Recommend a grade of steel

My three inch scale traction engine has broken the boiler pump rod between the eccentric and the pump ram and I need to make another. The rod is rectangular section 11mm x 4mm but has two 45 degree opposite bends in to clear the second shaft. The working pressure is 120psi so the rod is subject to a fair bit of stress. Could anyone please recommend a suitable grade of steel?

Hi Rob made mine from bright mild steel still going after 4 years !

H

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

Chris Gunn

Thanks Harry and Chris. No hydraulic lock so I am puzzled as to the cause. I certainly don't want it to recur as it is a major job to replace it. Rear wheel, gear guard, gear change levers (3 speed) 2nd shaft bearings and 2nd shaft itself all have to come off.

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

Thanks Mick. I have some 3/16 x 3 inch mild steel plate so rather than stress the steel by bending I'll mill from the plate.

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.

12mmx 4.5mm gives an increase in cross section area of well over 20%. Any possibility to slightly increase the size?

Could a gusset be added to reduce the bending (I’m guessing it failed on tone of the bends)?

Would surface hardening increase the stiffness/strength? Rounded corners a possible improvement?

The rod failed half an inch from the eccentric strap where the flange is welded to the rod itself. The break was very crytalline in appearance, not like mild steel at all. I suspect that after welding it was quenched too soon and that has caused a hard spot.

The pin attaching the rod to the pump ram also fractured. That pin was very hard and not possible to file so I suspect it was made from silver steel hardened and tempered. I intend to replace that with case hardened mild steel.

came apart in layers when i tried to put a bend in it avoid it it now

" 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!

Nigel, thank you for your response. In my post on 24 September, I described the fracture as being "not like mild steel at all". The fractured area, whilst not dead hard, is not easily filed unlikely to be MS.

The rod broke on a straight length about half an inch from the welded on piece which bolts to the eccentric. As I did not build the engine I have no idead as to the steel used but as I will be making the new rod I want to ensure no repeat. It took me a full day just to remove the second shaft!

The first thing I checked was the ram clearance and there is no way it contacted the end of the barrell as it had a good half inch of further travel.

The pin was hard and fractured at the shoulder of the 2BA thread and 1/4" pin. The tempering colour is still visible on part of the pin.

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

Sounds like a classic fatigue crack caused by shoulder acting as a stress raiser. Best to avoid sharp transitions especially where heat-treatment is applied too: on purpose or accidental tempering is liable to cause micro-fractures, and these are particularly likely to occur at sharp transitions. Parts become much weaker than they should and are very liable to fail when stressed repeatedly.

In this example one side of the groove is chamfered to relieve stress leaving the other side seriously weaker.

So manufacturing defect(s) rather than using the wrong steel.

Watching 'Forged in Fire' on telly two knives made with saw-backs both failed catastrophically at the first tooth when the blade was whacked into a 4" chain: no stress relief at the teeth and a hefty blade about an inch deep broke...

Dave

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.

Clack valve already checked and pipes blown through and no isolation valves present. Thank you for your post though and all help is much appreciated.

The sharp transition from minor to major diameter of the pin will be addressed when I machine up a new one.

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.

Finally bottomed it. I refitted the old broken rod after it had been welded to check alignment etc. The topside of the rod had been contacting the the bottom edge of the horn block in a kind of "stroking" motion hence no warning knocks. The contact point on the block was directly over the fracture point and bears the scars, fortunately not visible when the second shaft is installed.

I will mill a new rod with a tad more clearance and "move" the removed area to the opposite side of the rod where there is ample clearance. This way the rod strength is preserved.

My only problem then will be remembering how it all goes back together!

Following from the above, as I need to replace the pin which articulates the eccentric rod with the pump piston, I am considering replacing it with a case hardened mild steel version. The original was hardened and tempered silver steel and that of course broke.

I am doubtful that I could hold a precise tempering temperature long enough to achieve the correct temper all through. Could I please solicit opinions as to whether case hardened mild steel is suitable?