Spring Bars: An Engineer's Perspective

Spring bars are an incredibly important component on any watch, so why are they rarely mentioned? Like most things, it's a case of 'out of sight, out of mind' but in reality they are solely responsible for keeping your watch where it's supposed to be, on your wrist! Recently, while finishing some work on my Squale 2002 1000m 'Black Round Dots' I noticed that one of the original spring bars had developed a crack in the outer tube. On further inspection, it became apparent that Squale had decided to use the cheapest, nastiest, poorest quality spring bars that they could find! The watch has had no more than 1000 hours of actual wrist time and a failure of this kind should not be encountered. Could it have been 'just one of those things' that happen on occasion? I'd really like to give Squale the benefit of the doubt but with all the other problems I've had with this watch I'm more inclined to say no, definitely not; it's a quality control issue. I'll pen an article on higher prices and lower quality from some of the 'big boys' when I get ten minutes!

So, without further ado, and armed with no more than a mug of tea and a Garibaldi, out came the engineering hat and the post mortem began! Some of you will know of my background as a Rolls-Royce/Bentley Tool & Prototype Maker, and for quite a few years 'engineering perfection' was the daily norm, in fact, whenever I dabble in anything mechanical it still is! Closer inspection revealed a number of shortcomings that should have been blindingly obvious to a company like Squale. The Squale 101 Atmos. is no shrinking violet by any means and it appears that the decision to fit spring bars of dubious quality had been taken by the office junior! So what's actually going on? Let's take a closer look at the humble spring bar in order to try and make some sense out of it all.

The 'T' Bar

Let's get this one out of the way first. ORIS, along with a few other companies, go for 'T' bars on many of their diving watches. The reason for this is that they are much, much stronger than spring bars; they consist of a solid shaft with either a retaining screw at both ends, or a fixed head at one end and a retaining screw at the other. This is the way an engineer would do it!

The Spring Bar

The humble spring bar consists of a tube that has two sliding end pieces separated by a spring. The end pieces or 'trunnions' to give them their correct title, are retained by crimping over the ends of the trunnion tube. This design, although common, is fraught with all sorts of inherent problems that eat away at what is trying to be achieved. We used to have a saying at Rolls-Royce, 'Perfection is the road, not the destination.' and that holds true here. The fact that you have sliding parts means that there is inherent slack and 'wobble'. So what can be done?

The Engineering

The goal here is to get as close as possible to the 'T' bar. It's worth remembering that a superbly engineered spring bar may get close to the 'T' bar, but it will never equal or exceed its strength, assuming similar materials. As with most things in life the quality of the same thing varies considerably, and spring bars are no exception. Even with the high-quality spring bars, their design prevents a truly satisfactory solution. A few years ago many manufacturers changed the design of their cases, moving away from drilled lugs for aesthetic purposes; this started the rot, so to speak, and weakened an already flawed design even further. The reason for this is that the main tube in which the trunnions slide has to be short enough to allow sufficient room for one of the trunnions to be retracted in order to remove the spring bar. This causes additional problems when using certain straps. Another shortfall is the force that's required to compress the spring, lower quality spring bars have noticeably weaker springs. In an ideal world this should not make a difference as the trunnion spigot is in shear and should apply no compressive force to the trunnion; however, this is not an ideal world and case machining can often fall short. Drilled lugs offer a much better solution that allows the trunnion tube to be virtually the same length as the lug width, needing only around 0.1mm – 0.2mm clearance to 'beef' the job up nicely. Let's move on to see how the strap we choose affects the situation.

Machined End Link Bracelets

Here's where it starts to get a little complicated! Rolex spring bars are good, but they are by no means the best in the business; however, their strength and integrity are derived from the bracelet. The end links are drilled so that the clearance between the trunnion tube and the wall of the hole is very small, this makes sure that the trunnion tube cannot flex and that the trunnion spigots are kept on axis; this in turn ensures that the trunnion spigots are kept in shear and not being 'bent'. All this goes a good way to reducing the possibility of the trunnion spigots developing premature stress cracks in the corners.

Does Size Really Matter?

