在汽车行业中，塑料一直都是一种很理想的减重材料，长期以来都在大量广泛地应用于汽车的各个部分，比如车身面板、内饰材料和发动机舱部件等，但从未想过要把变速箱外壳和齿轮组也改为塑料。

不过，这种情况可能要发生改变了。最近，有两家欧洲公司正在进行联合研究工作，旨在显著扩大塑料复合材料在汽车传动系统中的应用，而且他们还计划借助电动车研究进行技术改良。

这对合作伙伴分别为英国变速器设计工程咨询公司Drive System Design(DSD)，以及广泛活跃在汽车、航空、能源和环境等领域的布鲁塞尔国际化学集团Solvay。

为了优化未来纯电动车的NVH特性（噪声、振动和不平顺性），这两家公司开始联手研发塑料材质的变速器外壳，并同时探索通过这种材料提高齿轮啮合效率的可能性。考虑到噪声问题，使用金属材料的可能性基本可以被直接排除在外。

DSD公司总经理Mark Findlay解释说：“利用塑料替代传统的金属铸件不但可以立刻获得减重效益，而且还有潜力实现效率的提升。聚合材料的固有阻尼特性允许厂商设计更多的有效档位，比如采用更小的螺旋升角或正齿轮等。由于产生的噪声过大，这种设计根本无法应用在传统材料的变速箱外壳中。而采用更小的轮齿可以减少滑动、增加滚动，从而提高齿轮组的效率。”

Findlay认为，机轴、套管和液压阀体均有希望被纳入塑料复合材料（或增强塑料复合材料）的应用范围。对常规乘用车变速器而言，如果完全使用塑料材料则最高可获得45%的减重效益。即使在为了保证车辆NVH特性而为车辆增加一层“皮肤”之后，使用塑料材料的减重比例仍可达到25%。此外，由于采用了塑料材料，每次齿轮啮合过程中的动能损耗最高可以减少0.5%。

事实上，这两家公司的合作，并不是汽车行业第一次试图拓展塑料复合材料在基础动力系统中的应用，但理想变为现实总是不那么容易。

早在20世纪60年代末，通用汽车就开始进行复合材料变速箱的研究，并成功打造了一个原型。F1方程式和航空航天行业也开展了类似的研发项目，探索复合材料带来的更多可能性。

20世纪80年代，复合材料应用的发展开始停滞不前，当时的复合技术无法满足管套和轮齿等变速器元件的高容量需求，但如今的技术可能会实现改变了。

Findlay对塑料材料可能应用范围的看法相当务实。他强调，为了协助提升车辆的NVH表现，这种技术可能最先在顶级电动车领域发挥作用。“由于电动车并不使用内燃机，因此传动系统的任何NVH问题均会更容易暴露在相对安静的车舱之中，这对能够发挥自身固有阻尼优势的塑料变速器外壳来说非常有利。”

此外，由于电动车行驶时的温度也比传统内燃机车更低，因此也能更好地适应低成本聚合材料120°C(248°F)左右的温度极限。Findlay的一个观点非常有趣，他认为由于现阶段电动车的产量远远低于内燃机车，刚好可以给相关制造技术一定的发展时间，逐步从原型制造阶段进化至量产阶段。

Findlay认为现在的挑战是：“汽车行业对很多新材料的机械属性并不熟悉，很容易掉入一些‘陷阱’之中，而且由于这些材料的非线性特性，随着温度变化，其属性发生改变的程度高达50%。举例而言，聚合物在到达玻璃化温度(glass transition temperature)后会逐渐变软，这会极大地影响该材料的机械属性，有时甚至吸点水也会影响聚合物的属性。通常来讲，在设计研发过程中寻求与材料供应商的合作，是个不错的做法，但如果设计研发的对象是聚合物这类属性多变的材料，那么与材料供应商的合作就是必不可少的了。”

DSD公司和Solvay集团正是为了实现这样的强强联手，开始了他们的复合变速器外壳联合研发项目。

考虑到结构性功能和量产制造的成本，这两家公司在塑料变速器研发中引入了低成本的复合技术。DSD公司认为，如果按照目前$10/kg的平均行业减重补贴计算，塑料复合材料变速器外壳的成本完全可以与现有的铝制产品竞争。

