Rethinking Constant Velocity Joints
Christopher A. Sawyer
GKN Driveline has two new technologies for the constant velocity joints (CVJs) that significantly reduce weight, NVH levels, size, and improve efficiency. “We’ve thrown out the rulebook for constant velocity joint design,” says Al Deane, global engineering manager, GKN Driveline Driveshafts, “because the generic limits of the Rzeppa joint currently in use have been reached while our customers continue to ask for more torque capacity in a smaller package.” The new designs lean heavily on GKN’s in-house forging and machining capabilities to produce both the S-shaped and radial offset tracks used on its Countertrack CVJ and the combination of angled and straight tracks found on its Crosstrack ball plunging CVJs.
Countertrack CVJ designs are used on the fixed joints of all propshafts, and on the wheel-end CVJ of front sideshafts of FWD, AWD and 4x4 vehicles. They follow a basic design principle: combining unique track profiles with opposed ball tracks that provide countervailing forces to reduce frictional losses and heat generation. The sideshaft design is based around opposing S-curve bearing ball tracks that allow steering angles of up to 50?, with an option for 52? for vehicles in need of a tighter turning circle. On a typical 3.0-liter V6 mid-size sedan, this two-degree difference can shave just over three feet, or approximately 10%, off the turning circle.
The SX sideshaft joint design was driven by OEM desires for maintaining common suspension and platform dimensions while offering engines with widely varying power outputs. “With the Countertrack fixed sideshaft design,” says Rob Rickell, global engineering director, sideshafts, GKN Driveline Driveshafts, “the CVJs on that same mid-size V6 sedan will be more than 30% more efficient, more than 8% smaller, and 15% lighter than a conventional design.” In addition, the joint operates approximately 30?C cooler than a conventional Rzeppa-style joint.
Countertrack propshafts CVJs utilize offset radial tracks that are circular in form, but very flat. Designed to solve endurance and temperature issues on propshafts with medium to high installed angles, the PX joint design also uses opposed ball tracks to keep frictional pressures and temperatures low. Dan Booker, global engineering director, propshafts at GKN, says the PX joint not only increases joint efficiency by more that 40%, reduces its size by more than 8%, and cuts weight by greater than 25%, “it improves vehicle NVH levels by reducing backlash in the system.”
“It may look simple, but it took a lot of modeling work to come up with the right combination of straight and angled ball tracks,” says Rickell. He’s referring to GKN’s Crosstrack ball plunging CVJ for both propshafts and sideshaft inner CVJs. Straight ball tracks have the least resistance to plunge efforts, but greatly increase backlash through the driveline. Angled tracks have much lower backlash levels, but greater plunge forces. Combining the two creates a joint that is almost as good at reducing maximum rotational lash as an angled CVJ with ball bearings, and has almost the same sliding force as a straight-track CVJ with roller bearings.
“The Crosstrack arose out of concerns where low plunge efforts had to be combined with low lash or play,” says Booker. And while idle backlash is not a problem with manual transmission vehicles, those with automatic or automated manual gearboxes have the power engaged at a stop. This causes vibrations to make their way up through the steering column and pedals, and creates a ‘clunk’ when the vehicle takes off from rest or shifts gears. Adding this design to sideshaft inner joints further reduces this phenomenon.
Al Deane admits GKN’s new CVJs can be up to 20% more costly to produce, but remarks that this disparity “is overcome on an engineering basis by the weight savings.” When combined with tubular shafts, the total driveshaft weight for the typical mid-size sedan drops 35%, or 8.8 lb., which increases fuel economy by 0.2 mpg. Says Deane: “Based on our calculations, that results in a $250 fuel savings at an average fuel price of $2.50 per gallon, which has increased significance these days.”
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