Between eight and 20 conrods in each engine translate the linear, reciprocating movement of the pistons into rotational motion at the crankshaft. The moving connection at each end is seated in bearings and each conrod is subject to tension and pressure as well as torsional and bending stresses. “If the quality is right, the very special design of a conrod gives it enormous stability that enables it to withstand these extreme external forces,” explained Frank Schneider who heads conrod production at the Rolls-Royce mtu facility. “However, if a conrod breaks, the result can be total engine failure. A broken conrod can literally blow out the entire contents of the crankcase.” To make sure that does not happen, production of these components demands the very highest levels of precision. “The conrods for our Series 4000 engines are produced to a tolerance of 10μ,” said Schneider. The Greek symbol ‘μ’ (pronounced ‘myu’) indicates a unit of length equal to 0.001 millimeters. For comparison, a human hair is roughly 50μ thick. In terms of conrod production, missing precision specifications by a hairsbreadth is therefore way off target.
From raw blank to high-precision mtu conrod
The blanks are produced at a forge in line with Rolls-Royce's specifications and are then machined to their final form at the mtu facility. And the process produces a whole lot of machining chips. For example, the original blank for a Series 4000 conrod starts out at almost 15 kilos. After preliminary machining, that drops to a mere 9.75 kilos. Absolute ‘μ-precision’ is then confirmed at the second station in the process where an unusual device is used to determine the mass of the conrod in motion. Because the conrod performs a rotating movement at one end and a reciprocating movement at the other, perfect balance during motion is vital. At this stage, the conrod also has to pass a hardness and distortion test. The component then comes under fire in the shot-peening unit where it is blasted with a stream of 1mm-diameter balls to compact the surface and enhance residual compressive stress.
Products with memory and identity
Next, every conrod gets its very own individual identity in the form of a serial number and a code that are engraved twice on each rod. This is essential because the next step involves cutting the component in two so that it can be secured on the crankshaft. Duplicating the engraved ID-signatures avoids the danger of mixing up the halves by mistake. The code contains information identifying the forge, batch, day and time – in short, the product memory. After a co-worker has cut the conrod and ground the razor-sharp serrated mating faces, both parts are assembled in accordance with strictly specified and standardized procedures. Then, before the two parts of the conrod are securely bolted together, two scanners check the ID-codes to ensure that they are a matching pair. If they do not fit perfectly, the screwing device shuts down automatically.
Complex but strong: the bearing
A conrod has an ‘eye’ at each end (generally called the big end and the small end). The small-end bearing is permanently joined with the conrod. The two-part, big end is bolted on a rod journal (crankpin) on the crankshaft. The bearings have a vital function. An exact combination of mobility and stability is essential and the more precise this is, the more engine power the conrod can cope with. With this in mind, eight years ago, funds were invested in a technology designed to substantially enhance this characteristic: ultrasonic shot-peening. Frank Schneider explained: “Ultrasound is used to propel tiny balls around for six minutes. As they fly around, they impact the inside face of the conrod eye and create a surface structure that gives the bearing shell exceptional grip and enables it to withstand extreme stress.” This means engine power can be increased with no loss of engine durability.
Not a single conrod leaves the plant without undergoing final visual inspections, hardness checks and crack tests. Only after every test has been passed is a conrod deemed fit for service in an mtu engine.