July 10, 2011
The way to Constant Velocity Joints
Remembering those times when I was a child (and I am still...) playing with the old Gears sets, my favorite part was the red Universal Joint (U-joint).
Certainly you should have seen them before, either these or their respective modern versions (as pictured below). These are the so called U-joints (Hooke's Coupling or Cardan Joint) and allow a rotating shaft to transmit power through a variable angle.
These have been used at some Technic sets since 1972. The red version at left was used in the early 70s into Gears sets, predecessors to the actual LEGO Technic theme, and it was later replaced by the version at the middle (1977). The rightmost version was introduced much later (2008) and it is a shorter 3L redesign thus allowing to achieve more compact mechanisms.
Parts like these were commonly used in the past to drive the Technic Supercars rear wheels, from the respective half-shafts coming out from a differential, and across double wishbone suspensions.
It happens that U-joints, like those in real life machines, have a common drawback. Except for specific configurations like in Double Cardan Shafts, even when the input axle rotates at a constant velocity, the output shaft rotates at non-uniform speed. The driver and the follower parts complete each revolution at the same time, but the velocity ratio is not constant throughout the revolution.
This effect causes inconvenient vibrations in shafts at fast speeds, and inaccuracy when used in precision machinery.
Because of several other effects like axle torsion and backslash, the phenomenon described is not easily noticed in Technic models.
Despite that, LEGO later produced another type of parts which address this odd behavior and that have been used in some of its large cars since 2001.
These are named 'Steering CV Joints' and pair with the 'Steering Gear with 4 Ball Joints'. Both are pictured in the image below.
The 'Steering CV Joint' on the left resembles the real and way more sophisticated CV-joints, like Rzeppa joints (from its inventor's name) or Tripod joints, used in today's real cars.
These are extremely flexible and solve the problem by allowing a rotating shaft to transmit power through a variable angle, at constant rotational speed, and without any appreciable increase in friction. They can also accommodate large angles when the front wheels are turned by the steering system.