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Differentials Explained - Standard Open Differential

An article explaining how an open axle differential works.
A standard differential, also referred to as an “Open” differential, is the OEM standard for many vehicles.  The differential case holds the ring gear, which is spun by the pinion gear, and together these equal a vehicle's final drive axle ratio.  The main purpose of the differential carrier, is to provide power transfer from the drivetrain to the wheels.  

A standard differential consists of several components:

Differential Case:  This portion is the main body of the unit.  The ring gear is bolted to one side, and the spider gears, or differential gears, are housed internally.

Differential Gears: Commonly known as Spider Gears, these transfer power to the axle shafts.

Differential Pin: This pin keeps the spiders gears aligned, and allows an axis point for 2 of the 4 spider gears, to allow a power split between left and right wheels.  

Other components within the axle housing, commonly called the “Pumpkin Housing” consists of the following:

Pinion Gear: This gear is the starting point for the internal transfer of driveline power within the axle housing.  

Ring Gear: Bolted directly to the differential case, this is the 2nd point of internal power transfer.

Misc hardware: Bearings, bearing caps, and shims will also be inside the pumpkin.  The bearing caps  are what holds the differential case in place, while the bearings give everything the ability to move freely.  Shims are necessary to properly set backlash for the ring and pinion.  

When the vehicle is driven in a straight line, an equal force is placed on both side gears.  Due to this, the spider gears will not rotate.  Once a tire starts to get under or over rotated, or enough force is applied to slow it down, the spider gears will start to transfer power to the tire with the least amount of resistance.  The spider gears design allows for different rotational speeds between tires.  This allows for a much easier time when turning the vehicle in front wheel drive, rear wheel drive, and four wheel drive.

Whenever a vehicle negotiates a turn, the outside wheel must travel farther (turn faster) than the inside wheel. This is accomplished by a series of gears that allow the outside wheel to turn faster than the ring gear, while the inside wheel turns slower than the ring gear. This differentiating action is very simple in a standard open differential and works great for most of the vehicles on the road today.  However, they do suffer greatly when encountering a low traction or slippery surface.

When a vehicle with an open differential encounters a low traction surface, it directs power to the wheel with the least amount of traction.  This results in the wheel on the low traction side spinning while the opposite wheel with high traction receives little to no power.  This is due to the low traction wheel not providing proper resistance to the side gear to spin the carrier and transfer power to the opposing wheel.  The carrier pinion gears and the carrier simply rotate around the side gear with the most traction and nothing gets passed on.
Article Type: 
How It Works