Polymers find use in engine transmissions

Boston and Toronto based BPG Werks has created an innovative mode of personal transport, dubbed the 'Shredder'. Resembling the love child of Robot Wars' Sir Killalot and a Segway PT. The all-terrain stand-on dual tracked vehicle (DTV) definitely has the cool factor. The DTV allows users to go from zero to a top speed of 25mph, even on steep uneven terrain or through water, in just a few seconds.

It is currently being used for extreme sports, although other uses do include agricultural and forestry work with the possibility of it being used as a personal transport system for troops.

One of the enabling technologies for the DVT is its patented 'track-biasing' Continuously Variable Transmission (CVT) that enables its acceleration, torque response and manoeuvrability. A key part of this is the driveshaft gear segment that was initially made of a coated brass. However, problems around durability soon became apparent as the power from the 13hp, 4-stroke engine, to the heavy duty rubber tracks quickly wore the brass gear to near failure.

As the driveshaft gear segment was pivotal to the overall performance of the transmission, and the DTV overall, BPG needed to replace the coated brass with a material that would provide improved durability and wear characteristics. In addition it needed to deliver solid and reliable performance at engine speeds over 5000rpm, and torque levels up to 20Nm to each track, for the life of the vehicle.

After some initial research, BPG teamed up with polymer material suppliers Victrex and US-based China Array Plastics. BPG Werks chose to specify a high-performance polymer to replace the brass as it was able to provide all the mechanical performance necessary, and was likely to be able to cope with the demanding applications the vehicle would be put up against. Engineering plastic has the additional advantage of being easily processed in a cost effective manner.

After some initial development, the team was able to construct a driveshaft gear segment using a polyaryletherketone over moulded around steel that was robust enough to replace the coated brass.

The team used a Victrix WG (wear-grade) polymer for two components within the driveshaft gear assembly, the sliders and also insert keys. Despite exposure to centrifugal and linear forces at high temperatures, the thermoplastic offers a much more durable, and lighter, solution.

"We looked at a couple of other materials, including the MoS2 coated bronze," says Russell Johnson, president of China Array Plastics. "But, it did not demonstrate the same durability as the (WG) wear resistant material from Victrex.

"The Victrex WG polymer overmould, although in constant contact with the belt drive, helps to provide stable acceleration and torque characteristics, with exceptional friction and heat resistance. This is due to the polymer's durability, impact and tensile strength, and low friction coefficient."

The eight slider mechanisms produced from the polymer overmoulded sliders and insert keys are a critical element of the transmission system, sliding back and forth thousands of times within the transmission while under load. The thermoplastic offers high wear resistance while maintaining dimensional integrity, resulting in solid synchronisation. This led to improvements in the longevity of the transmission as well as the overall riding experience.

Challenging the preconceptions
Another example of polymer being applied to a transmission system comes from Bradley University in the US, which has produced a Lightweight Urban Vehicle. A key part of the development was its use of polymer to form the gears.

Its three-wheeler trike, the Lightweight Urban Vehicle, uses transmission gears made of polyoxymethylene acetal, specified to reduce weight while able to withstand the torque produced by its 25hp motor.

Several acetal gears were used within the vehicle's planetary transmission system, all produced using DuPont's Delrin acetal resin with the gears facing conditions that many engineers have previously considered off limits for standard acetal material.

However Mike Cassata, a design engineer at consulting gear firm Winzeler Gear, says: "The high performance of a homo-polymer acetal can manage the torque and load, while delivering low friction and reduced noise.

"And we can proceed from design to machined models in weeks, with just a small tooling investment."

Dupont is now considering further exploration into gears for smaller transmissions, such as those found in ride-on lawn mowers and golf carts. And the successful use of Delrin for gears in the Lightweight Urban Vehicle has helped challenge the preconception that polymers are not able to withstand the harsh conditions associated with transmission applications. And while it is perhaps more unusual for larger gears and transmission parts to be made from polymers, there are increasing examples of ever larger gearing and transmission parts being made from high performance plastics to exploit advantages in durability, production costs as well as reduce weight. Development in this area continues at speed, with larger viable transmission gears likely to appear before long.

Justin Cunningham

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