Singer Owner July - August 2005

Gears, Gear Ratios and Torque

John Dowding

There seems to be some confusion and uncertainty about the terms "high ratio and low ratio" when applied, in particular, to drive axle (final drive) ratios and to a lesser extent to gear boxes. Gear ratios are used throughout a motor vehicle to increase torque (turning effort).

Low Ratio/ Gear Reduction/ Underdrive - is used to increase torque output. Input speed is high and output speed is low.

In this example torque output has been increased by a factor of 4 (ignoring friction). The driver gear is revolving 4 times faster than the driven gear. This is a typical gearbox ratio for first gear (high engine speed, low road speed).

High Ratio/ Gear Increase/ Overdrive - is used to reduce torque output - so input speed is low and output speed is high.

In this example torque output has been decreased to of the input (ignoring friction). The driver gear is revolving _ of the speed of the driven gear. This is too high for motor vehicle application.

Direct Drive - Is where no gear ratio is involved in the transmission of torque or if a gear set is used, both gears have the same number of teeth.

In this example there is no increase of decrease in torque (ignoring friction). 1 to 1 (direct drive) is top gear (4th in a 4 speed gearbox, 3rd in a 3 speed gear box).

Gearbox Types

1. Sliding Mesh (Fig 4) - Gears are selected by sliding the gear on the main shaft into mesh with the appropriate gear on the layshaft cluster.

When selecting 4th (top) gear, 3rd gear is slid forwards so that its dog teeth engage with the primary shaft dog teeth. Thus the primary shaft and the main shaft are locked together, giving direct drive (1 to 1) Note that the primary shaft gear is in constant mesh with the largest gear on the layshaft cluster. Since the primary shaft gear has fewer teeth that its layshaft gear there is a gear reduction (with torque multiplication) to be taken into account when 3rd, 2nd, 1st or reverse are selected.

Sliding Mesh Gearbox Notes

i. Often referred to as a "crash" gearbox. This is because it requires skill on the part of the driver to change gear silently, by synchronising the speeds of the layshaft cluster and main shaft gears by "double de-clutching".

ii. Tooth loadings on the spur teeth are very high because in any gear set only two teeth are transmitting the torque (Fig 5).

iii. The rolling action of the spur teeth produces gear whine.

iv. Superceded in the middle of the 1930s by synchromesh gearboxes.

2. Constant Mesh (Fig 6) - In this arrangement all gears are in constant (permanent ) mesh. Since they are all rotating at different speeds they have to be free to rotate on the main shaft.

This is in contrast to the sliding mesh gearbox where they are splined to the main shaft. The dog clutches (3rd and 4th) and (1st and 2nd) are splined to the main shaft and are a "sliding fit" (Fig 6; 2 and 5). In order to select a gear the appropriate dog clutch is slid along the main shaft so that its dog teeth engage with the dog teeth on the gear. Thus the gear is locked to the main shaft via the dog teeth clutch. Torque flow through the gearbox is the same for both sliding mesh and constant mesh typed as are the gearbox layouts.

Constant Mesh Gearbox Notes

i. This type of gearbox still requires the driver to double declutch in order to achieve a silent gear change.

ii. Because the teeth are helical and in constant mesh (Fig 5) tooth loading is lower. (Two teeth in mesh with parts of teeth in mesh either side).

iii. Gear noise reduced significantly because of sliding action of the teeth. (Rolling still takes place).

3. Synchromesh Gearbox (Fig 7) - Is a constant mesh gearbox with the addition of synchronizing devices (relying on friction) to ensure the correct alignment of the dog teeth on selection, thus ensuring a silent gear change. Torque flow through the gearbox and gear selection are exactly the same as for the constant mesh gearbox.

There are a number of different types of synchromesh mechanisms. The types relevant to Singers are the constant load type and baulk ring type.

Constant Load Synchromesh (Fig 8) - This type, which is obsolete, was the first generation and is found, as far as I am aware in all products of Singer Motors Ltd. Movement of the gear lever and selector fork causes synchro hub (4 and 5) to slide as one unit towards one of its mainshaft gears (1). Friction, produced by the cone faces (7) and on 5, equalizes the rotational speeds of the gear and synchrohub (mainshaft) so aligning the dog teeth (2) and those on the inside of sleeve 4. Further movement of the gear lever and selector fork causes sleeve (4) to overcome the spring pressure of the ball bearings and sleeve (4), then meshes (silently in theory) with dog teeth (2) on the gear.

The gear is now locked to the mainshaft by sleeve (4) of the synchro hub and it is now able to transmit the driving torque. The action of the cone faces to apply a high friction level to produce effective synchronising is determined by the ability of the springs and balls pressure before being overcome by sleeve (4). Because the springs pressure is constant, gear changing must be slow to give the cone faces time to synchronise. Quick changes cause the dog teeth to crash - commonly referred to as weak synchromesh.

Baulk Ring Synchromesh (Fig 9) - Is universally used today and was introduced by the Rootes Group on its products in the early 1950s. Therefore all Rootes Group Singer cars feature baulk ring synchromesh gearboxes. Figures 9 and 10 depict a baulk ring synchrohub. The main differences are that this type features a separate baulking ring (usually made from phosphor bronze) and three shift plates and two circlips instead of three balls and springs in the constant load type.

Initial movement of the gear lever and selector fork moves Synchro hub sleeve (6) shift plates (5) and circlips (7) causing cone faces (9) to make contact. Further movement causes the shift plates to apply a high axial force at (9) thus synchronising the rotational speeds between 6, 2 and 1.

Finally sleeve (6) overcomes the shift plates detents (5) which compress circlips (7) and sleeve (6) engages silently with baulking ring dog teeth (2) and gear dog teeth (1). The advantage of baulk ring synchromesh is the faster the driver moves the gear lever the higher the axial pressure applied to cone faces (9), the quicker 6,1 and 1 are synchronised.

Since both types of synchromesh rely on friction to be effective, so anti friction additives must not be added to the gearbox lubricant. Similarly the viscosity of the oil in the gearbox determines how quickly the knife edges of the cones (11) are able to wipe away the lubricant. Many mechanics incorrectly add gear oil to Rootes Group synchromesh gearboxes instead of the recommended engine oil.



Continued in Singer Owner November/December 2005