Think of it as a ‘suspension system’ for your accessory belt drive.

If there were no suspension systems on cars, except for the cushioning effect of the tires, you can imagine that under many conditions the ride would be rough and difficult to control. This is similar to the standard accessory drive belt and alternator pulley system. The alternator is a very heavy mass which, once rotating, cannot readily change its speed without absorbing considerable force to do so. Only the belt adds some cushioning effect, like the tires on a suspension-less car. But in this case, the ‘bumps’ seen by the belt are actually generated by the engine itself.

Whenever a spark plug fires, the engine crankshaft accelerates briefly, and then decelerates until the next plug fires, and so on. So while an engine might appear to be running steadily at a given engine speed, it’s actually continuously speeding up and slowing down. This effect is compounded as engine speed is changed via the throttle. You cannot see this effect since it occurs many times per second, on a continuous basis. As a consequence, the belt drive system speed also fluctuates continuously in concert with the crankshaft drive pulley.

OADs synchronize the belt drive system for improved performance and component life.

When the belt is accelerated by the engine, the alternator’s mass creates a substantial resistance against the belt. Due to the drive ratio between the crankshaft pulley and the alternator pulley, the alternator speed is typically about three times faster than the engine speed. The alternator’s rotating mass is also large. A momentary increase in engine power and belt tension is required to ‘absorb’ the acceleration. Similarly, on the deceleration everything reverses. Various problems ranging from annoying to severe can occur when the accelerations and tension reversals get out of sequence, or ‘out of phase’. This is typically referred to as ‘rope-tow effect’. In this case the belt may be accelerating while the alternator is still decelerating, and vice versa, continuously. Often you will see (and hear) the belt tensioner vibrating under this condition. Ultimately, the belt tries to absorb this impact-like behaviour while transmitting residual noise, vibration and harshness (‘NVH’) through the engine and into the car’s passenger compartment. The resultant effect is very similar to the analogy of the car with no suspension.

We know that vehicles with suspensions will absorb road bumps to smooth the ride. And if you load a car with people, the suspension will deflect a little more but still provide a smooth controlled ride. This is what the OAD does. Again, think of the alternator mass as the car body and the belt as the tires. But in this case, the ‘bumps’ are caused by the normal firing of the engine. So a ‘suspension system’ needs to be fitted between the belt (as per car’s tires) and the alternator (as per the car’s body).

Absorbing belt speed fluctuations, the OAD smooths the drive system.

The OAD actually connects, or ‘couples’ the alternator and the belt via unique internal spring and clutch mechanisms. Every time the engine fires and the belt is accelerated/decelerated, the OAD spring absorbs the speed fluctuation similar to the way a car’s suspension absorbs bumps through the compression and extension of its springs. When the car’s lights are turned on or the electrical load on the alternator is otherwise increased, the effect parallels adding weight to the car. The OAD spring deflects more, but it continues efficiently driving the alternator. Also like a car’s suspension system, the OAD contains a damping element to control resonance.

Overrunning or “free-wheeling” function – reduces slip & noise.

A significant additional feature of the OAD is its ‘overrunning’ function. This special function allows the alternator to effectively disconnect, or ‘decouple’, its operation from the belt and coast freely (like a bicycle wheel) if the belt speed slows suddenly. Sudden belt deceleration often causes slip and noise, because the heavy alternator rotor (mass) takes a longer period to decelerate than the belt. This condition usually occurs when shutting off the engine or when the transmission is shifted aggressively. Again, the OAD’s coast function eliminates damaging peaks in belt tension along with noises due to slippage.

An increasing number of new vehicles are now being equipped with a ‘Litens’ OAD as standard equipment in order to provide a greater level of NVH isolation, to the vehicle and its occupants, than would otherwise be possible.

Our OADs are now available as replacements, as well as retrofits, for various vehicle engine applications. In particular, where an OAD is retrofitted to an engine/vehicle not previously equipped with an OAD, noticeable and sustained NVH and belt drive system performance and durability improvements have been demonstrated.