Rob, the same as Harry above, did you have a hydraulic lock to cause it to break?

Chris Gunn

B&Q do lengths of black mild steel flat bar that's about 11 x 4mm. It machines nicely, but I've not needed to try bending it.

Edited By Mick B1 on 22/09/2021 11:54:56

Have you tried working out the stress? Can't be too difficult, pump diameter, pressure and offset in rod needed. Then you can tell what grade steel you need.

as long as you get it red hot, bending shouldn't be in issue.

" I suspect that after welding it was quenched too soon … caused a hard spot. "

"Quenched"? If both the rod and strap are mild-steel, quenching them won't harden them, but might distort them a bit. Let them cool naturally, which won't take long in that size.

I'm not clear if the weld itself was what broke, but if so, had you used leaded free-cutting mild-steel for the eccentric-strap? Welds in that are prone to fail under load, more so a shock whose forces exceed what should be expected.

If the break was on one of the bends, the bending might have put a stress-raising imprint on the inner face, or over-stretched the outer surface of the bar.

The failure of the pin suggests it was hardened but not tempered, or only very lightly.

As for what loaded everything to breaking-point, I would suggest carefully investigating the clearances to see if the pump ram had struck the end of the barrel.

+++

The steel sold in places like B&Q is not intended for fine engineering but for things like garden fittings and fixtures. I have some angle from that or a similar emporium, and it really is rough old stuff – but to be fair it was sold as plant-stake material!

Try doing a 'spark test' on the old rod.

en.wikipedia.org/wiki/Spark_testing

This might help determine what the original problem was.

Neil

You say there was no hydraulic lock, how can you be sure of this? Have you removed the clack valve to the boiler and checked there is free passage? I have seen several boiler clack valves for pumps "fur" up on the boiler side reducing the area of flow, the worst down to a small hole little bigger than a pin, can be particularly problematic on steel boilers using treatment, especially if you add it to the tender. The reduced area significantly loads the pump. The fact you have effectively 2 failures simultaneously (pin and rod) suggests to me there is something else going on. You suggest the rod failure is about half inch from the weld – that suggests it the point of failure is well on the edge of any HAZ. Lastly if your engine has isolation valves at the boiler for the clack I have seen some clowns close these valves when the driver isn't looking! That never goes well if the bypass is closed, I know many people who keep the handles in the tool box to prevent bystanders "fiddling" with them!

Paul.

Went through a Q & A scenario a few years ago when a mate had managed to wrangle the con rod between boiler feed pump rod and eccentric drive on a Simplex. Hydraulic lock etc. and all points east (and west). Transpired that looking vertically downwards on drive everything in line BUT, horizontally, the pump rod and eccentric centre lines weren't. Worth checking.

Regards Ian.

Only if you divulge the load it is going to see. What is diameter of pump ram and boiler pressure, diameter of pin and length of bearing?

From the comfort of my armchair, I think Silver Steel was the right choice. Some yield strengths from the Model Engineer's Handbook:

• Aluminium, 5 tons per square inch
• 60:40 Brass, 8 tons per square inch
• Malleable Iron, 8 to 10 tons per square inch
• Dural, 15 to 19 tons per square inch
• Free Cutting Mild Steel, 20 tons per square inch
• Mild Steel, 24 tons per square inch
• Stainless Steel, 36 tons per square inch
• High Tensile Steel, 40 tons per square inch
• Silver Steel, 35 to 50 tons per square inch

Bearing pins are worth hardening if they are subject to high wear AND are difficult to replace.

Unhardened Silver Steel is stronger than most common metals and could be used as a pin without heat treating it. Not the best possible, but pretty good compared with common alternatives.

The performance of Silver Steel can be improved by heat treating it, and this is probably where Robert's pin went wrong. Working with small parts and a blowlamp, it's quite easy to botch the heat-treatment. For best results it's necessary to heat the part to the correct temperature, (cherry red in dim light whatever that is), hold that temperature steady for a time related to the weight of the part, quench the part without allowing the it to cool en-route, and then remove the part at about 200°C and put it straight into a pre-heated oven to slowly anneal it.

Plenty of opportunities to get it wrong:

• Steel not hot enough or too hot for the incorrect time. (Too long or too short)
• Steel cools on the way to the quench
• Insufficient quench liquid (bucket rather than cupful)
• Failing to stir the part if the coolant is water
• Cooling too quickly in the quench causes Silver Steel to become brittle and – in the worst case – to crack. Problem here is microcracks, invisible to the eye, which severely weaken the part, especially if it's repeatedly stressed as a pump pin would be.

My inexpert experience of silver steel suggests it's fairly forgiving of mistakes. However, early on I had a high failure rate – either not hard, or super-brittle. Better now. I think the main difference is not flapping about, instead: smoothly heating up to about the right temperature, holding it roughly for the right length of time, not too hot, and then putting the part straight into the quench and stirring vigorously.

How the part is held can be problematic too. Pliers act as a heat-sink, likely to make one end of a short pin much cooler than the other on the way to the quench. Various ways of avoiding that, for example making the pin much longer than required and grinding it to length at the un-held end.

If the part comes out very hard, it needs to be annealed. Small parts are fairly easy – they can held at 200°C for half an hour in a domestic oven. Might be necessary to get a divorce before this is allowed…

My advice, persist with Silver Steel.

Dave

Edited By SillyOldDuffer on 05/10/2021 10:16:38