As a male I'm immediately inclined to say no, but in this particular case I'm afraid that it does. Assuming that the wall thickness of the trunnion tube stays the same, increasing its diameter will make it much, much stronger. If you double the diameter of a tube, its torsional rigidity increases sixteen times assuming no change in wall thickness. While this particular formula doesn't relate directly to the load on a spring bar, it does go some way in getting the point across. So, the bottom line is 'big is best' but don't mention it to the wife as doing so will, almost certainly, instigate a somewhat lengthy (and equally tedious) 'I told you so!' lecture.

Zulu and Military Style Straps

Soft straps when put under load tend to focus pressure toward the centre of the strap, with the outer edges easing out of the way, it also means that this pressure is transferred to the centre of the spring bar. The centre third of the spring bar is the least supported and it tends to flex, this in turn tries to bend the trunnion spigots taking them outside of their design remit of being in shear. If that's not bad enough, the bending moment does its very best to open the crimps that secure the trunnions and this, more often than not, leads to the cracking of the trunnion tube, resulting in catastrophic failure. The upside is that these types of strap pass through both spring bars and that being the case it's unlikely to end in tears or imminent suicide.

Rubber Straps

Rubber straps offer a little more support than the Zulu and military style straps but still place the spring bar under additional stress due to the flex in the rubber. The main problem here is that if one of the spring bars were to fail, the watch falls to the floor. As rubber straps are more common on dive watches, the 'floor' could well be as much as 36,069 feet down; borrowing a snorkel at this point is unlikely to cut it! The Seiko rubber straps, as fitted to the 1000m 'Tuna', have a steel tube fitted inside the strap that gives the spring bar the same type of support offered by the solid end link. The spring bars on the 'Tuna' are also 2,5mm in diameter with a trunnion spigot diameter of 1,1mm. This system is exceptional and I've never heard of one failing, indeed, I have dived extensively in a 'Tuna' and never experienced any problems whatsoever. A little silicon grease on the spring bar in order to ensure its smooth rotation within the tube is all that's required on assembly. Other manufactures 'trap' a hard insert within the strap during the manufacturing process to support the spring bar; this 'insert', like Seiko's tube, is invisible on the finished strap.

The Solution to our Problem

I could have put this at the beginning but if I did that, no one would bother reading the article. Joking apart, this type of failure needs to be addressed. Our watch is not just a timepiece, it's a trusted companion and it needs to stay on our wrist. For those with bracelets that have solid end links nothing really needs to be done. For the rest of us, the solution is often quite simple, and that's to fit better quality, larger spring bars. Care is required as the 'beefier' spring bars often have larger trunnion spigot diameters and may not fit the holes in your lugs. The trunnion spigots can be reduced to fit using a battery drill, a needle file, some abrasive paper (P600 grade) and a Vernier caliper or micrometer. Care is needed when undertaking this as too much pressure will deform the trunnion tube, resulting in 'sticking' trunnions.

Trunnion Spigots

A quick word on trunnion spigots. The 'switched-on' among you will be asking yourself how reducing the diameter of the trunnion spigots to fit a certain case will make them stronger than the original. Well, we've already covered that the increase in the trunnion tube diameter helps the situation out very well indeed. The fact of the matter is that the actual trunnion spigots only break in exceptional circumstances, and that is normally due to being levered at an angle. If we keep them on-axis so that they are in shear, the force required to shear them will be more than that needed to seriously damage the trunnion tube. In essence, we've just 'beefed' the whole thing up!

Which Spring Bars?

Those that read my ramblings will have realised that I don't endorse something unless I've actually used it myself. The first thing I do when I get a new watch is to check, and more often than not, replace the spring bars with the ones that I trust. The Seiko 'fat' spring bars are my recommendation for 22mm lug widths but if you want something that's really above and beyond, HELM spring bars are the ultimate. The downside is that HELM watches have drilled lugs, and as such, their spring bars are of a slightly different design. This design makes them super-strong, even the springs are noticeably stronger. I have chosen to fit these to the Squale 101 Atmos. 1000m, along with a HELM rubber strap just to be safe. HELM spring bars are available in 20mm and 22mm lug widths and, under normal use, are a 'fit and forget' part.

The Last Word

At the end of the day I'm a 'belt & braces' kind of guy and prefer not to leave anything to chance. Spring bars are incredibly cheap and when you consider what's at stake it really is a no-brainer. I tend to change my spring bars annually as a matter of course, but if I catch my watch on something I'll check, and possibly change them as soon as possible. So there you have it! 1900 words on the humble spring bar; who would have thought it? Better than a sleeping pill and no prescription required!??