Blog / musings...

19^th Aug 2018 – Unsecured collets on very high pressure air pumps I came across Yong Heng compressors last year. Manufactured in China, these very high compression machines generally cost around £250 and provide air shooting enthusiasts with the facility to compress their own air, up to around 300 BAR. I bought one last December because I was having to visit local diving (Scuba) shops way too often to fill my air tanks - which is both expensive and inconvenient. I now use this pump regularly to fill my pre-charged pneumatic (PCP) air rifles whenever I shoot metal plate targets at the club I belong too. They are very useful little tools to have, but they do need a little fettling and care to make work well and safely. I've been using the pump for the last 9 months without any problems. I regularly service it by changing the oil and replacing the filters and O-Rings. I also use an additional molecular sieve filter in order to remove moisture - which over time would otherwise damage the internals of an air rifle. Last week, I noticed that instead of taking around 6 minutes to fill a 0.35 litre container from 200 to 300 BAR, it was now failing to exceed 275 BAR, no matter how long it was left running. I stripped the filters, changing all the cotton tampon elements just in case they were clogged and I also replaced the O-Rings on the molecular sieve just in case there was a leak - however after running the pump for 12 mins I confirmed that it still couldn't exceed 275 BAR. I always use a blower fan to provide extra cooling for the piston cylinder as the pump runs – so after turning that off, I ran the pump into a short hose with a blanking plug... at which point I immediately heard a leak around the burst protection safeguard bolt screwed into the main output manifold (see pic above). After unscrewing that protective device I found that it is made from three parts – the first is the threaded body (left), the 2nd is a thin flat burst-disc fitted inside the body and the 3rd is a brass collet designed to compress the burst disk into place when the body is tightened into the output pressure manifold. The three are shown below – with the burst disk in the middle (it had to be gently punched out in order to remove). In this case, the disc (in the middle of the above pic) had over time deformed into a slightly domed shape. As soon as the pump pressure exceeded around 200 BAR, there was enough force to bypass the deformed disk and vent to the atmosphere. Changing the burst disk should be easy (the manufacturers of the pump ship it with five or so spare). Unscrew the threaded body, remove the brass collet from the head and replace the flat burst-disk. Refitting is the reverse... apart from one really huge gotcha, which unfortunately, I walked straight into...

The collet (arrowed in red) is a relatively lose fit in the end of the main body. In normal use the body of this bolt threads horizontally into the manifold. Can you see what the problem was? Unless the collet is somehow locked into place, then as the body is screwed home there is a risk that the collet will rattle free and drop unseen into the bore of the manifold. At that point if you tighten the head, as I did, you'll crush the collet. Oh dear, what a rookie mistake to have made...

I removed the threaded burst disk protector and realised that not only had I crushed the collet but I’d also damaged the countersunk body of the manifold. So at that point I removed the manifold from the pump completely so I could get proper access to it. A 5mm HSS drill bit in a pillar drill was used to clean the bore of the manifold... removing the bare minimum required to clean the base of the bore. The crushed collet was ruined, completely misshapen, there would be no compressing it back into shape. I didn’t have any suitable brass stock to machine my way out the problem, but I did have some 10mm copper bar – and so started the process of turning a replacement collet.

The newly machined copper collet is 0.2775” in diameter, centre drilled with a 4mm bit and with a front side shoulder cut to 45 degrees leaving a flat of just 0.0255” to meet with the drilled shoulder in the aluminium manifold. The overall height isn’t shown on the paper - but in the end I found 0.12” worked well. I generally tend to use inches rather than metric - simply because I'm more used to working in thousands of an inch of precision. On inspecting the main body – I realised that the base of the threaded end of the body was a little rough – and so turned that to cut off the bare minimum required to clean it up. That resulted in a body length (without collet) of 0.5370”.

With the burst disc fitted into the main threaded body and the new copper collet fitted into the head the end result looked as follows:- After carefully refitting all the hardware – I reverse pressurised the manifold to 280 BAR using my large 3 litre reservoir to check for leaks. Satisfied that the manifold was holding, I then replaced the oil. The pump had only actually run for around 4 hours since the last oil change which should occur on a 10 hour cycle, but as it was accessible and on the bench anyway, it made sense to tackle this at the same time. I then ran a pressurisation cycle up to 300 BAR with just a closed pipe... without any problem. The following day I pressurised my 0.35 litre bottle from 180 BAR to 300 BAR in 7 mins – which is slightly longer than I’d expect – but only by a minute or so. We'll keep an eye on things over the next couple of weeks - but fingers crossed this newly machined collet will do the trick. Comment | Back to Quick Links...