Ball differential
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A ball differential is a type of differential typically used on radio-controlled cars. It differs from a geared differential by using several small ball bearings rotating between two plates, instead of bevel gears.
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[edit] History
The first ball differential was patented in the early 20th century by an American engineer, for use in a lawn mower. In 1982 Cecil Schumacher, a British motorsport engineer, designed a ball differential for radio-controlled model cars. This was believed to be a brand new invention at the time, and the original American design was not found until Cecil Schumacher tried to patent the design. However radio-controlled cars were still a brand new application for the ball differential and Cecil Schumacher is regarded by many as the modern day inventor of the concept. Such was the popularity of the ball differential, originally applied in 1/12th on-road cars that Cecil Schumacher went to form a company to sell, Schumacher Racing Products.
To this date ball differentials remain very popular in the radio-controlled car market. They are used on almost every 1/12th on-road, scale touring car and electric off-road produced by many manufacturers. In these classes they are regarded as the industry standard. Schumacher Racing Products even use ball differentials on their nitro truck range, these however use strong materials and larger and harder ball bearings.
[edit] The Basic Principles
All radio-controlled car manufactures use the same basic design that Schumacher designed in the 1980’s. The main part of the diff is the gear (or pulley in a belt transmission). This component has multiple holes cut through it, following its outside diameter. These holes are slightly larger then the width of the ball bearings. These small balls, commonly around 2 mm diameter in a model car, sit inside the holes of the gear/pulley.
Outside of this there are the differential out-drives, and sometimes large washers. The out-drives connect to the axle in some way, sometimes via a direct mechanical linkage or a slotted cup. A bolt then travels through one outdrive, through the centre of the gear/pulley and screws into the outdrive on the other side. There is also a thrust bearing in place to stop the differential from loosening the screw in operation.
The outdrives have flat plate sides, so sit well on the washer (where used). As the bolt is tightened it pushes the plates on the outdrive onto the gear/pulley. As there are ball bearings in the gear/pulley, of diameter greater than the gears/pulley thickness, the outdrive plates come into contact with these instead of the gear/pulley.
As there is a now a force exerted on the outdrives pushing them onto the ball bearings, there is a friction. Very commonly aided with grease (commonly silicone grease) the friction means as one outdrive moves, the ball bearings rotate. Any ball rotating will have opposite sides moving in opposite directions. For example for a disk spinning clockwise, the right hand side moves down whilst the left moves up.
So the ball inside the differential is now rotating due to an outdrive rotating. As the other outdrive is on the opposite side of the gear/pulley, the edge of the balls movement causes the opposite outdrive to rotate in the opposite direction.
Thus differential movement is achieved. As the bolt is exerting the pushing force on the outdrives onto the gear/pulley, the differential can be easily adjusted by tightening of loosening the bolt, consequently changing force. This makes the differentials more adjustable, and more easily adjustable then geared differentials.
[edit] References
[edit] External links
- Schumacher Racing Products, inventors of the ball differential
- The Ball Differential, detailed information, (German language) CGI-graphics and animation that show the ball differential's inner workings
