Fatal Break? :(

[dougrail]

Nicely done Dan, thanks for the technical info. Very informative

James - sort out that engine of yours!! Suggestion: restore the blue cylinder covers and paint the wheels blue to match?

[DolwyddelanLightRail]

Nicely done Dan, thanks for the technical info. Very informative

James - sort out that engine of yours!! Suggestion: restore the blue cylinder covers and paint the wheels blue to match?

Will do when I can be bothered, they need repainting still but I'm planning to get them put back on

and blue wheels? nah, black with white rims!

Oh, and the piston is now back in the cylinder, I've let the glue cure overnight (it said it needed an hour, but those are always wrong) so I'll head back over to York tonight and steam test it on blocks (as I've been told it's going to be windy this afternoon)

[Pretoria]

Physics time !!!
A Mamod running at standard pressure of 14 PSI exerts a force of 6.91 x 10^-5 KGm^2 onto the piston and hence the end cap, in accordance with Newton's third law. At 40 PSI the force is 1.97x10^-4 KGm^2, nearly three times the force of the standard loco. The Dream steam cylinders still use a friction fit for the front and back end caps which have to with stand a huge amount of force on each stroke. The front cap has to resist more pressure, the force being the same but the surface area is smaller due to the piston rod, which is why the front end is usualy blown off. A loco running at 40 PSI has to resist the equivelent of a light hammer blow to each cylinder end cap on every power stroke. In short, stop trying to tow your Edrig and you will be fine. For those that do run with boilers at high pressures, as long as you do not just "dump" the steam into the cylinders but actually regulate the flow no harm will come to your pistons.

Engineering time:

  KGm^2 are wrong on three counts -- (1) it is kg, (2) it is kgm^-2, and far more importantly, kg are units of MASS, not FORCE.  The correct units of force are Newtons, or multiples thereof (eg kN)

Not sure what size the pistons of a Mamod are, but they  look about 3/8" -- which, at 14 psi, gives a force of 1.5 lbf; and at 40 psi = 4.4lbf. But either of these only occurs under full load -- regulator wide open with sufficient load to nearly stall the engine !

An equally (if not more) likely cause is hydraulic locking where water is carried over with the steam (priming) -- if sufficient is carried across to fill the clearance volume (ie the remaing volume in the cylinder after the exhaust port closes), then the remaining compression instead of trying to "squash" steam is trying to squash water -- which it cannot do as it is (to all intents and purposes) incompressible.  The resultant pressure ( x area = force) is a now a dynamic calculation and depends on the piston speed.  At low speed your engine will just lock up.  At higher speeds the forces become huge and you blow the cylinder cover off !  

So, moral (2)  --  warm your cylinders properly before engaging hyper-drive, and don't over-fill your boiler !!

Here endeth the lesson !  

[DLRdan]

Ah yes, I always miss out the minus sign, however I was always told that force could be writen both ways, as N or kgm^-2. Is this not the case ?

[Pretoria]

Ah yes, I always miss out the minus sign, however I was always told that force could be writen both ways, as N or kgm^-2. Is this not the case ?

Not at all ! Mass is a measure of "stuff" -- it is universal. Force is a measure of what work mass can do -- but this depends on gravity. Therefore more gravity = more potential to do work.

Force (Newtons) = Mass (kg) x Gravity (m^s-2 -- which for us = 9.81)

So, 1 kg will give rise to a Force of 9.81 Newtons at sea-level (if you take the same mass to the top of Everest, where gravity is slightly less, and use a spring balance -- it will "weigh" just under 9.81N).

The confusion arises because in the imperial system lbs and lbf are the same (eg psi). PSI is really lbf/inch^2. Things become exciting when you do imperial force calculations because gravity is 32.2 ft/s^2 and you have to keep a very careful eye on whether you are talking about lbs mass or lbs weight, otherwise your answer is 32 times wrong !!

Similarly when you buy food in the shop it "weighs" 250g -- it is our education system that cocks it up and fails to distinguish between mass and weight.

[DolwyddelanLightRail]

and don't over-fill your boiler !!

Well, it was priming to hell cause I was trying to stop the safety valve going off, that probably didn't help

[Pretoria]

and don't over-fill your boiler !!

Well, it was priming to hell cause I was trying to stop the safety valve going off, that probably didn't help

Well I dare to suggest that was the problem rather than boiler pressure (especially having seen the speed that the blue-peril whizzes round at !). And neither loctite or araldite (although I would recommend loctite by far as the better of the two) will cope with the hydraulic pressures that can be generated.

Slide valve cylinders don't suffer the same problem as when the pressure in the cylinder rises above the chest pressure, the slide valve can lift off and vent the water back into the chest.

On the other hand piston valve cylinders (and oscillators) can't. Full size and larger scale models therefore need cylinder relief valves to prevent blowing the covers off (although I seem to remember they managed to do so on Blanche on the FR). If the spring is weak enough on an oscillator it can lift off -- but then it is also likely to leak when running. So you can't have it both ways.

Hope that makes sense.

[DolwyddelanLightRail]

Well, the araldite didn't work Was worth a try anyway! Next option





Where the hell can you get some locktite 603?

[Pretoria]

Where the hell can you get some locktite 603?

Google Loctite 603 -- even Amazon sells it (~£6 for 10ml)

Or have you got a "Tooltastic" (ex Buck & Hickman) centre near you -- you can buy over the counter there.

Remember you need to thoroughly clean both surfaces first -- not ideal, but clean with meths and let it dry thoroughly. Careful with the 603 or you'll glue the piston and piston rod in too !!

[DolwyddelanLightRail]

Thanks Pretoria, thankfully there's a shop in sheffield so I'll pop down there tomorrow or friday!

[DLRdan]

Good news everybody, Ive got a cylinder end cap off too. This wasn't under steam, I was rolling it up and down my desk and the thing came out. I am completely stunned they they are only held in with glue, no interference fit as per what I had thought. I have also found that the bore is smaller than the standard cylinders.

[DolwyddelanLightRail]

I think this thread is going to just see how many people's cylinders fall apart here