd, 4th & Reverse)
EP Valve commoning block
Rear of E.P. valve block shewing electrical commoning block
Gear selector connector
Connector on main 'bus loom

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© South Devon Railway Trust 2007-11

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