Varibox CVT Technologies, a South African Intellectual Property (IP) company, has recently received search report feedback from a PCT (Patent Cooperation Treaty) application for its RADIALcvt design in which all 12 claims have been granted without modification. MechChem Africa’s Peter Middleton talks to Jan Naude of Varibox, the company’s managing director and principle inventor.
Varibox CVT was set up in 2007 to develop ‘out of the box’ continuous variable transmission (CVT) solutions: identifying the shortcomings in main stream CVT system and addressing these shortcomings at a fundamental level by inventing patentable design alternatives.
“The first CVTs were invented in the 1960s in The Netherlands. These were based on using two variable diameter pulleys connected by a thick rubber belt. Each pulley consists of two interconnected conical halves that slide towards and away from each other. When the cones are apart, the belt runs closer to the shaft axis and vice versa. By synchronising the driver and the driven pulley so that the driver pulley gets larger or smaller while the driven pulley gets smaller or larger, the speed ratio can be continuously varied,” begins Naude.
When connected to an engine management system, CVTs offer an alterative to fluid-based automatic transmissions or automated manual transmissions (AMTs), but CVTs are stepless and do not require individual gears to be engaged and disengaged.
Fast forwarding to 2016, Naude says Bosch now owns the intellectual property for pulley-based CVTs that now use metal bands instead of the rubber belts. These run using a traction fluid that separates the metal band from the metal pulleys. An alternative is available from LUK, which uses a metal chain instead of the belt. “All current CVT systems available in modern motor vehicles use one of these two pulley-based systems,” he tells MechChem.
Identifying the shortcomings of these systems, he says, at any time, the two halves of each pulley are kept at the required distance apart by an automatic hydraulic clamping system. “The position and the clamping force has to be very accurately controlled, so hydraulic pumps and control systems are required to continuously vary the effective drive- and the driven-pulley diameters.
Since the pulley radii both vary, the hydraulic clamping forces also have to change depending on the steel belt’s distances from the rotating shaft axes. This adds a level of control complexity to the hydraulic system, raising its costs.
“These CVTs also have two friction drive systems operating in series. The power from the engine comes into the first pulley set and has to be transferred to the band or chain. This is then transferred to the driven pulley at the second friction interface,” Naude explains.
The use of auxiliary hydraulic clamping and control systems and the friction interfaces both lead to losses. “Losses are usually evaluated at the maximum power point, which is a bit misleading,” says Naude. “A 100 kW CVT might be 95% efficient when transferring 100 kW, but if only transferring 20 kW, its efficiency is much less. On average, across the normal load profile for a pulley-based CVT, an 85% transfer efficiency is typical. Running a hydraulic pump off the drive absorbs a further 5% of the output power. So the accumulated losses can amount to 20% or more,” he says.
Further describing the transmission mechanism for pulley-based CVTs, Naude says that the traction stresses at the friction interfaces are another limiting factor of this technology. “At the microscopic level, the traction fluid solidifies at the steel-on-steel contact point, keeping the band or chain from directly contacting the pulley. But because all of the traction power has to pass through these two friction points in series, the contact pressures are very high. The highest currently possible is about 4.5 GPa, but this requires high-strength steel and operates at high temperatures. Reducing this contact stress is another key driver that underpins our alternative designs,” he informs MechChem Africa.
Jan Naude has been developing alterative CVT configurations since 2007. “We focus on transmissions for vehicles and for variable speed industrial applications. We generally start with low power options for small passenger vehicles and then we strive to scale them up. To date we have developed three different CVT products: the icvt (incremental); the ROTORcvt; and, most recently, the RADIALcvt,” he says …