How does a chain drive differential work?
A chain or belt drive differential consists of a sealed housing containing a differential.
The differential can be a conventional ‘Open’ , ‘Limited Slip’ or ‘Automatic Torque Biassing’ action differential.
1. Open Differential
The open action differential consists of a pair of planet gears running on a cross pin linking a pair of side gears which attach to the output shafts to the drive shafts (axles). The internal gears are normally lubricated with gear oil or grease.
The input to the differential is via a sprocket or belt pulley attached to the differential housing.
The motor drives the differential via a chain or toothed belt.
The differential housing runs in a pair of sealed roller bearings fitted to bearing carriers which attach to your vehicles chassis.
The differential allows the output shafts to rotate at different speeds relative to one another so that the drive wheels of your vehicle can turn at different speeds under cornering conditions.
An ‘open’ differential is good for use on the road but may not give the desired amount of traction on performance vehicles or off-road vehicles.
This is where a ‘limited slip’ differential can be used.
A limited slip differential can work on one of several different principles.
2. Viscous coupled LSD.
This can use planet and side gears as per the open differential but with a viscous coupling fitted between one output shaft and the differential’s housing.
When there is a differential speed between the two output shafts (as in a wheel-spin situation) the plates in the viscous coupling act on a special silicone fluid which causes the plates to lock up. This effectively locks the two output shafts and stops one wheel spinning relative to the other. Once traction is restored the differential reverts to its normal state.
Viscous coupled differentials are good all round units which should give drive in all conditions.
In cases of prolonged and extreme wheel-spin, the viscous fluid in the viscous coupling can overheat and degrade.
It should be noted that the fluid in the viscous coupling is separate from the gear oil which lubricates the internal gears.
3. Plate & Ramp LSD.
A plate and ramp LSD employs sets of clutch plates on each output shaft and these are activated by the cross pin riding up on ramps on the side gears which clamp up the clutch plates when there is differential rotational speeds of the wheels.
Different numbers of clutch plates and pre-load pressure settings and the angles of the ramps dictate how much torque is applied to the spinning wheels.
The ramps can be selected to operate in the forward drive direction only or in forward and overrun conditions. A 45/90 degree ramp would give the LSD effect only in the forward direction, a 45/45 degree ramp would give the LSD effect in the forward direction and on the overrun (useful for tail-out driving styles in off-road conditions).
Plate and ramp LSDs can give the ultimate traction on the track or off-road.
Most plate & ramp LSDs require special gear oils or gear oil modifiers.
4. Automatic Torque Biassing differentials (ATB).
An ATB uses combinations of axial and radial gears to bias the amount of torque going to each wheel when there is limited traction available to either wheel relative to the other.
The ATB differentials are smooth in action and are ideal for road and track use.
They do not produce the pronounced torque steer effects that a plate & ramp LSD can induce on a front wheel drive vehicle or the understeer effects on a rear wheel drive vehicle.
Their only drawback is that if one wheel looses all traction then no torque is transmitted to the opposite wheel. This situation is only likely to occur if one wheel leaves the track surface or finds very slippery off road conditions.
Most ATB differentials can run on engine, gear oil or on grease. Special additives are not required.
5. Other Differentials
Spur gear differentials – These can be ‘open’ type differentials as used in the original Austin Seven rear axle but are also used in the Ford Fiesta RS/Escort RS Turbo, viscous coupled LSD.
Cam & Pawl LSDs – found in some competition cars using “Jack Knight” units
Tend to be ‘sticky’ in their ability to switch on/off. Not very common.
Locker differentials – mostly used in off-road 4wd vehicles.
These can be automatic locking differentials or can be manually, electrically or pneumatically actuated.
Auto lockers tend to be clunky/noisy in operation.
Final Drive Ratio
The final drive ratio of a chain or belt drive differential is based solely on the ratio of the number of teeth on the differential sprocket to the number of teeth on the motor’s output sprocket.
There is no effect from the internal gearing within the chain drive differential unit on the final drive ratio.