The V belt or fan belt has virtually been replaced by the serpentine belt. Fast on the trail of the serpentine belt was the automatic belt tensioner.
Wikipedia defines a mechanical belt as “ a loop of flexible material used to link two or more rotating shafts mechanically.” Belts are looped over pulleys. They are the least expensive method to drive shafts that are not perfectly aligned.
The automobile has employed mechanical belt drives from the very beginning. Changing a fan belt was one of the tasks frequently performed by your mechanic. Early fan belts were V-belts and typically one belt was used to drive one, two or three major accessories.
These accessories being alternators, water pumps, fans, air pumps, power steering pumps and air conditioning compressors.
Mechanical energy from the motor was used to drive these necessary or optional accessories. The water pump and radiator fan (keeping the engine cool), and alternator (keeping the battery charged) were mandatory. The power steering pump and air conditioning compressor were options not every vehicle had and many times each of these had their own belt drive.
The cross section of a V-belt was “you guessed it” vee shaped but missing the pointed end of the vee. This belt sits in a pulley with a vee shaped groove. The sides of the belt in contact with the groove create enough friction when the belt is properly tensioned to drive the required accessory.
As the belt material (rubber and cord like a tire) wears it falls deeper in the vee.
Once it reaches the bottom of the vee it cannot carry enough friction to keep the parts rotating and the belt starts to slip.
This is the time the customer starts to complain.
“What is that squealing noise?”
Time for a new belt.
The serpentine belt used almost exclusively now is a much thinner belt made up of a series of small vees laid parallel to each other. These belts ride in pulleys with the same amount of thin vee shaped grooves as the belt. There are serpentine belts with seven and even eight vees but six is the most common.
The design of these belts is such that one belt can drive all the accessories on a vehicle. The path of a serpentine belt drive on a vehicle can be so circuitous that a diagram is required to remember how to put the belt on.
Many vehicles in fact have a diagram of the belts path to assist installation. Similar to the standard V-belt, the serpentine belt material wears in the vee and then reaches the point of slipping. That squealing noise again is the belt slipping.
Up until the mid eighties most belts were tensioned by a mechanical adjustment mechanism manipulated by a master technician. Too loose and the belt would slip and prematurely wear itself out. Too tight and the belt could ruin the bearings in any of the accessories. Yes a tight belt can destroy your alternator or a water pump.
The mid eighties brought us the automatic belt tensioner. Accurate tension is now built into the system provided that automatic tensioner is doing its job. Unfortunately automatic tensioners fail opposite to what you might think.
Instead of letting the belt get too loose a worn automatic tensioner over tensions the belt. Many times the first indication of a worn tensioner is a failed alternator.
So many times I see a new alternator and a new serpentine belt and the old tensioner. That new alternator is going to live a very short life.
Trail’s Ron Nutini is a licensed automotive technician and graduate of mechanical engineering from UBC. E-mail: firstname.lastname@example.org