Power Transmission

5-2001

Technical Report

The chain of events in a redesign

Designing from first principles has paid dividends to one company as it claims to have "created the first new chain design in over 100 years". Mark Fletcher investigates

Designing by 'looking outside the box' is something that we all claim to be able to do. However, saying it and putting it in to practice are two totally different animals. When this approach is put into practice, and works as well as intended, it is amazing what can be achieved.

The first question you must ask yourself in many design situations is: "Why do we make this like we do?" If you can find the answer to this question then you will have given yourself a good base camp from which to launch a major redesign. Not so long ago Renold asked this very question to itself with regard to fork lift truck (FLT) chains.

It turns out that FLT chains – although not designed to run on sprockets – are still based on standards developed for transmission chains. The questions Renold asked were why, and do they need to be? The answers being: "Because that is how they have always been made and no, respectively." These responses immediately gave it the ability to 'think outside the box' and evaluate the concepts and designs factors which make a good FLT chain from a new direction.

FLT chains are very simple beasts. In their simplest guise they consist of two outer plates connected by an interference-fit pin to two inner plates which are able to rotate about the pin. They are designed to transfer load, but not by means of sprockets, instead they are more analogous to belts in that they run over pulleys or sheaves. Based on unitary pitches they can be made to carry more load by adding more plates and longer pins – a practice which has existed for years but is still flawed in certain situations.

More plates demand a wider chain which leads to the end product requiring more space for the chain to run. The main problem however, is not the size of the chain, but the size of the pin which experiences more deflection the longer it gets. Another problem is variety. With pitch, plate count and load carrying ability being specification factors the designer has a huge choice of offerings, from several manufacturers. Some pitch/plate combinations overlap with others, in terms of load carrying, making choice that little bit more difficult. It was this factor that started the ball rolling for Renold.

It started by assessing what sizes of chain had the lion's share of the market in terms of volume and based its initial performance criteria on chain performance. Once this was established it then looked at the actual design, analysed what each part of the chain was doing (good and bad) and whether it could be redesigned to create a better product.

Traditional design dictates that the outer and inner plates should be of the same thickness. Renold immediately challenged this as both sets perform different functions. As the outer plates have an interference fit with the pin they are already in compressive stress which counteracts a modicum of any tensile stress the chain experiences. By making them thinner they would not take as much load, however Renold argued that no chains are used near their breaking point so this would not be a problem, however fatigue would be a major design point. It decided that the chain should be optimised for fatigue and working performance and not breaking load.

The problem with wider chains (and hence longer pins) is that, as the pin bows, the central inner plates will take less load than the others as they are at the point of maximum pin deflection. The 'outer' inner pins will exhibit a higher load/unit area which leads to greater wear. Another problem is that the higher load/unit areas causes the inner plates to 'grip' the pin and turn it, compromising the outer plate transition fit and removing some of the compressive pre-loading. This is especially true as the late count rises.

It was at this stage, once many of the design factors had been identified, that Renold turned to FEA techniques to create the optimum design.

Using the ability of Ansys software to cater for non-linear deformation, the company created a programme which instructed the software to analyse chain designs which exhibited two inner plates, two outer plates, specific fatigue figures and three differing load capabilities (15, 22 and 34kN). It had to consider pitch, plate size, pin size, pin and hole diameter and variety of interference fits. The software was then set to run in a loop and after two weeks of processing time the results materialised.

stanza fea.jpg (43420 bytes)

After certain design tweaks, the chain deigns were built and tested and the fatigue performance was found to be exactly what the software had predicted. The new chains exhibited thinner outer plates (good for fatigue life) and thicker inner plates (larger bearing surface). With a simple four-plate design the pin length was kept to a minimum – removing all of the problems associated with bending pins.

The new 3 -chain range, christened Stanza by Renold and launched at this year's Hannover show, will replace 26 (about 3/4) of its current range. They will not be offered with specific pitch sizes, instead they will come simply as Stanza 1, 2 and 3. Prices are not expected to be much different from those they replace.

With regard to their applications, fork lift designers will have the luxury of thinner chains which will lead to the creation of less cluttered masts which, in turn, will increase driver visibility. For applications that demand a much higher load carrying ability to chains can be used instead of one big one. Although not a glamorous product, the final chain is used in a massive array of applications all of which will benefit from the redesign. It proves that no matter how simple or established a product is there is still room for improvement.

'Looking outside the box' is something that has been covered in Eureka in numerous articles with January's leader and cover feature being classic examples. The leader explained how Seymour and Powell managed to redesign the bra – a design which has not changed in years, it also pointed to Dyson and the design of his vacuum cleaner – once again another essentially fresh approach. The cover article highlighted the benefits of clean design and how a fresh approach can actually make products more competitively priced.

The process itself is as simple or as difficult as you want to make it. The basic staring block for any redesign is usually a blank sheet of paper. What ever direction you decide to take from there you may not break new ground but you will certainly learn something.

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