Solvay集团表示，通过高效的FE（有限元）分析技术，结合使用现有的机械属性数据，以及模流特性和纤维定向等参数，针对复合材料耐久性的预测水平可以得到大幅提升。一般而言，纯塑料元件的回收相当简单明了，而且关于复合材料回收的相关研究也在不断进行之中，所有材料厂商都要面对回收塑料材料的问题。这些都可以成为塑料传动系统的研发的推动力。

Findlay表示：“我们比较倾向于设计一种复合结构的变速器外壳，具体是在结构性框架周围注塑聚合材料，从而形成一层连续的屏障，防止由于机油侵入造成内嵌与外层聚合材料之间的结合强度减弱。

DSD公司和Solvay集团正在与多家汽车厂商探讨适用于各种未来变速器和传动系统的潜在替代材料。目前，复合材料变速器技术仍处于研发阶段，仍需继续寻找最合适的材料和工艺，必须经过进一步的优化后才能进入接近量产的阶段。

DSP公司和Solvay集团预测，复合变速器外壳可能会在未来5到10年内进在市场上推出。

Solvay集团全球汽车市场经理Mark Wright强调，必须通过提供一系列解决方案，不断争取新的潜在客户，这点非常重要：“每个客户都有不同方面的需求，比如减重、NVH优化或能效提升等。我们必须针对每家客户的特定要求，为他们提供最合适的解决方案。”

他解释说，Solvay集团积极参与了“多项”具有很高知名度的项目，z展示公司在材料科学方面的潜力：“目前，Solar Impulse正在进行环球飞行挑战，我们为这驾实验性零碳太阳能飞行器提供了15种我们自己的产品；我们还为Polimotor 2全塑料赛车发动机项目提供了多种热塑材料。”

Polimotor 2项目大量采用复合材料，还将利用Solvay公司的高级聚合技术研发至多10种发动机零部件，包括水泵、油泵、进水口/出水口、节气门、油轨等多种高性能部件。目前，Solvay集团可以提供的材料包括Amodel聚邻苯二甲酰胺(PPA)、AvaSpire聚芳基甲酮(PAEK)、Radel聚亚苯基砜(PPSU)、Ryton聚苯基硫醚(PPS)、Torlon聚酰胺酰亚胺(PAI)，以及Tecnoflon VPL氟橡胶等。

作者：Stuart Birch
来源：SAE《汽车工程》杂志
翻译：SAE上海办公室

DSD, Solvay 'sink their teeth' into plastic transmission advances
 

For the automotive industry, plastics have long been a weight-saving material of choice, with a wide range of high-volume applications from body panels to interiors and underhood components—but transmission housings and gears are not among them.

Now that may change. Two European companies are collaborating in a study to achieve solutions that could herald a much wider role for plastic composites across transmission applications, and they are using electric vehicle (EV) research to help refine the technology.

The companies are U.K.-based Drive System Design (DSD), an engineering consultancy specializing in transmission design, development, and control, and Brussels-headquartered Solvay, an international chemicals group operating in sectors that include automotive, aerospace, energy, and the environment.

Based on their joint initiative to create a plastic transmission housing to improve NVH characteristics of a future pure EV, both companies are also exploring the possibility of using the material to improve the efficiency of meshing gears via tooth. In terms of noise, that would rule out using metals.

DSD Managing Director Mark Findlay explained: “There is an immediate weight saving from substituting plastic materials for conventional metal castings, but equally important is the potential for improved efficiency. The inherent damping provided by polymeric materials permits the use of much more efficient gears, such as reduced helix angles or spur gears, that would have unacceptable noise characteristics in a conventional casing. By using shorter teeth, typical tooth profiles for higher efficiency would have reduced sliding and increased rolling.”

He believes there is potential for shafts, casings, and hydraulic valve bodies to be made from plastic (suitably reinforced where appropriate), and states that full implementation could produce savings of up to 45% in casing weight for a typical passenger car transmission. With an NVH “skin” added, the saving would still reach 25%. A reduction in transmission losses would be “up to 0.5% per gear mesh.”

