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How does it fit at the end(s)?

pushrodsIn the pushrod section of RET-Monitor the reader has been given an insight into the different aspects of pushrod design. Much information has been shared on the specifics of the pushrod concerning its shape, material and contact area of cup and/or bowl.

The specifics of the connection between the either hollow or solid centre part of the pushrod has been briefly touched on. In this article the different concepts of connecting the pushrod ends to the centre part are explored further.

In general, there are three concepts - welding, pressing-in or forming from one piece - that have appeared in more or less chronological order in the pushrod's development history. We will take a closer look at the main differences between them.

The welded solution is the most widespread concept, and has been around for many years, originating from the roadcar market of the early 1900s. Here, the ball or cup ends are welded to the rod, which is almost always a tube and either straight of tapered.

The weld is typically a friction weld, mainly because of the low-cost nature of the process and man independent process quality, making the process stable and reliable - at least for OEM purposes.

Unfortunately, the rod ends, which consist of hardened steel, are not ideal for welding. Over time, lots of solutions have been developed to improve the quality of the weld, and significant improvements have been made. In principle, however, the welding process remains less than ideal.

The heat generated by the welding process degrades the material properties of the rod ends and may lead to cracks near the welded zone, which can result in the ball or cup breaking off. This goes even more so for performance engines, running at higher engine speeds and peak firing pressures, increasing the load on the valvetrain system.

In order to minimise these risks, a thorough knowledge of the welding design and process, as well as stringent quality checks on the welded areas, are of major importance.

Pushrods that use 'pressed-in' rod ends are considered a feasible alternative for the welded concept, because there is no heat-influenced zone and the rod ends can be designed in different shapes and specs to accommodate different customer requirements. These can vary from the type of pushrod ends (ball or cup), and of course adjustable length ends can be included as well.

One disadvantage of pressed-in rod ends though is the stress region introduced by the press fit between rod end and tube. At the depth where the pressed-in rod ends stops, a stress concentration will occur, but by detailed engineering it should be able to reduce these stresses to below the critical level. The pressed-in design is a very cost-effective and flexible solution, which can provide a wide range of combinations of tube diameter, shape (straight or tapered), materials and combination of rod ends, without the process uncertainties of the welding process.

Both this and the welding concept have the advantage that drillings through the rod ends can provide quite a simple means to transport oil through the pushrod to the critical contact areas between ball and cup, which is a lot more difficult to achieve with the third concept, the one-piece formed pushrod.

These solid pushrods might become standard practice in extremely loaded drag race engines, but the added value for other race classes, specifically longer-duration race series, is not (yet) apparent. The advantage of these solid pushrods with machined ends is the absence of the external stresses introduced by pressing or welding, as explained above. This enables better predictability and understanding of the mechanical behaviour of the pushrod.

Apart from the increase in mass over tube-style pushrods, which, on the pushrod side of the valve train, is not that significant for the performance of the engine, the machining of the ends is somewhat more extensive in comparison to machining separate rod ends. The requirement to secure oil transportation to the critical contact areas in the valve train might be overcome by using a small diameter, long drilling through the length of the pushrod. But this drilling remains a critical process operation, leading to a cost disadvantage.

Summarising the different pushrod concepts, it will be a matter of customer requirements as to which concept is most suitable for the application. Currently, the pressed concept seems to have the better predictability over the welded design, where the solid pushrod design seems to be feasible only for extremely high loaded applications. And although the pushrod market will not significantly grow - in fact a slight decrease is more likely - race engineers will continue to balance their requirements against what can feasibly be produced.

Based on the multi-race requirement for the engines, one could expect that the reliability of the valvetrain system, together with more stringent cost directives, will play a more significant role in decision-making in the future. That might just lead to a shift in the choice of pushrod concepts.

Fig. 1 - Pressed-in hardened rod end in aluminium pushrod

Written by Dieter van der Put

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