GEAR CHANGING 2 – INTRODUCTION – EXTERNAL CLUTCHES
In Gear changing 1 we revealed how gear members within the same gear train can be connected with each other. The question of how gear members of a gear train can be coupled with gear members outside of that gear train will now be answered. Often it is about sliding couplings and the specific activating and deactivating of pawls. External clutches can be divided roughly into three basic groups:
- A driver – if existing – as the input member of a gear transfers the power flow from the sprocket to different gear members. There are of course some hubs without a driver – as already mentioned before.
- Some hubs with several gear trains allow to couple gear members from a gear train with gear members of another gear train. This is applied in some historic Sturmey-Archer hubs with a differential planetary gear train for example as well as in the Rohloff Speedhub.
- Finally ring gear and planet carrier as output members transfer the power flow to the hub shell as output members via several clutches thus driving the bike.
COUPLING THE DRIVER WITH PLANET CARRIER / RING GEAR
Many geared hubs contain a driver holding the sprocket and transferring the power flow to different gear members – mostly to the ring gear or the planet carrier. Many drivers comprise a sliding clutch, others use a cam disk for controlling the pawls elegantly. In the first historic 3-speed hubs the entire planetary gear train was moved. In the following sections different versions of clutches within the driver will be described.
Drivers without sliding coupling
The first historic hubs with driver did not contain any sliding clutches. Sturmey-Archer did not connect the driver with the gear train by a clutch but they applied the reverse principle: in 1902 (Three-speed gear) and 1905 (Model X) they launched two 3-speed hubs which allowed the entire gear train to be moved back and forth by a toggle chain. Both hubs were similar in structure comprising a long sun gear to ensure that the planets of the planet carrier always kept meshing with the sun when being moved back and forth. As usual, both hubs provided a transmission output at the planet carrier to the hub shell and at the ring gear to the shell – via pawls (Model X) or toothings. Moving the gear train allowed to couple the planet carrier with the shell (hill gear, shifting to the brake side) or to couple the ring gear with the shell (fast gear, shifting to the sprocket side). The gear train allowed to be pushed into a toothed driver while being shifted causing either the planet carrier or the ring gear to be coupled with the driver from the inside. Additionally, the output from the ring gear to the shell could have been deactivated – for achieving the hill gear – by disengaging two toothings in the 1902 model and by pushing pawls inwards in the 1905 model.
Unfortunately, these hubs will only be found in a museum, but in 1957 Shimano put the model 333 (Trimatic) on the market according to a patent of Mr. Keizo Shimano himself. The Trimatic has a lot in common with these early Sturmey-Archer hubs, since it contains a gear train as well, which allows to be moved as a whole – and a driver without sliding coupling. Planet carrier and ring gear both have internal toothings situated directly next to each other. The ring gear’s toothing is discontinuous and smooth at the edge to the plant carrier’s toothing. The driver contains two pairs of pawls (for planet carrier and ring gear) are arranged in such a way that a pair of pawls either meshes with a certain internal toothing when the gear train is moved (activated clutch) or it touches the discontinuous area of the ring gear and gets pressed inwards (deactivated clutch). The gears are changed as follows:
Hill gear: The gear train is shifted completely to the brake side and the driver’s pawls for the planet carrier touch the discontinuous area of the ring gear (deactivated). Due to the left position of the gear train the planet carrier’s external pawls are meshing with the toothing of the brake side (activated) and the external pawls of the ring gear are pulled out of the toothing of the driver side (deactivated). Thus the power flow is transferred from the driver to the ring gear and from the planet carrier to the shell.
Direct gear: The gear train is shifted in the intermediate position and the driver’s pawls for the planet carrier and the ring gear now touch its corresponding internal toothings (both activated). Due to the intermediate position of the gear train the planet carrier’s external pawls are still meshing with the toothing of the brake side (activated), but the ring gear’s external pawls are still pulled out of the toothing of the driver side (deactivated). Thus the power flow is transferred from the driver to the planet carrier and from the carrier to the shell.
