Cost-efficient hollow transmission shafts

　　Conventional solid transmission shafts have,until now, been a major contributing factor in high vehicle weight and thus in increased fuel consumption. In vehicle weight and thus in increased fuel consumption. In view of this, lightweight construction measures are being developed using hollow shafts.

　　Another reason for the increasing demand for hollow shafts is a technological one. In automatic and double-clutch transmissions, two or more shafts are arranged concentrically, necessitating a hollow design of the outer shafts.

　　There is also a trend towards an increase in motorisation and thus towards high loads in the powertrain. This means that transmission shafts have the task of transferring greater torques while demonstrating the lowest possible part weight. Figure 1 shows that hollow shafts are highly suitable for torsional load and that they enable weight-savings to be made at the same time.

　　Currently, transmission shafts are usually hollow drilled at the end of the production process chain. This is a particularly time-consuming and costly operation. Now, efforts need to be made to minimise these additional costs associated with lighweight design. The high steel prices and alloy surcharges also put pressure on manufacturers to use the minimum amount of raw material possible. As hollow drilling of the finished part generates maximum waste, alternative production processes need to be established.

　　In order to identify suitable process chains, it is necessary to classify the shafts according to design, as the most suitable production process may differ depending on the geometry.

　　Types of shafts

　　The first geometrical group is formed by the “tubular shafts”, which demonstrate uniform wall thicknesses of between five and 10mm with no local increases. This type of shaft is mainly used in automatic transmissions.

　　Another group of shafts is classified as the DCT shafts (hollow double-clutch transmission shafts). These shafts require one or two collars for the running gears that need to be milled at a later point. The inner diameters are large (25 to 40mm), so that a concentrically aligned slender (solid ) shaft may be inserted within them.

　　The collar shafts are the classic manual transmission shafts, equipped with one or more collars into which the running gears are later milled. The hollow volumes of the collar shafts are mostly low and used for lightweight design. This type of shaft is also found in light-and heavy-duty trucks.

　　The next group is the pinion shafts. At 3 to 5kg, these shafts add considerable weight to the axle drive. The main cause of this high weight is the solid pinion head, which sometimes has a big diameter of up to 120mm.

　　Finally, the group of side shafts needs to be considered. These have a profiled flange at one end and need to transfer a high torque across a long distance (300 to 600mm).

　　Production costs

　　Due to the low wall thicknesses of tubular shafts, producing them from seamlessly rolled tubular material is currently the least expensive method of manufacture. However, seamless tubes within a diameter range of 50 to 70mm cannot be delivered with wall thicknesses above 15mm. This means that tubular material cannot be used for all other hollow shaft categories, where thick-walled collars need to be forged.

　　The shaft geometry is produced by means of rotary swaging or cold forging process. Swaging enables somewhat more flexible and specialised geometries to be produced. However, at cycle times of between 30 and 60 seconds, the swaging process is considerably slower and thus more expensive than cold forging on automated multi-stage presses (cycle times of between 5 and 8 seconds).

　　The collar-shaft geometry represents the standard shaft shape in manual transmissions in both passenger and commercial vehicles. The high degree of tapering at the shaft end generally only allows for hollow spaces with small convenient production process starts with the billlet and a simple drilling operation. In the following cold forging process, the desired outer and inner contour is then generated.

　　For the group of hollow double-clutch transmission shafts, it is particularly advisable to generate the cavity by hollow forging instead of machining, as turning of the large inner diameters, which may be up to approx. 40mm , is quite expensive. As many of these shafts are designed using highly priced steel alloys, it is important all the more to spare raw material.

　　Weight reduction is also an important topic in the case of pinion shafts. By using hollow geometries, around 0.5 to 1.5kg of material can be saved without any impairment to the strength properties. With most parts, huge weight savings can be achieved by producing a hollow pinion head. Strength calculations show that the stresses in the tooth root are relieved within only a few millimetres. This allows the full torque to be transferred even by a hollow pinion head.

　　Even if side shafts are not transmission shafts, they still deserve a brief mention. Due to the great shaft lengths of between 300 and 600mm, a hollow geometry allows the part weight to be reduced significantly. The current production process begins with the forging of the profiled flange at 1,200℃. This preform undergoes deep-hole drilling and is then elongated to double the length by means of swaging. During this process, shoulders and wall-thickness variations are incorporated.

　　Depending on the shaft geometry, the optimum hollow production sequence requires a different combination of several forging and machining steps. The Hirschvogel Automotive Group has many years’ experience in these processes and has established a cost-efficient production of hollow geometries, which was possible through a close working relationship whith its automotive customers.