There is nothing new in wanting to extrapolate plastic’s roles into fundamental powertrain technology, but wanting and achieving are not the same things.

In the late 1960s, General Motors considered composite gearboxes and created prototypes. Formula One and aerospace industries have also embraced R&D programs that looked at possibilities.

In the 1980s, when such advances were seriously mooted, contemporary composites’ technology could not deliver radical powertrain application solutions such as casings and gear teeth for high-volume requirements; now it may be able to.

Findlay is pragmatic about these possible developments and stresses that it is in the premium EV category that the technology is likely to find its first application to help counter NVH: “The low cabin noise levels in a vehicle without an IC engine expose any NVH issues arising from the driveline, making the inherent damping of a plastic housing advantageous.”

Temperatures encountered in an EV are lower than an IC engine powertrain, so are more compatible with lower cost polymer temperature limits of around 120°C (248°F). An interesting point made by Findlay is that current production EV production volumes are hugely lower than those of conventional vehicles, making it easier for manufacturing technology eventually to migrate from prototype quantities to series production levels.

There are challenges, he said: “New and unfamiliar materials bring pitfalls for the unwary because of the subtleties of the mechanical properties, which can change by up to 50% over the operating temperature range due to non-linear behavior. Polymers soften above their glass transition temperature, which can significantly affect mechanical properties; even the moisture absorption of polymers can influence properties. It’s always good practice to work with a material supplier from the earliest stage of design but, when the material properties are as different as polymers and metals, it is absolutely essential.”

That is why DSD and Solvay are busy cooperating to meld their individual specialist capabilities.

For the plastic transmission study, low-cost composite technology is being incorporated from the outset to combine structural capability with volume-feasible manufacturing costs. Including the typical industry allowance for weight reduction at $10/kg saved, DSD believes composite transmission casings can be engineered to be competitive in price with existing aluminum products.

Solvay states that durability prediction has been greatly enhanced by effective finite element (FE) analysis, backed by proven data on mechanical properties and appreciation of the influence of parameters such as mold flow characteristics and fiber orientation (for composites). The recycling of plastic-only components is regarded as being straightforward, and research into composite recycling is ongoing; an issue that is common to all material manufacturers. All this is germane to the possible drivetrain developments.

Said Findlay: “Our preferred approach for a transmission casing is composite construction involving overmolding a polymer around a structural frame to provide a continuous barrier against any ingress of oil, which could otherwise infiltrate and weaken the bond between the inserts and the polymer.”

DSD and Solvay are currently discussing with vehicle manufacturers the areas within transmission and driveline systems that offer the best potential for material substitution in the future. Currently, the technology is in the development phase to optimize and prepare the most suitable materials and processes in a near-production-ready state.

DSD and Solvay anticipate a five- to 10-year timescale before the first applications come to market.

Solvay’s Global Automotive Marketing Manager, Mark Wright, underlines that it is important to approach potential customers with a range of alternative ideas: “Each customer has individual priorities, whether for weight reduction, NVH improvement, or increased efficiency. We have to reflect that by presenting the most appropriate options for their particular case.”

He explained that Solvay has taken part in “a number” of high-profile projects to demonstrate the potential of its materials: “We supply the Solar Impulse—an experimental zero-carbon, solar-powered aircraft attempting to fly around the world—with 15 different Solvay products, and also support the Polimotor 2 race engine program by providing several different thermoplastic materials.”

Polimotor 2 is composites intensive and will use Solvay’s advanced polymer technology to develop up to 10 engine parts, including a water pump, oil pump, water inlet/outlet, throttle body, fuel rail, and other high-performance components. Solvay materials targeted for use encompass Amodel polyphthalamide (PPA), AvaSpire polyaryletherketone (PAEK), Radel polyphenylsulfone (PPSU), Ryton polyphenylene sulfide (PPS), Torlon polyamide-imide (PAI), and Tecnoflon VPL fluoroelastomers.

Author: Stuart Birch
Source: SAE Automotive Engineering Magazine