Fast gear: : The gear train is shifted completely to the sprocket side, the driver’s pawls for the planet carrier are meshing with its corresponding internal toothing (activated), but the driver’s pawls for the ring gear now touch the discontinuous area (deactivated). Due to the right position of the gear train now the planet carrier’s external pawls are pulled out of the toothing of the brake side (deactivated), but the ring gear’s external pawls are now meshing with the toothing of the driver side (activated). Thus the power flow is transferred from the driver to the planet carrier and from the ring gear to the shell.
Drivers with sliding coupling (radiating clutch)
Since 1918 the ‘K’ hub from Sturmey-Archer contained a driver with a sliding coupling (radiating clutch) which could be moved by a sliding block. The claw-like driver was slotted in a characteristic way allowing the radiating clutch also to be rotated with the sprocket while being moved. The lugs of the radiating clutch mainly had three functions:
- They allowed to mesh with a cam-like toothing of the planet carrier when the toggle chain was not pulled. Thus the faster rotating ring gear was the driving gear member and the fast gear was achieved.
- When pulling the toggle chain one time they meshed with a wedge-like toothing of the ring gear. This led to a direct gear with the ring gear as driving member.
- Pulling the toggle chain a second time finally led to a deactivation of the pawls of the ring gear while the clutched remained coupled. This caused the planet carrier to be the driving member and a hill gear was achieved.
Sturmey-Archer applied this principle of a radiating clutch within a claw-like driver until 1987 with marginal modifications only. In 1970 they launched the S3C containing a driver without slots and with built-in pawls (in backward direction). From 1987 all Sturmey-Archer drivers were unslotted and with built-in pawls. Prior to this, drivers and the corresponding radiating clutches were manufactured in several versions: The driver of the K hub e.g. had six slots and frontal cams at its planet carrier for the clutch to mesh with. The notably successful model AW (since 1936), however, used a clutch with four lugs only meshing with the protruding journals of the planet pinions instead of using cams of the planet carrier.
With the ‘Universal Dreigang’ – becoming extremely rare meanwhile – during the years before the Second World War Fichtel & Sachs introduced its first hub containing a driver on the market. Its claw-like driver reminds a lot of the Sturmey-Archer design. The model 55 from Sachs still contained a slotted driver and a coupling block which could be moved similar to a radiating clutch. The ring gear, however, did not have an internal wedge-like toothing but an inner contour allowing the block to engage with. Pulling the toggle chain a step further led to a deactivation of the ring gear’s pawls – similar to Sturmey-Archer.
The driver was splined internally similar to the Sram P5 which was already described regarding the hub elements. Its sliding coupling could mesh with a toothing of the planet carrier and a toothing of the ring gear. The ring gear was permanently connected with a pawl carrier which transferred the power flow to the hub shell. The pawl carrier could be moved for achieving a hill gear. Its pawls then were pulled out of the internal toothing of the hub shell causing the slower rotating planet carrier to be the driving gear member. Since then all Fichtel & Sachs hubs containing a driver were built according to that principle: the sliding clutch moved within an internally splined driver and it allowed the pawls of the ring gear to be pulled out of the hub shell’s internal toothing. Sturmey-Archer, however, pulled pawls out of a toothing only in very few ancient hubs, later on they always deactivated the pawls by pushing them inwards. More about these principles will be told in other section.
Drivers with sliding clutch and pawls
In 1970 the S3C coaster brake hub came onto the market with three innovations: firstly, its driver and clutch were splined similar to the model 515 from Fichtel & Sachs. Secondly, the driver provided two rear-facing pawls which could mesh with the ring gear’s internal toothing similar to the radiating clutch. When pedaling forward the clutch was allowed to turn slightly within the driver causing to the pawls to retract inwards, when pedaling backward they were extended. Thirdly, the cams at the planet carrier for contacting the radiating clutch now contained additional inclined areas. These chamfers caused the radiating clutch to be connected normally with the planet carrier when choosing the fast gear in traveling direction, but pedaling backward led the radiating clutch to release from the cams of the planet carrier. When back pedaling further the rear-facing pawls of the driver extended, engaging with the internal toothing of the ring gear and turning the ring gear backwards – not the planet carrier. This ensured to get the brake force of a hill gear in all the other speeds: the slower rotating planet carrier provides more torque for the brake screw due to the gear ratio.