　　参考译文：

　　到目前为止，常规实心传动轴在车辆总重中占有很大的比重，因此也增加了燃油消耗。鉴于此，使用空心轴的轻重量结构解决方案正在开发中。

　　空心由需求增长的另外一个原因来自技术因素。在自动变速箱和双离合器变速箱中，两个或以上轴同心布置，使得外轴必须使用空心设计。

　　机械化方面的需求增长也是目前的一种发展趋势，因此要求的动力系中的载荷也越来越高。这就意味着传动轴在传递更大扭矩的同时，还应尽可能降低部件重量。表明空心轴非常适合于扭转载荷，同时它们也降低了重量。

　　目前，传动轴通常是在生产工艺链的最后阶段钻成中空。这不仅非常费时，而且成本很高。如今，人们需要采取措施，结合轻重量设计，降低这些额外成本。钢材的高价格和合金附加费用也增加了制造商的压力，迫使他们尽量采用最少的原材料。由于空心钻取制成零件产生很大的浪费，因此需要寻求替代生产工艺。

　　为确定适用的工艺链，必须根据设计将轴进行分类，因为最适宜的生产工艺可能因几何结构的不同而大相径庭。

　　轴的类型

　　第一类轴由具有几何形状的“管状轴”形成，它们的臂厚均匀，为5至10mm，而且局部没有增加。这种类型的轴主要用于自动变速箱。

　　另一种轴被分类为DCT轴（空心双离合器传动轴）。这种轴需要一个或两个轴环用于稍后铣出的传动装置。内径较大（25至40mm）,这样在它们之间可能要插入一个同心对正细长（实心）轴。

　　轴环轴是传统的手动传动轴，配有一个或更多的轴环，用于装入稍后铣出的传动装置。轴环轴的空心体积大部分都很小，用于轻重量设计。这类轴也可在轻载和重载卡车上找到。

　　下一类轴是小齿轮轴。这些轴重3至5公斤，对车轴传动增加了相当大的重量。重量大的主要原因是齿轴齿顶，有时它的最大直径达120mm。

　　最后一类需要考虑的轴是边轴。它们在一端有异形法兰，并且需要长距离（300至600mm）传递高扭矩。

　　生产成本

　　由于管状轴的壁厚很小，目前最廉价的生产方式是用无缝轧制管材生产。但是，直径在50至70mm范围的无缝管的壁厚无法大于15mm。这意味着管材并不适用于所有其它类空心轴，因为其它类别的厚壁轴环需要进行锻造。

　　轴的几何外形经由回转模锻或冷锻制成。花键的成形可以在锻造工艺的早期整合。模锻可以用于锻制相对更为灵活的专用几何形状。但是，在30至60秒的循环时间内，模锻工艺相当慢，因此其成本也高于自动化多级压力机上的冷锻（循环时间为5至8秒）。

　　轴环轴几何形状代表了客车和商用车手动变速箱中的标准轴形状。轴端的高锥度通常只允许较小直径的空腔。便利的生产工艺从毛坯钢坯和简单的钻孔操作开始，在之后的冷锻工艺中形成所需的内外轮廓。

　　对于空心双离合器传动轴类，特别推荐使用中空锻造而不用机加工生成空腔，因为大内径车削（可能高达约40mm）成本非常高。由于许多这些轴都采用高价合金钢，因此节约原材料就越发重要。

　　减轻重量在小齿轮轴上也是一件非常重要的事项，通过使用空心几何形状，能够节省约0.5至1.5公斤的材料，并且不会影响强度属性。对于大多数部件，采用空心轴齿顶也能降低重量。强度计算表明，齿根的应力在仅仅几个毫米内就缓和了。这样，即使使用中空的齿轴齿顶，也能传递全部扭矩。

　　即使边轴不是传动轴，它们仍然值得一提。由于轴比较长（300至600mm），中空几何开关使部件重量显著降低。目前的生产过程从1，200℃异形法兰锻造开始。这种预成形经过了深孔钻，然后通过模锻延展至原长度的两倍。在此过程中，轴肩和壁厚变动合并完成。

　　根据轴的几何形状的不同，最优的中空生产工序需要以不同方式组合几个锻造和机加工步骤。Hirschvogel自动化集团在这些工艺方面有多年的经验，并且已建立了一种经济高效的模式生产中空几何形状轴，并通过与其自动化客户进行紧密合作而得以实现。