In 1987 the AWC hub came onto the market with three innovations: firstly, the chamfers for decoupling the clutch from the planet carrier when back pedaling now were applied within the clutch itself. Secondly, the driver contained forward-facing pawls in addition to the rear-facing pawls. Thirdly, the clutch did not have the radiating lugs anymore for engaging with the internal toothing of the ring gear. When pedaling forward the forward-facing pawls of the driver extended and the rear-facing pawls retracted. Thus the ring gear was driven from the inside in all three speeds. When pedaling backward the rear-facing pawls of the driver extended and the forward-facing pawls retracted thus causing the increasing of the brake force which was already mentioned. The driver’s clutch only has the task to mesh with the planet carrier and to disengage the external pawls of the ring gear when touching them with its outer diameter (for the hill gear). Sturmey-Archer’s subsequent drivers were all designed in that way. The driver of the Shimano Nexus Inter 3 is similar.
Drivers with shifting ring, cam ring and pawls
With the Shimano Nexus Inter 7 in 1992 several innovation were brought onto the market: finally a hub with rotary gear shifter! Two planetary gear trains connected in series and the shift drum were two further highlights. Its function was already explained in another section. The Shimano also contained a complex and interesting driver. The ring gear is driven by a pair of external pawls and two additional pairs of internal pawls meshes with the toothing of the common planet carrier. The internal rear-facing pawls ensure that the torque from back pedaling the sprocket will be transferred 1:1 to the coaster brake via the common planet carrier – the ‘coaster brake booster’. The forward-facing pawls can be deactivated by a disengageable shifting ring pressing them outward leaving the toothing. The shifting ring engages (activating the clutch) when both its journals slide into a recess of the cam ring at a defined rotation angle. This clutch allows the driver to be coupled with the ring gear or directly with the planet carrier.
When pedaling backward the ring gear also revolves backward (faster) since the rear-facing pawls of the driver touch the planet carrier’s toothing. Shimano came up with an interesting mechanism to prevent the ring gears toothing from colliding with the pair of external pawls: the rear-facing pawls are not held in place allowing them to be turned a bit after touching the toothing of the planet carrier. While being turned they contact a ring which will retract the pair of external pawls during its rotation. The Sram I motion 9 hub is designed similarly concerning the driver, but it contains a multi-stage cam sleeve. This sleeve allows controlling a shifting ring for activating or deactivating pawls – and it additionally controls other internal and external clutches.
COUPLING THE SUN GEAR WITH THE PLANET CARRIER
There were some interesting historic hubs from Sturmey-Archer with differential coupled planetary gear trains, as already described. They allowed coupling the differential sun gear 1 with planet carrier 2 – for achieving closer gear ratios or with the axle. Sturmey-Archer provided a second radiating clutch which was movable within the slots of sun gear 1 also containing an internal toothing. Both clutches were shifted by sliding blocks and block 2 was arranged in such a way that it was not yet moved at the beginning of the gear shifting (as block 1), but only at the end. For gears 2, 3 and 4 the second clutch remained locked in the toothing of planet carrier 2 (by a spring) thus causing sun gear 1 to revolve permanently and achieving a closer gear ratio. The hill gear finally required to pull the toggle chain completely causing sliding block 2 to pull the second clutch out of the toothing of planet carrier 2 and to engage with the axle toothing. Now sun gear 1 was fixed with the axle (larger speed reducing gear ratio) and planet carrier 2 revolved unloaded. This coupling principle was applied in the AF, FM and FC hub (4-speed) as well as in the ASC (3 speed fixed gear).
COUPLING THE RING GEAR WITH THE HUB SHELL
The following sections reveal how the ring gear can be connected with the hub shell. Mostly the ring gear is the shiftable gear member whereas the planet carrier mostly is connected non-shiftable.
Shifting ring gear pawls by pulling them out of engagement
When shifting into a hill gear, the power flow is transferred from the driver to the ring gear and from the planet carrier to the hub shell following the typical ‘3-speed principle’. Several principles for preventing the ring gear from meshing with the hub shell directly via its pawls were already mentioned. Fichtel & Sachs above all mostly resolved that by moving the ring gear with the driver’s clutch thus pulling its pawls out of the toothing of the hub shell. This principle is pretty simple and it was applied successfully e.g. in the Torpedo H3111 as well as in 5-speed and 7-speed hubs. However, it needs a small, un-toothed area within the hub shell and the ring gear’s toothing needs to be a little bit longer for still meshing with the planet wheels. The typical Fichtel & Sachs pawls are relatively small and it needs only a spring ring to keep them under tension.
Shifting ring gear pawls by pushing them out of engagement
Sturmey-Archer did not deactivate the pawls of ring gears by moving them – besides from some ‘ancient’ hubs – but by turning them to the inside. These pawls were mounted turnable with pins and kept under tension by torsion springs. One end of a pawl (chamfered) extends into the ring gear, the other end meshes with the toothing of the hub shell (screw-in ball ring sprocket side). When moving the clutch across the inner end the pawl is turned out of the toothing of the ball ring. This interesting concept was applied in some 3-speed hubs of Shimano later on, you will also find pawls mounted turnable in pins.
In the above described hubs the pawls of the ring gear were always shifted by a driver’s clutch. From 1982 to 1984 there was a 2-speed hub from Shimano, the ‘Change hub’, without a driver, having the sprocket directly placed on the ring gear. The ring gear was toothed and meshed with pawls installed inside the hub shell. The pawls of the planet carrier meshed with a toothing of the hub shell. The change hub contained a clutch plate for pressing the hub shell’s pawls out of engagement (inactive clutch), thus leading the slower rotating planet carrier to be the output member for the hill gear. With activated clutch (pawls released) the power flow was transferred from the ring gear directly to the hub shell (direct gear).
Shifting ring gear pawls by a control ring
The legendary Duomatic and Automatic hubs from Fichtel & Sachs contained a turnable control ring allowing the ring gear’s pawl to shift when pedaling back or to shift at a certain speed. The old version of the Duomatic included a pawl carrier with rear-facing and forward-facing pawls and a ring gear with a special inner contour. The rear-facing pawls were completely embedded within the ring gear’s inner contour and they were permanently driven. The forward-facing pawls were only partly embedded in the inner contour with the remaining part meshing with the toothing of the hub shell. When turning the ring gear in comparison to the pawl carrier by pedaling back the forward-facing pawls were pressed inward (inactive clutch) or kept released (active clutch) depending on the inner contour.
The new version of the Duomatic didn’t contain a pawl carrier any more since the ring gear had built-in pawls meshing with a control ring and with the toothing of the hub shell. In this version the ring gear’s pawls were turned within the control ring which forced the pawls to be pressed inward or to be released depending on its inner contour. In the Automatic hub a control ring was turned by flyweights. The control ring allowed to release the flyweights at a specific speed causing them to engage with the toothing of the hub shell then (fast gear). Since 2011 Sram offers the Automatix 2 speed hub which resembles the design of the Automatic pretty much. Sturmey-Archer has a (new) S2 Duomatic in its program, since from 1966 to 1997 a S2 ‘kick back’ hub was offered.
Shifting by indexing spring and indexing sleeve
From 1960 to 1964 the Bendix Automatic hub already existed allowing to switch between two speeds by back pedaling even before Fichtel & Sachs and Sturmey-Archer launched their Duomatic and Automatic. This pretty complex hub did not contain any pawls but drive cones and drive screws similar to the already described parts in another section. While pedaling back an indexing sleeve with a toothing placed on the screw of the ring gear revolved one switching position further. When pedaling forward again the toothing of the indexing spring revolved one switching position further. The drive cone was pulled toward the ring gear by the drive screw. In the first switching position the drive cone was pulled against three cams of the indexing spring – which prevented the cone to get connected with the hub shell (deactivated clutch, planet carrier as output). In the second switching position the three cams extended into the three cut-outs of the cone – which allowed the cone to be pulled against the ring gear completely and to transfer its torque to the hub shell activated clutch, ring gear as output with direct gear).
Coupling the ring gear by a sliding clutch
The Sachs Orbit hubs were described in another section. It contains an externally toothed sliding clutch with internal and external pawls. Its external pawls are not shiftable, they only provide the connection to the hub shell. The ring gear gets coupled with the hub shell when moving the clutch into the corresponding internal toothing of the ring gear. In this way a direct gear is achieved.